Impact of esterification with malic acid on the structural characteristics and in vitro digestibilities of different starches

This research focused on the structural characteristics of resistant starches (RSs) that were obtained from corn, potato, and sweet potato and esterified by L-malic acid. Further, the unique effect of the degree of substitution (DS) on the crystalline properties was studied. Different starches were allowed to react with 2 M malic acid (pH 1.5) for 12 h at 130 °C. The shapes of the granules and the Maltese-cross shapes of samples were maintained and visible under an optical microscope. The FT-IR spectrum displayed evident carbonyl peaks at 1740 cm⁻¹, and the onset temperature (T_o) and gelatinization enthalpy (∆H) gradually decreased as DS increased. The malic acid-treated starches exhibited an increased RS content compared to those of the control. The RS contents of potato, sweet potato, and corn, which were 65.5%, 70.0%, and 89.8% in the uncooked MT-samples, decreased to 57.3%, 63.8%, and 86.7% in the cooked MT-samples, respectively, and exhibited high heat stability; corn starch yielded the highest RS among them. The thermal and malic acid treatments resulted in the partial hydrolysis and rearrangement of the helix structure of crystalline area, which was affected by esterification. The result revealed that the RS content increased as that of DS escalated.