School Science Lessons
Topic 16b Proteins, amino acids, esters, aromatic hydrocarbons,
breakdown of molecules, organic chemistry terms
2009-09-18
Please send comments to: J.Elfick@uq.edu.au
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16.6.1.1 Proteins,
peptides, amino acids
16.5.1.0
Esters, derivatives of fatty acids, (RCOOR') Esters group: (-COOR)
suffix: -oate
16.8.0 Aromatic hydrocarbons
16.10.0 Breakdown of
large
molecules to small molecules
16.11.0 Organic chemistry terms
16.6.1.1 Proteins,
peptides, amino acids
16.6.3
Prepare protein solutions
16.6.9 Nitrogen in an organic compound, Kjeldahl
method
16.6.12 Proteins are amphoteric
16.6.13 Urea forms biuret
16.6.14 Reactions of urea with sodium
hypochlorite
16.6.15 Reactions of urea with nitrous acid
16.6.16 Reactions of urea with soda lime
16.6 17 Hydrolysis of urea with urease
16.6.18 Urea acts as a base
16.5.1.0
Esters, derivatives of fatty acids, (RCOOR') Esters group: (-COOR)
suffix: -oate
16.5.1 Prepare ethyl chloride
16.5.1.1 Ethyl acetoacetonate
(ethyl
3-oxobutanoate)
16.5.2 Prepare ethyl acetate (ethyl
ethanoate) 1
16.5.4 Hydrolysis of esters
16.5.5 Prepare methyl salicylate
(oil of
wintergreen)
16.5.6 Prepare amyl acetate (pear
oil)
16.5.7 Prepare ethyl butyrate
16.5.8 Prepare ethyl acetate (ethyl
ethanoate) 2
16.5.9 Prepare methyl chloride
16.5.10 Rubbing alcohol, surgical
spirit
19.2.9
Pectin in jelly and jam
19.2.9.1
Jelly using fresh pineapple and
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 Prepare 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, trimethylamine, choline
19.2.21a Choline
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 harmful substances
in
cigarette smoke
19.2.29 Toxic effect of common drugs
on Daphnia
19.2.30 Tests for chewing gum
quality by
comparing bubbles
10.5.5 Steam distillation to
find water and fat content of food
16.8.0 Aromatic
hydrocarbons
16.8.1
Reactions of benzene
16.8.2
Prepare ferric tannate with
tea leaves
16.8.3
Extraction of caffeine and
benzoic acid from soft drinks, e.g. cola and lemonade
16.10.0 Breakdown of
large
molecules to small molecules
16.10.2
Breakdown of sugar with yeast
16.10.4
Prepare
wood gas and wood tar
16.10.4.1
Distil wood (destructive
distillation)
16.11.0 Organic
chemistry terms
16.11.1 Alkyd resin
16.11.2 Alkylation
16.11.4 Catenation
16.11.5 Citric acid cycle, Krebs cycle
16.11.6 Conjugated
16.11.7 Denature
16.11.8 Epoxy
16.11.9 Esterification
16.11.10 Fume cupboard, fume hood, fume cabinet
16.11.11 Hydrogenation
16.11.12 Hydrolysis
16.11.13 Peptides
16.11.14 Phosphorylation
16.11.15 Racemic
16.11.16 Sulfonation
16.11.17 Tautomer
16.5.1.0 Esters,
derivatives of fatty
acids (RCOOR') Esters group: (-COOR) suffix: (-oate)
Esters (RCOOR', R(C=O)OR') (-oate) derivatives of fatty acids: ethyl
ethanoate (ethyl acetate) (CH3COOC2H5,
CH3C=OOCH2CH3) [glyceride (acyl
glycerol) fatty acid ester of glycerol: HOCH2CH(OH)CH2OH]
Esters include methyl buranoate apple, ethyl methanoate rum essence,
ethyl butanoate pineapple oil, pentyl ethanoate banana, octyl butanoate
orange, methyl salicylate oil of wintergreen, amyl acetate pear oil.
16.5.1 Prepare ethyl chloride
Ethyl chloride (C2H5Cl) is an alkyl halide.
See 16.2.2: Halogen compounds,
haloalkanes
(alkyl halides) | See
diagram 1.13:
Smelling chemicals
Pour
ethyl alcohol or methylated spirit into a test-tube. Note the odour.
