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Revision questions

Revision questions. Classify carbohydrates according to their structure. Give two examples of each. Write a note on (a) pectin, (b) starch, (c) glycogen and (d) cellulose. Give an account of the properties of sugar; cite a culinary example in each case.

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Revision questions

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  1. Revision questions • Classify carbohydrates according to their structure. Give two examples of each. • Write a note on (a) pectin, (b) starch, (c) glycogen and (d) cellulose. • Give an account of the properties of sugar; cite a culinary example in each case. • Give an account of the properties of starch; cite a culinary example in each case. • Give an account of the properties of cellulose. • What is the RI of fibre? • List four ways of increasing fibre in the diet. • Enumerate the biological functions of carbohydrates. • Outline the effects of heat on sugars and starches. • What is the energy value of carbohydrates? • Differentiate between gel formation and gelatinisation. • With regard to current healthy eating guidelines, suggest three ways that food sources of carbohydrate in the diet should be adjusted. Give a reason in each case. • Outline the digestion and absorption of carbohydrate.

  2. Q1. Classify carbohydrates according to their structure and give two examples in each class

  3. Q1. Classify carbohydrates according to their structure and give two examples in each class There are three types of carbohydrates: Monosaccharides Disaccharides Polysaccharides

  4. 1. Monosaccharides A monosaccharide contains one sugar unit. C6H12O6 is the chemical formula of a monosaccharide. Glucose, fructose and galactose are all monosaccharides.

  5. 2. Disaccharides These are formed when two monosaccharides join together with the elimination of water (condensation). There are three disaccharides: maltose, sucrose and lactose. The chemical formula is C6H12O6.

  6. 3. Polysaccharides • Polysaccharides 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 include starch, pectin, cellulose, gums and glycogen. • Pectin, cellulose and gums are also known as non-starch polysaccharides. • Starch is made up of glucose units arranged as follows: • Straight chains are known as amylose • Branched chains are known as amylopectin.

  7. Q2. Write a brief note on (a) pectin, (b) starch, (c) glycogen and (d) cellulose (a) Pectin Pectin is a polysaccharide found in fruit and vegetables. It is involved in setting jams and 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: Use fruit rich in pectin, e.g. blackcurrants and apples Heat needs to be applied to the fruit Add an acid, e.g. lemon juice changes protopectin to pectin.

  8. (b) Starch Starch is a polypeptide chain made up of glucose units. (c) Glycogen Glycogen is the carbohydrate that is stored in the liver of an animal or human. Glycogen is the only carbohydrate from an animal source and acts as an energy store. Q2. Write a brief note on (a) pectin, (b) starch, (c) glycogen and (d) cellulose

  9. (d) Cellulose Can absorb large amounts of water. Cannot be digested, but it adds bulk to the diet, i.e. gives a feeling of fullness. Aids the removal of waste from the body. Is insoluble in water. Q2. Write a brief note on (a) pectin, (b) starch, (c) glycogen and (d) cellulose

  10. Q3. Give an account of the properties of sugar; cite a culinary example in each case 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.

  11. 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. 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 ̶ 177°C. Eventually, the heat will cause carbonisation (burning).

  12. 5. The 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.

  13. 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 and 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 sucrose. Invert sugar is used in jam production.

  14. Culinary uses of sugar Sweetener – desserts, drinks Preservative – jam, relish Caramelisation – caramel custard, crème brûlée Fermentation – yeast bread, wine-making Gel formation – sugar combines with pectin to form gel (jam-making) Colour – a sugar solution preventsdiscolouration of cut fruit

  15. Q4. Give an account of the properties of starch; cite a culinary example in each case • 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 airtight.

  16. 4. Dextrinisation Dextrins are shorter chains of starch. On heating, dextrins form longer chains and become brown-coloured substances called pyrodextrins. An example of dextrinisation is toasting bread. 5. Hydrolysis Hydrolysis is a chemical breakdown of a molecule caused by adding water to produce smaller molecules. Disaccharides become monosaccharides partly due to hydrolysis.

  17. 6. Gelatinisation • Gelatinisation is based on the principal that when starch is heated in the presence of water, starch grains swell, burst and 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.

  18. 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.

  19. Q5. Give an account of the properties of cellulose Cellulose: Can absorb large amounts of water Cannot be digested, however it adds bulk to the diet (gives a feeling of fullness) Aids the removal of waste from the body Is insoluble in water.

  20. Q6. What is the RI of fibre? The RI of fibre is 25–30 g per day.

  21. Q7. List four ways of increasing fibre in the diet Change from white to wholemeal bread. Eat plenty of fruit, vegetables and pulses. Choose fibre-rich breakfast cereals, e.g. ‘All Bran’. Add some wholemeal flour when baking.

  22. Q8. Enumerate the biological functions of carbohydrates Carbohydrates are used for heat and energy for the body. They are protein sparers, so can be used for growth and repair. Excess carbohydrate is changed to glycogen and stored in the liver and muscle as an energy reserve or it is changed to body fat (adipose tissue), which insulates the body. Cellulose moves food through the intestine, preventing constipation.

  23. Dry heat Carbohydrate foods browns due to the presence of dextrins, e.g. toast. Sugar caramelises, e.g. caramel slices. The Maillard reaction occurs because of the interaction between sugar and amino acids, e.g. roast potatoes. Q9. Outline the effects of heat on sugars and starch

  24. Moist heat Cellulose softens, e.g. cooked vegetables. Starch grains swell, burst and absorb liquid, e.g. flour used to thicken sauces. Pectin is extracted by heating fruit in water with sugar and acid, e.g. jam-making. Sugar dissolves in warm liquid, e.g. making syrups.

  25. Q10. What is the energy value of carbohydrates? The energy value of carbohydrates is: 4 kcal / 17 kJ

  26. Q11. Differentiate between gel formation and gelatinisation 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 jam-making. Gelatinisation • Based on the principal that when starch is heated in the presence of water, starch grains swell, burst and absorb the liquid, resulting in the thickening of the liquid.

  27. Q12. With regard to current healthy eating guidelines, suggest three ways that food sources of carbohydrate in the diet should be adjusted and give a reason in each case Eat less sugar: Change high-sugar foods for starch food, therefore exchanging short, quick energy sources for slow, sustained energy sources. Eat more fibre: Change processed cereal products for the wholegrain variety, e.g. brown bread instead of white bread. Replace white pasta, rice and cereal products with the wholegrain variety.

  28. Q13. Outline the digestion and absorption 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 glucoseSucrase breaks sucrose into glucose and fructose. Lactase breaks lactose into glucose and galactose.

  29. Absorption:Monosaccharides are absorbed through the villi of the small intestine into the bloodstream and are carried to the liver in the portal vein. Assimilation: Monosaccharides can be oxidised to produce energy (cellular respiration). Some monosaccharides are changed to glycogen and stored in the liver and muscles as an energy reserve. Excess carbohydrate is changed to body fat and stored in the adipose tissue under the skin. Vitamin B1, B2 and pyridoxine are needed to metabolise carbohydrates.

  30. Key words to revise • Hygroscopic • Invert sugar • Maillard reaction • NSP • Pectin • Peristalsis • Photosynthesis • Starch Adipose tissue Caramelisation Crystallisation Dextrinisation Empty kilocalories Gelatinisation Gel formation Glycogen Hydrolysis

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