Download carbohydrates

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Date: __________________________________
CARBOHYDRATES
1.
2.
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Carbohydrates are the cheapest and most abundant nutrients available.
Their main function is the production of heat and energy.
Animals cannot synthesise carbohydrates.
Carbohydrates are chiefly found in plant foods where they are manufactured by
photosynthesis.
Functions of Carbohydrate
1. Provide _____________________ for physical activities.
2. Supply ______________. Excess carbohydrate is converted into fat and stored
as adipose tissue beneath the skin. This has the advantage of reducing heat loss
but too much causes obesity.
3. Cellulose stimulates the peristaltic movements of intestine. It aids ___________
and prevents ________________.
What is the elemental composition of
carbohydrate?
 ______________________________
 ______________________________
 ______________________________
Types of Carbohydrates
There are _______________ main types of carbohydrates:
1. _________________________________
2. _________________________________
3. _________________________________
Monosaccharides and disaccharides are often called ________________ sugars
whereas polysaccharides are often called ________________ sugars.
Monosaccharides
A monosaccharide is made of a single sugar molecule. They are ____________
_______________ __________ which dissolves easily in water to form
________________ _______________ solutions.
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Examples of monosaccharides are ___________________, _________________
and ____________________. Glucose and fructose can be found naturally in fruit.
Galactose can be found in milk products.
(a) Fructose
 Fructose is sometimes called ‘fruit sugar’ because it is found predominantly in
fruits, plant juices and honey.
(b) Glucose
 Glucose is used by the body for energy. All other carbohydrates are converted
into glucose during digestion.
 Glucose is found in ripe fruits and some vegetables e.g. onions, beetroot.
 It is also available commercially in powdered, liquid or tablet form. It provides
a fast source of energy and is often taken by athletes for this reason.
(c) Galactose
 Galactose is found in the milk of mammals where it forms part of the milk
sugar, lactose.
Disaccharides
Disaccharides contain two sugar molecules. Dissacharides are _________
____________ ____________ that are ___________ and ________________ in
water. Each disaccharide is made of ___________ ________________ molecules.
Examples of disaccharides are _________________ (cane sugar or household
sugar), ________________ (milk sugar) and __________________ (malt sugar).
Sucrose and lactose can be found naturally in fruit and milk respectively. Maltose is
obtained from cereal grains.
(a) Sucrose
 Sucrose is formed from one unit of glucose and one unit of fructose.
 Sucrose is most commonly used in cookery and is obtained by refining sugar
cane or beet.
 It is also present in some fruits and vegetables.
(b) Lactose
 Lactose is formed from one unit of glucose and one unit of galactose.
 Lactose is found in the milk of mammals to supply the infant with a source of
energy.
 It is not as sweet as sucrose.
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(c) Maltose
 Maltose is formed from two units of glucose.
 It is sometimes called ‘malt sugar’ and is found in cereals such as barley
where it is formed during germination.
Polysaccharides
Polysaccharides are made up of _________ ___________________ molecules that
are linked together. Unlike monosaccharides and disaccharides, most
polysaccharides are not ______________ or ______________ in cold water.
Examples of polysaccharides are ___________________, ___________________,
___________________ and ___________________. Glycogen, pectin and cellulose
are collectively referred to as ________________ _________________.
(a) Starch
 After photosynthesis, plants store their reserves of glucose by converting
them to starch.
 Starch is made up of many glucose molecules
linked together.
 Starch is plentiful in cereals such as rice,
wheat, oats and is present in potatoes, pulse
vegetables and some root vegetables.
 Starch is insoluble in cold water and difficult
digest.
 Cooking will penetrates or breaks the outer
covering, gelatinising the starch granules
making it more digestible.
(b) Dextrin
 Dextrin is formed when foods containing starch e.g. bread are baked or
toasted. The dextrin forms part of the crust on such foods and is more soluble
than starch.
(c) Cellulose
 Cellulose consists of hair-like fibres which are not sweet and which do not
dissolve in water.
 Like starch, cellulose is also formed by many glucose molecules that are
linked together. However, in cellulose, the molecules are linked differently.
 Cellulose is found in the cell walls of plant cells. It is a strong structural
material that provides support to plants. Thus, all plant foods contain
cellulose.
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(d) Pectin
 Pectin is a polysaccharide that acts as a cementing agent between plant cells.
 It can be extracted for use as a setting agent for making spreadable gels.
 Pectin is commercially extracted from citrus peel and apple pulp to make fruit
jellies, jams and marmalades.
(e) Glycogen
 Some of the glucose in the body that is not used up immediately is converted
to glycogen.
 Glycogen is temporarily stored in the liver and muscle cells and used as an
energy reserve.
 When the body suddenly requires a supply of energy, glycogen can be quickly
converted to glucose.
 However, if the energy is not used up, the glycogen will be converted to fat.
Sources of Carbohydrates
Simple carbohydrate foods are foods that
are naturally rich in simple sugars such as
glucose, fructose, sucrose, lactose and
maltose. Honey produced by bees and most
varieties of fruits such as apples and bananas
are good sources of the simple sugars
glucose and fructose.
Complex carbohydrate foods are high in
polysaccharide
starch.
These
include
underground stem and root vegetables such as
potato, tapioca and yam as well as cereals such
as rice, wheat and corn. Cereal grains can also be
milled into flour and used to make a wide range of
products including bread, pasta and noodles.
These are also high in complex carbohydrates.
Processed foods such as candy, ice cream, cakes
and biscuits are also sweetened with sucrose.
However, excess intake of these processed sources of simple sugars is discouraged
as they tend to be stripped of other nutrients.
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Carbohydrate Requirements
 About two-thirds of the average diet consists of a mixture of carbohydrates.
 In poor countries, over 80% of the diet may be composed of carbohydrate foods.
 In wealthy countries, carbohydrates form less than 50% of the diet.
Effects of Deficiency
Except in cases of actual starvation, it is rare to find insufficient
carbohydrate in the diet, as it is the cheapest food available.
Dietary Problems due to Excess Intake of Carbohydrates
 If someone eats more carbohydrate than the body requires, the
excess is converted into fat and stored under the skin. This is
one of the major causes of ________________.

