Amylopectin in potato starch

Figure 5.16 Schematic model for the arrangement of amylopectin in potato starch. Crystalline layers containing double helical linear segments in amylopectin molecules form a continuous network consisting of left-handed helices packed in tetragonal arrays. Neighboring molecules are shifted relative to each other by half the helical pitch. (Adapted with permission from reference 3)...

P. A. M. Steeneken, Reactivity of amylose and amylopectin in potato starch, Stdrke/Starch, 36 (1984) 13-18. 

Svegmark, K., Hermansson, A.M., 1991. Distribution of amylose and amylopectin in potato starch paste — effects of heating and shearing. Food Strucmre 10, 117—129. 

Starches can be separated into two major components, amylose and amylopectin, which exist in different proportions in various plants. Amylose, which is a straight-chain compound and is abundant in potato starch, gives a blue colour with iodine and the chain assumes a spiral form. Amylopectin, which has a branched-chain structure, forms a red-purple product, probably by adsorption. 

The variation between the starch from different plants is considerable. The percentage of amylose varies from 27% in maize starch through 22% in potato starch to 17% in tapioca starch. The waxy maizes are unusual in that they are almost pure amylopectin. This is extremely convenient because it avoids the need to separate amylopectin from amylose chemically. 

The data19 summarized in Figure 1 show that the extent of the hydrolysis of soluble potato starch by barley beta amylase reaches a limit which is independent of the concentration of the amylase. The data are typical of the action of beta amylases on unfractionated starches, when the hydrolyses are carried out at or near pH 4.5.1 3 6 19 20 Under these conditions, the hydrolysis of unfractionated starches usually ceases when 60 to 64% of the maltose theoretically obtainable from the substrate has been formed. The exact value of the limit obviously will depend upon the concentration of amylopectin in the starch and upon its structure. 

Swinkels29 collected published characterization data for tapioca starch and compared it to that for other starches of commercial significance (Table 12.4). Tapioca starch is differentiated from other starches by its low level of residual materials (fat, protein, ash), lower amylose content than for other amylose-containing starches, and high molecular weights of amylose and amylopectin. The small amount of phosphorus in tapioca starch is partially removable30 and, therefore, not bound as the phosphate ester as in potato starch. It is also common to find protein and lipid values of zero, as reported by Hicks.31 The very low protein and lipid content is an important factor which differentiates tapioca starch from the cereal starches. 

Many starches contain small proportions of phosphorus which is not solvent-extractable but is present as phosphate ester of the C-6-hydroxyl group of a few n-glucose residues. Potato starch contains about 0.1% of phosphorus, most of which (0.08%) is present in the amylopectin component. Cereal starches may contain only 0.02% of phosphorus. The origin of the phosphorus in potato starch is not known. One possibility, wliich has not been examined experimentally, is that traces of n-glucose 1,6-diphosphate are incorporated into the polysaccharides by P-enzyme or by starch-UDPglucosyl transferase. 

Due to the natural phosphate content present in the amylopectin fraction, potato starch has an anionic character. A cationic potato starch is actually a blend of cationic amylose and amphoteric amylopectin. 

A higher amylopectin content of some starches leads to their higher gelatinisation temperatures, but the lowering which is observed in potato starch indicates the influence of phosphorylation in the latter. The relatively high pasting temperature and low peak viscosity of wheat starch has been attributed to its relatively high content of phospholipid impurities, which form helical complexes with the amylose chains as indicated above. 

Crystalline human salivary a-amylase produced 6 -phosphomaltotetraose from potato amylopectin, demonstrating that the covalently linked phosphate in potato starch was attached to the C-6 hydroxyl group by a phosphoester linkage in amylopectin [23]. 

Com and rice starches have been oxidized and subsequently cyanoethylated (97). As molecular size decreases due to degradation during oxidation, the degree of cyanoethylation increases. The derivatized starch shows pseudoplastic flow in water dispersion at higher levels of cyanoethylation the flow is thixotropic. Com and rice starches have been oxidized and subsequently carboxymethylated (98). Such derivatives are superior in the production of textile sizes. Potato starch has been oxidized with neutral aqueous bromine and fully chemically (99) and physically (100) characterized. Amylose is more sensitive to bromine oxidation than amylopectin and oxidation causes a decrease in both gelatinization temperature range and gelatinization enthalpy. 

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