Mastering Carbohydrates: Chemistry and Importance in Food

1. ITP 210 FOOD CHEMISTRY Carbohydrates

2. Scope of Courses • The role of carbohydrates • Sources • Definition and classification • Carbohydrate chemistry (structure, stereoisomer), functional properties and chemical reaction • Monosaccharide, disaccharide, oligosaccharide and polysaccharide

3. Overview of Carbohydrate • One of the most classes of chemical compounds • The macromolecules in which energy from the sun is stored by photosynthesis. • An essential structural component of living cells: • Simple sugars: monosacharides • Complex carbohydrate: polysacharide

4. Carbohydrate Production through Photosynthesis 6nCO2 + 6nH2O (C6H12O6)n + O2 + Energy Hexose = 675 kcal/mol

5. The Role of Carbohydrate • Play a major role in human diets, comprising 40-75% of energy intake. Energy value : 4 kcal/g • The basis for the manufacture of sugar, starch, paper, wood pulp, pharmaceuticals, special chemicals • Play important role in food characteristics (taste, color, texture, etc)

6. Carbohydrate-containing Foods • Cereals : wheat, maize, rice, barley, sorghum • Sugar crops : sugar cane, sugar beet, corn • Root crops : sweet potato, potato, cassava • Pulses : mungbean, kidney bean, soybean • Fruits : banana, grape • Vegetables • Milk products

7. Definition • Organic compounds that consist of carbon (C), hydrogen (H) and oxygen (O) • The polyhydroxy aldehydes, ketones, alcohols, acids, their derivatives, and the polymers derived from these compounds. • They vary from simple sugars containing from three to seven carbon atoms to very complex polymers.

8. Classes of Carbohydrates Based on Functional Groups

9. Degree of Polymerization (DP) • DP - The average number of monomer units in the molecule. When applied to starch, maltodextrin or glucose syrup molecules it refers to the average number of anhydroglucose units in the molecule. • Carbohydrates can be classified according to their DP and may be divided initially into three principal groups, namely sugars, oligosacharides and polysaccharides.

10. Classes of Carbohydrates Based on DP

11. Monosaccharides • Parent monosaccharides are polyhydroxy aldehydes H-[CHOH]n-CHO or polyhydroxy ketones H-[CHOH]n-CO-[CHOH]m-H with three or more carbon atoms • The CHO, C=O and –OH are reactive, important in carbohydrate structure and reactions • The generic term ‘monosaccharide’ denoted to a single unit, without glycosidic connection to other such units • Cannot be converted hydrolytically into smaller molecules • Plays as sweeteners and instant energy source

12. Monosaccharides • Triose (3C) : Glyceraldehide • Tetrose (4C) : Erythrose • Pentose (5C) : Arabinose, ribose, xylose • Hexose (6C) : Galactose, glucose, fructose

13. Aldoses and Ketoses • Aldoses: monosaccharides with an aldehydic carbonyl (-CHO) at C1. Example: Glyceraldehyde, erythose, ribose, glucose • Ketoses: monosaccharides with ketonic carbonyl (-C=O) at C2. Example: fructose, sorbose

14. Nomenclature • Based on: • Functional group: ketose, aldose • Number of carbon atoms attached: triose, tetrose, pentose, hexose • Position of OH group in Cn-1: D-glucose, L-glucose, D-fructose, L-fructose

15. Structural Representation(Fischer Projection) ALDOSES KETOSES C1 C2 C3 C1 C4 C2 C1 C5 C2 C3 C6 C4 C3

16. Reducing sugar • Sugar with aldehydic carbonyl group has reducing power • Reducing property is determined by the presence of free OH in C1 • Example: Glucose, lactose

17. Structural Representation • Fischer projection formula (acyclic/linear systems) • Haworth projection formula (cyclic systems: pyranoid, furanoid)

18. Stereoisomers • The orientation of other groups attached to the carbon chain of carbohydrates affects the properties and interactions of monosaccharides. • The alternative forms of a molecule in which atoms or groups attached to the carbon chain point in different direction are termed steroisomers. • Steroisomers are identified as D- or L- depending on the directions pointed by atoms attached to the carbon chain.

