Functions of Food
• to maintain life
• to supply energy & give warmth
– carbohydrates
– fats
– proteins
• for growth and repair
– Proteins
• keep healthy & help to fight against disease
Carbohydrates
• elements : C, H, O
– ratio of H : O
=2:1
• monosaccharides
– C6H12O6
– all are sweet & soluble in water
– all are Reducing Sugars
– include Glucose, Fructose & Galactose
Carbohydrates
 disaccharides (from 2 monosaccharides)
– C11H22O11
– all are sweet & soluble in water
– Maltose ( 2 glucose molecules )
– Sucrose ( glucose + fructose)
(non-reducing sugar)
– Lactose ( glucose + galactose)
surcose
Carbohydrates
• polysaccharides (NOT sugar)
• for energy storage
– starch (store in plants)
glycogen (store in animals)
• Hydrolysis:
Polysaccharide +H2O
 Disaccharides +H2O
 Monosaccharides
Functions of Carbohydrates
 main source of energy
 cellulose:
- fibrous material of Plant Cell Wall
- dietary fibre: stimulates Peristalsis
 excess carbohydrates
- stored as glycogen in liver & muscle
- stored as fats under skin
Test for Reducing Sugars
(Benedict’s Test)
A
distilled
water +
Benedict’s
solution
B
glucose
solution +
Benedict’s
solution
water
bath
Is there any colour change in
tubes A and B ?
Ans: Only the mixture in tube
A has a colour change.
A
distilled
water +
Benedict’s
solution
B
glucose
solution +
Benedict’s
solution
water
bath
What is the sequence of
change when there is a colour
change ?
Ans: The blue solution changes
first to green, then to a
yellow coloration and
eventually a brick-red
precipitate is produced.
Test for Starch
(Iodine Test)
What is the final colour in
tube A ?
iodine solution
Ans: The solution in tube A
changes from brown
to blue black.
distilled
water
starch
solution
A
B
What is the purpose of
setting up tube B ?
iodine solution
Ans: To act as a control.
distilled
water
starch
solution
A
B
Proteins
• elements: C, H, O, N,
sometimes S, P
• components : amino acids
forms dipeptides & polypeptides
• Condensation:
Amino acids –H2O  Dipeptides
–H2O  Polypeptide
Proteins
 cannot be stored
- excess proteins are deaminated by liver
~ to Urea which will be excreted away by
Kidney
~ to Carbohydrates (Glycogen) which will
be stored in Liver
Functions of Proteins
• for growth and repair
(as structural components of cells)
• to produce hormones and
enzymes
• to give energy
• for making Haemoglobin in blood
• for making Antibodies
Test for Proteins
(Biuret Test)
What colour changes in
tubes A and B ?
Ans: Mixture in tube A
changes from blue
to purple while
mixture in tube B
remains blue
without any change.
copper sulphate solution
A
egg white +
sodium
hydroxide
solution
B
water +
sodium
hydroxide
solution
Lipids ( fats & oils )
• elements : C, H, O
• components of 1 lipid molecule:
1 glycerol + 3 fatty acids
• insoluble in water
• soluble in organic solvent
Functions of Lipids
 give energy
 component of cell membrane
 form fatty tissues under skin
> to store energy
> acts as insulator to keep warm
 to transport fat-soluble vitamins (A, D, E, K)
Spot Test for Fat
Which substance, oil or water,
leaves a permanent
translucent spot on the filter
paper ?
Ans: Oil.
Emulsion Test for Fat
2 drops of
cooking oil
A
2 drops of
cooking oil
B
2 cm3 of
alcohol
2 cm3 of
distilled
water
after shaking to
form a clear
solution
shake and then
allow to stand
2 cm3 of
distilled
water
2 cm3 of
distilled
water
shaking
shake and then
allow to stand
Which test tube has an emulsion formed ?
