Application of Differential Thermal Analysis to Examination of Thermal Behavior of Starch

Abstract

It is supposed that gelatinization of starch, a typical thermal reaction, can be studied effec-tively using techniques of thermal analysis. We applied differential thermal analysis (DTA) to examine thermal behavior of starch. The DTA curve of starch heated in an air-tight cell on DTA apparatus, indicated single endothermic peak accompanied by disappearance of polarization pattern of starch. From this result it was concluded that the DTA curve was suitable to determine gelatinization process of starch. In DTA, gelatinization characteristics of starch could be represented in three temperatures; onset temp. (To), peak temp. (Tp) and recovery temp, (Tr) of the DTA curve, To was neither influenced by varying heating rate (2-15 deg/min), varying amount of sample (0.8-14.3 mg) nor varying concentration of starch (2, 6-50, 6%) within a standard deviation of it. Ta was relatively near the value of photopastegraphy and Tp was near values of initial and 20 BU of amylography. However, Tr was higher than temp. at maximum viscosity of amylography. The gelatinization temp. (GT) of starch immersed in many kinds of salts solutions could be determined successfully by DTA in aqueous system. From these results, difference between these salts in the influence on the GT could be demonstrated quantitatively by the critical concentration index defined as the index of minimum concentration required to make a change of the GT. This index was in linear relation with the lyotropic value and the hydration number of ions, and furthermore correlated very closely with the B-coeficient of salts, The GT of starch treated with a salt of high B-coefficient increased as the content of salt retained by starch granule increased, while the endothermic heat of gelatinization (ΔHg) was unchangeable essentially. However, in the case of a salt with low 13-coefficient, the GT and ΔHg were decreased, which were accompa-nied by the corresponding decreases of the intensity of X-ray diffraction. The results suggested that the salt retained by starch granule affected the structural stability of starch micelle. The DTA curves of specimens of potatoes, rices, wheat flours and processed noodle foods indicated single endothermic peaks specific to species. These patterns agreed with those of starches isolated from foods, except for slight shifts of some degrees to higher temp, region, and accom-panied by disappearance of polarization pattern of starch. The DTA curve of buckwheat indicated a broad peak of endotherm. It was declared that the DTA curve of those foods reflected the endothermic signals of the gelatinization of starch in food without confusion with the signal of other matrix components. Potato and milled rice indicated some increases of the GT after storage. The GT of wheat flour Boughs increased with addition of sodium chloride or carbonate. The GT of processed noodle foods determined successfully.