Topic 19 Chemicals in the home, home chemistry
Updated: 2008-07-17
Please send comments to: J.Elfick@uq.edu.au
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Table of contents
19.1.0 Household chemicals, chemical reactions in the home
19.2.0 Composition of food
19.3.0 Cooking
19.3.6 Food preservation (File name: topic19a.html)
19.4.0 Food chemistry
19.4.1 Checklist of chemicals in the home, household chemicals
19.5.0 Fabrics in the home
19.6.0 Hardware, laundry, painting, cleaning, preserving
19.7.0 Beauty and skin care products
19.8.0 Common measures
9.228 Body Mass Index (BMI)
4.2.11 Glycemic index (GI), GI value and GI load
4.2.0 Fermentation processes in food production
19.1.0 Chemical reactions in the home
19.1.0.1 Sequestrants
19.1.0.2 Stabilizers and thickeners
19.1.0.3 Emulsifying (surface active) agents
19.1.0.4 Polyhydric alcohols
4.38 Calorific value of fuel
19.1.0.5 Water retention agents
19.1.0.6 Acidulated water
19.4.1 Checklist of chemicals in the home
19.4.2 Kitchen hints
19.1.1 Solid acids, solubility
19.1.2 Solid acids, pH
19.1.3 Solid acids, add sodium carbonate
19.1.4 Taste of acids, taste
19.1.5 Acid-base indicators
2.35 Carbon dioxide in the home
19.1.6 Baking powder
19.1.7 Prepare carbon dioxide with sour milk and baking soda, sodium hydrogen carbonate
19.1.8 Prepare self-leavened flour, "self-raising flour"
19.1.8.1 Plain flour and self-raising flour
19.1.9 Prepare a chemical sponging agent, baking powder
19.1.16 Table salt and rock salt
19.1.17 Cooking fats
11.3.3 Triple scale wine hydrometer
6.6.18 Alcoholic fermentation
19.1.20 Dipsticks to test the vitamin C, ascorbic acid, content of food
19.1.20.1 Tests for metallic copper
19.1.20.4 Tests for glucose
19.1.20.5 Multiple reagent strip
19.1.20.6 Tests for nitrate / nitrite with dipsticks
19.1.22.7 Tests for sulfite
19.1.20.8 Tests for tartaric acid
19.1.20.9 Tests for adulteration of food by borax with turmeric paper
19.1.20.12 Tests for urine
19.1.20.13 Tests for water
19.2.0 Composition of food
19.2.0.1 Colloids
7.8.0 Colloids, sols, emulsions, gels, aerosols, foams, types of colloids
7.8.3.2.1 Gels in the home kitchen
9.127 Elements in foods
19.4.2.2 Food allergies and intolerance (File name: topic19a.html)
19.2.1.0 Fats in our food
19.2.1.1 Fats in animals and plants
19.2.1.1.1 The cis and trans forms of linoleic acid
19.2.1.2 Classification of fats
19.2.1.3 Hydrogenation, cis - trans fatty acids
19.2.1.4 Rancidity
19.2.1.5 Heating fats
19.2.1.6 Antioxidants, antioxidant phenols, vitamin E, beta-carotene
19.2.1.7 Cholesterol
19.2.1.8 Omega-3 fatty acids
19.2.1.9 Free radicals
19.2.1.10 Margarine
19.4.3 Margarine label
19.2.1.11 Coconut oil
19.2.1.12 Fish oils
19.2.1.13 Ice-cream
19.1.1.14 Chocolate
16.3.3.1 Waxes
19.2.9 Pectin in jelly and jam
19.2.9.1 Gelatine in jelly with fresh or tinned pineapple
19.2.10 Egg white, albumen, and egg yolk
19.2.10.2 Eggs in a cake mix
19.2.11 Yeast, fermentation, brewing, whisky, fish sauce
19.2.12 Salad dressing and mayonnaise emulsions
19.2.13 Fruit salts
19.2.14 Food colouring liquids and detergent
J1. Prepare yoghurt and sauerkraut (for primary grade 4 students, about 9 years old)
J2. Prepare sauerkraut  (for primary grade 4 students, about 9 years old)
19.2.15 Heat starch, glycemic index
19.2.17 Glycoalkaloids, avoid bruised or green potatoes
19.2.18 Extract iron, Fe, from breakfast cereal
19.2.21 Fish smell
19.2.22 Laundry starch
19.2.22.1 Wheat starch and gluten
19.2.23 Milk
19.2.24 Butter
19.2.26 Custard
19.2.27 Garlic
19.4.2.3 Caffeine, extraction with supercritical carbon dioxide
19.2.28 Tests for the harmful substances in cigarette smoke
19.2.29 Toxic effect of common drugs on Daphnia
19.2.30 Test chewing gum by comparing bubbles
10.5.5 Steam distillation to find water and fat content of food
19.3.0 Cooking
19.3.1 Taste, smell, flavour
19.3.2 Anatomy and physiology of meat
19.3.3 Boiling, test the cooking water of boiled vegetables.
19.3.3.1 Mashed potato, pommes purée
19.3.4 Baking and retention of nutrients
19.3.4.1 Tests for dextrins in toast
19.3.4.2 Browning reactions of fruits and vegetables
19.2.20 The effect of lemon juice on the browning of an apple
19.3.4.3 Non-enzymatic browning, caramelization
19.3.4.4 Non-enzymatic browning, the Maillard reaction
19.3.4.5 Roasting meat
19.3.4.6 Meat treatments, marinades, salting meat, marbled beef
19.3.5 Microwave cooking
19.1.0 Household chemicals, chemicals in the home
Acids are used give tartness to foods, or to alter the acidity of the medium, i.e. to lower the pH in canned products, to prevent the crystallization of jams and jellies. Bases are used as ingredients of baking powders used in pastry production, and in powders for effervescent beverages.
Improving agents includes chemical compounds that enhance the quality criteria of foods, e.g. flavour and consistency, and substances used for polishing and glazing confectionery products.
19.1.0.1 Sequestrants, sequestering agents, either removes an ion or makes it ineffective by forming a complex with it, e.g. a chelate complex. An example is the sequestration of Ca2+ ions in water softening. A sequestrant binds with metal ions to prevent them from catalysing chemical reactions that spoil preserved food. Metals such as copper, iron and nickel get into food from processing machinery or because of chemical reactions with the container. The sequestrant citric acid acts as a synergist (increases the effect) for antioxidants. Sequestrants are used in shortenings, mayonnaise, lard, margarine, cheese.
19.1.0.2 Stabilizers and thickeners are added to improve the texture and blends of foods, e.g. carrageenan (E407, from seaweed used in icings, frozen desserts, salad dressing, whipped cream, confectionery, and cheeses.
19.1.0.3 Emulsifying (surface active) agents, "food soaps" (E433-444) are used to stabilize emulsions of oil and water components in foods.
19.1.0.4 Polyhydric alcohols are used as humectants to keep from drying. they may also be sweet, e.g. sugarless chewing gum may contain mannitol (E421), sorbitol (E420), and glycerol (E422, and have the same calorific value as cane sugar, 16.5 kj / g.
19.1.0.5 Water retention agents, e.g. polyphosphates (E450-452), are used in processing poultry, fish and mammalian meats to bind water and minimize "drip". Phosphates are also used in soft drinks. However, excessive intake of phosphates from processed food may harm bone growth in children.
19.1.0.6 Acidulated water is water that has been made slightly acidic by the addition of an acid substance such as lemon juice or vinegar (about one teaspoon to half a litre of water). Peeled fruit and vegetables such as apples, pears, celeriac, globe artichokes and salsify are immersed in acidulated water to prevent them from discolouring. It can also be used for cooking. Cauliflower, for instance, will be snowy white if boiled in acidulated water.
19.1.1 Solid acids, solubility
See Appendix: Citric acid | See appendix: (+)tartaric acid | See appendix: Trioxyboric (III) acid (boric acid)
Shake different solid acids in separate test-tubes half filled with water. Which of the acids are the most soluble and the least soluble in water?
19.1.2 Solid acids, pH
See appendix: Methyl orange | See appendix: Phenolphthalein
Divide the solution in one test-tube into three portions in three different test-tubes. Test the first solution with litmus paper. Add drops of methyl orange solution to the second solution. Add drops of phenolphthalein solution to the third solution.
19.1.3 Solid acids, add sodium carbonate
See appendix: Sodium carbonate | See diagram: 3.34.9: Limewater test for carbon dioxide
Add a little solid sodium carbonate to a sample of each acid solution. Note what happens in each case. Pass gases from the reaction through limewater. Shake the test-tube so that the gas mixes with the limewater. The milky precipitate shows that carbon dioxide forms when acids react with sodium carbonate.
19.1.4 Solid acids, taste
See appendix: Citric acid | See appendix: Acetic acid | See appendix: Cream of tartar
Do NOT taste these acids in the laboratory. Each acid has a sour taste that is a characteristic of acids.
Note the taste of: lemon juice contains the white crystalline citric acid. Vinegar contains ethanoic acid (acetic acid, CH3COOH), Cream of tartar contains the acid salt potassium hydrogen tartrate, the purified form of argol that occurs as brown crystals in fermenting wine.
19.1.5 Acid-base indicators
See 3.53 pH and acid-base indicators, acidity and alkalinity | See appendix: Sodium hydrogen carbonate, baking soda
Use grape juice or red cabbage juice for acid-base indicators. Note the sour tastes of fruit and vinegar and the taste of baking soda. Grape juice turns red in acid lemonade and blue in alkaline dishwater. Use flower pigments as pH indicators.
19.1.6 Baking powder
See also: 13.7.7: Prepare carbon dioxide by heating hydrogen carbonates | See appendix: Cream of tartar
Formerly, bakers added sodium bicarbonate and sour milk, lactic acid, to bread dough to make bread rise as carbon dioxide gas bubbles formed during baking. Later dry cream of tartar from the wine industry was substituted for sour milk to make a dry mixture. Later, calcium acid phosphate. CaHPO4, was substituted for cream of tartar. Nowadays corn starch or rice flour is added to keep the mixture dry. Baking powder is a mixture of sodium bicarbonate with cream of tartar, tartaric acid, acid phosphate or sodium aluminium phosphate or any combination of these without any farinaceous (wheat) substance, so it can be labelled "gluten free". It must yield >10% of carbon dioxide and may contain permitted colouring substance. Baking powder contains compounds called food aerators, to be added to dough to make it rise during cooking as bubbles of carbon dioxide gas form. Baking powder can be used as a substitute for yeast which is used in sour dough.
Baking powder contains:
1. A leavening agent as a sources of carbon dioxide (dry solids): baking soda (sodium bicarbonate, sodium hydrogen carbonate, NaHCO3) or ammonium hydrogen carbonate,
2. Acidic substances to form acids when water is added: 1. cream of tartar (potassium hydrogen tartrate, acid tartrate) 2. phosphates to replace cream of tartar
(b1) Acid phosphates, e.g. calcium hydrogen phosphate (calcium acid phosphate, CaHPO4), sodium dihydrogen phosphate V (sodium dihydrogen orthophosphate, sodium orthophosphate NaH2PO4.2H2O)
(b2) Phosphate aerators, e.g. food additive E450 Diphosphates (Sodium and potassium phosphates), food additive E541 Sodium aluminium phosphate, basic (emulsifier, acidity regulator) 3. Rice flour or corn flour to keep the mixture dry.
