In order to control digestion rate, changes in structural properties of starch, relative crystallinity, gelatinization enthalpy, swelling properties, and RDS, SDS, RS content were observed by HMT and malic acid-treatment. Also, RS content according to HMT and malic acid-treatment level was proved.
Modified alkaline steeping method was employed in order to apply HMT on sweet potato starch. In this research, influence of moisture, temperature, and time on SDS yield was evaluated based on Box-Behnken design under three different condition sets: moisture(20%, 25%, 30%), temperature(90℃, 110℃, 130℃), and time(2hr, 9hr, 16hr). Statistical analysis was conducted by SAS software. Maximum level of SDS content (50.9%) was obtained under the following condition: 22.8% of moisture, 113℃, and 8 hours and 35 minutes of process time. Analysis on SDS content generation suggested that moisture level, temperature, and interaction between them casted significant influence (p < 0.01) on the SDS content level.
Electron micrograph results of RS fractions indicated hollow regions and growth ring structure, which suggested that HMT caused higher enzyme susceptibility in all starches. In SDS+RS and RS fractions, X-ray diffraction patterns were not influenced by HMT. In the SDS+RS fraction, gelatinization enthalpy decreased more than that of the RS fraction. The outcome showed that the SDS fraction comprises fewer perfect crystallites and amorphous regions, while the RS fraction had crystalline regions mostly.
2M malic acid solution with different pH values (1.5, 3.5, 5.5, 7.0, 8.5) was applied in malic acid treatment to corn starch. With FT-IR spectroscopy, carbonyl groups were detected in malate starch, indicating formation of cross-linking by esterification. As pH value of malic acid-treated sample decreased from 8.5 to 1.5, the RS content increased from 18.2 to 74.8%, which did not change after gelatinization due to the DS increase. Granular structure of malate starch was left unaffected, and the starches maintained its birefringence.
Corn, potato, and sweet potato starch was reacted with 2M malic acid (pH 1.5) at 130℃ for 12 hours under the forced convection oven condition. Shapes of granules and Maltese Cross shapes were still maintained and visible through optical microscope from the samples. FT-IR spectrum displayed clear carbonyl peaks at 1740 cm-1, while To (onset temperature) and H (endothermic enthalpy) gradually decreased as DS (degree of substitution) increased. Malic acid-treated samples maintained high RS (resistant starch) content after 30 minutes of cooking at 100℃. After cooking of the samples, RS content of potato starch decreased from 65.5% to 57.3%, sweet potato starch from 70.0% to 63.8%, and corn starch from 89.8% to 86.7% with relatively higher thermal stability. Among the samples, heat-treated sample of corn starch formed the highest RS content by 57.3–86.7%.
The result indicates that thermal and malic acid-treatment of starch caused partial hydrolysis and rearrangement of the crystalline area and helix structure of starch as well via esterification. It was also observable that RS content increased with higher DS in the same result. Even though thermal treatment alone could not control content of enzyme resistant starch, applying malic acid-treatment led to increase in RS content after gelatinization as well. From the observation, it is safe to conclude that it is possible to obtain certain amount of enzyme resistant starch via malic acid-treatment.