Carbohydrates
© PDST Home Economics
Photosynthesis
• The process by which green plants use energy from the
sun to change carbon dioxide and water into glucose and
oxygen.
• Carbon dioxide + Water → Glucose + Oxygen.
•
6CO2
+ 6H2O → C6H12O6 + 6O2
Classification of Carbohydrates
There are 3 types of carbohydrates
1. Monosaccharides
2. Disaccharides
3. Polysaccharides
Monosaccharides- e.g glucose
CH2OH
C
O
H
H
H
C
OH
C
OH
H
C
C
H
OH
OH
Monosaccharides
• A Monosaccharide contains one sugar unit
• C6H12O6 is the chemical formula of a monosaccharide
• Glucose, fructose and galactose are the 3
monosaccharides
Disaccharides
• Are formed when two monosaccharides join together
with the elimination of water (condensation)
• There are three disaccharides: maltose, sucrose &
lactose
• The chemical formula is C12H22O11
C6H12O6
+C6H12O6
C12H24O12
- H2O
C12H22O11
Condensation reaction
Polysaccharides
• These are formed when three or more
monosaccharides join together with a loss of a water
molecule each time.
• They may be straight or branched
• Examples: Starch, pectin, cellulose, gums & glycogen
• Pectin, cellulose & gums are also known as Non-Starch
Polysaccharides
• Starch is made up of glucose units arranged as
follows:
1.Straight chains are known as amylose or
2.Branched chains are known as amylopectin
Polysaccharides continued….
• Formula: (C6H10O5)n
C6H12O6
--- H2O
(C6H10O5)n n=the number of times a bond is formed
Chemical structure of a polysaccharide
Classification of Carbohydrates
Class
Chemical
Formula
Example
Source
Monosaccharides C6H12O6
Glucose
Fructose
Galactose
Fruit
Honey
Digested
milk
Disaccharides
C12H22O11
Maltose=Glucose+Glucose
Barley
Sucrose=Glucose+Fructose Table sugar
Lactose=Glucose+Galactose Milk
Polysaccharides
(Complex Carbs)
(C6H10O5)n Starch
Cellulose
Pectin
Glycogen
saccharides
non-starch
poly-
Bread, pasta
Whole
cereals
Fruit cell wall
Liver and
muscle cells
Non-Starch Polysaccharides
• These are also known as NSPs, dietary fibre and
roughage
• NSPs cannot be digested in the body and absorb
large amounts of water
• They aid the removal of waste from the body by a
process known as peristalsis
• Peristalsis is the muscular movement of food along
the gut
• Sources of NSPs include wholemeal bread, brown rice
& wholemeal pasta
• Refined foods contain few if any NSPs
Properties of
Carbohydrates
1. Sugar
2. Starch
3. Non-Starch
Polysaccharides
1.Properties of Sugar
1.Solubility
 Sugars are white crystalline
compounds that are soluble in
water
 Solubility is increased by
heating the water
 A syrup is formed when sugar
is heated
2. Assists Aeration
 Sugar denatures egg protein,
enabling aeration to occur, e.g.
in the making of sponge cakes
– the egg when whisked with
sugar becomes aerated
3. Crystallisation
 This occurs if more
sugar is added than can
be absorbed by a liquid
 Crystal particles are
formed when the
mixture cools
 Crystallisation is used in
the confectionery and
sweet industry
1.Properties of Sugar cont….
4. Caramelisation
 When sugars are heated, they produce a range of
brown substances know as a caramel
 There are ten gradual changes in sugar between
melting and caramelisation
 These stages occur between 104°C & 177°C
 Eventually, the heat will cause carbonisation (burning)
1.Properties of sugar cont….
5. Maillard Reaction
Sugar (Carbohydrate) + Amino Acid + Dry Heat
= Browning of foods, e.g. roast potatoes
6. Sweetness
Sugar has varying degrees of sweetness based
on a point scale using the tasting method
Sucrose has a relative sweetness of 100
Fructose has a relative sweetness of 170
Lactose has a relative sweetness of 15
1.Properties of sugar cont….
7. Hydrolysis
 Hydrolysis is the chemical breakdown of a molecule by
adding water to produce smaller molecules
 This occurs when water is added to a disaccharide to
produce two monosaccharides
 Hydrolysis is the reverse of the condensation
reaction
1.Properties of sugar cont….
