Optimization of Pullulanase-Assisted Hydrolysis to Produce Resistant Starch - Enriched Instant Rice From St25 Cultivar

ST25 rice, a premium Vietnamese cultivar known for its high eating quality, contains primarily carbohydrates (82.4%), with moderate protein (6.74%), and low lipid (0.76%) content. This study optimized the enzymatic hydrolysis of ST25 rice using pullulanase to enhance its resistant starch (RS) content. Key variables-substrate concentration (10-50% w/v), enzyme activity (10-50 U/g), hydrolysis temperature (45-65 °C), time (2-10 h), and pH (3.5- 5.5)-were evaluated. Optimal conditions were identified using response surface methodology (RSM) with a Box-Behnken design, yielding a maximum RS content of 9.25% at 39.09% substrate concentration, 18.28 U/g enzyme activity, and 5.94h hydrolysis. The experimental RS value (9.32%) closely matched the model prediction (R² = 94.45%). Following hydrolysis, the rice underwent gelatinization, dual-stage enzymatic degradation, microwave treatment, and retort sterilization. The final instant rice product contained 45.9% starch, 5.17% protein, 1.7% lipid, and 44.1% moisture, while meeting food safety standards (no detection of E. coli, Salmonella, or spoilage organisms).