Optimization of malic acid and ultrasound dual modification conditions to reduce starch digestibility of cooked whole rice grains.

Rice is a staple food for a large portion of the global population, while it is often associated with a high glycemic index. In this study, rice grains were for the first time dually treated with various concentrations of malic acid and different ultrasound durations to reduce starch digestibility. Results showed that a wide range of digestibility (up to ~30 % difference in the amount of starch digested after 120 min) reached after the treatments. Especially, Tai rice treated with 4 M malic acid and 30 min of ultrasound showed the slowest starch digestibility. Notably, ultrasound increased starch digestibility at 2 M malic acid, while decreased digestibility at 3 M and 4 M concentrations. The reduced digestibility by ultrasound at higher acid concentrations could be attributed to a higher degree of esterification, more compact internal structures in cooked rice grains, increased V-type crystallinity, and greater amount of long amylopectin and short amylose chains. Collectively, it suggests that rice variety, malic acid concentration, and ultrasound treatment conditions all influence starch digestibility and should be considered together when developing rice with slower digestibility.