Effects of heat treatment methods on anthocyanin content and starch digestibility of purple sweet potato flour

Abstract

Purple sweet potato (Ipomoea batatas L.), which is one of the most primary global food, contains high amount of anthocyanin pigments and starch. This study is designed to evaluate the effect of various heat treatment methods on anthocyanin, antioxidant and resistant starch contents of purple-fleshed sweet potato flour. Dried powder was produced by drying, whereas steaming was applied to produce steamed flour. Freezing was done after steaming process to form steamed-frozen sample. Heat-moisture treated flours were made by heat-moisture treatment at different temperature, 70oC and 100oC, and 35% moisture content. The highest anthocyanin composition was obtained, at 101.96 ± 1.80 mg cyanidin-3-glucoside/100g db, under the steaming-freezing process. However, steamed flour, rather than steamed-frozen sample, contained the largest percentage of DPPH radical scavenging among 5 types of powders, with 48.79 ± 4.13%. Meanwhile HMT 100 sample provided the smallest values with 3.22 ± 0.22 mg cyanidin-3-glucoside/100g db and 11.21 ± 8.74% DPPH scavenging. Out of the other aspects, it is undeniable that heat-moisture treatment was the most appropriate method to enhance starch digestibility in purple sweet potato. Having lower rapidly digestible starch content (65.67 ± 1.39%w/w, db), released glucose peak (67.67 ± 2.52mg/dl), glycemic index value (79.56 ± 0.91%), and higher resistant starch content (16.56 ± 1.01w/w, db); steamed-frozen flour had positive effects on starch fractions. Thus, the results of this project can contribute to finding starch digestibility of flours made by steaming and freezing, and anthocyanin content and antioxidant capacity in heat-moisture treated powder. Keywords: purple sweet potato, Ipomoea batatas L., dried flour, steamed flour, steamed-frozen flour, heat-moisture treated flour.