Eating too many carbohydrates in the form of sugar can lead to
_____________
________________.
This
also
leads
to
___________________.

Eating over-processed foods such as white bread, white sugar and white rice can
lead to _______________ ____________________ and _________________ of
the _____________________. This is because processed food lacks roughage
(dietary fibres) which aids digestion.
Carbohydrates in Food Preparation
Caramelisation
When sugars are heated at high temperatures in the presence of water, they can
undergo caramelisation. During caramelisation, the water molecules evaporate,
leaving behind a concentrate brown substance known as caramel. Caramel has a
toffee-like aroma and flavour. Caramelisation contributes to the attractive flavour and
brown colour of toasted bread and roasted onions.
Maillard Browning
When heated at high temperatures, some sugars can undergo a browning reaction in
the presence of nitrogen-containing compounds such as proteins. This is known as
Maillard Browning (refer to Protein notes).
Dextrinsation
When starch is heated or treated with hot acid, the long starch chains become
hydrolysed, that is, they break down into shorter chains called dextrins. This process
is called dextrinisation. Dextrins are flavourless compounds that are often used as
fillers and flavour carriers in food seasonings.
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Gelatinisation
When starch is heated in a solution that contains water, the starch granules undergo
gelatinsation. In this process, the starch granules lose their ordered structure and
absorb water. This results in the thickening of the whole solution. The gelatinisation
of starch is important in the preparation of many food products, such as custard
sauce.
Gelation
If the proportion of starch molecules to liquid is high enough, a gel may form
after cooking. A gel is a semi-rigid substance consisting of a liquid trapped in
a network of molecules. In a starch gel, the amylase molecules form a net-like
structure, entrapping the liquid. Starch gels can be seen in custards, where
corn flour is often used as a gelling agent. Many sauces and stews are
thickened with gelatinised starch and may also form gels when cooled.
Carbohydrates
Written exercise
1. Name the three chemical elements found in carbohydrates.
2. What is the body’s main use for carbohydrates?
3. State three properties:
(a) Monosaccharides
(b) Disaccharides
(c) Polysaccharides
4. What happens when you eat too much carbohydrate food?
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