If baking powder contains 36% phosphate aerators it could contain about 10% aluminium. Baking powder must be stored dry. Mix the leavening agent baking soda, sodium bicarbonate, with acidic ingredients to make it work in cooking. However, baking powder contains baking soda and a powdered acid, so it can work without other acidic ingredients.
19.1.7 Prepare carbon dioxide with sour milk and baking soda, sodium hydrogen carbonate
See appendix: Sodium hydrogen carbonate, baking soda
Add acid buttermilk or soured unpasteurized milk or vinegar or fruit juice to sodium hydrogen carbonate. The reaction forms carbon dioxide.
19.1.8 Prepare self-leavened flour, "self-raising flour"
See appendix: Sodium dihydrogen phosphate V | See appendix: Potassium hydrogen tartrate | See appendix: Aluminium potassium sulfate (potassium alum)
Use self-leavened flour to make steamed bread. Mix the flour with water without addition of any baking soda. Knead the dough and let it stand for 10 to 15 minutes. This kind of flour is made by blending a small quantity of chemical sponging agent, also called baking powder, with ordinary flour. The sponging agent contains 20- 40% of sodium hydrogen carbonate, and 35-50% of acidic substances such as sodium dihydrogen phosphate, potassium hydrogen tartrate and aluminium potassium sulfate (potassium alum, Al2(SO4)3.K2(SO4).24H2O), and filling agents such as starch and aliphatic acids. Sodium hydrogen carbonate reacts with acidic substances to produce carbon dioxide, while the acidic substances decompose the carbonate to lower the basicity of finished products. The filling agents are used to prevent the flour from moisture absorption, agglomeration and loss of effects. They can also regulate the forming rate of gas or make the bubbles be evenly produced. When water is added to self-leavened flour, the hydrolysis of sodium hydrogen carbonate shows basicity, while hydrolysis of sodium dihydrogen phosphate shows acidity. The reaction results in release of carbon dioxide. Heating decomposes sodium hydrogen carbonate to make spongy steamed bread.
19.1.8.1 Plain flour and self-raising flour
See appendix: Baking soda
"Plain flour" is "wheat flour" made from the endosperm "kernels" of wheat grains by grinding and sifting. "Self-raising flour" contains plain flour and baking soda, sodium hydrogen carbonate.
In the kitchen, to test whether flour is plain flour or self-raising flour, place a little on your tongue. If you feel a tingle, this indicates that the flour is self-raising flour.
19.1.9 Prepare a chemical sponging agent, baking powder
See appendix: Baking powder
To prepare 10 g of chemical sponging agent, weigh 3 g of sodium hydrogen carbonate, 2 g of starch and 0.7 g of calcium phosphate. Mix them with 5.3 g of sodium dihydrogen phosphate in a small beaker. Weigh commercial flour and the prepared sponging agent in the ratio of 50 to 1, and mix them thoroughly to make 20 g of self-leavened flour. Add 15 mL water to the prepared self-leavened flour and knead the dough. Lay aside the dough for 5-10 minutes (leaven dough) and then make the dough into spongy, delicious steamed bread by steaming for 15-20 minutes.
19.1.16 Table salt and rock salt
1. Common salt is sodium chloride crystals. Common salt, rock salt, comes from the naturally occurring mineral of sodium chloride called halite. It is often found as cubic crystals and associated with gypsum in Triassic rocks. Sea salt is extracted from evaporated sea water. Table salt may be made from rock salt or naturally evaporated sea salt and contain iodine (iodized salt), anti-caking agents, e.g. anti-caking agent (554) and potassium iodate. Kosher salt is a coarse salt with large crystals used for drawing blood from meat. Pickling salt is a fine grained salt used for pickling and it contains no additives, e.g. anti-caking agents. Grey sea salt, "Sel gris", is unprocessed, and has minerals from the sea. Indian black salt, kala namak, has a brown black in colour and a smoky, sulfur flavour. Rock salt is a grey colour, contains minerals and impurities, and is used in ice-cream machines and for melting ice and snow on the roads using brine and sea sand.
2. Table salt may be "iodized" by the addition of potassium iodide or potassium iodate. About 0.01% potassium iodide in table salt is added as a nutrient for the thyroid hormone thyroxine for those on an iodine deficient diet, which leads to goitre. However, the iodide will oxidize in air to iodine that is lost through evaporation. Thiosulfate was formerly used as a stabilizer but now it is normally glucose, dextrose. Because alkaline conditions prevent oxidation, bases such as sodium bicarbonate or phosphates may also be added. Potassium iodate may be used to avoid these problems.
3. Atmospheric moisture may cause the cubic crystals of sodium chloride to stick together and the salt does not flow. This problem can be solved by using about 0.5% drying agents, e.g. carbonates (E501-4), and sodium aluminium silicate (E554). Another method is to change the shape (habit) of the cubic salt crystals to a form that does not provide large flat surfaces to pack together. Salt normally crystallizes as cubes because the octahedral faces of the crystal consisting of either all Na+ or all Cl-  grow faster than the cubic faces with alternating Na+ and Cl-. If an impurity is absorbed onto the surface of the fast growing octahedral faces, e.g. urea, the reverse happens, and octahedral crystals are formed instead of cubes. So more than 13 ppm potassium ferrocyanide K4Fe(CN)6.3H2O) (E536) is added to table salt. This compound is quite safe, however, to avoid using the word "cyanide" on labels, the compound may be referred to as "yellow prussiate of potash" or the IUPAC name "hexacyanoferrate".
4. Sprinkling salt on water causes the surface to contract momentarily towards the crystals, while with pepper, the opposite tends to happen.
1. Examine the label on a contained of table salt and note the contents in addition to sodium chloride.
2. Make a freezing mixture and measure its temperature. A mixture of ice and sodium chloride, freezing mixture, has temperature -20oC. The salt lowers the melting point of ice. The salted ice is still at 0oC but above its new melting point so it melts.
19.1.17 Cooking fats
Shortening is solid, white fat made from hydrogenated vegetable oil. Copha is a solid fat derived from coconuts and is quite saturated. Lard is the rendered fat from pig abdomen. Deep frying requires fats / oils with heat tolerant properties, e.g. corn and peanut oils but not butter, margarine, lard and olive oil.
19.1.20 Dipsticks to test the vitamin C, ascorbic acid, content of food
1. Use dipsticks to measure vitamin C content in fruit juices. You may find more than in the original fruit because the processor adds the minimum to replace any vitamin naturally present that has not survived processing and storage. 2. Test the effect of boiling vitamin C in water 3. Test the effect of cooking at different pH values by adding sodium carbonate. of soda. 4. Test the effect of boiling in the absence of oxygen. If blend vegetables and measure vitamin C content before and after, you will find a large increase because the boiling extracts the soluble vitamin from the food. (e) Test your urine and establish how much you excrete after taking a dose (1 -2 g) over a period of one day? Measure the volume of urine and the concentration of vitamin C. Plot the amount of the original vitamin remaining, and the rate of excretion during the day
19.1.20.1 Tests for metallic copper 1. Copper in water 2. Copper in ice confectionery Sensitivity: 10-5000 mg / L 3. Legal limits for food (mg / kg).
19.1.20.4 Test for glucose
This reaction is not given by fructose or galactose nor by the non-reducing disaccharides, sucrose and lactose, but maltose does react. 1. Tests for hydrolysis of sucrose to glucose (invertase or H+). 2. Tests for formation of glucose in germinating seeds (about 1 minute for halved barley grains against wetting) 3. Tests for glucose in urine. Benedict's test, which is commonly used, detects only the total reducing substance and does not predict the amount of glucose present. Compared to Benedict's test, which detects the total sugar present in urine, the strip test detects semi-quantitatively the amount of glucose present in urine.
Completely immerse the reagent area of the strip in fresh urine for 1-2 seconds and remove. Gently tap the edge of the strip against the side of the urine container to remove excess urine. Compare the test area closely with a colour chart exactly 30 seconds after dipping the strip in the urine. Hold the strip close to the colour chart and match carefully.
The Clinistix strip is impregnated with the enzymes glucose oxidase and peroxides, and the indicator substance O-toluidine. The O-toluidine is oxidized to a blue green substance (Schiff's base) with varying shades of colour, which is then compared with the standard chart provided in the kit to report the approximate level of glucose present in the urine.
The Diastix strip has an area impregnated with the above enzymes together with potassium iodide and a blue background dye. The oxygen liberated in the final reaction binds with the dye to produce a series of colour changes 30 seconds after wetting the strip with urine.
19.1.20.5 Multiple reagent strip
It is a firm plastic strip to which are affixed several separate reagent areas. Sugar, albumin, urobilinogen and bilirubin are the four biochemical substances tested in a random urine sample. Although the heat and acetic acid test detects the presence of proteins such as albumin, only a semiquantitative test will be really useful.
Glucose: It makes use of the same principle as described above for the Diastix strip. The final colour ranging from green to brown.
Bilirubin: It is based on the coupling of bilirubin with diazotized dichloronaniline in a strongly acid medium. The colour ranges through various shades of tan.
Ketone: It is based on Rothera's reaction principle and on the development of colours, ranging from buff pink for a negative reading to purple when acetoacetate reacts with nitroprusside. It also detects acetone but not beta-hydroxybutyrate.
Specific gravity: In the presence of an indicator the polyelectrolytes present in urine give colours ranging from deep blue green in urine of low ionic concentration through green to yellow green in urine of increasing ionic concentration.
pH: This test is based on the double indicator principle that gives a broad range of colours covering the entire urinary pH range. Colours range from orange through yellow and green to blue.
Proteins: The test area of the reagent strip is impregnated with an indicator, tetrabromophenol blue, buffered to pH 3.0. At this pH it is yellow in the absence of protein. Protein forms a complex with the dye turning the colour of the dye to green or bluish green. The colour is compared with the colour chart provided, which indicates the approximate protein concentration. It is based on the protein error of the pH indicator. At a constant pH, the presence of protein leads to the development of any green colour. Colours range from yellow for "negative" through yellow green and green to green blue for "positive" reactions.
Uroblilinogen: This test is based on a modified Ehrlich reaction, in which p-dimethyl amino benzaldehyde in conjunction with a colour enhancer reacts with urobilinogen in a strongly acid medium to produce a pink red colour.
