Effect of Starch on the Equilibrium of Boric Acid

Abstract

We have reported that behavior of starch depended on the existing of boric acid. Noteworthy, gelation behavior of soluble starch, one of the modified starch, was different from other native starches. Critical concentration of starch gelation (CCSG), in other words, the minimum concentration of starch to form gel, was increased as the amount of boric acid increased when soluble starch was employed. On the contrary, CCSG of native starches (corn, wheat, and potato) was decreased as the amount of boric acid increased. We have also tested drying process of gelatinized starches to evaluate the effect of boric acid on the water-holding ability of starch. When soluble starch was employed, no difference of the rate of water loss from gelatinized starch at 100 °C was observed, irrespective of the existing of boric acid. In contrast, the drying rate of native starches were strongly affected and suppressed in the presence of boric acid. Based on these fact, we assumed that the effect of boric acid changes depending on the type of starch. It is well known that boric acid B(OH)3 is shifted to B(OH)4 - above pH 9.24. Therefore, dry tolerance of gelatinized starch was expected to be shifted by pH value. To confirm the effect of pH on the dry tolerance of gelatinized starch in the presence of boric acid, we measured the rate of water loss from gelatinized starch at 100 °C. The rates of the water loss from all tested starches were shifted to be smaller by increasing pH value. The rate of water loss from native starches were gradually shifted to be smaller, while that from soluble starch dropped between pH 9 and 10. Kinetic analysis of water loss from gelatinized starch revealed that there were two phases in the process of water loss from gelatinized starch until complete water loss. Then, we analyzed the initial stage of the dry process by regarding this process as the first order reaction. Plots of the rate constant, k value versus pH value showed like titration curves in every dry process of starches. Interestingly, inflection point of the dry process using soluble starch differed from native starches. The inflection point was around pH 9, whereas those of native starches were around pH 8. This fact indicates the equilibrium of boric acid may vary by depending on the type of starch. To investigate that change of the equilibrium of boric acid in the presence of starch, gelatinized starch in the presence of boric acid were measured by Raman spectroscopy. Raman spectra of gelatinized starch in the presence of boric acid revealed much amount of starch inhibited change of three-coodinated boric acid to four-coodinated even in high basic aqueous solution. Detailed analysis is currently being investigated and will be presented at this meeting.