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due to penetration of water molecules into its branchedstructure. As mentioned above, long boiling makes it dissolvein water or in weak acids, as well as in solutions with hydroxidesof potassium, rubidium, cesium, or francium and concentrated Studieson starch include examination of water absorption, chemicalmodifi cation of molecules, behavior under agitation, andhigh-temperature thermomechanical abrasion resistance.Although starch is a polymer, its strength under stress appearsto be low. At temperature above 150°C, the glycoside bondsstart cracking and over 250°C starch granules subside endothermally.At low temperatures, however, some reorganizationof hydrogen bonds is observed together with straightening ofthe molecule chains during the cooling process (retrogradation).In some extreme cases, under 10°C, precipitation isreported. Starch may be hot water soluble and formed in thinfilms; its molecular orientation causes brittleness in both foilsand solid packages. Both amylose and amylopectin consist ofglucopyranosis molecules, yet the structural differences betweenthese two polymers determine their different properties. Amyloseis mostly a linear polymeric molecule, consisting of α-1,4-linked D-glucopyranose (Fig. 4). The molecular weightof amylose varies from 500 anhydroglucose units in highamylosemaize starch to more than 6,000 anhydroglucoseunits in potato starchsolutions of chloral hydrate. Soluble starch (amylumsolubile) is obtained as a result of long boiling of starch with
water or weak acid; link cleavage at the amylopectin chain
branching sites is then observed, and eventually a watersoluble
product is formed. It is employed as an indicator in
chemical analysis.
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