19. Stereoisomers • In most cases, the D-form of the monosaccharide, is much more common than L- forms among cellular carbohydrates. • Many C atoms of monosaccharides are asymmetric each of their four covalent bonds links to a different atom or chemical group. • For example: the middle C of glyceraldehyde is asymmetric because it shares electrons in covalent bonds with -H, -OH, -CHO, and -CH2OH.

20. Stereoisomers • The groups attached to asymmetric carbon atoms can take up either of two fixed position with respect to other C atoms in a chain. • In glyceraldehyde the -OH group can extend either to the left or to the right of the C chain with reference to the -CHO and -CH2OH groups. • The -OH extends to the right is called D-glyceraldehyde (from the Latin dexter= right) • The -OH extends to the left is L-glyceraldehyde (from laevus = left).

22. CHO CHO CHO CHO CHO HOCH HCOH HCOH HOCH HCOH HOCH HOCH HOCH HCOH HCOH HCOH HOCH HCOH HCOH HOCH HCOH HCOH HCOH HCOH HCOH CH2OH CH2OH CH2OH CH2OH CH2OH D-Allose D-Talose D-Glucose D-Mannose D-Gulose ALDOSES CHO HOCH HCOH HOCH HOCH CH2OH L-Glucose

23. CHO CHO CHO CHO CHO CHO HCOH HOCH HCOH HOCH HOCH HCOH HOCH HCOH HCOH HOCH HOCH HCOH HOCH HOCH HCOH HCOH HCOH HCOH CH2OH HCOH HCOH CH2OH CH2OH CH2OH CH2OH CH2OH D-Idose D-Galactose D-Ribose D-Arabinose D-Xylose D-Lyxose ALDOSES

24. CH2OH CH2OH CH2OH C=O C=O C=O HOCH HCOH HCOH HCOH HCOH HOCH HCOH CH2OH HCOH CH2OH CH2OH D-Fructose D-Pentulose D-Sorbose KETOSES CH2OH O=C HCOH HOCH HOCH CH2OH L-Fructose

25. =  =  HO H H OH C C CHO HCOH HCOH HCOH HOCH HOCH HOCH O O HCOH HCOH HCOH HC HC HCOH CH2OH CH2OH CH2OH -D-Glucose -D-Glucose D-Glucose The structure is not linear  Oxygen bridge between C1 and C5 (because of H-bond)

26. Structural Representation(Haworth Projection) • In the six-carbon monosaccharide (glucose), a covalent bond can form through a reaction between the aldehyde at C1 and the -OH at C5  produces glucopyranose ring structures. • The ketone at C2 (in fructose) can also react with the OH at C5  produce a glucofuranose ring

27. 6 H2COH 6 O 1 CH2OH HOCH2 5 O H H 2 5 H 4 1 H OH HO OH H H OH OH 3 4 2 3 OH H H OH -D-glucopyranose ( -D-glucose) -D-glucofuranose ( -D-fructose) Pyranose and Furanose

28. HCH2OH CH2OH CHO O O C=O H HO CH2OH HOCH2 H HCOH HOCH OH H HOCH HO H OH H H OH HCOH HOCH HCOH H OH OH H HCOH CH2OH CH2OH D-Galactose D-Fructose

29. 6 CH2OH = D 5 O H H H 4 1 OH H OH OH 2 3 H OH -D-glucopyranose ( -D-glucose) Nomenclature • In -D-glucopyranose, the “pyranose” portion of the name refers to the pyranoid ring in the molecule • The D designator specifies the configuration at C5 (the chiral center farthest from the reference group)

30. 6 HCH2OH 5 O H H H 4 1 OH H OH OH 2 3 =  H OH -D-glucopyranose ( -D-glucose) Nomenclature • Stereochemistry at C1 is given by  • Prefix: “gluco” • If the configuration at C1 is inverted, -D-glucopyranose is formed

31. 6 O 1 CH2OH HOCH2 2 5 H HO OH H 3 4 OH H -D-fructofuranose ( -D-fructose) Nomenclature • A five-membered ring: furanose, portion of the name refers to the furanoid ring in the molecule • The D designator specifies the configuration at C5 (the chiral center farthest from the reference group)

