Carbohydrates

1. Carbohydrates

2. Outline - Classification - Monosaccharides - Chiral Carbon Atoms - Structures of Important Monosaccharides • Cyclic Structures - Disaccharides - Polysaccharides

3. Carbohydrates • Major source of energy from our diet • Composed of the elements C, H and O • Produced by photosynthesis in plants

4. Carbohydrates • Polyhydroxy compounds (poly-alcohols) that contain a carbonyl (C=O) group • Elemental composition Cx(H2O)y • About 80% of human caloric intake • >90% dry matter of plants • Functional properties • Sweetness • Chemical reactivity • Polymer functionality

5. Carbohydrates Carbohydrates are aldehydes or ketones with –OH groups on the carbons that make up the chain. Classified based on Length of the carbon chain Location of the C=O aldehyde=aldo ketone=keto Family ending ose

6. Types of Carbohydrates • Monosacchrides • Disaccharides Contain 2 monosacchride units • Polysacchrides Contain many monosacchride units

7. Monosacchrides • Three Carbons = Triose • Four Carbons = Tetrose • Five Carbons = Pentose • Six Carbons = Hexose

8. Monosacchrides • Aldoses are monosacchrides with an aldehyde group and many hydroxyl (-OH) groups. • Ketoses are monosacchrides with a ketone group and many hydroxyl (-OH) groups. • Monosaccharides are categorized by the number of carbons (typically 3-8) and whether an aldehyde or ketone • Most abundant monosaccharides are hexoses (6 carbons) • Most monosaccharides are aldehydes, i.e. aldoses

9. Chiral Objects • Geometric property of a rigid object (or spatial arrangement of atoms) of being non-super-imposable on its mirror image

10. Isomers • Isomers are molecules that have the same chemical formula but different structures • Stereoisomer differs in the 3-D orientation of atoms • Diastereomers are isomers with > 1 chiral center. • Pairs of isomers that have opposite configurations at one or more of the chiral centers but that are not mirror images of each other. • Stereoisomers with more than one chiral center which differ in chirality at only one chiral center. • A chemical reaction which causes a change in chirality at one one of many chiral center is called an epimerisation.

11. Mirror Images • The three-dimensional structure of a chiral compound has a mirror image. • Your hands are chiral?

12. D and L Notation (Enantiomers) • D,L tells which of the two chiral isomers we are referring to. • If the –OH group on the next to the bottom carbon atom points to the right , the isomer is a D-isomer; if it points left, the isomer is L. • The D form is usually the isomer found in nature.

13. Fisher projections D-galactose

16. Cyclic Structures • Monosaccharides with 5-6 carbon atoms form cyclic structures • The hydroxyl group on C-5 reacts with the aldehyde group or ketone group

17. Carbonyl Group • Carbonyl groups subject to nucleophilic attack, • since carbonyl carbon is electron deficient: • – -OH groups on the same molecule act as nucleophile, add to carbonyl carbon to recreate ring form

19. Haworth Structure for D-Isomers The cyclic structure of a D-isomer has the final CH2OH group located above the ring.

20. Haworth Structure for D-Glucose • Write –OH groups on the right (C2, C4) up • Write –OH groups on the left (C3) down • The new –OH on C1 has two possibilites: down for  anomer, up for  anomer

21. Haworth Structure for D-Glucose   -D-Glucose-D-Glucose

22. Cyclic Forms

23. Mutarotation • Mutarotation: A small amount of open chain is in equilibrium with the cyclic forms. • The most stable form of glucose is β-D-glucose . -D-glucose D-glucose (open) β-D-glucose (36%) (trace) (64%)

24. Mutarotation • The α- and β- anomers of carbohydrates are typically stable solids. • In solution, a single molecule can interchange between • straight and ring form • different ring sizes • α and β anomeric isomers • Process is • dynamic equilibrium • due to reversibility of reaction • All isomers can potentially exist in solution • energy/stability of different forms vary

25. MUTAROTATION • For example, in aqueous solution, glucose exists as a mixture of 36% α - and 64% β - (>99% of the pyranose forms exist in solution).

