Subcritical Water Mediated Hydrolysis of Rice Husk Lignocellulose and Upcycling Applications of the Hydrolysates

Subcritical water (SW) has been recognized as a promising technology for converting lignocellulosic byproducts such as rice husks (RH) into biofuels. However, there has been limited research on the utilization of RH hydrolysates as a food ingredient. Hence, this study evaluated the hydrolysis efficiency of RH under varying SW temperature conditions (150 ~ 350 °C) and their physiological activities for potential utilizations in the food industry. Based on brix, reducing sugar content and precipitate content, the maximum yield of RH hydrolysates was obtained by SW at 300 °C; however, a steep decrease in reducing sugar content was observed with further increasing temperature to 350 °C. Simple sugar compositions of the RH hydrolysates indicated that hydrothermal hydrolysis of hemicellulose occurred at 200 °C; thereafter, hydrolysis of cellulose occurred at 250 °C. However, simple sugars were not detected at greater than 300 °C. Thermal derivatives such as 5-hydroxymethylfurfural (HMF) and furfural were steeply increased at 250 ~ 300 °C, whereas these products were not detected at 350 °C. The antioxidant activity of the hydrolysates increased gradually with increasing SW temperature and reached to maximum at 300 °C, thereafter the activity was not affected by SW temperature. Based on the MTS assay, SW temperature did not influence the cytotoxicity of RH hydrolysates, and those prepared at 150 °C exhibited the highest prebiotic potential. Alternately, the hydrolysates prepared at temperatures greater than 250 °C prevented the growth of probiotic strains. Electronic tongue measurement revealed that SW temperature influenced the taste attributes of RH hydrolysates, resulting in increased sourness and umami with increasing SW temperature. Consequently, the results of this study suggested that RH hydrolysates prepared by SW had potential applications in the food industry for various purposes such as an antioxidant agent, prebiotics and a taste enhancer based on the applied SW temperature.