Acid modification of starch granules in alcohols: reactions in mixtures of two alcohols combined in different ratios

The acid modification of starch granules has been studied in different ratios of mixtures of the following combination of alcohols, methanol and 2-propanol, methanol and ethanol, methanol and 1-butanol, ethanol and 2-propanol, and ethanol and 1-butanol, from 1000 to 0100 in 10% (v/v) intervals. The time-course of the reaction of potato starch with 0.36% (w/v) HCl in methanol and 2 propanol at 25 °C showed that a series of limit dextrins were formed in 48 to 72 h. The dp of the limit dextrins decreased as the content of the higher alcohol increased. Limit dextrins were formed from the reaction of potato, waxy maize, and amylomaize-7 starches with 0.36% (w/v) HCl in various combinations and ratios of the alcohols. Plots of the dp of the limit dextrins vs. the ratio of each alcohol mixture gave curves that resembled titration curves with different plateau regions in which the dp of the limit dextrins changed very slowly or not at all. These plateau regions were characteristic and dependent on the types of alcohols, the volume ratios of the two alcohols in the mixture, the temperature, and the type of starch. It is proposed that the formation of limit dextrins and the formation of plateaus in the plots of dp of the limit dextrins vs. the alcohol ratios are due to the presence of physically different “kinds” of α-(1 → 4) glycosidic linkages in the starch granules. It is further proposed that the mechanism by which the alcohols or combination of alcohols are producing the different kinds of linkages to become more susceptible to acid hydrolysis involves the conversion of crystalline regions in the granule into amorphous regions.