Test the liquid with litmus paper. No colour change occurs. Add dilute
hydrochloric acid. Heat the mixture gently by putting the test-tube in
hot water. Smell
any gas coming from the test-tube.
Be careful! Do not inhale gases directly from the test-tube. Fan the
gas towards the nose with the hand and sniff cautiously. If no odour is
detected, move closer and try again.
Cool the mixtures and add drops of concentrated sulfuric acid. Heat the
mixture gently by putting the test-tube in hot water.
Be careful! Smell for not more than one second. The gases may cause
general anaesthesia. Note the sweet "ethereal" smell.
The heated sulfuric acid acts as a catalyst.
HCl (aq) + C2H5OH (l) --> C2H5Cl
(g)
+ H2O (l)
hydrochloric acid + ethanol --> ethyl chloride + water
16.5.1.1 Ethyl
acetoactonate (ethyl 3-oxobutanoate)
ethyl ethanoate + sodium ethoxide --> ethyl acetoacetonate
(3-oxobutanoate), CH3COCH2COOC2H5
(acetoacetic ester) (hydrolysis + acid) --> acetoacetic acid
(3-oxobutanoic acid), CH3COCH2COOH (unstable
beta-keto acid)
CH3COCH2COOH → CH3COCH3 + CO2
acetoacetic acid --> acetone + carbon dioxide
16.5.2 Prepare ethyl acetate
(ethyl ethanoate) 1
Alcohols react with organic acids to produce esters and water. Esters
are non-electrolytes, so they must be heated to speed the reaction.
Sulfuric acid is used for a dehydrating agent and catalyst to join the
other portions of the reactant alcohol and acid to produce the ester.
Ethyl ethanoate (ethyl acetate, acetic ether [CH3COOC2H5])
is a colourless liquid with a fruity smell used as a solvent for
lacquers and paints. Esters of low molecular mass have fruity smells
and are found in flavours and perfumes. The semi-structural formula is R1COOHR2.
R = alkyl groups, e.g. R1 = CH3 and R2 =
C2H5. When you heat a mixture of ester and water
it produces a mixture of alkanoic acid and alkanol in an equilibrium
mixture.
Mix other organic acids with an alcohol.
Add drops of concentrated sulfuric acid then heat the test-tube gently
in hot water. Note the odour of the ester, e.g. pentyl ethanoate smells
of apricots and octyl acetate smells of oranges. Add five drops of
ethanoic acid (glacial acetic acid) to five drops of ethyl alcohol with
one drop of concentrated sulfuric acid as a catalyst. Heat the
test-tube gently. Note the fruity odour of ethyl acetate and the sharp
odour of acetic acid.
alkanol + alkanoic acid <--> ester + water
R1COOH + R2OH <--> R1COOR2
+
H2O
organic acid + alcohol <--> ester + water
CH3COOH + C2H5OH <--> CH3COOC2H5
+
H2O
ethanoic acid + ethanol <--> ethyl ethanoate + water
(acetic acid + ethyl alcohol <--> ethyl acetate + water)
16.5.4 Hydrolysis of esters
See 12.12.0: Soaps and
synthetic detergents
1. Add acid to an ester.
The H+ of the acid catalyses the hydrolysis.
CH3COOC2H5 + HOH (H2O)
--> CH3COOH + C2H5OH
2. Add alkali to an ester. This is called
saponification because the
reaction is used to prepare soap.
CH3COOC2H5 + NaOH --> CH3COONa
+ C2H5OH
16.5.5 Prepare methyl salicylate (oil
of wintergreen)
Methyl salicylate (oil of wintergreen, HOC6H4COOMe)
has the odour of "oil of wintergreen" used for liniments. Add 1 g of
salicylic acid to a mixture of 1 mL of methyl alcohol
and three drops of sulfuric acid in a test-tube. Heat the test-tube
gently and note the odour of the ester produced by the reaction.
methyl alcohol + salicylic acid --> methyl salicylate (oil of
wintergreen)
16.5.6 Prepare amyl acetate (pear oil)
Amyl acetate (pear oil, pentyl ethanoate, CH3COOC5H11)
has the odour of bananas or pears. Mix 5 mL of ethanoic acid (acetic
acid) 3 mL of pentan-1-ol (amyl
alcohol, n-pentyl alcohol, C5H11OH) and 1 mL of
sulfuric acid in a test-tube. Heat the test-tube gently and note the
odour of the ester produced by
the reaction.
amyl alcohol (l) + acetic acid --> amyl acetate (banana or pear
oil)
16.5.7 Prepare ethyl butyrate (pineapple oil)
Ethyl butyrate has the odour of pineapples. Mix in a test-tube 1 mL of
concentrated sulfuric acid and 2 mL of
ethanol. Add 2 mL of n-butyric acid (butanoic acid, C3H7COOH).