Ans: Test tube A.
emulsion
oil
water
2 drops of
cooking oil
A
2 drops of
cooking oil
B
2 cm3 of
alcohol
2 cm3 of
distilled
water
after shaking to
form a clear
solution
shake and then
allow to stand
2 cm3 of
distilled
water
2 cm3 of
distilled
water
shaking
shake and then
allow to stand
emulsion
oil
water
What happens to the other tube ?
Ans: The mixture separates into two layers because
fats do not dissolve in water.
Vitamins
• no energy value
• essential for small amount to
maintain good health
• water soluble vitamins
( B, C )
• fat soluble vitamins
( A, D, E, K )
 excessive of some vitamins
be harmful
may
Vitamin A
 formed in the body from Carrotene
(a yellow pigment in carrots)
 destroyed at high temperature
 essential for forming visual purple
(maintain dim light vision)
Vitamin C
 Destroyed after prolonged cooking
 Necessary for wounds-healing
Vitamin D
 Formed in Skin from Ultraviolet Light
 Help to regulate Ca & P metabolism
Detection of Vitamin C in Lemon
Juice by using DCPIP
syringe
What colour change has
occurred ?
lemon juice
Ans: The blue DCPIP
decolourizes.
DCPIP solution
syringe
lemon juice
DCPIP solution
What conclusion can you draw ?
Ans: Lemon juice contains
vitamin C which
decolourizes blue DCPIP.
Vitamin
A
C
D
Sources
Egg yolk, milk,
cheese, carrot,
green
vegetables
Fresh fruits &
green
vegetables
Cod liver oil
& egg yolk
Deficiency
Disease
Night blindness
Scurvy
Rickets
Food Tests
Substances
Test
Original
Colour
Positive Result
Reducing
Sugar
Benedict’s
Blue
Orange ppt
Starch
Iodine
Brown
Blue-black
Protein
Biuret
Blue
Violet
Fats/Oils
Spot
---
Translucent
spot
Fats/Oils
Emulsion
Clear
Milky emulsion
Vitamin C
DCPIP
Blue
Colourless
(decolourize)
Mineral Salts
 regulate body metabolism
 essential for healthy growth
 necessary for construction of certain
tissues
 needed in small amount
 include Ca, S, K, Na, Mg, Fe, I
Mineral(s)
Sources
Functions
Making bones &
teeth
Cheese, milk,
Calcium &
Important for
vegetables
Phosphorus
blood clotting &
muscle
contraction
Liver, eggs,
Structural
beef, green
Iron
component of
leafy
Haemoglobin
vegetables
Deficiency
Disease
Rickets
Anaemia
Dietary Fibre
 mainly cellulose
 indigestible material for human
 give bulk to food & stimulate peristalsis 
prevent Constipation
 lack of dietary fibre: Large Intestine Cancer
 sources: wholemeal cereals, unpolished
rice, fresh vegetables & fruits
Balanced Diet
• have enough food to supply enough
– energy
• carbohydrates, fats, proteins
– body building materials
• proteins
– substances to maintain health
• vitamins, minerals, water &
dietary fibres
• malnutrition : not having balanced
diets for long time
Energy Contents in Food
• Calorimeter is used to measure the
amount of energy contained in a
particular type of food
• Carbohydrate (17kJ/g)
• Protein (18kJ/g)
• Fat (39kJ/g)
Factors affecting energy
requirement
•
•
•
•
•
Sex
Age
Occupation
Physical Activities
Stage of individual
(pregnancy, breast-feeding)
Measure the Energy
Value of Food
Explain why the energy value
of the peanut is lower than
those from standard tables.
Ans: Because there are a
number of inaccuracies
associated with this
method due to
incomplete combustion
and heat loss.
thermometer
boiling tube
water
burning
peanut
Basal Metabolic Rate (BMR)
 minimum amount of energy needed by an
individual lying awake in bed to maintain
breathing, body temperature & heartbeat
 varies from one individual to another
 daily energy requirement > basal
metabolic rate
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