Production of cold water-viscous corn starch by heat-moisture treatment and alcoholic-alkaline treatment

Abstract

The heat-moisture treatment (HMT) has been suggested to employed on various starches to change the proportion of starch digestion fraction which is generally classified into three groups, rapidly digestible starches (RDS), slowly digestible starch (SDS), and resistant starch (RS). The inherent physicochemical properties such as low swelling power and paste viscosity of HMT starch limited its industrial use. In addition, studies of the HMT conditions that maximize the content of RS in normal corn starch were lacking. The alcoholic-alkaline treatment (AAT) has been applied to natural starches and carried out under various conditions such as heating at room temperature (25 °C), 35 °C, and above the gelatinization temperature as a measure of increasing viscosity and solubility in cold water. On the other hand, there has been no study on the changes in physicochemical properties of starch when the concentrations of ethanol and sodium hydroxide, which are used for AAT, have not been excluded in the structural change of starch by heat. In this study, a full factorial method of 2-factor 3 level was introduced, and a total of nine different HMT conditions were performed on the normal corn starch to maximize the RS content. Further, three different levels of AAT were introduced to HMT starch to improve the low cold water viscosity and solubility of HMT starch. The changes in X-ray diffraction pattern and thermal properties were compared according to the heat treatment conditions. As the moisture content increased to more than 25%, the RDS content increased sharply and the relative crystallinity and gelatinization enthalpy decreased. The heat-moisture treated sample H2, of which the moisture content was 15% and the heating temperature was 100 ℃, showed the highest content of the RS at 57.2%. In HMT-AAT sample (H2A2) which was treated with 17.2% and 3.2% of ethanol and sodium hydroxide, respectively, the cold water viscosity showed the highest at 1542 cP in the 8% starch suspension. The SUN-PREGEL, a commercial pre-gelatinized normal corn starch product, showed a cold water viscosity of 565 cP at 8% starch suspension and was 0.37 times lower than H2A2 sample. The RDS, SDS, and RS contents of sample H2 were 43.2%, 48.8% and 1.90%, respectively. The RDS, SDS, and RS content of SUN-PREGEL were 85.5%, 2.6% and 11.9%, respectively. Therefore, it was confirmed that the SDS content of H2 was significantly more than that of the commercial pre-gelatinized normal corn starch (p < 0.05). Scanning electron microscopic observation showed that HMT enlarged the size of the cavity inside the starch granule. The particles were irregular in shape and the surface area increased when the AAT was applied to starch. Furthermore, the starch granules formed agglomerates with high porosity at a certain alcoholic-alkaline treatment concentration (A2). The high cold water viscosity and SDS content of H2A2 starch could be explained by the formation of agglomerates with high porosity. Regarding the physical properties such as cold water solubility and cold water viscosity, H2A2 sample showed much better properties than these of SUN-PREGEL did. It was confirmed that the dual-modified starch through HMT and AAT on normal corn starch had high cold water viscosity and simultaneous slow digestion characteristics, which have not previously been reported. When such modified starch (H2A2) is added to food as a substitute for conventional pre-gelatinized starch, it shows the same thickening ability with less than half amount of the starch. This dually modified corn starch could be used as a health functional ingredient providing the low glycemic response.