It smells like rancid butter. Heat the test-tube gently and note the
odour of the ester produced by the reaction.
ethyl alcohol + butyric acid (l) --> ethyl butyrate (pineapple
oil)
16.5.8 Prepare ethyl acetate 2
1. Repeat the above experiment with two drops of glacial ethanoic acid
(acetic acid) in place of the ethanol. Connect a delivery tube from the
test-tube to a solution of limewater. Tests for carbon dioxide during
the reaction. The odour of acetic acid disappears. The reaction
produces ethyl acetate.
2. Mix 2 mL ethyl alcohol with 3 mL acetic acid in a test-tube. Add 3
drops concentrated sulfuric acid. Heat the mixture gently by immersing
the test-tube in hot water. Cautiously note the odour of the ethyl
acetate produced and compare it with the odours of ethyl alcohol and
acetic acid. Ethyl acetate has a fragrant odour different from the
wine-like odour of ethyl alcohol and the sharp odour of acetic acid.
[heated with sulfuric acid] ethyl alcohol (l) + acetic acid (aq)
-->
water (l) + ethyl acetate (aq)
16.5.9 Prepare methyl chloride
Pour 5 mL methyl alcohol and 5 mL ethyl alcohol into separate
test-tubes. Note their odours. Drop small pieces of red and blue litmus
paper into each liquid. Add to each about 5 mL of dilute hydrochloric
acid. If you see no visible signs of reaction, warm each mixture gently
by standing the tube in hot water for 5 minutes. Cautiously smell any
gas that may be coming from the test-tubes.
Be careful! Sulfuric acid is a corrosive chemical!
Cool
the mixtures and add a few drops of concentrated sulfuric acid to each
test-tube. If you see no visible signs of reaction, warm each
mixture gently by standing the tube in hot water for 5 minutes.
Cautiously smell any gas that may be coming from the test-tubes. Avoid
smelling for more than one second any gases liberated, since one of
them can cause general anaesthesia when inhaled in sufficient quantity.
Although there is no apparent reaction when hydrochloric acid is mixed
with either of the two alcohols, after heating the mixture with
concentrated sulfuric acid, a reaction occurs shown by the production
of a gas with a sweetish "ethereal" smell.
[heated with sulfuric acid] methyl alcohol (l) + hydrochloric
acid (aq) --> water (l) + methyl chloride (g)
16.5.10 Rubbing alcohol, surgical spirit
In many countries, rubbing alcohol is not isopropyl alcohol
(propan-2-ol) but is specified mixture of ethanol and water. Surgical
spirit is a methylated spirit, i.e. ethyl alcohol denatured with methyl
alcohol to prevent its use as an alcoholic beverage. Different brands
of surgical spirit may also contain other liquids. It is used to clean
body surfaces before surgery. However, some people use the term
surgical spirit for isopropyl alcohol.
16.6.3 Prepare protein solutions
Shake the white of an egg in its own volume of water. Squash peas
in water, filter and use the filtrate. Make a gelatine solution from
a commercial "jelly" preparation Collect solid proteins, e.g. hair,
feathers. Test on a microscope slide: plant juices, meat, soup, pieces
of tissue, sunflower seed.
16.6.9 Nitrogen in an organic compound,
Kjeldahl method
Be careful! Do this experiment in a fume cupboard.
Add 10 mL of concentrated sulfuric acid to 0.5 g of urea in a long
necked flask (Kjeldahl Flask). Add potassium hydrogen sulfate to raise
the boiling point of the acid and complete the decomposition of the
protein. Boil in a fume cupboard for 10 minutes. Leave to cool then add
100 mL water. Add strong sodium hydroxide solution (30%) and
anti-bumping granules. Distil the mixture. Tests for ammonia in the
distillate. For volumetric titration, pass all the ammonia through 1 M
acid solution. The ammonia neutralizes some acid. Titrate the acid left
over with an alkali to find how much acid used by the ammonia.
Repeat the experiment with 5 g egg albumin (egg white).
