Spent yeast reuse as inoculum: a strategy to boost cellulosic ethanol productivity

For the implementation of an integrated biorefinery into a pulping mill, the valorization of bark through cellulosic ethanol production and simultaneously the valorization of the residual yeast from each fermentation run should be considered to increase sustainability and pulp and paper profitability. Spent yeast recycling during cellulosic ethanol production is an opportunity to reduce waste according to environmental issues for disposal at the industrial level, boosting the circular economy. This work evaluated the reuse of spent yeast in successive fermentation experiments at Erlenmeyer and bioreactor scales for cellulosic ethanol from Eucalyptus globulus bark. The spent yeast recovered through centrifugation at the end of each experiment was used as inoculum for subsequent batch fermentation. Across five sequential fermentations, wherein only a fraction of spent yeast was recycled, a discernible decline in ethanol volumetric productivity was observed throughout successive fermentation cycles. Conversely, employing the complete spent yeast for recycling along 11 consecutive trials resulted in a notable enhancement of ethanol productivity, as evidenced by a reduction in fermentation duration to approximately one-third remaining the conversion efficiency above 70%. Four successive experimental trials were conducted at the bioreactor scale, resulting in volumetric productivities ranging from 2.00 to 5.83 g L-1 h-1. The reuse of spent yeast in consecutive fermentation cycles demonstrably augmented productivity, thereby validating a promising strategy for the industrial-scale production of cellulosic ethanol. This significantly contributes to advances in the circular economy principles within the overarching framework of biorefinery operations.