Integrated production of bioethanol and biomethane from rice waste using superior amylolytic recombinant yeast

[Display omitted] •Conversion of unripe rice to biofuels was performed using a biorefinery approach.•Yeast strain showed hydrolysis of unripe rice equal to commercial enzyme cocktail.•Consolidated bioprocessing eliminates need of chemical pretreatment of substrate.•Spent fermentation slurry after ethanol production was used for methane production.•Efficient carbon utilization was achieved by producing both ethanol and methane. This study utilized a circular economy approach to convert unripe rice, a low-cost by-product of the rice milling industry, into biofuels using a biorefinery process. The recombinant yeast Saccharomyces cerevisiae ER T12.7 strain was tested for its ability to produce ethanol from unripe rice. In hydrolysis trials with 20 % (dw/v) unripe rice, ER T12.7 showed superior saccharification yields comparable to the commercial enzyme, STARGENTM 002. In 1-L bioreactor tests, ER T12.7 produced ethanol as efficiently as the parental ER V1 strain under simultaneous saccharification and fermentation conditions. The spent fermentation broth from both amylolytic strains was evaluated for biomethane production, achieving high yields of up to 373.61 mL CH4/g volatile solids. This research is the first to demonstrate process integration to produce ethanol and methane from rice waste sequentially, highlighting the potential of unripe rice in biorefining for a circular economy.