Physicochemical properties of corn starch from microwave-assisted modification in acetic acid solution
Date Issued
2010
Date
2010
Author(s)
Li, Yi-Jen
Abstract
Acid treatment and acetylation are two common modifications used in food industry. Acid modification reduces the paste viscosity of starch and acetyl esterification decreased pasting temperature and increased clarity and stability of paste and of its freeze-thaw cycles. The traditional acetylating reaction conducts in alkali condition and it is a time consuming process and generates large volume of waste solution. This thesis attempted to establish a microwave assisted reaction model for starch modification in acetic acid solution. The hypothesis is using microwave energy that causes a rise in the temperature within starch granules as a result of rapid changes of the electromagnetic field at high frequency to achieve the goal of modification. The reactions were conducted on damped powders of starch granules that were impregnated with different concentration of acetic acid solution (0 (water as control), 0.05, 0.1, 0.5, 1.0, 2.0, 5.0, 10.0, 15.0, 17.44 N) and were heated in a domestic microwave oven for 30 seconds. All samples showed low hydrolysis, the loss ratio of all starch samples to be smaller than 2%, even in glacial acetic acid (17.44N, 99.7%). By microscopic examination, swelling and birefringence loss of starch granules significantly appeared as concentration of acetic acid solution in the range of 0 ~ 10.0 N. The change extent increased as the concentration of acetic acid increased. The majority of starch granules lost their birefringence in the experimental group of 5.0 and 10.0 N. The decreasing of crystallinity was verified by x-ray diffraction and differential scanning calorimetric (DSC) analysis. This microwave assisted modification in acetic acid solution caused partially gelatinization of starch granules indicated by enthalpy reducing and the appearance of retrogradation endothermic peak in DSC analyses. The enthalpy of starch gelatinization significantly decreased as the increasing of concentration of acetic acid in the range of 0 ~ 10.0 N. The microwave assisted modification also reduced the pasting viscosity of starch with different extent. The pasting profile of starch prepared from 1.0 and 2.0 N experimental groups were similar to the one of thin-boiling starch. Dynamic rheological analysis showed that starch granules of 5.0 and 10.0 N had been swollen under low temperature (<55℃), starch molecules degraded more significant when compare to other treatments, but still maintain high molecular weight. In the experimental groups of high acetic acid concentration (15.0 and 17.44 N), the effects on starch became less significant and the granules remained intact. The acetic acid was used as solvent to conduct an acetylation reaction of starch with acetic anhydride. The degree of substitution of starch acetate was 0.004 and 0.008 for using 10.0 and 17.44N acetic acid, respectively, with 30 seconds microwave heating time.
Subjects
corn starch
microwave heating
acetic acid
esterification
Type
thesis
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