Carbohydrates © PDST Home Economics. Photosynthesis The process by which green plants use energy from the sun to change carbon dioxide and water into.

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. Monosaccharides2. Disaccharides3. Polysaccharides

Monosaccharides- e.g glucose

CH2OH

C

H

OH

H

O

C

C C

C

H

OH H

OH

H

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 C12H22O11C6H12O6 +C6H12O6 C12H24O12

- H2O - H2O

C12H22O11 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 or2.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

ClassClass Chemical Chemical FormulaFormula

ExampleExample SourceSource

MonosaccharidesMonosaccharides CC66HH1212OO66GlucoseGlucose

FructoseFructose

GalactoseGalactose

FruitFruit

HoneyHoney

Digested Digested milkmilk

DisaccharidesDisaccharides CC1212HH2222OO1111 Maltose=Glucose+GlucoseMaltose=Glucose+Glucose

Sucrose=Glucose+FructoseSucrose=Glucose+Fructose

Lactose=Glucose+GalactosLactose=Glucose+Galactosee

BarleyBarley

Table sugarTable sugar

MilkMilk

PolysaccharidesPolysaccharides

(Complex Carbs)(Complex Carbs)(C(C66HH1010OO55))nn StarchStarch

Cellulose Cellulose non-starchnon-starch

Pectin Pectin poly- poly-

Glycogen Glycogen saccharidessaccharides

Bread, pastaBread, pasta

Whole Whole cerealscereals

Fruit cell wallFruit cell wall

Liver and Liver and muscle cellsmuscle 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. Sugar2. Starch3. 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)

5. Maillard ReactionSugar (Carbohydrate) + Amino Acid + Dry

Heat = Browning of foods, e.g. roast potatoes6. SweetnessSugar has varying degrees of sweetness based

on a point scale using the tasting methodSucrose has a relative sweetness of 100Fructose has a relative sweetness of 170Lactose has a relative sweetness of 15

1.Properties of sugar cont….

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

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

1.Properties of sugar cont….

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

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

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

6. Hydrolysis Hydrolysis is a

chemical breakdown of a molecule by adding water to produce smaller molecules

Disaccharides become monosaccharides partly due to hydrolysis

2. Properties of Starch

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 & Apples2. Heat needs to be applied to the fruit3. 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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