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Fst 326 Module 2: Carbohydrates

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Fst 326 Module 2: Carbohydrates MODULE 1: CARBOHYDRATES CARBOHYDRATES • Definition: -Originally defined as compounds containing C, O and H in certain proportions, or as hydrates of carbon. • However, some CHO may contain N2, S2, or both. • Simple CHOs were given the empirical formula (CH2O)n, or Cn (H2O)n, or Cn (H2O)y • The building block of CHOs is the simple sugar -n ranges from 3-7 for simple sugars, where 3C = trioses, 4C = tetroses, 5C = pentoses, 6C = hexoses etc • These sugars also exist CHO as aldehydes (aldoses) e. g. CHOH glyceraldehyde, CH2OH D-glyceraldehyde or • As ketones (ketoses) CH2OH e.g. dihydroxyacetone C=O CH2OH Dihydroxyacetone • CHOs are classified on the basis of the number of sugar units they possess e.g. • Monosaccharides –single sugar molecules that are not bonded to other sugar molecules. Consist of trioses, pentoses and hexoses. • Glucose is the most common monosaccharide in food. It is an aldose. Fructose is a ketose, while galactose is the only other food monosaccharide, which occurs in the disaccharide lactose. Sugar molecules glucose, fructose, galactose structure • Monosaccharides exist in more than one structural form –as straight chain or Fischer projection and as ring or cyclic Hayworth projection forms • In solution, the ring form predominates. (See figs). Monosaccharide Straight Form Conversion to Ring Form Monosaccharide ringed Form • Disaccharides –two monosaccharides are joined together by a glycosidic bond to form a disaccharide • The 3 important food disaccharides are sucrose, maltose and lactose. Important Disaccharides • Sucrose, composed of glucose and fructose is common table sugar. • Lactose, milk sugar, is composed of glucose and galactose. • Maltose comprises two glucose molecules • Oligosaccharides –comprises of 3-10 sugar molecules e.g. raffinose (3) and stachyose (4) molecules • Raffinose –consists of one molecule each of galactose, glucose and fructose • Stachyose –consists of one molecule each of raffinose and galactose Stachyose • Raffinose and stachyose are found in legumes, nuts seeds and dry beans. They are not digestible by man, but passed into the lower intestine, where they are broken down by bacteria to gas (flatulence). • Heteropolysaccharides –Yield a mixture of monosaccharides when hydrolysed e.g. Hemicelluloses Glycogen molecule [Homopolysaccharide] of glucose • Similar to the whorl structure of amylopectin, except for the branching at α 1-6 position. Homopolysaccharides 1. Cellulose –most abundant polysaccharide [glucose homopolysacch] 2. Starch –consists of amylose and amylopectin molecules. Amylose is a straight chain polymer with repeating glucose units joined by α 1-4 bonds. -Amylopectin consists of chains of glucose units joined by α 1-4 linkages with branches at α 1-6 positions 3. Dextrins –Produced when starch is partially broken down by enzymes, acid or dry heat e.g. when corn syrup is made, bread is toasted, gari is made and flour is browned. It has less thickening power than starch. Amylose molecule Amylopectin Molecule Amylopectin 4 Glycogen –storage form of glucose in animals (animal starch). The structure is similar to amylopectin’s. It is stored in the liver and in muscles temporarily to maintain normal blood sugar levels. • Heteropolysaccharides • Give a mixture of monosaccharides when hydrolysed (e.g. hemicelluloses) 5. Fructosans (levans) are repeating fructose units e.g. Inulin. Inulin is the carbohydrate reserve of plants which do not accumulate starch e.g. sweet potato. Inulin is directly water-soluble, without forming a paste, and is not coloured by iodine. Levans have 40- 100 residues of fructose with one glucose molecule. Inulin Structure Hemicelluloses • Are heterogeneous polysaccharides found in plant cell walls. • In many cases, the molecules have branching side chains • Xylose and arabinose, which are pentoses are common components of hemicelluloses. 6. Plant Fibres • Also called dietary fibres, roughages or bulk is a complex mixture that includes cellulose, hemicelluloses, β-glucans and pectin as the polysaccharide components. Xylose and Arabinose structures • Some polysaccharide components found around plant cell walls e.g. fibre, play important structural roles, while gums are non-structural. • Lignin, a non-CHO molecule may be part of the fibre complex of woody parts. • Dietary fibre is indigestible to humans, but are important in the diet for the prevention of chronic diseases including colon cancer, diabetes and cardiovascular diseases Cellulose • Is composed of many glucose units linked together to form long fibres • It may be chemically modified (methylated or carboxymethylated) to make it more soluble and able to form gels. • Methylcellulose and carboxymethylcellulose are used to