Principal components of food

1. Principal components of food

2. Principal components • Proteins • Carbohydrates • Fats and oils

3. Proteins • Function – maintains the structure and proper functioning of all living organisms. • Structure – a polymer formed from about 20 amino acids by peptide linkage.

4. COOH COOH COOH H H H H2N C H2N H2N C C CH3 H CH2COOH 2-Aminopropanoic acid (Alanine) Aminobutanoic acid (Aspartic acid) Aminoethanoic acid (Glycine) Some amino acids

5. H R H R’ H OH N CH C N CH C OH H O O -H2O H R H R’ N CH C N CH C Peptide linkage O O Peptide linkage

6. Hydrolysis of proteins • Breaks the peptide linkages in a protein molecule • The composition of the protein molecule may be deduced by using paper chromatography

7. acid H R H R’ N CH C N CH C H R H R’ O H N CH C N CH C OH O O O Hydrolysis of proteins

8. Past paper questions • 2001 6c • 2001 6d • 2000 7b

9. Carbohydrates • Function – provide energy • Formula – Cx H2yOy • Monosaccharides C6H12O6 (glucose, fructose) • Disaccharide C12H22O11 (sucrose, maltose) • Polysaccharides (C6H10O5)n (starch, cellulose)

10. H H H H H H C O C* C* C* C* H C OH OH OH OH OH Stereoisomers of C6H12O6 4 chiral carbons 24 = 16 stereoisomers

11. Glucose

12. Glucose • Open chain (acyclic form) • Two Ring forms (cyclic form) • M.p. 146oC , 150oC • Optical rotations • +112o , +19o +52.7o

14. Fructose

15. Reducing sugars

16. Reducing sugars

17. Disaccharides

18. Maltose

19. Maltose

20. Sucrose

21. Polysaccharides • Carbohydrate polymers • Storage polysaccharides • Energy storage – starch and glycogen • Structural polysaccharides • Use to provide protective walls to cells - cellulose

22. Starch

23. Cellulose

24. H+ glucose fructose Hydrolysis of sucrose • Sucrose, like all disaccharides, is hydrolysed by dilute mineral acids to two monosaccharides, glucose and fructose • C12H22O11 + H2O  C6H12O6 + C6H12O6 • The reaction can be effected by enzyme

25. Hydrolysis of starch • A solution of starch can be hydrolysed in the presence of an enzyme to a disaccharide, maltose. • 2(C6H10O5)n +nH2O  nC12H22O11(maltose) • It starch is boiled with dilute sulphuric acid, it is hydrolyzed to a monosaccharide, glucose. (C6H10O5)n +nH2O  nC6H12O6

26. Past paper questions • 2001 7c • 2000 6c • 1998 7b

27. Fats and oils

28. Triglyceride

29. Fatty acids

30. Fatty acid

31. Fatty acid

32. Hardening of vegetable oil • Unsaturated oils usually have a lower melting point and exist as a liquid. • Hydrogenation of some of the C=C bonds converts to solid fats. • E.g. Margarine

33. Hydrolysis of fats and oils • Fats and oils are hydrolysed into carboxylic acid and glycerol in human body. • The substances are then used as fuel, or used in building cell membranes and fatty tissues. • In laboratory, hydrolysis can be carried out in an alkaline medium (i.e. saponification)

34. Iodine value • Unsaturated fat is considered desirable in our diet. • Iodine value is defined as the number of grams of iodine that reacts with 100 grams of fats/oils.

36. Rancidity • Rancidity is caused by reactions of fats / oils which release foul smelling aldehydes and fatty acids. • Two types: • Hydrolytic rancidity • Oxidative rancidity

37. Hydrolytic rancidity • Occurs as a result of hydrolysis of glyceride molecules to glyerol and free carboxylic acids by the presence of moisture in the oils. • E.g. Frying of chips in oil at high temp. • The reaction is speeded up in the presence of certain micro-organisms or enzymes.

38. Oxidative rancidity • Occurs when fats/oils are exposed to air and undergo oxidation. • Fats/oils have a high degree of unsaturation are more susceptible to oxidation. • The oxidation is a free radical mechanism and is accelerated by trace metals, light and free radical initiators.

39. Past paper questions • 2000 6a • 1999 7d