Insight into rice (Oryza sativa L.) cooking: Phenolic composition, inhibition of α-amylase and α-glucosidase, and starch physicochemical and functional properties

Abstract

This study provides a comprehensive evaluation of the effect of the phenolic composition in addition to the thermal treatments (i.e., roasting and frying) and the subsequent cooking on the starch physicochemical characteristics of red and brown rice. Native starch (Ns) was isolated to provide a theoretical and practical basis for starch functional properties. Red rice was characterized by superior phenolic contents and the presence of proanthocyanidins, which were enhanced by roasting and declined after all cooking methods. However, brown rice showed greater phenolic stability compared with red rice. Chromatographic separation showed that red rice was dominated by protocatechuic acid and (−)-epicatechin, whereas brown rice showed high contents of p-hydroxybenzoic acid, (−)-epicatechin, and syringaldehyde. Our results showed that processing profoundly influenced the color parameters of rice grain. Fourier transform infrared spectroscopy (FTIR) indicated intermolecular hydrogen bonds formed between native starch and phenolic compounds in rice influenced by cooking methods. Brown rice showed lower amylose (AAC), which influenced the grain's thermal characteristics and digestibility. Cooking and frying enhanced the AAC while roasting led to a significant reduction. Cooked red rice phenolic extract showed higher enzyme inhibition for α-amylase and α-glucosidase compared to brown rice extract. Notably, Ns showed considerably higher AAC, gelatinization enthalpy, and digestibility rate than native rice grain. Oil-frying significantly increased damaged starch, reduced gelatinization enthalpy, enhanced resistance starch (RS), and reduced the estimated glycemic index (eGI), which are favorable nutritional qualities.