19.1.20.6 Test for nitrate / nitrite with dipsticks
Sensitivity for nitrate = 10-500 mg / L. Sensitivity for nitrite = 1 -50 mg / L. Interference from nitrite removed by adding aminosulfonic acid so separate nitrite strip not needed. 1. Tests for oxides of nitrogen in air. Sensitivity 1 mL of NO2 / m3 of air. 2. Tests for nitrite in saliva, average 7 mg / L, except after foods with high nitrate level, e.g. celery, beets, where you obtain elevated levels for 24 hours. 3. Tests for nitrate / nitrite in fermented raw meat, e.g. salami, legal limit 500 mg / kg, nitrate; Cured meat (corned beef), legal limit 125 mg / kg, nitrite; Canned ham, legal limit 50 mg / kg, nitrite. 4. Tests for nitrite in vegetable, e.g. Conventional carrots 40-100 mg / kg; Organically grown carrots 200-400 mg / kg; Fresh spinach 5 mg / kg, if refrigerated for two weeks 300 mg / kg. (e) Tests for denitrification in waterlogged soils, soil + nitrate + glucose --->N20, Sensitivity: nitrate 10-500 mg / L nitrite 1-50 mg / L
Nitrates and nitrites (E249-253) occur naturally in many vegetables. Additional nitrite can be derived from nitrate by bacterial activity in the gut.
19.1.20.8 Test for tartaric acid
Grape juice and wine (added acetic acid ensures total tartrate is measured) Less than 1 g / L indicates very poor quality. Sensitivity: 0.5-10 g / L
19.1.20.9 Test for adulteration of food by borax with turmeric paper
19.1.20.12 Tests for urine: dipsticks Reagent dipsticks ("Dip-stix") can be used to test for the following chemicals in a fresh urine sample: blood, protein, glucose, ketones, nitrite, N-acetyl-B-glucosaminidase, bilirubin, robilinogen
19.1.20.13 Tests for water: pH, free chlorine and total chlorine, chlorine / chloramine, ammonia (NH3 / NH4+), nitrite and nitrate, oxygen.
19.1.22.7 Tests for sulfite
1. Tests for air pollution - Sensitivity: 5 mL (13 Mg) of SO2/ m3 air 2. Tests for sulfite preservatives - Legal limits: Fruit juices 115 mg / L; concentrated 600 mg / kg, Gelatine 1000 mg / kg, Dehydrated carrots 1000 mg / kg, Cheese 300 mg / kg, Sausages 500 mg / kg, Wine 300 mg / kg, - Sensitivity: 10-500 mg / L.
19.2.0.1 Colloids
Most foods and their components of lipids, proteins and carbohydrates are colloidal, e.g. milk consists of fat particles dispersed in water. Margarine is an emulsion of water, flavours, colours, and vitamins in a semi-solid fat; mayonnaise is oil, vinegar and egg yolk. Blood, enzymes, muscle tissue, bone skin and hair all involve colloids. Lotions, creams and ointments are mostly emulsions of oils dispersed in water or vice versa. Emulsions are one form of colloids. Other examples of colloids are paints, rubbers, oils, pigments, plastics, gels, starches, air pollution and clouds.
19.2.1.0 Fats in our food
See diagram 19.2.1: Fats in our food
Fats, oils and some waxes are the naturally occurring esters of long, straight chain carboxylic acids. These esters are the materials from which soaps are made. At room temperature, fats are solid or semi-solid and oils are liquids alcohol + organic acid ---> ester + water glycerol + fatty acid ---> fats or oils + water All fats are formed from glycerol, glycerine, propan-1,2,3-triol, CH2OHCHOHCH2OH. The fatty acid part of the fat differs 1. in the length of the chain, which controls the molecular mass, and 2. the number and position of the double bonds, unsaturation. The 3 main groups of fatty acids are 1. saturated fatty acids, e.g. stearic acid 2. the straight chain unsaturated fatty acids, e.g. oleic acid, and 3. the polyunsaturated fatty acids, e.g. linoleic acid. The normal saturated fatty acids have the general formula CH3(CH2)nCOOH, where n is usually an even number from 2 to 24, e.g. stearic acid (n=16), lauric acid (n=10). Milk contains short chain fatty acids, n < 10. The building block for fatty acids is the acetate ion, CH3COO-. The most important unsaturated fatty acids have 18 carbon atoms with one double bond in the middle of the chain, called mono-unsaturated fatty acids. Polyunsaturated fatty acids have more double bonds between the middle double bond and the carboxyl group, COOH. Atoms can rotate about single bonds but not about double bonds, so two arrangements are possible called "cis" and "trans". Most double bonds in natural fats and oils are cis, e.g. oleic acid in olive oil. Fatty acids with the cis double bond do not pack together easily so have a low melting point of double bond containing material (i.e. oils). Substances made up of shorter chains also melt at lower temperatures. Chemists describe polyunsaturaed fatty acids as having more than one cis-methylene interrupted double bond.
19.2.1.1 Fats in animals and plants
Fats and oils are used to store, transport and utilize the fatty acids that an organism requires for its metabolic processes. Energy storage in animals: fat 38 kj /g, carbohydrates 17 kj /g, protein 23 kj /g. Fats store water and when metabolized in the body to produce energy, they also produce water, e.g. fatty hump of the camel. Plants, fungi, yeasts and bacteria, can synthesize both fats and their component fatty acids. Animals can synthesize most of their fatty acid needs, but they prefer to ingest plant foods and modify them to their own needs. Only plants can synthesize linoleic and linolenic acids, but animals can increase the chain length and further increase unsaturation, e.g. fish oils, that are rich in unsaturated acids.
Saturated fatty acids are predominantly present in fats which are solid at room temperature, for example, milk, butter and animal fats. Saturated fats may raise the level of "bad" cholesterol leading to hardening of the arteries, high blood pressure, heart disease and strokes. Animals produce mainly saturated fats because their fats also have a structural support function and must not be too fluid. Some animals can maintain a high temperature through internal heating, insulation and behaviour. Unsaturated fatty acids may be mono-unsaturated or polyunsaturated. Mono-unsaturated fatty acids, e.g. oleic acid, are found in most animal and plant fats and oils, especially olive oil. Unsaturated fatty acids occur mainly in oils. Most fats and oils contain a mixture of saturated and unsaturated fatty acids but in widely varying proportions. An intake of fat in the diet is essential as some fatty acids are required for important functions in the body. Fat soluble vitamins A, D, E and K must also be provided by food containing fat. A fat free diet is not only difficult to prepare but is also very unpalatable.
The so-called "bad cholesterol" is the LDL (low density lipoprotein) cholesterol used to build body cells but excess can form plaque on the walls of arteries to the heart and brain causing atherosclerosis. The so-called "good cholesterol" is HDL (high density lipoprotein) cholesterol produced in the liver and intestines that removes excess cholesterol from atherosclerosis plaques and my protect from heart attack.
Polyunsaturated fatty acids, e.g. linoleic acid, linolenic acid, are found mainly in vegetable oils. Polyunsaturated fats are essential to animals as building blocks and for controlling the cholesterol content of the blood. Plants produce mainly unsaturated oils which allow them to withstand extremes of temperature because their fats or oils are fluid at low temperatures. Polyunsaturated fats lower "bad" cholesterol but also lower "good" cholesterol. Polyunsaturated fats are found in margarine, vegetable oils and seed oils. Some research claims that polyunsaturated fats may be are oxidized into "free radicals" which contribute to the development of some cancers and accelerate ageing.
Mono-unsaturated fats are the "good" fats and should make up most of the fats in a diet, up to about 30% of a diet.
Saturated fat in the diet can raise the level of blood cholesterol to increase the risk of heart disease from atherosclerosis, fatty plaques on the walls of blood vessels.
Unsaturated fat can form free radicals by lipid peroxidation, leading to cancer and accelerated ageing. So both saturated and unsaturated fat can have health hazards!
Oleic acid has been found to increase good cholesterol and lower bad cholesterol. Proponents of olive oil claim that in countries where oleic acid is the principle fat in the diet the people have the lowest incidence of heart disease and strokes and the longest life span and that only olive oil is high in mono-unsaturated fats and low in both polyunsaturated and saturated fats. However,  it seems that people living in different countries where the components of fats in their diets are almost identical may have very different rates of the incidence of cancer, so perhaps other factors are involved.
19.2.1.1.1 The cis and trans forms of linoleic acid
See diagram 19.2.1: Linoleic acid
In the cis configuration, the four hydrogen atoms adjacent to the double bonds occur on the same side of the carbon axis. In the trans configuration, the four hydrogen atoms adjacent to the double bonds occur on alternate sides of the main carbon axis with two on one side and two on the other. The more stable trans configuration may be produced from the cis configuration during partial hydrogenation of polyunsaturated vegetable oils to improve their texture. However, trans fatty acids tend to raise the level of low density lipoproteins (bad LDLs) and lower the level of high density lipoproteins (good HDLs) resulting in changes in cholesterol levels that may increase the risk of the heart disease atherosclerosis. So mono-unsaturated, unhydrogenated oils, e.g. olive oil, are preferable to the trans fatty acids in french fries, chips, and donuts. The first double bond is on carbon #6, counting from left to right so this is an omega-6 fatty acid, typical of the unsaturated fatty acids in plant oils and seeds. However, fish oils contain omega-3 fatty acids, i.e. the first double bond in on carbon #3.
19.2.1.2 Classification of fats
1. Saponification value from hydrolysis of a fats into component fatty acids, as their anions or soaps, and glycerol. Saponification value = number of milligrams of potassium hydroxide to saponify one gram of fat (or oil). It is a measure of the average chain length, molecular mass of the fatty acids. Fat and saponification value: Coconut oil 250-260, Butter 245-255, Lard (pig fat) 193-200, Peanut oil, 185-195, Linseed oil, 189-196.
2. Iodine value measures the number of double bonds in the fat. Iodine reacts with the double bond. Iodine value is the number of grams of iodine that react with 100 g of fat or oil. Fats with low iodine values are saturated. Fats with high iodine values are polyunsaturated. Fat and iodine value: Coconut oil 8-10, Butter 26-45, Lard 46-66, Peanut oil 83-98, Linseed oil 170-204.
3. Acid value measures how much glycerides in the fat or oil have been decomposed to free acid. This regulated by food standards codes.
4. Peroxide value measures the oxygen taken up by the oil to form peroxides and is a measure of freshness of the oil. This regulated by food standards codes.
(e) Oxygen uptake. If polyunsaturated fats are incubated at 60oC, they gain weight from oxygen uptake.
19.2.1.3 Hydrogenation, cis - trans fatty acids
3[CH2O(CO)(CH2)7CH==CH(CH2)7CH3] + 3H2 --->3[CH2O(CO)(CH2)16CH]
glyceryl trioleate + hydrogen (nickel catalyst) + heat ---> glyceryl tristearate
Hydrogenation means to add hydrogen to a molecule. Unsaturated fats can be saturated by adding hydrogen to the double bonds with a nickel catalyst. Hydrogenation converts a substance with the properties of a liquid vegetable oil into a substance with the properties of a solid animal fat, e.g. linoleic and oleic acids turn into stearic acid. Margarine is made from pure vegetable oils but the manufacturing process may cause some hydrogenation of unsaturated fatty acids. Processed oils such as shortenings may contain a high proportion of fats changed by hydrogenation. In nature, most unsaturated fatty acids are cis fatty acids, i.e. the hydrogen atoms are on the same side of the double carbon bond. In trans fatty acids the two hydrogen atoms are on opposite sides of the double bond. Trans double bonds can occur in nature as the result of fermentation in grazing animals so people eat them in the form of meat and dairy products. Trans double bonds are also formed during the hydrogenation of vegetable or fish oils, e.g. French fries (fried potato chips), donuts, and other snack foods are high in trans fatty acids. Manufacturers may hydrogenate polyunsaturated oils to help foods to stay fresh or to obtain a solid fat product, e.g. margarine. Trans fatty acids, i.e. hydrogenated fats, tend to raise total blood cholesterol levels, and raise LDL bad cholesterol and lower HDL good cholesterol.
