Phosphorylated potato starches

This ehapter eonsiders the reeent view on moleeular structures of the amylose and amylopectin components in potato and how they are organized to form characteristie structures inside the stareh granules. The phosphorylation of starch, and the synthesis of its eomponents in normal and genetieally modified potatoes, is also discussed. 

Muhrbeck, R, Tellier, C. (1991). Determination of the phosphorylation of starch from native potato varieties by 31P NMR. Starch, 43,25-27. 

In a similar use of a-amylase, Parrish and Whelan249 treated potato starch with crystalline human salivary a-amylase and obtained a phosphorylated maltotetraose that had previously been reported by Postemak250 and that was the smallest phospho-dextrin formed. They determined its structure to be 63-phosphomaItotetraose, similar in structure to the smallest a-limit dextrin, 63-a- D-glucopyranosylmaltotriose, formed by this enzyme and porcine pancreatic a-amylase. 

Various partially phosphorylated or crosslinked derivatives of (10.20) have been reported and a slight degree of phosphorylation is known to exist in some natural products (e.g. potato starch). Phosphorylation is now believed to play a central role in plant starch metabolism with both C3 and C6 positions being involved [17]. 

Starch contains various impurities including proteins, phospholipids and glucose-6 phosphate, but these seldom exceed in total about 3%. The nature and level of impurities is, however, believed to influence the behaviour and properties of the various starches. Wheat, maize and potato starches all have different lipid compositions. Wheat starch has a greater phospholipid content than the maize variety and potato starch has the highest degree of phosphorylation. 

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. 

Cereal starches usually contain only -0.05% P, but potato starch contains about double this amount. In the latter, most of the P is in monoester form on the amylopectin chains. About 1 in 300 glucose residues are believed to be phosphorylated, but this can be increased by suitable treatment. In cereal starches, most of the P is accounted for by the phospholipid impurities. 

Starch phosphates with a fairly low degree of -OPO3 substitution for -OH, are obtained by heating starch with phosphoric acid at about 60°C. Starches from com, wheat or rice (the main sources) are usually not phosphorylated, but some natural varieties such as potato starch already contain a few phosphate ester groups (up to about 0.1% P) (Chapter 10.1). 

Fortuna T, Juszczak L. Palasinski, phosphorylation of potato starch classified in terms of grain size. Zywnosc 2000 4 (25) 91-98. (in polish). 

The only naturally occurring covalent modification of starch (apart from the hypothetical link to primer) is phosphorylation. In potato, phosphorylation of... 

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