Microwave subcritical water pre-treatment and enzymatic hydrolysis of geographical identification (GI) tag Indian black rice (Chakhao Poireiton) straw for fermentable sugar production

Abstract

Rice straw is an important feedstock for second-generation biorefineries, the majority being conventional white rice. This research investigated the potential of the straw of black rice Chakhao Poireiton, a rice variety of increasing importance, for biorefining. Pre-treatment of black rice straw was carried out in a pressurised microwave reactor and free sugar release was measured following digestion with Cellic® CTec3. This was compared to white rice straw. Pre-treatment (100 − 200° C for 5 min) did not drastically impact the sugar composition but brought about an enhanced release of glucose after enzymatic hydrolysis from 25 to 50% (black rice) and from 26 to 55% (white rice). For xylose digestibility, the increase was around seven-fold for black rice (8 to 57%) and five-fold for white rice (13 to 64%). This improvement in digestibility of the straw samples could have come from modification in the lignocellulose structural features making the polysaccharides more susceptible to hydrolysis. Black rice could be a suitable feedstock for a combined biorefinery – food and nutraceuticals from grain (much higher than white rice) and the straw for fermentable sugars comparable to white rice straw for biofuels.