In some countries, governments have required fast-food companies to commit to reducing trans fats in their cooking and listing trans fat content on labelling. Some companies have claimed that consumers do not like the taste of products if all trans fats are eliminated. However, apparently, if only a small proportion of trans fats are used, taste is not a problem. In other countries, trans fats in cooking have been banned altogether by legislation.
19.2.1.4 Rancidity
Oxygen in the air oxidizes unsaturated fats adjacent to the double bond to produce smaller easily evaporated volatile compounds with a rancid smell. Most of the fatty acids in butter are C16 -C18 but shorter chain fatty acids are also present. The acid from rancid butter is 1,3-butadiene: CH2=CH-CH=CH2, a butane with two double bonds, bivinyl. butyric acid: C3H7COOH. Cheeses made from milk with more short chain fatty acids have a stronger smell. Margarine rarely becomes rancid because the longer chain fatty acids must first be broken before the short chain, rancid smelling compounds form. Commercial fats and oils have added antioxidants to prevent rancid compounds from forming. The same short chain acids in rancid butter are present in human perspiration
19.2.1.5 Heating fats
Smoke point is the temperature at which a fat breaks down into visible gaseous products and thin wisps of bluish smoke begin to rise from the surface. Smoke point, smoking point, falls with the continued use for cooking because the oil or fat decomposes and the free fatty acids have a lower smoke point. So the higher the initial smoke point, the longer the fat is usable before it starts to smoke. Smoke point of an oil or fat is an important piece of information for consumers and should be listed on food labels. Flash point is the higher temperature when bursts of flame start. Ignition temperature, is the higher temperature at which the entire surface of the frying medium becomes covered with flame. P/S ratio is the ratio of polyunsaturated fatty acids to the saturated fatty acids present. Although heating may not change the P/S ratio of polyunsaturated oils, it causes the formation of oxidized compounds, which tend to destroy the vitamin E content and make oils unpalatable. Changes in the peroxide value of oils after heating reveal how heating oxidizes oils. Olive oil is mainly mono-unsaturated oleic acid is the most stable cooking oil because it also contains a steroid stabilizer. So it needs no refining, preservatives or refrigeration. 1. Safflower oil: Approx. smoke point: 246oC Approx. P/S ratio: 6.0 2. Sunflower oil: Approx. smoke point: 229oC Approx. P/S ratio: 4.7 3. Maize oil: Approx. smoke point: 229oC Approx. P/S ratio: 3.1 4. Peanut oil: Approx. smoke point: 246oC Approx. P/S ratio: 1.9 (e) Soybean oil: Approx. smoke point: 256oC Approx. P/S ratio: 3.7 (f) Olive oil: Approx. smoke point: 204oC Approx. P/S ratio: 0.5
19.2.1.6 Antioxidant phenols, antioxidants, vitamin E, beta-carotene
See diagram 19.2.1.6: Antioxidants, BHT, BHA, TBHQ, Propyl gallate, Vitamin E
Antioxidants are preservatives for fatty products and oils that are themselves oxidized instead of the added substance. Antioxidants inhibits oxidation or reaction with oxygen. They are soluble in oil and cheap to produce. They prevent the occurrence of oxidation, i.e. rancidity. Vitamin C (ascorbic acid E300-301) is an antioxidants for water-soluble products. The fat soluble antioxidant butylated hydroxy anisole (BHA) E320 is added to edible oil and fat products in some countries. However, the antioxidant butylated hydroxy toluene (BHT) E321 is not usually added to foods but it is used in polythene film used to wrap food. It is added to petrol, lubrication products and rubber. Some antioxidant esters allowed in edible oils, margarine, table spread, salad oils include mono-tert-butylhydroquinone (TBHQ) and propyl gallate, propyl, octyl and dodecyl of gallic acid (3,4,5-trihvdroxvbenzoic acid, E310-312}
Antioxidants are related to the "natural" antioxidant, vitamin E, alpha-tocopherol and have similar properties. Vitamin E occurs in vegetable oils, e.g. wheat germ oil. It prevents the oxidation of unsaturated fatty acids in cell membranes and removes toxins. Lack of vitamin E may cause liver damage and infertility. The amount of vitamin E needed in the human diet depends on the amount of polyunsaturated fat consumed. However, excess vitamin A is harmful, as with any fat soluble vitamin.
Red wine is said to contain polyphenol and anthrocyanidin anti-oxidants and the anti-oxidant reservatol.
19.2.1.7 Cholesterol
See diagram 16.3.5.3: Cholesterol | See diagram 19.2.1.7: Steroids
Cholesterol is a fat-like molecule, an alcohol, base of all steroids, e.g. sex hormones, bile acids, vitamin D and cortisone. Cholesterol is not a fat but a steroidal alcohol. It has 27 carbon atoms so it is not a terpene. It is essential for the blood and cell membranes and is found in all the cells of the body. It is produced in the liver and also comes from foods of animal origin. Cholesterol in the blood becomes coated with a phospholipid protein envelope called lipoprotein in a high density (HDL) and low density (LDL) form. LDL carries cholesterol. Special receptors on the cells favour use of LDL. In countries where people eat large amounts of meat and dairy products their diets are high in cholesterol and saturated fats and so the mortality rate from heart disease is high. In countries where diets are low in cholesterol and rich in the polyunsaturated fats found in vegetable oils and fish, the death rate from coronary disease is lower. Vegetable oils contain the phytosterols instead of cholesterol. Isolation of ergosterol used to be employed as evidence proving the addition of vegetable oil to animal products. Atherosclerosis occurs when excess LDL cholesterol circulates in the blood, accumulates in the inner walls of the arteries to the heart and brain, and reacts with other substances to form a plaque that can clog those arteries. A blood clot can form to block a narrowed artery and can cause a heart attack or stroke. So the levels of HDL cholesterol and LDL cholesterol in the blood are measured to evaluate the risk of heart attack. A level < 130 mg/dL is optimal for most people. As high LDL level reflects an increased risk of heart disease LDL cholesterol is called "bad" cholesterol. Up to one fourth of blood cholesterol is carried by high density lipoprotein (HDL). It is called "good" cholesterol because it may protect against heart attack. by carrying cholesterol away from the arteries and back to the liver, to be excreted from the body. What is called LPG cholesterol is a genetic variation of plasma LDL that may cause fatty deposits in arteries. People with heart disease, diabetes or who are obese are likely to have high triglycerides level, high LDL cholesterol level and a low HDL cholesterol level. Triglyceride levels < 150 mg/dL are normal. Most people can raise their HDL (good cholesterol) levels by exercising, not smoking and staying at a healthy weight.
LDL cholesterol of less than 100 mg/dL is the optimal level. A LDL level more than 130 mg/dL reflects an increased risk of heart disease. That is why LDL cholesterol is often called "bad" cholesterol. Lp(a) is a genetic variation of plasma LDL. A high level of Lp(a) is an important risk factor for developing fatty deposits in arteries prematurely. The way an increased Lp(a) contributes to disease is not understood. The lesions in artery walls contain substances that may interact with Lp(a), leading to the buildup of fatty deposits. People with high triglycerides often have a high total cholesterol, a high LDL cholesterol and a low HDL cholesterol level. Many people with heart disease also have high triglyceride levels. People with diabetes or who are obese are also likely to have high triglycerides. Triglyceride levels of less than 150 mg/dL are normal; levels from 150-199 are borderline high. Levels that are borderline high or high (200 mg/dL to 499 mg/dL) may need treatment in some people. Triglyceride levels of 500 mg/dL or above are very high. Doctors need to treat high triglycerides in people who also have high LDL cholesterol levels.
19.2.9 Pectin in jelly and jam
See also 16.3.1.8: Pectin
If it contains too much pectin, it flows slowly, so add sugar. If the pectin is low add apples that are high in pectin to assure gelation. Measure fruit juice pH. Best gelation if pH between 3.1 and 3.4. If pH is too high, the jelly is watery and will not set.
19.2.9.1 Gelatine in jelly with fresh or tinned pineapple
Jelly, "Jello", is not recommended with pineapple Ananas comosus, papaya Carica papaya, figs Ficus carica, guava Psidium guajava, kiwi fruit Actinidia chinensis, and ginger root Zingiber officinale because they contain proteolytic enzymes which prevent the gelatine from setting.
Prepare two small jellies, one containing crushed fresh pineapple, the other containing crushed tinned pineapple. Leave to set. When the tinned pineapple jelly is firmly set, shake the jelly containing the fresh pineapple. It has not set well because the fresh pineapple contains enzymes that digest protein in the jelly. The enzymes in the tinned f pineapple have become inactive because of heating and processing.
19.2.10 Egg white, albumen, and egg yolk
The texture of egg yolk and egg white, albumen, is because of the dissolved globular proteins with outer charges that attract water molecules and prevent other proteins from clumping them together. However, when egg are heated, as in making scrambled eggs, the globular proteins unravel, denature, exposing the inner charges on the proteins causing the S-H groups on the amino acid cysteine to oxidize and from covalent disulfide bonds, disulfide bridges. So the scrambled egg becomes hard and loses water.
1. An egg beater forms foam better when at room temperature than when chilled. If beaten too much, the foam breaks and becomes liquid. Add sugar and cream of tartar to stabilize the egg foam. Add fat to cause the foam to collapse.
2. Heat the white albumen of an egg. Observe how it turns from slimy water white to chalk white in a firmly firm cooked egg. The protein loses surrounding water and shrinks.
3. Proteins can be denatured by heat or weak acid solutions which destroy the hydrogen bonds and cause tertiary proteins to uncoil, e.g. vinegar, acetic acid, can "cook" an egg white, albumen, without heat.
19.2.10.2 Eggs in a cake mix
Use a cake recipe that requires use of one egg. Use the same recipe for making four cakes but with no egg, one egg, two eggs and three eggs. Eat the four cakes and describe the taste and texture of each cake.
19.2.11 Yeast, fermentation, brewing, whisky, fish sauce
See also 3.38: Carbon dioxide and fermentation for brewing
To make whisky, barley is soaked in water then allowed to germinate until roots and shoots form. During this germination enzymes are produced that can convert starch to fermentable sugars. The germinated grain (green malt) is then dried over a smoky peat fire to stop further germination. The malt is ground to form grist that is mixed with hot water then put in a large container, a mash tun, for further germination to form a weak alcoholic solution, to be distilled into casks to form whisky. The taste of whisky comes mainly from the smoke of the peat fire.
"Dried yeast"  (active granules) baker's yeast, living yeast, contain "bakers' yeast", an emulsifier, e.g. emulsifier 491, potato flour and a vegetable gum, e.g. 414. During bread making, diastase converts starch into maltose, sucrase converts sucrose to invert sugar (fructose + (+) glucose), zymase converting (+) glucose to alcohol and carbon dioxide gas that causes the baking to rise. In sauerkraut manufacture, lactic acid bacteria convert sugar in cabbage into 2-hydroxypropanoic acid (lactic acid).
