Experimental cookery

In general the longer the starch chain the less its solubility. Very short chains broken from a long starch chain, i.e., glucose and maltose, or short chains of these units, are completely soluble. Dextrins, which according to Haworth may be composed of 12, 16, 17, or 18 glucose units, are more soluble than starch and give reddish colors with iodine. The paste- or gel- forming characteristics are also affected by the chain length, shorter chain products having less and less ability to form pastes. Alpha- and beta-amylose. Generally starch is said to be composed of two substances, though sometimes more are listed, a soluble portion and a more insoluble one. Samec says there is a continuous transition between the soluble and less soluble portions. The nomenclature used for these two CONSTITUT^ OF STARCH 405 substances is varied and confusing both in the earlier and in the present-day literature. Thus at present the terms most often used for the more insoluble fraction are alpha-amylose, amylbpectin, and amylo-cellulose, whereas the soluble portion is designated as beta-amylose or just amylose. The propor- tion of alpha- and beta-amyJose varies in starches from different sources, and even in different samples from the same species. In addition, different methods of separating the two fractions and different pre-treatment of the starches would undoubtedly lead to varying relative proportions' being reported by different investigators. In starch pastes or solutions the beta- amylose often changes back to the insoluble form, a process called retro- gradation. Richardson, Higginbotham, and Farrow state that natural or "unmodified starches contain chains of varying lengths and it appears plausible to suggest that the shorter, more easily soluble chains constitute the amylose (beta-amylose) fraction, whereas the longer chains make up the amylopectin (alpha-amylose). The amylose retrogrades because the original separation has been made on material not in equilibrium or at such a temperature that on cooling or concentrating its solutions the solubility of some of the chains is exceeded and precipitation must follow. Amylo- pectin does not retrograde because it is already precipitated, though heavily hydrated." Thus differences in starch gels might be expected at boiling and room temperatures because the solubility of the soluble starch increases with higher temperatures. There is evidence that in natural or unmodified starches not more than 20 per cent of the starch and often less is soluble, though this varies with the source of the starch. Many investigators have reported that alpha-amylose is the starch com- ponent to which paste-forming properties of starch gels are due, but others have reported that beta-amylose imparts the mucilaginous qualities. Phosphoric acid. Samec states that in some species of plants both alpha- and beta-amylose are combined with phosphoric acid; in starches from other species these two components are linked with nitrogen and phos- phorus. Other investigators have reported that the phosphorus is combined with only the alpha-amylose fraction. Alsberg states that in wheat starch the phosphorus is found only with the alpha-amylose fraction. Alsberg states that Fernback found that small granules of potato starch contained a larger proportion of phosphoric acid than the larger granules, but this proportion has not been determined for wheat and some other starches.