H2SO4 (aq) + 2NH3 (g) --> (NH4)2SO4
(aq)
16.6.12 Proteins are
amphoteric
Amino acids are amphoteric in that they contain both acidic and basic
groups in their molecules. Proteins dissolve in alkalis
and in concentrated solutions of acids. In alkaline solutions proteins
are
negatively charged. In strongly acid solutions proteins are positively
charged.
They are uncharged at the iso-electric point and are precipitated. At
pH higher than the iso-electric point, a
protein acts as an acid. At pH lower than the iso-electric point the
protein acts
as a base. When acting as an acid a protein forms a fast colour with a
basic dye, e.g. methylene blue. When acting as a base, a protein reacts
with an acid dye, e.g. eosin.
To show the amphoteric nature of a
protein, prepare four test-tubes, two containing eosin, and two
containing Millon's reagent methylene
blue.
1. Add a white feather or white wool to each test-tube. Add 3
drops of acetic acid to one eosin solution and 3 drops of concentrated
ammonia solution (ammonium
hydroxide) to the other eosin solution. Leave to stand for five
minutes. Wash the feather or wool and note the fast dyeing in the eosin
and acid solution. The
protein acted as a base in the presence of the acid, and reacted with
the
acid dye eosin.
2. Add 3 drops of acetic acid to one of the methylene
blue
solutions and 3 drops of concentrated ammonia solution (ammonium
hydroxide) to the other methylene blue solution.
Leave to stand for five minutes. Wash the feather or wool or
feather
and note the fast dyeing in the alkaline solution. The protein acts as
an acid in alkaline solution and reacts with the basic dye methylene
blue.
16.6.13 Urea forms
biuret
Heat some crystals of dry urea slowly in a test-tube until the liquid
which forms solidifies again as the white solid biuret. Dissolve the
biuret in water for use in the biuret reaction.
2NH2.CO.NH2 --> NH2.CO.NH.CO.NH2
+ NH3
16.6.14 Reactions of
urea with sodium hypochlorite
Dissolve crystals of urea in the minimum amount of water. Add drops
of
sodium hypochlorite solution. Nitrogen gas forms. Carbon dioxide gas
also forms but it dissolves in the alkaline solution.
NH2.CO.NH2 + 3NaOCl --> N2 +
2H2O
+ 3NaCl + CO2
16.6.15 Reactions of
urea with nitrous acid
Add an equal volume of dilute hydrochloric acid to a saturated solution
of sodium nitrite. Cool under the tap. When the effervescence has
moderated, add drops of a solution of urea. Nitrogen gas forms.
NH2.CO.NH2
+ 2HNO2 --> 2N2 + CO2 + 3H2O
16.6.16 Reactions of
urea with soda lime
Heat a mixture of urea and soda lime. Test the gas formed for ammonia.
NH2.CO.NH2 + 2NaOH --> 2NH3 +
Na2CO3
16.6.17 Hydrolysis of
urea with urease
Dissolve urea crystals in water.
Add a tablet of urease or soya flour and keep at 40oC.
for a minute. Tests for ammonia. The enzyme, urease, hydrolyses the
urea
NH2.CO.NH2 + H2O --> 2NH3
+ CO2
16.6.18 Urea acts as a
base
Add an equal volume of concentrated nitric acid to a saturated solution
of urea. The
white precipitate is urea nitrate, (NH2.CO.NH2.HNO3).
16.10.2 Breakdown of sugar with yeast
See 3.38:
Carbon dioxide and
fermentation for brewing
16.10.4 Prepare wood gas and wood tar
See diagram:
16.10.4
When heating sawdust strongly in a hard-glass test-tube, the gas coming
out of the test-tube can be ignited. This gas is called wood gas. It
contains carbon monoxide, hydrogen gas, methane, and other gases. The
oily dark
brown liquid left in the bottom of the test-tube is called wood tar. It
contains wood alcohol, propanone (acetone) ethanoic acid (acetic acid)
and other substances. If sawdust is heated without any air, the residue
will be wood
charcoal.
Fill a sidearm test-tube one half full of wood chips (or sawdust)
and fit a one-hole stopper with a delivery glass tube into the
test-tube.
When heating, observe that the wood chips gradually become black and an
oily dark brown liquid flows through the side arm of the test-tube. Use
a lighted match to ignite the gas coming out of the delivery tube. The
gas can burn steadily. Note that the volume of the product is very
small. The residue is charcoal (carbon).