Fermented fish sauce, garum,  made usually from anchovies, tuna, eel and  mackeral, was popular during the Roman empire and is still made in Vietnam and other Asian countries.
19.2.12 Salad dressing and mayonnaise emulsions
1. Mix oil and water then shake. The oil and water separates and settles according to the different densities. Add a surfactant and shake. An emulsion forms. If the surfactant is egg yolk, i.e. lecithin, the emulsion is salad dressing. Beat the mixture hard to obtain small droplets so that the emulsion becomes mayonnaise.
2. Beat an egg yolk until it becomes thick. Add lemon juice or vinegar. Slowly add olive oil while stirring. A stable emulsion forms.
19.2.13 Fruit salts
Mix 0.45 kg icing sugar (fine sugar), 113 g cream of tartar, 113 g tartaric acid, 113 g carbonate of soda, 2 g Epsom salts. Mix thoroughly and sift twice. Put in glass jar and seal jar tightly. Add one teaspoon fruit salts to 0.28 L water and drink without delay!
19.2.14 Food colouring liquids and detergent
Use a dish of milk, three food colour liquids and a little detergent. Put two drops of three different food colouring liquids into the three corners of a dish of milk. Quickly add two drops of detergent into the fourth corner and observe. Look for common features, especially patterns that are seen when this experiment is least or most active. Later, another drop of detergent can be added.
19.2.15 Heat starch, glycemic index
The glycemic index is a ranking of carbohydrates based on their immediate effect on blood glucose (blood sugar) levels. It compares foods gram for gram of carbohydrate. Carbohydrates that break down quickly during digestion have the highest glycemic indexes. The blood glucose response is fast and high. Carbohydrates that break down slowly, releasing glucose gradually into the blood stream, have low glycemic indexes
Heat a mixture of starch and water by pouring boiling water on it while stirring. The colour turns from chalk white to nearly water white because the starch grains have burst and let the starch out. This is similar to making the starch more soluble and more digestible.
19.2.17 Glycoalkaloids, avoid bruised or green potatoes
Some glycoalkaloids, e.g. alpha-solanine and alpha-chaconine, are toxic compounds in plants from the Solanaceae family, e.g. potato, tomato, capsicum, tobacco. Toxicity is caused by anticholinesterase activity on the central nervous system and membrane disruption which affects the digestive system. Damaged potatoes should be avoided, especially by women who are pregnant. Potatoes can be irradiated to delay sprouting and prevent greening but not prevent the production of solanine. At temperature 300oC, the wet potato chips sizzle, dry out and go brown on the outside.
19.2.18 Extract iron, Fe, from breakfast cereal
Powdered iron is added to breakfast cereal. We can digest some of it.
1. Crush a cup of breakfast cereal into a fine powder with a mortar and pestle. Put the crushed cereal in a plastic bag and add hot water. Stroke the bag with a magnet towards one corner of the bag. The black fur in the corner of the bag is iron.
2. Put a cup of cereal in a food blender. Add hot water to submerge all the cereal. After 20 minutes, hold a magnet to the side of the blender and turn it on.  See the iron deposit in the blender next to the magnet.
2. Crush a serving of dry breakfast cereal, e.g. corn flakes, add water and stir in a magnetic stirrer. Observe iron powder sticking to the magnetic follower. Calculate the concentration of iron in the cereal, e.g. "Special K" 20 mg per 100 g dry cereal
3. Float flakes of breakfast cereal in water in a Petri dish on an overhead projector. Use a strong magnet to pull the flakes across the dish.
19.2.21 Fish smell, trimethylamine
Proteins in raw fish are denatured by citric acid, lemon juice. Freshly caught fish have no odour. However, the end products of enzyme reactions accumulate when the fish is to give the characteristic fish smell. If fish is not fresh, it give off trimethylamine, N(CH3)3, the source of fish smell. The cooked fish is less tasty and the cooking smell is offensive. To stop fish smell, soak fish in soy bean paste or milk so that proteins in them absorb the smell. Use ginger or green onion during cooking. Lemon juice, vinegar, wine, and rice wine can neutralize fish fat which contains trimethylamine. Soak freshwater fish in vinegar water before cooking. Trimethylamine is produced by bacteria in our intestines but it is broken down by an oxidation reaction in the liver. The reaction requires a certain enzyme. If people do not have the enzyme, due to a genetic fault, they may smell fishy! They suffer from a metabolic disorder called Trimethylaminuria (TMAU). Such people can be relieved of this embarrassing problem by avoiding foods rich in the amino alcohol choline, CH2OHCH2N(CH3)3OH. Choline is found in egg yolk, liver, kidney, soya beans, peas, and whole grain wheat. Trimethylamine is found in beetroot and herrings so some people say it has a "herring smell". At the end of rigor mortis bacterial action may decompose the fish protein and add to the offensive smell. So fish should be eaten fresh and cooked for only ba short time to denature tissue between the fibres and heat the fish to an acceptable temperature for eating.
19.2.22 Laundry starch
Prepare a suspension of laundry starch. The starch breaks up, but does not dissolve. Boil the suspension. The starch turns from chalk white to nearly water white because boiling burst the starch grains and let the starch out. The starch is now more soluble and more digestible.
19.2.22.1 Wheat starch and gluten
Gluten is a protein complex formed by kneading of the wheat flour dough proteins gliaden and glutenin. Gliaden is soluble in alcohol but glutenin is not.
Tie plain wheat flour in a fine cloth. Bang it repeatedly in a dish of water. Let the white suspension of starch settle in the dish and decant the water. The sticky mass left in the cloth is mainly gluten and cellulose.
19.2.23 Milk
Homogenize by breaking the fat globules. Heat milk to form a skin of protein and calcium compounds.
19.2.24 Butter
Heat butter. The fat separates from salt and water. Heat too hot, as in deep drying. The fat cracks to form the unsaturated hydrocarbon acrid smelling tear producing acrolein.
19.2.26 Custard
If you cook custard at too high or too low a temperature it becomes either watery or curdled.
19.2.27 Garlic
When a clove of garlic (Allium sativum) is crushed the enzyme allinase acts on alliin (S-allylcysteine) to produce unstable allicin (diallyl thiosulfionate) that degrades to diallyl sulfide CH2.CH.CH2.S-S.CH2.CH.CH2 and other sulfur compounds called ajoenes and dithiins. Diallyl disulfide can also be prepared by steam distillation. All these compounds are said to have health benefits owing to their anticlotting, antifungus, antibacterial and antioxidant properties. However, garlic should be eaten in oil preparations, e.g. olive oil, or cooked. Raw garlic may damage the digestive system.
19.2.28 Tests for harmful substances in cigarette smoke
Burning, leaves of the tobacco plant give off smoke in which more than one thousand chemical substances have been identified. These substances include tobacco tar, nicotine, carbon oxide and aldehydes considered harmful to human health. Tobacco tar contains many kinds of carcinogens, e.g. benzopyrene; nicotine is similar to hydrocyanic acid in toxicity; excessive carbon oxide will weaken oxygen carrying capacity of blood, and lead to an oxygen deficit in body tissues. Put 10 mL of 95% alcohol in a sidearm test-tube. Fit the test-tube with a 1 hole stopper carrying a glass delivery tube, one end of which is kept under the alcohol and close to the bottom of the test-tube. Insert the other end of the delivery tube in a lighted cigarette. The smoke is drawn down through the delivery tube into the alcohol solution by an air extractor attached to the side arm of the test-tube. Some substances such as nicotine and benzidine are dissolved in alcohol to make colour of the solution change from colourless to yellow, and finally to brown along with an increase in the number of lighted cigarettes.
19.2.29 Toxic effect of common drugs on Daphnia
Be careful! Children must not taste the test solutions! Young children may be distressed by the sight of Daphnia struggling under the influence of these substances. However, such a sight can send a powerful deterrent message about substance abuse. Collect Daphnia in spring from ponds or purchase from goldfish supply shops.
1. Prepare the following test solutions in test-tubes:
1.1 10 mL coffee from a coffee cup containing 1 teaspoon of coffee powder, or the usual way you make coffee, active ingredient caffeine
1.2 10 mL cooking sherry, 17% alcohol / volume active ingredient.
2. Stir the following substances into 10 mL water at 37oC:
2.1 300 mg aspirin tablet, active ingredient acetylsalicylic acid,
2.2 1 g pipe tobacco or the contents of discarded cigarette butts, active ingredient nicotine,
2.3 1 Benadryl allergy caplet, active ingredient diphenhydramine.
3. Use an eye dropper to transfer a Daphnia to 5 test-tubes containing 10 mL pond water. Transfer 1, 2, 3, 4 drops of test solution into test-tubes 1, 2, 3, 4. Put no test solution in the control test-tube
4. Use a microscope to observe movement, heart rate and gill movement of Daphnia in control test-tube 5, then in test-tubes 1 to 4. Record the least number of drops of test solution to kill the Daphnia. Tobacco causes quick death at the lowest doses. Alcohol first slows the heartbeat rate, then is lethal at higher doses. Aspirin and allergy capsules are lethal at the highest doses. Coffee causes "racing" of the heart, heart palpitations, but is not lethal.
5. Repeat the experiment with 5.1 decaffeinated coffee, 5.2 red wine 12.5% alcohol / volume, 5.3 100 mg low dose "baby" aspirin, 5. 4. "lite" low nicotine cigarettes. This experiment does not compare the relative effects of the active ingredients because the concentration as mg/mL in the test solutions varies.
19.2.30 Test chewing gum by comparing bubbles
Chew different samples of chewing gum until the taste has gone. Apply the same exhalent force to make a chewing gum bubble. Measure the diameter of the chewing gum bubbles. Note whether the samples of chewing gum are made from chicle based on gutta-percha plasticized by triterpenes or made from poly (vinyl acetate), PVA.
19.3.0 Cooking
16.3.6.0.1 Fibrous proteins and globular proteins, collagen
1. The four basic methods of cooking are as follows:
1.1 Wet heating - boiling, steaming, pasteurization 100oC to 120oC
1.2 Dry heating - baking, roasting, up to 250oC
1.3 Hot oil frying, up to 300oC
1.4 Microwave cooking, up to 120oC.
2. Cooking breaks down long molecules to smaller molecules that are more easily digested. Cooking may also release flavours and destroy unwanted flavours. Cooking may also raise temperature so that enzyme reactions can occur.
2.1 Braising is done after browning the food in some fat then half immersing in liquid in sealed pot and cooked slowly.
2.2. Broiling is the same as grilling in England but in USA it means grilling food under direct heat source as in an electric oven.
2.3. Caramelizing is browning of sugars in solution when heated above 150°C. The sugars break down to many compounds, including sugars, and carbon. Onions are commonly caramelized.
2.4. Frying is putting food into a bath of hot fat or oil. It involves sealing and browning. If the temperature does not exceed 180°C, the fat or oil used should not break up or decompose. The food should be dry or dipped into flour, bread crumbs or batter. Potato is commonly deep fried. If a fire occurs, drop in baking soda and cover the pan with a lid.