27. Isomerization

28. Oxidation/Reduction • Carbonyl group can be oxidized to form carboxylic acid • Forms “-onic acid” (e.g. gluconic acid) • Can not form hemiacetal • Very hydrophillic

29. Oxidation/Reduction • Also possible to oxidize alcohols to carboxylic acids • “-uronic acids” • Galacturonic acids • Pectin • Reactivity • Aldehydes are more reactive than ketones • In presence of base ketones will isomerize • Allows ketones to oxidize

30. Reducing sugars • Reducing sugars are carbohydrates that can reduce oxidizing agents • Sugars which form open chain structures with free carbonyl group • Reduction of metal ions • Fehling test: CuSO4 in alkaline solution

31. Reduction • Carbonyl group can be reduced to form alcohol • hydrogenation reaction • Forms sugar alcohol (“-itol”) • glucose glucitol (aka sorbitol) • mannose mannitol • xylose xylitol • Sweet, same calories as sugar, non-cariogenic • Very hydrophillic • Good humectants

32. Acetal Formation • In acid solution, sugars can react with alcohols to form acetals known as glycosides • Reaction is a nucleophilic addition of two alcohols to aldehydes

33. 1. Protonation of OH group 2. water removal toform carbocation 3. alcohol addition andrelease of proton Acetal Formation

34. Disaccharides and polisaccharides

35. Disaccharides • Two monosaccharide units linked together Glycosidic Linkage (1→4) Glucose Glucose Maltose

36. Important Disaccharides • Maltose • Glucose + Glucose • Malt sugar • Found in fermenting grains

37. Maltose   -1,4-glycosidic bond - D-Maltose

38. Lactose • Glucose + Galactose • Milk sugar

39. Lactose Intolerance • Enzyme Lactase low or absent • Lactose fermented in the intestine • Nausea, cramps, bloating, gas, and diarrhea

40. Sucrose • Fructose + Glucose (1→2) • Found in many plants (especially sugar cane, sugar beets) glucose fructose

41. Polysaccharides • Polysaccharides are polymers of monosaccharides. • They are composed of glycosyl units in linear or branched arrangements, much larger than the 20-unit limit of oligosaccharides. • The number of monosaccharide units in a polysaccharide, termed its degree of polymerization (DP), varies. • Only a few polysaccharides have a DP less than 100; most have DPs in the range 200–3000. • The larger ones, like cellulose, have a DP of 7000–15,000. • It is estimated that more than 90% of the considerable carbohydrate mass in nature is in the form of polysaccharides. • Polysaccharides can be either linear or branched. • The general scientific term for polysaccharides is glycans • If all the glycosyl units are of the same sugar type, they are homogeneous as to monomer units and are called homoglycans. Examples : cellulose and starch amylose which are linear, and starch amylopectin.

42. Polysaccharides • Heteroglycan: When a polysaccharide is composed of two or more different monosaccharide units. • A polysaccharide that contains two different monosaccharide units is diheteroglycan, a polysaccharide that contains three different monosaccharide units is a triheteroglycan,….. • Diheteroglycans generally are either linear polymers of blocks of similar units alternating along the chain, or consist of a linear chain of one type of glycosyl unit with a second present as single-unit branches. • Examples of the former type are algins and of the latter guar and locust bean gums

43. Polysaccharide Solubility • Most polysaccharides contain glycosyl units that, on average, have three hydroxyl groups. • Polysaccharides are thus polyols in which each hydroxyl group has the possibility of hydrogen bonding to one or more water molecules. • The ring oxygen atom and the glycosidic oxygen atom connecting one sugar ring to another can form hydrogen bonds with water. • Glycans possess a strong affinity for water and readily hydrate when water is available. • In aqueous systems, polysaccharide particles can take up water, swell, and usually undergo partial or complete dissolution. • Polysaccharides modify and control the mobility of water in food systems, and water plays an important role in influencing the physical and functional properties of polysaccharides. • Together polysaccharides and water control many functional properties of foods, including texture.

