Combined Enzyme-pretreatments and drying techniques on rehydration kinetics, digestibility and physicochemical properties of instant rice

Abstract

This study aimed to investigate the combined effects of various drying techniques namely oven drying (OD) and freeze-drying (FD), along with enzymatic pretreatments including α-amylase (AA), β-amylase (BA), and amyloglucosidase (AMG), on the textural characteristics, physical properties, rehydration kinetics, and digestibility of instant rice, using Sembada 188 hybrid rice variety as the study model. Our study demonstrated five significant findings: (a) Freeze-dried instant rice (FDIR) samples exhibited lower bulk density (0.24–0.28 g/ml) and higher porosity (58.6 – 62.9%) compared to oven dried instant rice (ODIR) samples (bulk density: 0.47–0.53 g/ml, porosity: 52.3 – 60.4%), attributed to the porous structure of FDIR samples as evidenced by the microstructural observations. Enzyme pretreatment, particularly with BA and AA, further reduced bulk density and increased porosity. (b) AMG-treated samples displayed significantly higher rehydration ratios (p < 0.05) than other enzyme-treated samples, regardless of drying technique. This is followed by BA-treated samples, while AA-treated and control samples had lower rehydration ratios. (c) Enzymatically treated cooked instant rice samples displayed comparable hardness to freshly cooked rice (FCR), with no significant difference (p > 0.05). (d) Freeze-dried instant rice with BA-treatment (BAFD) samples showed the highest digestion rate, likely due to increased surface area, suggesting potential for rapid absorption and energy release. (e) The Peleg model accurately predicted water abruption behaviour for different hydrating temperatures and time (r² > 0.98). FDIR samples exhibited a lower Peleg rate (K₁) than ODIR, indicating faster rehydration. In conclusion, the combination of freeze-drying and AMG enzymatic treatment yielded the most promising results, producing instant rice with superior rehydration and textural properties. Specifically, BAFD outperformed other treatments in terms of rehydration ratio and texture.