2.5. Poaching is cooking by simmering in water. Unlike boiling, the water temperature is kept below boiling point. Eggs are commonly poached.
2.6. Roasting is cooking food with dry heat and can be done in an oven or over an open fire. Meat and poultry are commonly roasted. Fish and vegetables can also be roasted.
2.7. Sautéing and deglazing is briefly cooking food in a shallow pan for quick sealing and browning of small pieces of food. However, The Concise Larousse Gastronomique defines "Saute" as to cook meat, fish, or vegetables in fat until brown, using a frying pan (skillet), a saute pan or even a heavy saucepan and "Deglaze" as to heat wine, stock or other liquid together with the cooking juices and sediment left in the pan after roasting or sautéing to make a sauce or gravy.
3. Woks are used mainly in Chinese cooking where all the food is first chopped and peanut oil or other oils are added to swirl around to coat the bottom of the wok. When the oil starts to smoke, add the chopped food. The salt in cooking water increases the osmotic pressure of the water and stops the food losing desirable flavour molecules by diffusion. Food cooked in salt water is no more salty than food cooked in unsalted water, unless you drink the liquid in which the food was cooked.
4. Cooking time is proportional to the square of the smallest radius of the food rather than its weight.
4.1 Taste raw and cooked to note the difference in flavour of different foods raw, cooked without salt, and cooked with salt, rice or wheat flour, banana, peanut, bean curd, onion, beef, fish.
4.2. Try to eat raw potato, raw rice and raw meat! Cooking breaks down inedible and indigestible large molecules and fibres to smaller structures that can be eaten and digested. Similarly cooking destroys some bad flavours, poisons, bacteria and other organisms that may be harmful if eaten in raw food.
19.3.1 Taste, smell, flavour
See also 9.246: Sense of taste | See also 19.4.4.25: Sweeteners, food additives | See also 19.4.4.18: Food acids, food additive
1. The 4 basic tastes are sweet, sour, bitter and salt, also "umami"; the savoury taste.
Sweet: Different sugars have different sweetness.
Sour: Acids always taste sour
Bitter: Most alkaloids taste bitter, e.g. purgative aloes (Aloe sp.), wormwood (Artemisia absinthium) used in the illegal (in some countries) alcoholic drink absinthe, lupulin in hops (Humulus lupulus) used in making bitter beer, amygdalin in bitter almonds (Prunus amygdalus), colocynth in the bitter apple (Colocynthis sp.), quinine in bark of Cinchona sp. and used in tonic water, caffeine in Coffea sp. and used in coffee.
Salt: Most sodium salts and most chlorides taste salty.
2. Umami is the taste of monosodium glutamate used in Asian cooking and Parmesan cheese. The "trigeminal sense" allows us to "taste" chillies and onions.
3. Flavour is the quality perceived by the sense of taste assisted by the sense of smell. Smell depends on small molecules that can be carried in the air, e.g. from ripening fruit and flowers but also air from the back of the mouth carry molecules from the food being chewed up to the olfactory epithelium of the nasal cavities, the "after taste". Cooked food tastes differently from uncooked food, e.g. uncooked meat is relatively tasteless. People who lack the sense of smell are called anosmic. People who lack the sense of taste are called ageusic.
Test blindfolded students for their ability to identify food with some students with nose clips or holding their noses:
3.1 Bland and tasty foods, e.g. potato crisps,
3.2. Whole foods and pureed foods.
4. Raw and cooked vegetables. Be careful if using raw onions or chillies. Some students may be allergic to raw onions.
5. Tomato and monosodium glutamate
6. Table salt and sugar.
19.3.2 Anatomy and physiology of meat
Meat consists of muscle fibres, connective tissues and fats. The muscle fibres largely consist of two proteins, myosin and actin. extended along the fibres and can move past one another, to cause a similar contraction. Meat is muscles attached to bones by tendons. Muscles contain long protein fibres actin and myosin. These fibres slide along each other when stimulated by nerve impulses, which make the muscle shorter and fatter. If muscles are used for short, fast bursts of energy, then glucose from the blood provides the fuel. If the muscles have to give long sustained activity, then fat provides the energy and in this case another protein, called myoglobin, is needed to help oxidize the fat and provide the energy. Myoglobin is a single chain protein of 153 amino acids, containing a haeme that is the main oxygen carrying pigment of muscle tissues. Myoglobin is bright red, so muscles that work a lot are red while those that are used for less regular, sudden movements are pale or even white. Fibrous tissues that surround the muscles and connect them to the tendons and bones are made from collagen. The more collagen in the meat, the tougher it is. There are three main types of connective tissue: collagen, reticulin and elastin. Collagen is the most abundant and the most important for the cook to appreciate. Collagen is a complex molecule made up from three strands that are twisted together rather like a rope. Collagen derives its stiffness and strength from the arrangement of these intertwined helices. However, if collagen is heated to temperatures above about 60oC, the three strands can separate and the material loses its strength. Once denatured into single strands, collagen becomes a very soft material, and is given a different name, gelatine. You already know that gelatine is a soft, tender, material, since it is used as the basis of all jellies. The collagen is mostly found around bundles of muscle fibres and helps to hold them together. The muscles are then joined to the bones with yet more sinews (yet more "connective tissue"), which cooks recognize as tough "gristly" material. These sinews are made from the proteins reticulin and elastin; reticulin and elastin can only be denatured and softened by heating for very long times at temperatures above 90oC. Muscles consist of bundles of long cells called muscle fibres or myofibrils. Resting myofibrils contain two types of proteins actin and myosin arranged side by side. The muscle contracts when the actin and myosin proteins slide together. like the finger from each hand to form the protein actomysin the presence of calcium ions Ca2+ powered by the energy carrying molecule ATP (adenosine triphosphate).
After an animal is slaughtered, blood circulation stops, and muscles exhaust their oxygen supply. Muscle can no longer use oxygen to generate ATP and turn to anaerobic glycolysis, a process that breaks down sugar without oxygen, to generate ATP from glycogen, a sugar stored in muscle. The break down of glycogen produces enough energy to contract the muscles, and also produces lactic acid. With no blood flow to carry the lactic acid away, the acid builds up in the muscle tissue. If the acid content is too high, the meat loses its water binding ability and becomes pale and watery. If the acid is too low, the meat will be tough and dry. As glycogen supplies are depleted, ATP regeneration stops, and the actin and myosin remain closed in a permanent contraction of actomysin called rigor mortis. Freezing the carcass too soon after death keeps the proteins all bunched together, resulting in very tough meat. Individual protein molecules in raw meat are wound in coils, which are formed and held together by bonds. When meat is heated, the bonds break and the protein molecule unwinds. Heat also shrinks the muscle fibres both in diameter and in length as water is squeezed out and the protein molecules recombine, or coagulate. Because the natural structure of the protein changes, this process of breaking, unwinding, and coagulating is called denaturing. The protein of meat congeals when heated. If it is further heated after it has congealed, its combination is broken and it becomes a compound called melanoidin with a chemical amino carbonyl reaction to oxygen in air and others.
Most animal muscle is roughly 75% water, 20% protein, and 5% fat, carbohydrate s, and assorted proteins.
Observe the arrangement of bone muscle, tendons and connective tissue is a piece of meat selected for roasting. Mature beef should have areas of muscle separated by fat, an arrangement called marbling.
19.3.3 Boiling, test the cooking water of boiled vegetables
Meat cooked in hot water has no roasted flavours because the temperature does not rise above 100oC. Vegetables with large surface / mass ratios (e.g. spinach) are especially sensitive to loss of vitamins. Vitamin C is a most unstable nutrient in neutral and alkaline conditions, and in the presence of oxygen; folic acid may be even more unstable; and thiamine, riboflavin, carotene and niacin are sensitive under certain conditions. Nutrient losses increase if large amounts of cooking water are used. Cooking for longer times also results in a greater loss of nutrients.
Test cooking water of boiled vegetables
1. Boil green vegetables in water, e.g. spinach brussels sprouts, spring greens. Removed the cooked vegetables and filter the cooled cooking water. Observe the green colour of the cooking water because of beta carotene, vitamin A precursor. Test the cooking water for vitamin C with a Vitamin C dipstick Tests for the presence of iron in the cooking water with a mixture of 1% potassium ferrocyanide and 1% hydrochloric acid to make potassium ferrocyanide. A deep blue precipitate shows iron. Boil water for a long time in a clean iron pot. The cooled water may test positive for iron and this iron may be an important source of iron in some countries.
2. Tests for iron in the water vegetables were boiled in. Add to a sample of the cooking water 1% potassium ferrocyanide and 1% hydrochloric acid, mixed just before use. A deep blue precipitate indicates iron. In some cultures the iron in the diet may come from the use of iron pots.
3. Put some unpeeled new potatoes into boiling water for different lengths of time. Remove them and cut them open. Observe the growth with time of a translucent ring from the outside. Inside the ring the potato still has an opaque white texture. The width of the ring is the distance that has reached a temperature of 60oC.
4. Put dry peas in a measuring cylinder and measure the volume of water needed to fill the cylinder to the 100 mL mark. Put the peas in boiling water for 15 minutes boiling. Put the boiled dry peas in the measuring cylinder and measure the volume of water needed to fill the cylinder to the 100 mL mark. Calculate the percentage change in the volume of the peas.
5. Use a microscope to observe the characteristic shape of starch grains, e.g. potato, rice, wheat, maize. Boil the starch grains until the cells burst and the starch forms a gelatinous mass leaving empty cell wall envelopes "ghosts". This gelating action occurs at different temperatures: barley 51oC to 61oC, tapioca 52oC to 64oC, pea 57oC to 70oC, potato 58oC to 66oC, wheat 50oC to 64oC, maize (corn) 62oC to 70oC, rice 68oC to 70oC.
6. Show the effect of the salivary enzyme amylase on raw and boiled starch measured using dipsticks specific for glucose.
7. Show that vitamins and minerals may be lost in the water vegetables are boiled in, e.g. spinach, Brussels sprout. Note the colouring of beta carotene, vitamin A precursor in the water used for boiling.
8. Drop small pieces of raw and cooked liver into test-tubes containing 10 vols (3%) hydrogen peroxide and compare the rate of effervescence because of the enzyme peroxidase catalase. Fresh pineapple contains a powerful proteolytic enzyme called bromelase, but in tinned pineapple it is inactivated. So a pineapple garnish for ham, gammon, should be made of fresh pineapple. Fresh pineapple interferes with setting gelatine and whipped egg whites because the enzyme is active.
