This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: 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 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 proportion of alpha- and beta-amylose 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 component 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 phosphorus. 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.
Samec states: "Esterification with phosphoric acid, thus introducing a group with ability to ionize, greatly increases the hydration capacity and does not essentially decrease the tendency to association.
"If a starch paste be heated under pressure to 120°C, the high viscosity of the paste, the proportion of the undialysable phosphoric acid, the quantity of substance which can be transported by the electric current in a given time, and the precipitability by alcohol diminish, whilst the electrical conductivity of the solution rises but the magnitude of the molecular aggregates does not change in the same degree."
Therefore Samec believed there was some correlation between the phosphoric acid content and the high viscosity of starch pastes. Hence he tried phosphating potato amylose with the following result:
Relative viscosity of a 1-per cent solution
Color with iodine
From these results it appears that phosphoric acid may have some relation to the viscosity of starch pastes. Other factors affecting viscosity will be discussed later.
Fatty acids. Taylor and Morris make a further distinction between the two starch fractions from the fact that the insoluble residue carries certain high molecular-weight fatty acids combined with its carbohydrate. This insoluble fraction is called alpha-amylose and the soluble fatty-acid free portion is designated as beta-amylose. In potato starch pastes or solutions all the starchy material may be dispersed and, because potato starch contains no fatty acids, Taylor and associates question the existence of alpha-amylose in potato starch as it exists in cornstarch. They state that because the beta-amylose retrogrades on standing to a residue which resembles alpha-amylose except in its fatty-acid content, many investigators report the presence of alpha-amylose in potato starch.
The fatty acids may be split from the carbohydrate by long acid hydrolysis or with lipase-free amylase. Taylor and associates report that unsaturated fatty acids are liberated before the saturated ones.
Samec states that "careful researches concerning the colloidal changes which occur because of esterification of the starch substance with fatty acids are not as yet available, but we assume that the hydration capacity will thereby be decreased since longer chains of C-H1 are introduced into the molecule. The hydration capacity is also decreased by compounds containing phosphorus and nitrogen."
Cereal starches contain a relatively higher percentage of fatty acids. Potato starch contains no fatty acids but has a relatively high phosphorus content. Wheat also has a relatively high phosphorus content. Taylor states that roughly, those starches having a relatively high fatty-acid content may have a relatively low phosphoric-acid content and vice versa.