thicken, stabilise and provide bulk in various processed foods. β-glucans: • Are made up of glucose units linked differently from those of cellulose such that the structures are less linear and more soluble in water than cellulose. Oats and barley are rich sources of β-glucans. Pectic substances • High mol wt. polysaccharides found in plant cell walls and middle lamellae. • They are composed of galacturonic acid units joined by α-1,4 glycosidic linkage. • Some of the acid groups along the galacturonic acid chains become methylated during fruit ripening. • The proportion of methyl esters in a pectic substance is referred to as degree of esterification (DE). • There are 3 distinct pectic substances a. Protopectin –Nonmethylated galacturonic acid polymers found in immature fruit b. Peptinic acid –Methylated galacturonic acid polymer produced during ripening a. Peptic acid –Short-chain demethylated derivative of peptinic acid associated with overripe fruit Protopectin pectin peptic acid Immature fruit ripe fruit overripe fruit (no gel) (gel) (no gel) • Pectins are classified based on degree of esterification into High methoxy pectin (HM >50% methyl) or low methoxy pectin (LM <50% methyl). • HM pectin occurs naturally in fruits • LM pectin is chemically modified pectin • Pectins form colloidal dispersions, sols and gels • High methoxy pectin needs acid (pH 3.5) and sucrose to form gel. • Low methoxy pectin needs Ca++ for gel formation. Gums • Composed of various hexose and pentose sugars and their derivatives. Plant gums are hydrocolloids, -long chain polymers of monosaccharides that dissolve or disperse in water, producing a thickening or texture-building effect. They are sourced from sea plants, land plants or microorganisms. Functions 1. Thickeners and texture builders 2. Retain water and reduce evaporation rates 3. Modify ice-crystal formation 4. In the preparation of low calorie and reduced fat foods. 5. Virtually non caloric fat replacers in food applications. • Proteins can be blended with gums to form gels with properties similar to that of fat. Gums thicken foods and add bulk, providing a mouthfeel similar to fat. Although they cannot be used for frying, many CHO fat replacers can withstand heat, and can be used in meat products. • High solubility, pH stability and gelling ability are desirable properties of food gum • Ingredients in reduced fat foods e.g. salad dressings and processed meats to increase viscosity and also behave as emulsifiers Sources of Gums Sources Examples Sea weed extract Agar, alginates, carrageenan, furcellaran Plant seed gums Guar gum, locust bean gum Plant exudates Gum arabic, gum tragacanth, gum karaya Microbial Xanthan gum, gellan gum, derivatives dextran gum Chemical Methyl cellulose (methylation) modification Starch Derived from plant sources e.g. corn, potato, cassava, rice, wheat. • Molecule consists of 200 or more glucose • Consists of amylose and amylopectin units • Ratio of amylose/amylopectin in starch determines starch property • Can be heat or chemically modified into modified starches for special applications e.g. film formation, freeze-thaw stability, pasting and gelling properties, enhanced solubility and increased/decreased viscosity. Starch Properties • Gelatinisation is an irreversible disruption of starch molecular structure by physical, thermal or chemical means. Gelatinisation temperature is an indication of how well structured is a starch granule and is indicative of starch swelling and solubilsation properties Starch pasting and gel formation • A starch paste is a viscoelastic starch and water system. Pasting encompasses three changes in starch: swelling, exudation and disruption. • Gelation is the formation of a gel from a cooled paste. A starch gel is a rigid thickened starch and water mixture that has the properties of a solid. Starch Retrogradation • Is the attempt by amylose, amylopectin and intergranule matrix and starch granules to reassociate, or as some claim, the tendency of amylose to precipitate from solution as starch paste cools. As the paste contracts, it squeezes out water, resulting in syneresis or weeping. Functional properties of Carbohydrates Sugars 1. Sugar molecules contain two important reactive functional groups, the carbonyl (-C=O), and the alcohol (OH) groups. The (OH) gp. is important for solubility and sweetness, while the (-C=O) gp. is important for reducing activity and the Maillard reaction (colour and flavour). • Solubility vary in sugars due to crystallisation behaviour, molecular size and molecular weight. 2. Reducing sugars are those that contain the aldehyde or ketone groups, including all the monosaccharides and some disaccharides. They act as reducing agents. • Dextrose equivalent (DE) is a measure of the % of the glycosidic bonds hydrolysed in disaccharides and polysaccharides. In
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