- Slides: 33
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. • 6 CO 2 + 6 H 2 O → C 6 H 12 O 6 + 6 O 2
Classification of Carbohydrates There are 3 types of carbohydrates 1. Monosaccharides 2. Disaccharides 3. Polysaccharides
Monosaccharides- e. g glucose CH 2 OH C O H H H C OH H C C H OH OH
Monosaccharides • A Monosaccharide contains one sugar unit • C 6 H 12 O 6 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 C 12 H 22 O 11 C 6 H 12 O 6 +C 6 H 12 O 6 C 12 H 24 O 12 - H 2 O C 12 H 22 O 11
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: (C 6 H 10 O 5)n C 6 H 12 O 6 --- H 2 O (C 6 H 10 O 5)n n=the number of times a bond is formed Chemical structure of a polysaccharide
Classification of Carbohydrates Class Chemical Formula Example Source Monosaccharides C 6 H 12 O 6 Glucose Fructose Galactose Fruit Honey Digested milk Disaccharides C 12 H 22 O 11 Maltose=Glucose+Glucose Barley Sucrose=Glucose+Fructose Table sugar Lactose=Glucose+Galactose Milk Polysaccharides (Complex Carbs) (C 6 H 10 O 5)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 3. Crystallisation v Sugars are white crystalline v This occurs if more compounds that are soluble in sugar is added than can water be absorbed by a liquid v Solubility is increased by v Crystal particles are heating the water formed when the mixture cools v A syrup is formed when sugar is heated v Crystallisation is used in the confectionery and 2. Assists Aeration sweet industry v Sugar denatures egg protein, enabling aeration to occur, e. g. in the making of sponge cakes – the egg when whisked with sugar becomes aerated
1. Properties of Sugar cont…. 4. Caramelisation v When sugars are heated, they produce a range of brown substances know as a caramel v There are ten gradual changes in sugar between melting and caramelisation v These stages occur between 104°C & 177°C v Eventually, the heat will cause carbonisation (burning)
1. Properties of sugar cont…. 5. Maillard Reaction v Sugar (Carbohydrate) + Amino Acid + Dry Heat = Browning of foods, e. g. roast potatoes 6. Sweetness v Sugar has varying degrees of sweetness based on a point scale using the tasting method v Sucrose has a relative sweetness of 100 v Fructose has a relative sweetness of 170 v Lactose has a relative sweetness of 15
1. Properties of sugar cont…. 7. Hydrolysis v Hydrolysis is the chemical breakdown of a molecule by adding water to produce smaller molecules v This occurs when water is added to a disaccharide to produce two monosaccharides v Hydrolysis is the reverse of the condensation reaction
1. Properties of sugar cont…. 8. Inversion v The hydrolysis of sucrose is also known as the inversion of sucrose (mixture of glucose & fructose), known as ‘invert sugar’ v Inversion may be brought about by either: (a) heating sucrose with an acid; or (b) adding the enzyme invertase, or sucrase v Invert sugar is used in production of jam
2. Properties of Starch 1. Flavour v Starch (a white powder) is not sweet in flavour 2. Solubility v Starch is insoluble in cold water 3. Hygroscopic v 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 v Dextrins are shorter chains of starch v On heating, dextrins form longer chains & become brown-coloured substances called pyrodextrins v 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 v 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) v On cooling, this becomes a gel v An example of this is using flour to thicken soups and sauces
2. Properties of Starch 6. Hydrolysis v Hydrolysis is a chemical breakdown of a molecule by adding water to produce smaller molecules v Disaccharides become monosaccharides partly due to hydrolysis
Properties of Non-Starch Polysaccharides 1. Cellulose v Can absorb large amounts of water v Cannot be digested, however adds bulk to the diet (gives a feeling of fullness) v Aids the removal of waste from the body v Is insoluble in water
3. Properties of Non-Starch Polysaccharides 2. Pectin v Pectin is a polysaccharide found in fruit and vegetables v It is involved in setting jams & jellies v The following shows the pectin change in the ripening of fruit: Under-Ripe to ripe to Over-Ripe Protopectin to Pectic Acid (pectose) v 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 v When pectin is heated in the presence of acid and sugar, water becomes trapped v The long chains of polysaccharides cool to form a gel v 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: 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 q Monosaccharides can be oxidised to produce energy (cellular respiration). q Some monosaccharides are changed to glycogen and stored in liver and muscle as an energy reserve. q Excess carbohydrate is changed to body fat and stored in the adipose tissue under the skin. q Vitamin B 1, B 2 and Pyrodoxine are needed to metabolise carbohydrates.
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