Carbohydrates and Dietary Fibre

Starch hydrolysis by amylases

Starch is a mixture of amylose, an essentially linear chain of \alpha-D-glucose units joined by \alpha(1\to4) linkages, and amylopectin, which also carries \alpha(1\to6) branch points. Amylases hydrolyse these bonds, and each enzyme has a fixed mode of action that determines the products it forms. Endo-acting enzymes cleave bonds inside the chain, whereas exo-acting enzymes remove units from one end. \alpha-Amylase is endo-acting: it cleaves internal \alpha(1\to4) bonds at random, rapidly lowers the viscosity of starch paste, and produces dextrins and small oligosaccharides. Because of this action it is used for liquefaction in starch processing. \beta-Amylase is exo-acting: it works from the non-reducing ends and releases maltose units one after another, so it produces maltose sequentially. It cannot pass the \alpha(1\to6) branch points, so it leaves limit dextrins behind. Glucoamylase removes glucose units from the non-reducing ends, and pullulanase specifically hydrolyses the \alpha(1\to6) branch linkages. Key terms:

• liquefaction — the early stage in which \alpha-amylase reduces starch viscosity by random chain cleavage.
• saccharification — the later stage in which dextrins are converted into glucose or maltose by glucoamylase or \beta-amylase.
• limit dextrin — the branched residue left after \beta-amylase has acted on amylopectin.

Example: treating a starch solution with \beta-amylase alone gives mainly maltose, whereas treating it first with \alpha-amylase gives a mixture of dextrins that other enzymes must then break down further.

Oligosaccharides classification

An oligosaccharide is a carbohydrate built from a small number of monosaccharide units, conventionally about three to ten. Oligosaccharides are classified by degree of polymerisation (DP): disaccharides have two units, trisaccharides three, and so on. A trisaccharide such as raffinose, which is composed of galactose, glucose and fructose, belongs to this group and occurs in legumes. Raffinose family oligosaccharides such as stachyose, which are also found in pulses, extend the same pattern to higher DP values. Oligosaccharides must be distinguished from polysaccharides, which have DP values in the hundreds or thousands. Xanthan, alginate and gellan are high-molecular-weight polysaccharides obtained from microbial fermentation or from seaweed, so they do not count as oligosaccharides. Example: a sugar with three monosaccharide residues and a DP of 3 is an oligosaccharide, whereas a chain with several thousand glucose residues is a polysaccharide even if its monomer is the same.

Cereal grain polysaccharides

Cereal grains store most of their carbohydrate as starch, which is a mixture of amylose and amylopectin, and they also contain non-starch polysaccharides such as arabinoxylans and \beta-glucans. Dextrins are low molecular weight polysaccharides formed when starch is partly hydrolysed by dry heating, often with a small amount of acid, and they are used in adhesives and coatings. Dextrin is therefore a product of partial starch breakdown and is not a separate polymer type. Amylose is an essentially linear chain of D-glucose units joined by \alpha(1\to4) glycosidic linkages, whereas \alpha(1\to6) linkages are the branch points of amylopectin. Amylopectin is a highly branched polymer, and its monomer is D-glucose, not D-galactose. Retrogradation is the reassociation of gelatinised starch chains, mainly amylose, through hydrogen bonding as the paste cools. This forms ordered crystalline regions that cause the firming and staling of bread and cooked starch products. Key terms:

• gelatinisation — the swelling and loss of crystalline order in starch granules when heated in water.
• retrogradation — the cooling-induced re-formation of crystalline junctions in gelatinised starch.