Impact of retrogradation on starch digestibility, predicted glycemic index, and nutritional composition in black and red rice

Pigmented rice, such as black and red rice, has garnered attention due to its unique bioactive profiles. Retrogradation is known to alter digestibility, glycemic response, and nutritional properties, yet its effects on pigmented rice remain less explored. This study aimed to evaluate the impact of starch retrogradation on the digestibility of starch, glycemic index, digestive enzyme inhibition, and nutritional characteristics of Sabah, Malaysia local black (Tadong) and red (Merah) rice varieties. Rice samples underwent retrogradation using a freeze-thaw-drying method: gelatinisation, cooling at 4 °C, drying at 60 °C, and then at 95 °C. Control samples were prepared without the cooling step. Starch fractions (rapidly digestible starch [RDS], slowly digestible starch [SDS], resistant starch [RS]), apparent amylose content (AAC), degree of retrogradation (DG), predicted glycemic index (PGI), enzyme inhibition (α-amylase, lipase), phenolic (TPC) and anthocyanin contents (TMAC), and overall nutritional composition were assessed using standardised analytical methods. Retrograded samples showed significantly lower RDS (20.93–25.47%) and higher RS (47.37–51.00%), AAC (24.83–26.43%), and DG (69.29–70.39%) than controls. The retrograded rice exhibited significantly reduced PGI and enhanced inhibition of pancreatic α-amylase (39.97–47.61%) and lipase (14.85–20.90%). Slight reductions in TPC and TMAC were observed, attributed to thermal processing. Retrogradation also significantly increased dietary fibre content and decreased moisture, both of which are beneficial for storage stability. The freeze-thaw-drying retrogradation method effectively improved starch digestibility, glycemic response, enzyme inhibitory activity, and dietary fibre content of pigmented rice, supporting its potential application in functional food formulations targeting metabolic and digestive health.