19.3.3.1 Mashed potato, pommes purée
Two types of potato are: 1. the floury type with cells that separate and that break down easily when cooked, e.g. "King Edward" and 2. the waxy type with cells that are firmly held together when cooked, e.g. "Charlotte". Most new potatoes are waxy type. Floury types make a mashed potato that is light, fluffy, and thick. Waxy types make mashed potato that is thick, smooth and silky textured. Floury types contain more starch granules per cell than waxy types and are more dense. The cells contain starch granules. The cell walls are a network of cellulose held together by hemi-cellulose. During cooking, the hemi-cellulose breaks down, the cell walls get weaker, the cells start to separate and break open. The starch granules absorb water and expand. Many granules are also released from the cells of the floury types. Prevent potatoes from absorbing water during cooking, which makes the mashed potato wet, by either cooking them in their skins or first peel then steam them. Also, you can cook them at a temperature below the boiling point, allow to cool, then cook again before they are mashed. The first cooking activates enzymes which allow the pectin to react with calcium to make a glue to hold the cells together during the second cooking. Mash potatoes so that the cells are opened without breaking the released starch granules to form an unpleasant sticky jelly.
Use iodine solution to examine slices of floury type and waxy type potatoes. Also, examine the starch grains before and after cooking.
19.3.4 Baking and retention of nutrients
Retention of nutrients may be low, e.g. vitamin C retention in baked apple 40%, thiamine retention in potatoes 41%. Baking powders destroy thiamine that needs slightly acid conditions to keep stable during baking. Bread loses little nutritional value during baking but toasting causes loss of protein and vitamins, depending on how much drying out and browning. The heating, rolling and flaking processes to manufacture breakfast cereals do not affect protein content, but explosion puffing, e.g. puffed wheat, puffed rice, reduces the nutritive value. However, you usually eat these foods with milk.
19.3.4.1 Test for dextrins in toast
Heated starch breaks into shorter length polymers called dextrins. Prepare an iodine solution with 1 g iodine, 2 g potassium iodide 100 mL of water. Starch gives a blue black colour. Dextrins give a brown red colour. Very small dextrins give no colour.
Prepare toast with a thin slice and a thick slice of bread. Grind the toast to fine crumbs. Shake the crumbs with water and filter with a Buchner funnel. Add ammonium sulfate to precipitate starch and leave dextrins in solution. Tests for dextrins with iodine solution. The thin slice produces more dextrins than the thick slice.
19.3.4.2 Browning reactions of fruits and vegetables
Phenolase browning occurs at the cut surface of light-coloured fruits and vegetables, e.g. apples, bananas, potatoes, where phenols oxidize to orthoquines which then polymerize to form melanin  brown pigment. The phenolases, the enzymes that catalyze the phenols oxidation, contain copper. Phenolase are active in the pH 5-7 range and can become inactivated below pH 3. Phenolase browning is an enzymatic-catalytic reaction. Other browning reactions of food are non-enzymatic reactions, e.g. Maillard reaction, caramelization, and ascorbic acid oxidation. When tissues are damaged, polyphenolic substances from the vacuoles of the plant cells contact the oxidase called phenolase (phenol oxidase enzyme) in the cytoplasm of plant cells and in the presence of oxygen produce substances that protect the plant and favour wound healing. However, these substances produce a brown coloration.                  .
Browning can be prevented by the following processes:              
1. Immersion in water or dilute salt solution or coating with syrup so that the substrate has no contact with oxygen. Concentrated sugar solution also depresses enzyme activity. Also, storage in carbon dioxide or nitrogen prevents the browning reaction.
2. Boiling or steaming to inactivate the protein enzymes that are denatured by heat. Browning does not occur in cooked fruits and vegetables.
3. Adding acid to lower the pH, e.g. lemon juice or cream of tartar (potassium hydrogen tartrate, KHC4H4O7). Ascorbic acid may also act as an antioxidant to reduce quinones.
4. Lower the temperature to depress enzyme activity. However, some fruits become brown even when frozen, e.g. banana skin.
5. Use sulfur dioxide or sulphites, e.g. sodium sulfite, sodium metabisulfite, that react with quinones formed from phenolic compounds to block further reactions. A sulfur dioxide concentration of 10 ppm. (10 mg per litre) inhibits phenolase. Sulfites are used in the processing of dried fruits, red wine, red and white grape juice. However, sulfites may cause medical problems to people who have sulfite sensitivity.
6. Dehydration makes phenolase inactive, even in bananas.
Browning may be a useful process as when plums become prunes, grapes become raisins and green tea leaves become black tea.
7. Lemon juice contains two components that can block the browning reaction:
7.1 Organic acids, mainly citric acid, with pH < 2, that lower the pH of the apple tissure below the best pH for the action of oxidases.
7.2 The biological antioxidant vitamin C that is oxidised to colourless substances.
Test the effects of anti-browning compounds by using cut slices of celeriac or celery.
Test the effect of lemon juice on the browning of an apple.
1. Put a slice of fresh apple on a sheet of wax paper. Examine the exposed moist surface with a magnifying glass.
2. Every minute, take another look at the apple and note any change in appearance. After 5 minutes, remove the skin of the apple and describe the appearance of this newly exposed fruit surface.
3. Use two new slices of apple. Place them side-by-side on a fresh sheet of wax paper. Soak the end of a cotton swab in lemon juice. Use this swab to paint the exposed surface of one of the apple slices. Every minute, re-examine the apple slices. Describe any changes and note whether both slices change at the same rate.
19.3.4.3 Non-enzymatic browning, caramelization
Non-enzymatic browning is a chemical process that produces a brown colour in foods without the activity of enzymes. Caramelization is the oxidation of sugar, a process used extensively in cooking to release volatile chemicals producing the characteristic caramel flavour and brown colour. The complicated chemical processes that are not fully understood include sucrose inversion to fructose and glucose, condensation to result in the formation of carbon, isomerization of aldoses to ketoses, dehydration , fragmentation reactions and unsaturated polymer formation, production of melanins. Caramelization temperatures of different sugars include the following: fructose 110oC, galactose 160oC, glucose 160oC, maltose 180oC, sucrose 160oC. The brown colour may be due to fructose dianhydride and carbon particles. Caramel is used as a flavouring and food colouring E150 in candy and Coca Cola and other foods.
The melting point of sucrose is 186oC. The solubility of sucrose is 2.59 g sucrose per g water at 50oC.
Be careful! "Toffee" burns are very painful and dangerous. Wear a full face mask and gloves. Sputtering of hot sugar may occur.
1. Add two measures of sugar to one measure of water. Stir and heat in a Pyrex beaker or non-stick deep pot until all the sugar is dissolved. Keep heating and adding more sugar until no more can dissolve. Remove the heat when the solution has a straw colour. Do not allow boiling to occur. You can check the temperature of the solution with a special "candy thermometer". Pour the hot solution into paper cups and immediately clean the Pyrex beaker or saucepan. Allow to cool. The glassy solid may be eaten or used in movie sets for imitation glass bottles and windows so that actors are not hurt when hit with a "bottle" of when they jump through a "glass window". After removing the heat, and while the solution is still hot, add whipped cream and butter to make the French desert "creme brulee" ("burnt custard").
2. Put granulated sugar or glucose in a shallow porcelain-lined evaporating dish or metal pan with a volume of ten times the volume of sugar used. Heat the sugar over an electric stove until it froths up suddenly then immediately turn off the stove or remove the dish or pan from the stove. Add water to bring the dark brown viscid mass to the consistency of a heavy syrup. The product will be insoluble in water if more than 15% of  the original weight of the sugar is lost. If the caramel dissolved in water is cloudy, carbonization has occurred so some of the sugar has been reduced to carbon.
19.3.4.4 Non-enzymatic browning, the Maillard reaction
The Maillard reaction (Louis Maillard 1912) refers to the chemical reactions between 1. the aldehydes and ketones from reducing sugars and 2. proteins or amino acid. The reactive carbonyl group of the sugar interacts with the amino group of the amino acid. The type of amino acid decides the many flavour compounds produced and these compounds may then break down to form more new flavour compounds. The Maillard reactions produce caramel candies made from milk and sugar, the browning of bread in toast, malting of barley to give the colour of beer, chocolate, coffee, maple syrup, lightly roasted peanuts, colour of condensed milk. The carbonyl group of the sugar reacts with the amino group of the amino acid, producing water and N-substituted glycosylamine that undergoes Amadori rearrangement to form ketosamines that react further to produce various short chain products, brown nitrogenous polymers and melanoidin. Melanoidin is light brown and has the good smell of grilled meat. However, melanoidin is easy to volatize, so its smell disappears if it is heated for a long period of time with low heat. his is why cooks grill meat for a short period of time over a high flame. Over 200oC carbonization occurs and the protein component causes a bad smell. Pentose sugars react more than hexoses, which react more than disaccharides. Different amino acids produce different amounts of browning.
1. Make toast from  white bread and  brown bread or  wholemeal bread
Put equal thickness slices of white bread and  brown bread or wholemeal bread in each side of a toaster. Turn on the toaster and note the time taken for each slic to start to burn. The brown bread or wholemeal bread slice burns faster than the white bread slice because the white bread contains less sugar and protein for the Maillard reaction. If the white bread slice is really white it may reflect more radiation from the toaster than darker slices.
2. Make caramel confectionery
Boil together sugar and milk or butter until the solution turns brown. You can check the temperature of the solution with a special "candy thermometer". To make toffee heat the solution to 160oC. Add baking soda and vinegar to make honeycomb toffee. Leave to cool.
19.3.4.5 Roasting meat
When muscle fibres are heated above about 40oC the proteins start to denature. In muscle proteins, which are extended along the muscle fibres, this change of shape involves the proteins coiling up. This coiling process inevitably causes some contraction of the muscle. Muscles contract during cooking so pieces of meat contract along the direction of the muscle fibres. When the protein molecules are denatured, the muscles contract so the meat becomes harder. The meat will be tough. The cooking time should be long enough to degrade the connective tissue present in the meat, without toughening the muscle proteins.
The toughening occurs in two stages. The first stage is caused by the muscle proteins that are extended along the muscle fibres contracting into coils as they denature. The pieces of meat should be getting thinner during this stage as the fibres contract. The second stage is the coagulation of the now denatured muscle proteins into lumpy, knotty masses. There should be a much greater degree of toughening in this stage, but no noticeable changes in the shape of the pieces of meat. Meat is roasted to 1. to make it tender to eat 2. to give it a roasted flavour 3. to kill any harmful bacteria 4. to create an acceptable warm temperature for eating when fats are still liquid
Low temperature roasting
Most cooks first brown the meat at a high temperature to seal in the meat juices during cooking before letting it cook through at a lower temperature. However, cooking at less than 70oC. can produce tender meat before the meat is browned on the outside. Slow cooking for a long time at a low temperature causes collagen in the meat to break down into gelatine and the meat fibres to shrink and release the meat juice to form a gravy. The fibres separate easily and are tasty. The high temperature surface searing of the meat following the slow cook creates lots of aroma molecules when the ribose and the amino acids released from the meat react together and these add to the rich flavour. This final heating also kills off any bacteria that may have survived the slow cooking.
1. Observe roasting meat
The meat consists mainly of three materials: water, proteins and fat. When a strong source of heat is applied to the surface of the meat, the protein coagulates and aggregates to form crust, possibly the result of the Maillard reaction as in the formation of crust on bread. The presence of heat and water, will make the collagen (the proteins present in meat) degrades to form a gelatine. This the reason why the meat is removed and this brown crust appears at surface. However, if the meat is warmed too long, the evaporation of the water associated to the coagulation of the other proteins in the surface, will have for inverse effect to dry and harden the meat. It is necessary here all the art and the experience (experiment) of the cook to find the equivalence completed between these two opposite phenomena.
