Optimisation of germination temperature and time to improve γ-aminobutyric acid, polyphenols and antioxidant activity in brown rice

Abstract

Rice (Oryza sativa L.) is a staple food for over half of the world's population. Brown rice, which is hulled directly from rough rice, consists of a bran layer, embryo, and endosperm. From a nutritional point of view, brown rice should be the preferred eating type as contains higher dietary fiber, B-vitamins and minerals than white rice as well as important phytochemicals such as tocopherols, tocotrienols, γ-oryzanol and polyphenols, suggested to have properties that may benefit health. Germination is an effective and common process used to improve the nutritional quality of cereals. Bioactive compounds with antihypertensive activity (γ-aminobutyric acid, GABA) and antioxidant activity (polyphenols) are accumulated in sprouts. During germination, the synthesis and accumulation of bioactive compounds is highly affected by operating parameters such as time and temperature. The objective of this research was to optimize temperature and time during the germination process of brown rice for the maximum GABA concentration, total phenolic content (TPC) and antioxidant activity using the response surface methodology (RSM). Long-grain brown rice varieties 15 and 17 were provided by the National Institute of Agricultural Research from Ecuador (INIAP). GABA concentration and TPC was determined by HPLC-MS and Folin-Ciocalteau method, respectively. Antioxidant activity was evaluated by the oxygen radical absorbance capacity method. Response surface plots showed that germination time and temperature significantly influenced the GABA concentration, TPC and antioxidant activity of germinated brown rice varieties. The longer the germination time the higher was the GABA concentration, TPC and antioxidant activity in germinated brown rice. In contrast, temperature had no influence on GABA synthesis during germination. Under the predefined conditions of the temperature from 28 to 34 ºC, and germination time from 24 to 120 h, the maximum GABA concentration, TPC and antioxidant activity were obtained at different germination conditions. The maximum GABA concentration (137.8 and 119.7 mg/100g dry weight in 15 and 17 brown rice varieties, respectively) was achieved at a temperature of 34 ºC and time of 120 h. The maximum TPC (3066,5 and 2878,9 μg gallic acid equivalents/g dry weight for 15 and 17 brown rice varieties, respectively) was achieved at a temperature of 28 ºC and 120 h of germination. Finally, the maximum predicted antioxidant capacity (28.7 and 27.4 μmol Trolox equivalents in 15 and 17 brown rice varieties, respectively) was obtained at a temperature of 34 ºC and 28ºC for variety 15 and 17, respectively and 120 h. In conclusion, the RSM proved to be useful for optimization of germination process to obtain maximum concentrations of GABA, TPC and antioxidant capacity in ecuatorian brown rice varieties.