Boosting second-generation ethanol titers from green coconut fiber by using high-concentration polyethylene glycol

Abstract

Efforts have been made to improve and modify lignocellulosic pretreatments, but this step corresponds to a high cost in second-generation ethanol production. Previous studies have shown that high-concentration polyethylene glycol (PEG) is a possible way to dispense chemical and physicochemical pretreatments and increase ethanol production from lignocellulosic material. However, no operational conditions of this strategy were optimized. The present study investigated the effects of operational conditions on simultaneous saccharification and fermentation (SSF) performance using high PEG concentrations and green coconut fiber (GCF) as a substrate. The high-concentration PEG (150 g.L−1) increased ethanol production compared to the PEG-free, 10, and 50 g.L−1 PEG conditions. Using 20 % (w.v−1) solid loading, the batch SSF cultivation with PEG 1500 and without supplementary nutrients reached ethanol production and ethanol yield equal to 22.21 g.L−1 and 64.0 %, respectively. The batch SSF cultivation with 30 % (w.v−1) solid loading presented mass transfer limitations. The addition of salts, yeast extract, or peptone improved ethanol production. Inhibition by ethanol on the Saccharomyces cerevisiae CAT-1 strain was indifferent to the presence of PEG 1500, while the addition of PEG ensured the cellulolytic activity reuse after three fermentation cycles. The fed-batch SSF with PEG facilitated the GCF liquefaction so that it was possible to operate with up to 30 % (w.v−1) solid loading without problems of viscosity and free water. Using 150 g.L−1 PEG 1500 and enzyme loading equal to 13.3 filter paper unit. g−1, the fed-batch SSF reached 35.1 g.L−1 ethanol at 48 h, representing an increase of 70 % compared to the fed-batch SSF without PEG.