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.

Keywords:

• Black rice
• Chakhao Poireiton
• straw
• microwave hydrothermal pre-treatment
• enzymatic saccharification
• biorefining

Acknowledgments

We would like to thank the University of Nottingham's Bioenergy and Brewing Sciences (BABS) technical team for the use of analytical facilities such as HPLC and the Microwave reactor. Dr Kamaljit Moirangthem is grateful to Dr Ranjana Rajkumari, Dhanamanjuri University (India) for providing the black rice samples; Dr Emily Fong, University of Nottingham (UK) for coordinating the import of the samples to the UK; and Novozyme A/S, Denmark for kindly providing the Cellic CTec3 enzyme.

Disclosure statement

The authors declare that they have no conflict of interest.