Pseudocatalytic acceleration of hydrothermal leaching of amylose from starch in acidic media

Abstract

Kinetic studies were conducted at 60–700C using buffer solutions with pH 3–5 to establish the effect of solution acidity on the efficiency of amylose leaching from starch. It was shown that the kinetic data are well described by the Kruger-Ziegler model. It was established that as the pH of the solutions decreases from 5.0 to 3.0, the activation energy of amylose leaching process decreases from 185 to 22 kJ/mol, and its dependence on hydrogen ion concentration is described by two linear segments within the pH ranges of 3.0–3.3 and 3.6–5.0. The effect of reducing the hydrogen bond energy upon addition of an additional proton to the water molecule was demonstrated through quantum-chemical modeling using the Gaussian 09 software. It was shown that protonation of the water molecule leads to a decrease in the effective charge on the oxygen atom from –0.626 to –0.142 unit of electron charge. At the same time, the linearity of the hydrogen bond along the –ОН–О– atoms is violated, the length of the hydrogen bond increases from 1.856 to 2.370 Å, and the binding energy decreases by 4.6 times. Thus, hydrogen ions can be considered as a kind of pseudo-catalyst for the process of amylose leaching from starch grains, since in their presence the activation energy of the process decreases and its rate increases. It was concluded that the process of hydrothermal leaching of amylose from starch can be intensified by carrying it out in acidic media without additional heating of starch suspensions.