16.10.4.1 Distil wood (destructive
distillation)
Distil wood in a furnace. Condense the products in copper tubing to
produce charcoal, pyroligneous acid, wood alcohol, propanone (acetone)
and ethanoic acid (acetic acid).
16.11.1 Alkyd resin
Adhesive and coating resins
made
from
glycerol and
unsaturated organic acids. They are rigid cross-linked polymers formed
when there are more than two functional groups on linear chain
monomers. They are use in paint enamels and making dentures.
16.11.2 Alkylation
Replacing a hydrogen on a
cyclic
compound with an alkyl CH3
or
longer chain group.
16.11.4 Catenation
Formation of chains of atoms.
16.11.5 Citric acid
cycle, Krebs cycle
A cycle
of
reactions in which ADP is
recharged to ATP as part of the energy conversion processes in the body.
16.11.6 Conjugated
Alternating double and single
bonds. Note that
polyunsaturated chains have "cis-methylene interrupted" or "skipped"
double bonds.
16.11.7 Denature
The tertiary structure of a
protein
collapses, denatured, by
heating, acid or agitation in air.
16.11.8 Epoxy
Oxygen directly linked to two
adjacent
bonded carbon atoms
forming a triangle.
16.11.9 Esterification
Forming an ester,
reaction
of organic acid with an
alcohol. Reverse process is ester hydrolysis, saponification, the
making of soap from fat.
16.11.10 Fume
cupboard, fume hood, fume cabinet
Enclosed
reinforced cupboard
with facilities for chemical
reactions and used under negative air pressure.
16.11.11 Hydrogenation
Addition of hydrogen to
a
molecule to convert
unsaturated molecules to saturated and reducing double bonds to single
bonds.
16.11.12 Hydrolysis
Splitting a molecule using
a
reaction with water.
16.11.13 Peptides
Amides derived from two or
more amino
carboxylic acid
molecules by formation of a covalent bond from the carbonyl carbon of
one to the nitrogen atom of another with loss of water. Peptides
include structures formed from alpha-amino acids and from any amino
carboxylic acid. C = any organyl group.
16.11.14
Phosphorylation
Adding a phosphate
group
to a molecule.
16.11.15 Racemic
A one-to-one mixture of left
handed and
right-handed, chiral,
forms of the same molecule. Most chemical reactions produce products as
racemic mixtures, whereas biological reactions generally produce one or
the other form only. R,R" The R designates an undefined organic group,
e.g. a hydrocarbon chain. The R it is not necessarily the same as R".
16.11.16 Sulfonation
Addition of the function
group
-SO3H to a
molecule.
16.11.17 Tautomer
When an atom, e.g. hydrogen,
moves
backwards and forwards
between different places on a molecule, the new and original molecules
form a tautomeric pair.
16.8.1 Reactions of benzene
See
16.3.4.0: Aromatics,
aromatic compounds
1. Add 1 mL bromine water to 5 drops of benzene in a test-tube. The
bromine water is not decolorized, unlike ethylene and acetylene with
bromine water.
2. Put 5 drops of benzene in 2 test-tubes. In one of the test-tubes
add iron filings. Add 3 drops of bromine water to each test-tube.,
Hydrogen bromide forms in both test-tubes but more in the test-tube
containing the iron filings ha acts as a catalyst.
C6H6 + Br2 --> C6H5Br
+ HBr
3. Add 5 drops of benzene to 1 mL of acidified potassium
permanganate solution. The permanganate solution decolorizes only when
the mixture is heated. The benzene is oxidized to lower molecular weigh
molecules.
16.8.2 Prepare ferric tannate with tea leaves
Tannin is a mixture of organic chemicals related to polyhydroxy-benzoic
acids. Tannin has a bitter taste and is astringent, i.e. it contracts
the mouth. It is found in the bark and other tissues of many plants
probably to control grazing. It is used to prepare black ink and
leather from animal hides.
1. Add 200 g (2 tea bags) of dried tea to 250 mL of boiling water.
2. Add an unused pad of steel wool to 100 mL of vinegar, boil for 10
minutes, then strain through cotton wool in a filter funnel. Leave to
cool then add 1 mL of hydrogen peroxide solution to produce a brown
red, indicating iron (III).
3. Add equal volumes of solution 1.
to solution 2. to produce a black solution of ferric tannate.