32. 6 O 1 CH2OH HOCH2 2 5 H HO OH H =  3 4 OH H -D-fructofuranose ( -D-fructose) Nomenclature • Stereochemistry at C2 is given by  • Prefix: “fructo” • If the configuration at C1 is inverted, -D-glucofuranose is formed

33. Structural Representation(Haworth Projection) The glucospyranose ring occurs in two forms that differ only in the orientation of the -OH group at C1: • -OH group points downward, in the alpha () form of the sugar, as in -glucose. • -OH group points upward from the ring in a beta () position of the sugar as in -glucose.

35. Structural Representation(Haworth Projection) • Starches, which are assembled from -glucose units, are soluble and easily digested. • Cellulose, synthesized from -glucose units, is insoluble and cannot be digested as a food source by most animals.

36. The difference between amylose and cellulose Amylose: -1,4 Cellulose: -1,4

37. Disaccharides • Monosaccharides in the ring form can link together to form disaccharides or in greater numbers to form polysaccharides. • Disaccharides are formed when two monosaccharides are coupled together. • The coupling is of a specific type: an oxygen atom forms a bridge between the units coupled together • This oxygen atom must be part of an acetal or ketal group.

38. Glycosidic bonds • These are the covalent chemical bonds which link anhydroglucose units in starch chains. • The glycosidic linkages can be alpha () or beta () 1-2, 1-3, 1-4, and1-6. • In starch structure, the bonds may be linked in the -1,4 configuration to form a linear chain or in the -1,6 configuration which gives a branch point.

39. Disaccharides • Linkage of two monosaccharide molecules to form the disaccharides such as maltose, lactose and sucrose. • The linkages are designated as alpha () or beta () depending on the orientation of the -OH group at the number C1 forming the bond. • The glucosidic bonds may be ruptured by enzymes at specific sites in the amylose and amylopectin polymers.

40. Disaccharides

41. Physical properties of sugars • Sweetness • When dissolved, all sugars are sweet to the tongue • Sugars contribute 4 kcal/g • Sugars have different sweetness • Very sweet sugar  smaller quantities  less calories • Relative sweetness: sucrose  used as a standard  1.0

42. Relative sweetness of selected sugar solutions (5% solutions) and other sweeteners

43. Physical properties of sugar • Hygroscopicity • Ability to attract and hold water, which is characteristic of sugars to varying degrees • Maintaining the freshness of some baked products • A source of potential problems in texture when RH is high • An elevation in temperature increases the absorption of moisture from the atmosphere

44. Hygroscopicity RH = relative humidity

45. Physical properties of sugars • Solubility • Sugar is soluble in water because of numerous OH groups. • Sugars have different solubility in water • As the temperature of water rises, the amount of sugar capable of being dissolved in water increases

46. H H H O O H H : : O CH2 O : : H CH O H H O : : : O CH CH H O H O CH CH H H H : : O O H H : : H O H O H H Glucose (Soluble in Water)

47. Solubility of selected sugars at 50oC

48. Chemical Reactions • Hydrolysis • Disaccharides undergo hydrolysis when heated • An acidic medium favors this degradative reaction as does the presence of water • Invert sugar: sugar formed by hydrolysis of sucrose, a mixture of equal amounts of fructose and glucose

49. Hydrolysis CH2OH O OH O H H CH2OH H OH H H HO O CH2OH OH H OH Sucrose OH H -D-glucopiranosida--D-fruktofuranosida CH2OH O OH CH2OH O H H H +HOH OH H + H HO CH2OH H OH OH H OH OH H -D-Glucose -D-Fructose Invert sugar

50. Chemical Reactions • Caramelization • When sugars are heated to such intense temperatures (170oC for sucrose)  melt  caramelization occurs • Caramelization requires very high temperatures. When a sugar is heated to temperatures above its melting point, dehydration occur resulting in the formation of furfural derivatives, which undergo a series of reactions ending with polymerization to brown pigments. • Creates pleasing color and flavor changes • Caramel colour: can be used as colorant in food processing