This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Starch paste. Starch granules added to cold water swell slightly. According to Alsberg the axes may increase as much as 15 per cent. Such swelling is not like that in hot water inasmuch as there is no change in structure observable under the polarizing microscope. If the starch is dried rifts may appear in the granule, and the dried starch granules are deprived of their power to reassume their former volume when wetted. When a small quantity of starch is mixed with cold water, a thin, milky, nonviscid suspension is formed. If left standing, the starch granules settle to the bottom of the container and the supernatant liquid appears clear. If the starch suspension is heated a change takes place. It becomes thick, forming a paste which is opaque or transparent. The viscosity of the warm paste is far greater than that of the water suspension, but on cooling it loses some of its fluidity, becoming less plastic and more viscid, firm, or stiff. Alsberg has suggested that the reason for the loss of plasticity of the cool paste is that it regains some of its former rigidity at lower temperatures and that the surface of the granules may be more sticky at the lower temperature, thus preventing one granule from moving or flowing so easily past another. It has also been suggested that some of the starch is less soluble at the lower temperature.
When cornstarch is heated with water the resulting paste is clear or transparent. When flour is used the paste is opaque. It is for this reason that a clearer color is obtained when cornstarch is used with colored fruit juices than when flour is used. Samec ascribes the opaque color with wheat starch pastes to the combination of the wheat starch with phosphorus and protein. In another reference he says, "Pastes from wheat starches are white, cloudy, and a little slimy. The starches of cereals do not contain as large quantities of phosphorus, but contain fatty acids. Pastes from starch of such starch are 'pasty' and not 'pectinous.' Elsewhere it has been stated that combination of starch with phosphorus increases, whereas fatty acids decrease, the hydration capacity, which may have some effect on the translucency of the paste.
Granule injury. If starch granules are injured or ruptured in milling or in their separation from other material, some soluble starch is found dispersed in cold solutions; and with increasing proportions of soluble starch the unheated solution becomes more viscid, the heated solution correspondingly less stiff or viscous. Long grinding appears to break some of the starch chains into shorter units.
Gelatinization. The change upon heating a cold water starch suspension, so that a transparent or opaque, plastic, viscid product that forms a gel results, is known as gelatinization. Gelatinization depends upon the starch's being heated to a definite temperature, but the temperature at which gelatinization starts depends upon (1) the length of time the granule is wet before it is heated, (2) the kind of starch, (3) the size of the granule, and (4) possibly the salts in the water or combined with the phosphoric acid of the starch.
During gelatinization some starches tend to disperse, that is the physical units become smaller or the granule may disintegrate. With other starches most of the granule may remain intact but may assume a typical hydrated shape. Thus the photomicrographs of Woodruff and of Sjostrom indicate that wheat starch when hydrated tends to become kidney-shaped.
Gelatinization occurs and extends over a wide range of temperature. Swelling may start and the paste becomes more viscid at a fairly low ternperature, but complete swelling and maximum stiffening ability may not occur until the starch is heated to a much higher temperature.
Samec says that, if starch granules have been wet or soaked for some time before heating, gelatinization begins at a lower temperature. This gives at least a partial explanation for the shortened cooking period of soaked cereals.
The kind of starch. Richardson, Higginbotham, and Farrow state that potato starch gelatinizes more readily than sago or cornstarch. They found that cornstarch gelatinizes slowly in water at temperatures up to 100°C. and in pastes prepared by heating for 30 minutes at this temperature the granules, although swollen, are far from completely dispersed in the solution. The gel formed on cooling such a paste is quite distinct in type from that of potato, in which the granules are dispersed to a much greater extent. A sago gel may be regarded as intermediate in type between those of potato and cornstarch.
Alsberg and Rask report that wheat and maize starches change little until a temperature of 65°C. is reached, when they begin to thicken slightly, thus increasing the viscosity.
Woodruff and Nicoli heated 5-per cent starch suspensions by weight in a water bath at such a rate that 38 minutes were required for the paste to reach 90° and 22 minutes longer to reach 99.5°C. They found the translucency increased quite suddenly at a temperature specific for each starch, this temperature not being sharp but covering a range of 1° to 2°. Heating a longer time to a higher temperature did not increase the trans-lucency. This temperature was as follows: Corn, 86-87°; wheat, 87-88°; rice, 84-85°; potato, 60-70°; arrowroot, 79-80°; and cassava, 74-75°C. It was necessary to heat each starch to 90° or higher for maximum gelatinization.
The size of the starch granules. Alsberg states that Nageli found that larger granules of potato starch begin to gelatinize at 55°C, whereas the smallest granules do not begin to gelatinize until a temperature of 65°C. is reached. Suggested reasons are that the walls of the larger granules may be thinner or that large granules in swelling necessarily develop greater internal pressures per unit of granule surface than small ones.
The effect of large granules gelatinizing at lower temperatures and swelling more rapidly than small ones is indicated by Alsberg. Large granules in gelatinizing first swell and absorb more water which may affect the plasticity of the baking dough. "If the average granule size is small, this effect would come later than if the average size is large." Other things being equal, the longer the dough remains plastic, presumably, the greater will be the volume of the baked loaf. The size of the starch granules might also apply to textures and volume obtained in cake.