8.Inversion
 The hydrolysis of sucrose is also known as the
inversion of sucrose (mixture of glucose & fructose),
known as ‘invert sugar’
 Inversion may be brought about by either: (a) heating
sucrose with an acid; or (b) adding the enzyme
invertase, or sucrase
 Invert sugar is used in production of jam
2. Properties of Starch
1. Flavour
 Starch (a white powder) is not sweet in flavour
2.Solubility
 Starch is insoluble in cold water
3. Hygroscopic
 This property relates to how starch absorbs moisture
from the air e.g. biscuits soften if they are not kept
air tight
2. Properties of Starch
4. Dextrinisation
 Dextrins are shorter chains of starch
 On heating, dextrins form longer chains & become
brown-coloured substances called pyrodextrins
 An example of dextrinisation is toasting bread
2. Properties of Starch cont…..
5. Gelatinisation is based on the principal that when
starch is heated in the presence of water, starch
grains swell, burst & absorb the liquid, resulting in the
thickening of the liquid
 As the temperature rises, this mixture becomes even
more viscous, forming a sol (A sol contains particles
that do not fully dissolve but are evenly dispersed
throughout the liquid)
 On cooling, this becomes a gel
 An example of this is using flour to thicken soups and
sauces
2. Properties of Starch
6. Hydrolysis
 Hydrolysis is a chemical
breakdown of a molecule
by adding water to
produce smaller
molecules
 Disaccharides become
monosaccharides partly
due to hydrolysis
Properties of Non-Starch
Polysaccharides
1. Cellulose
 Can absorb large amounts of water
 Cannot be digested, however adds bulk to
the diet (gives a feeling of fullness)
 Aids the removal of waste from the body
 Is insoluble in water
3. Properties of Non-Starch Polysaccharides
2. Pectin
 Pectin is a polysaccharide found in fruit and vegetables
 It is involved in setting jams & jellies
 The following shows the pectin change in the ripening of fruit:
Under-Ripe to ripe to Over-Ripe
Protopectin to Pectin to Pectic Acid
(pectose)
 For pectin extraction :
1. Use fruit rich in pectin, e.g. Blackcurrants & Apples
2. Heat needs to be applied to the fruit
3. Add an acid, e.g. Lemon juice changes protopectin to pectin
3. Properties of Non-Starch
Polysaccharides
3. Gel Formation
 When pectin is heated in the presence of acid and
sugar, water becomes trapped
 The long chains of polysaccharides cool to form a gel
 An example of this is in making jam
Effects of Heat on Carbohydrates
Dry Heat
 Carbohydrate foods
browns due to the
presence of
dextrins, e.g.Toast
 Sugar caramelises,
e.g. Caramel slices
 Maillard reaction
occurs because of
the interaction
between sugar &
amino acids, e.g.
roast potatoes
Moist Heat
 Cellulose softens, e.g.
cooked vegetables
 Starch grains swell,
burst & absorb liquid,
e.g. flour used to
thicken sauces
 Pectin is extracted by
heating fruit in water
with sugar & acid, e.g.
jam making
 Sugar dissolves in
warm liquid, e.g.
making syrups
Culinary uses of sugar
• Sweetener - desserts
• Preservative - jam
• Caramelisation – caramel
custard
• Fermentation – yeast bread
• Gel formation – sugar
combines with pectin to
form gel – jam making.
• Colour – a sugar solution
prevents discolouration of
cut fruit.
Culinary uses of starch
• Thickener – sauces, soups,
stews.
• Hygroscopic – absorbs
moisture to increase shelf
life of cakes, keeps baking
powder dry.
• Dextrinisation – browning
e.g.toast.
Culinary uses of
non-starch
polysaccharides
• Gel formation – jam pectin
forms gel with acid and
sugar.
• Cellulose absorbs moisture
and gives feeling of
fullness.
• Cellulose adds texture e.g.
breakfast cereals
Biological Functions of
Carbohydrates
• Carbohydrates are used for heat and energy for the
body.
• They spare protein so it can be used for growth and
repair.
• Excess carbohydrate is changed to glycogen and stored
in liver and muscle as an energy reserve or it is changed
to body fat (adipose tissue) which insulates the body.
• Cellulose moves food through intestine preventing
constipation.
Digestion of Carbohydrates
Mouth:
Physically broken by teeth.
Salivary Amylase breaks Starch into Maltose.
Stomach:
Physically churned up.
Intestine:
Pancreatic juice
Amylase breaks
Starch into Maltose.
Intestinal Juice:
Maltase breaks
Maltose into Glucose.
Sucrase breaks
Sucrose into Glucose & Fructose.
Lactase breaks
Lactose into Glucose and Galactose.
Absorption
Absorption:
Monosaccharides are
absorbed through the villi
of the small intestine into
the blood stream and are
carried to the liver in the
portal vein.
Assimilation of Carbohydrates
 Monosaccharides can be oxidised to produce energy
(cellular respiration).
 Some monosaccharides are changed to glycogen and
stored in liver and muscle as an energy reserve.
 Excess carbohydrate is changed to body fat and stored
in the adipose tissue under the skin.
 Vitamin B1, B2 and Pyrodoxine are needed to metabolise
carbohydrates.
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