Effects of Salts on the Reaction of Normal Corn Starch with Propylene Oxide

The objective of this research was to develop a greater understanding of the mechanism by which salts inhibit gelatinization of starch granules. Using hydroxypropylation of corn starch in the presence of sodium sulfate under conditions employed industrially as the control reaction, reactions with propylene oxide were done with different salts at levels that matched the control reaction in anion molal concentration, cation molal concentration, and total ion molal concentration. Salts used were selected based upon their position in the lyotropic series. Reactions were characterized by determining reaction efficiency and ion concentrations. Sodium and potassium citrates, salts exhibiting a strong lyotropic effect, were extremely effective as gelatinization inhibitors, but resulted in low reaction efficiency. Salts without a strong lyotropic effect (sodium and potassium chlorides) exhibited a weak protective effect, probably at least in part based upon generation of a Donnan potential. In the hydroxypropylation reaction, sodium citrate greatly reduced swelling; sodium chloride was ineffective as a swelling inhibitor, resulting in pasting of granules, and sodium sulfate allowed restricted swelling, most likely as a result of a moderate lyotropic effect and some generation of a Donnan potential. That reaction efficiency parallels granule swelling was confirmed.