44. Polysaccharide Solubility • The water of hydration is naturally hydrogen-bonded to and thus solvates polysaccharide molecules is described as water whose structure has been sufficiently modified by the presence of the polymer molecule so that it will not freeze. • The water is referred to as plasticizing water. • The molecules that make up this water are not energetically bound in a chemical sense. • While their motions are retarded, they are able to exchange freely and rapidly with other water molecules. • This water of hydration makes up only a small part of the total water in gels and fresh tissue foods. • Water in excess of the hydration water is held in capillaries and cavities of various sizes in the gel or tissue. • Polysaccharides are cryostabilizers rather than cryoprotectants • Thus both high-and low-molecular-weight carbohydrates are generally effective in protecting food products stored at freezer temperatures (typically -18°C) from destructive changes in texture and structure.

45. Polysaccharide Solution Viscosity and Stability • Polysaccharides (gums, hydrocolloids) are used primarily to thicken and/or gel aqueous solutions and to modify and/or control the flow properties and textures of liquid food and beverage products and the deformation properties of semi solid foods. • They are generally used in food products at concentrations of 0.25–0.50%, indicating their great ability to produce viscosity and to form gels. • The viscosity of a polymer solution is a function of the size and shape of its molecules and the conformations they adopt in the solvent.

46. Polysaccharide Solution Viscosity and Stability • Linear polymer molecules in solution gyrate and flex, sweeping out a large space. • They frequently collide with each other, creating friction, consuming energy, and thereby producing viscosity. • Linear polysaccharides produce highly viscous solutions, even at low concentrations. • Viscosity depends both on the DP (molecular weight) and the extension and rigidity, that is, the shape and flexibility, of solvated polymer chain. • A highly branched polysaccharide molecule will sweep out much less space than a linear polysaccharide of the same molecular weight • As a result, highly branched molecules will collide less frequently and will produce a much lower viscosity than will linear molecules of the same DP. • This also implies that highly branched polysaccharide molecules must be significantly larger than linear polysaccharide molecules to produce the same viscosity at the same concentration. • Likewise, linear polysaccharide chains bearing only one type of ionic charge (always a negative charge derived from ionization of carboxyl or sulfate half-ester groups) assume an extended configuration due to repulsion of the like charges, and thus the volume swept out by the polymer. Therefore, these polymers tend to produce solutions of high viscosity.

47. Gels • A gel is a continuous, three-dimensional network of connected molecules or particles (such as crystals, emulsion droplets, or molecular aggregates/fibrils) entrapping a large volume of a continuous liquid phase, much as does a sponge. • In many food products, the gel network consists of polymer (polysaccharide and/or protein) molecules or fibrils formed from polymer molecules joined in junction zones by hydrogen bonding, hydrophobic associations (van der Waals attractions), ionic cross bridges, entanglements, or covalent bonds, and the liquid phase is an aqueous solution of low-molecular-weight solutes and portions of the polymer chains. • Gels have some characteristics of solids and some of liquids. • A sponge-like structure is formed and can retain its shape. • The three-dimensional network structure offers significant resistance to an applied stress causing it to behave in some respects as an elastic solid. • However, the continuous liquid phase, in which molecules are completely mobile, makes a gel less stiff than an ordinary solid, causing it to behave in some respects as a viscous liquid. • Therefore, a gel is a viscoelastic semisolid; that is, the response of a gel to stress is partly characteristic of an elastic solid and partly characteristic of a viscous liquid. • Choice of a specific gum for a particular application depends on the viscosity or gel strength desired, desired rheology, pH of the system, temperatures during processing, interactions with other ingredients, desired texture, and cost of the amount needed to impart the desired properties.

48. Important Polysaccharides • Starch • Polymer of 30 to 1000 glucose units • Storage form of glucose in plants • Cereal grains (wheat, rice, corn, oats, barley) as well as tubers such as potatoes are rich in starch • Two forms: Amylose and Amylopectin

49. Amylose

50. Starch