2H+ + Fe --> Fe2+ + H2
2H+ + 2Fe2+ + H2O2 -->
2Fe3+ + 2H2O
Fe3+ + tannic acid --> ferric tannate
16.8.3 Extraction of caffeine and benzoic acid
from soft drinks, e.g. cola and lemonade
See
1.10.0A: Purine group of
alkaloids, caffeine
1. Isolation of caffeine
Add 2 g of sodium carbonate to 50 mL of a
cola (kola) drink in a 1 litre conical flask. Add 50 mL of
dichloromethane (methylene chloride) and swirl gently for five minutes.
Do not shake. Transfer into a separating funnel and leave to settle for
10 minutes). Drain the lower methylene chloride layer into a 250 mL
conical flask. Add 50 mL more dichloromethane to the separating funnel
and enclose with a stopper. Carefully invert the separating funnel 3
times to allow any remaining caffeine to be extracted into the
dichloromethane layer. Again drain the lower methylene chloride layer
into the 250 mL conical flask. Add 5 g of anhydrous magnesium sulfate
to remove the water when it forms insoluble hydrated magnesium sulfate.
Filter the now clear dichloromethane through cotton wool pad into a 250
mL beaker. Evaporate the dichloromethane on a water bath in a fume
cupboard or distil it off to recover the solvent. Weigh the remaining
precipitate. Test the precipitate by putting a small amount on a watch
glass and mix with 3 drops of concentrated hydrochloric acid. Be
careful! Add small crystals of potassium chlorate. Mix with a glass rod
and evaporate to dryness on a water bath in a closed fume cupboard.
Leave the watch glass to cool then moisten the residue with 2 drops 2 M
ammonia solution. The residue turns purple
2. Isolation of benzoic acid
Pour half a drink-can of lemonade is
poured into a 1 L conical flask and add 2 drops of dilute hydrochloric
acid. Add 50 mL dichloromethane then swirled gently for five minutes.
Pour into a separating funnel and leave to allowed to settle for 5
minutes. Drain the solvent layer into a 100 mL beaker and leave to
evaporate in a fume cupboard. A residue of benzoic acid remains.
19.2.9 Pectin in jelly
and jam
See 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 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
watery white to chalk white in a firmly cooked egg. The protein
loses surrounding water and shrinks. The reaction is irreversible. You
cannot dissolve the solid egg white in water.
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 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 Prepare
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 breakdown
quickly during digestion have the highest glycemic indexes. The blood
glucose response is fast and high. Carbohydrates that breakdown
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
See 16.3.6.4: Alkaloids produced
by plants from
amino acids
Some glycoalkaloids, e.g. alpha solanine and alpha chaconine, are toxic
compounds in plants from the Solanaceae family, e.g. potato, tomato,
capsicum, tobacco. Potato tubers in sunlight form glycoalkaloids when
amyloblasts change to chloroplasts. Potatoes should be stored in cool
dark places where air circulates, but not in a refrigerator. The alpha
solanine has a bitter taste and the alpha chaconine is the more
toxic. Toxicity is caused by anticholinesterase activity on
the central nervous system and membrane disruption which irritates the
gastro-intestine of the digestive system. Do not purchase green,
bruised, insect-damaged or cut potatoes. Pregnant women should
especially avoid eating damaged potatoes to avoid possible birth
defects due to glycoalkaloids.. Potatoes should not be displayed under
strong fluorescent light in supermarkets. Potatoes can be irradiated to
delay sprouting
and prevent greening but not prevent the production of alpha solanine.
Cooking does not destroy the glycoalkaloids.
19.2.18 Extract
iron, Fe, from breakfast cereal
Powdered iron is added to breakfast cereal to "fortify" it. We can
digest some of it when the iron filings are oxidised in the stimachand
later absorbed by the small intestine. A steelworks could make a
smallnail from the iron in your body.
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 for 10 minutes. 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, choline
See diagram 16.3.3.0: Lipids (choline)
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, 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.
19.2.21a Choline, CH2OHCH2N(CH3)3OH,
is found in egg yolk, liver, kidney, soya beans, peas, and
whole grain wheat. Choline is an amino alcohol, a component of
phospholipids, a water-soluble essential lipid in cell membranes,
associated with vitamin B complex.
19.2.22 Laundry
starch
Make 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 Tests for chewing gum quality 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.