The chemistry of the browning of meat is still the subject of disagreement and further research. Some chemists say that the Maillard reaction occurs when the denatured proteins on the surface of the meat recombine with the sugars present. The combination creates the "meaty" flavour and changes the colour, the browning reaction. When meat is cooked, the outside reaches a higher temperature than the inside, triggering the Maillard reaction and creating the strongest flavours on the surface. As many as six hundred components have been identified in the aroma of beef. However, if the Maillard reaction is strictly the reaction between an amino and and a reducing sugar, there are not many reducing sugars in meat so the browning of meat may be due more to the breakdown of tetrapyrrole rings in myoglobin, the muscle protein.
During roasting, different changes occur at different temperatures:
40oC Red muscle proteins denature, meat is still red
55oC to 60oC Red muscle proteins coil and shrink. Water bound to protein starts to flow. The myosin molecules start to shrink and squeeze out fluid from the muscle cells. Shrinking fibres makes the meat firmer.
60oC to 65oC Produces "rare meat" that is pink and juicy. Muscle proteins start coagulating. Collagen begins to denature and break down to form gelatine which dissolves in water.
65oC to 70oC Produces grey protein. Water bound to protein stops flowing. Kills bacteria on the surface of the meat. These bacteria may be harmless even if they cause meat to spoil. The food poisoning bacteria Salmonella and Escherichia coli (E. coli) are killed above 68oC. However, minced meat may have surface bacteria mixed throughout the meat so all the meat should reach that temperature.
80oC This produces "well done" meat that is brown grey and dry tough meat.
90oC Collagen turns into gelatine.
100oC - 120oC Roast and fried flavours and brown colours form as proteins and carbohydrates break down into smaller sugar molecules and aldehydes and react with amino acids sulfur gives agreeable aromas so cooks add garlic or onions because of their high sulfur content. The meat smell is because of the molecule bis-2-methyl-3-furyl-disulfide. Similar reactions gives the colour and flavour to bread crust, toast, biscuits and caramel toffee. All these reactions are called the Maillard reaction, browning reaction, and they occur at above 140oC. Caramelization of sugars occurs at about 150oC.
Above 200oC carcinogenic molecules may form in barbecued meat.
2. Toughness and flavour of cooked meat
Cut a slice of meat 10 equal size pieces. Start cooking the pieces every 2 minutes in a frying pan or under a grill
 until all the pieces are being cooked then turn all the pieces every minute. After the last piece has been cooking for 3 minutes turn off the heat. Note the colour of each piece. Test the pieces for toughness and flavour by 1. chewing 2. cutting with a blunt knife. Pieces cooked for up to 8 minutes are probably tender but pieces cooked for longer are increasingly tougher. Note an increasing of flavour with cooking time.
3. Cooked collagen turn into gelatine
Use meat with lots of connective tissue and still connected to the bones with tendons, e.g. shin beef, oxtail. Cut the meat and tendons into small pieces, and cover with water, heat to boiling and leave to simmer. Do not allow all the water to evaporate! After each 30 minutes remove 50 mL of the water and place in a container in a refrigerator. After each removal of water replace with 50 mL of boiling water. Water removed after about 2 hours simmering thickens on cooling until water removed after 3 hours forms jelly as it cools. Collagen in meat and tendons heated to above 60oC changes from its triple stranded helical form into a single stranded form called gelatine. Gelatine is soluble in water so its concentration increases as more collagen denatures during simmering. When cold, high concentrations of gelatine molecules aggregate together in a loose gel or jelly.
4. The browning reaction is affected by temperature
Cook 8 pieces of meat at different temperatures. Heat the oven until the temperature is steady at 100oC. Put in a piece of meat for the time listed below. Raise the temperature of the oven and cook the next piece of meat. Note the colour, and the occurrence of the cooked meat smells and tastes, and the burnt meat smells and tastes. Cook each piece for half the cooking time for each side. 100oC Cooking time 12 minutes; 120oC Cooking time 9.5 minutes, 140oC Cooking time 8 minutes, 160oC Cooking time 7 minutes, 180oC Cooking time 6 minutes, 200oC Cooking time 5.5 minutes, 220oC Cooking time 5 minutes, 240oC Cooking time 4 minutes
19.3.4.6 Meat treatments, marinades, salting meat, marbled beef
1. Marinades are made of acid, oil, and herbs. The acid is to denature the proteins and open the meat structure for flavour to enter. Marinades work best on meats that are not too dense or where the meat is cut into pieces.
2. Salting meat, brining, adds moisture to the meat through osmosis because the meat's cell fluids are less concentrated than the salt water. Water leaves the muscle cells and salt flows in to dissolve fibre proteins and concentrate the cell fluids so water is attracted back into the meat. So salting meat adds both salt and water to the meat muscle. Not all the water is squeezed out by cooking because of the added water.
3. Many flavour or aroma molecules cannot dissolve in water but can dissolve in fat. Muscles that are used often consume the stored fat, and so the meat from these muscles has little fat. Older the animals have fat energy reserves in their muscles. Meat with white streaks of fat is called marbled and gives beef with the best flavour. Ageing allows enzymes in the muscle cells to break down the overlapping proteins, which makes the meat tender.
19.3.5 Microwave cooking
See also 27.01: Electromagnetic spectrum
1. Microwaves are a form of high frequency radio waves similar to the waves used by AM and FM radio. Microwaves enter from the outside of food losing about half their energy every 3 cm. Microwaves are reflected by metals so the walls of the microwave oven are made of stainless steel or epoxy coated stainless steel. so that the microwaves can bounce around the inside and not escape into the kitchen. The holes in the steel door are too small to let the microwaves pass through. Microwaves are transmitted by paper, glass and some plastics which do not absorb microwave energy and do not become hot. Microwaves are absorbed by food containing moisture.
1. Radiation
The microwave region of the electromagnetic spectrum is from frequency 300 MHz, wavelength 1 m, to 300 000 MHz, wavelength 1 mm. In Australia, microwave ovens operate at 2450 MHz (Class B / Group 2, ISM equipment Standard CISPR11), wavelength 12 cm. This is a lower frequency and lower energy than ionizing radiation. Microwave radiation is generated in an electronic tube called a magnetron. For safety reasons, when the microwave door is opened,. switches must cut the power. The power flux density of microwave radiation should not exceed 50 W / M2 at any point 5 cm or more from the external surface of the oven. The inside of a microwave oven is coated with metal that acts like a Faraday cage and reflects the microwaves like a mirror.
2. Cooking
Microwave cooking occurs because water is a strong absorber of microwaves so the water in food boils and turns to steam. In a normal oven, the range of black body radiation is generated but only some longer sections of this radiation are absorbed by the food. The radiation penetrates the surface of the food for about one wavelength for infrared heat radiation < 1 / 10 mm. Only the outside of the food is directly affected and the inside of the food is heated by conduction. The 12 cm wavelength microwaves penetrate deeper into the food. Avoid exploding food, e.g. eggs, tomatoes, apples, by cutting into them before microwave cooking. Elevate the cooking pan in the microwave oven to allow more bounce of microwaves up from the floor of the microwave oven and get more even cooking.
3. Containers
Microwave cooking containers should not absorb the radiation so they should be made from material with low polarity, i.e. low dielectric constant and low dissipation factor at 2450 MHz. The material should be stable at high temperature and have good resistance to oil vapour. e.g. polystyrene and certain glass. Never use metal containers in microwave ovens because microwave cannot pass through metal and are reflected to damage the magnetron. Remove meal foil packaging and twisted ties or tags. Do not use dishes with metallic rims, cups with glued-on handles, delicate glassware, cut glass vases, jars with metal lids. However, you can use metal foil to protect food that might overcook in some areas. Do not cook in plastic food storage bags because they may melt. However, "GLAD WRAP" may be used to cover dishes but do not remove it immediately after cooking. Do not use brown paper bags or newspaper because they contain metallic impurities that may cause blue spark arcing and damage the oven. Do not put mercury thermometers in the microwave oven. Do not put a wet cat in the microwave oven. Use containers labelled "microwave oven safe". Use shallow round dishes because microwaves penetrate only 2 to 3 cm into most food. Food in the corners or dishes with corners will receive more energy and probably overcook. Arrange food containers in a ring at outside of the rotating turntable. Sometimes it pays to put the container containing thin food on top of another container on the turntable to get more bounce of microwaves through the food. Remember that if food is not cooked through, e.g. poultry, then bacteria may remain unharmed inside.
4. Superheating
When tap water is heated on a stove top, the maximum superheating is 100.75oC. However, in a microwave oven, superheating temperatures of 105oC to 110oC can occur. The microwave oven heats the water directly and the container indirectly, the opposite of what happens when heating a container of water on a stove. In the microwave oven the water is not heated near the beaker surface where nucleation is most likely to occur. Evaporation occurs only at the water surface and is not fast enough to cool the bulk water. There is much less superheating in plastic cups because water does not wet plastic and many more nucleation sites are available for boiling. It is dangerous to put a cup of still water in the microwave oven because when you try to remove the cup nucleation may occur and the water boils, forms bubbles and steam, and may scold your hand. To avoid the delayed eruptive boiling of liquids after cooking, leave to stand for at least 20 seconds before removing food or liquids from the microwave oven.
1. Test a container for potential microwave oven use. Fill a one cup glass measure with water and put it in the microwave oven alongside the container to be tested, e.g. a ceramic dish. Heat for on high for 60 seconds. If the container is microwave oven safe, it will remain cool but the water should be hot. However, you cannot use this test on plastic containers that must be labelled microwave "oven safe".
2. Cook a potato for 2 minutes then cut it open to see the translucent areas finger-like regions extending from the outside where the potato has been heated above 60oC. This show that microwave ovens do not provide a uniform density of microwave energy. So "hot spots" and "cold" spots may exist in microwave ovens unlike the conventional oven. This phenomenon is the reason cooks suggest leaving food cooked by microwaves to "stand" for some time after cooking to allow the temperature to equalize in the food.
3. Compare popcorn cooked in the microwave with popcorn cooked on the conventional stove. Use the same number of corn (maize) grains and later count the number of unpopped grains. Carefully open popcorn bags away from your face!
4. Do not put sealed tins or glass jars in a microwave oven, e.g. babies bottles fitted with a screw cap. Remove food from tin cans. If boiling liquids in a microwave oven use a wide moth container. Eggs cooked in the microwave may explode. Egg yolk contains fat so tends to cook more quickly than egg white. Even out of the shell, eggs may explode in the microwave because rapid heating causes a build-up of steam, so before cooking puncture egg yolks and whites. Also,  puncture skins of potatoes, apples, sausages and oysters to allow steam to escape. Food with a high fat content, e.g. sausage roils, may catch fire if overcooked.
5. To observe the effect of the wavelength of microwaves take out the turntable and put a one cup glass measure with water and a long stick of uncooked spaghetti. Heat on high for one minute. The spaghetti will have an uncooked area every 12 cm.