Ethanol production from alkali- and ozone-treated cotton stalks using thermotolerant Pichia kudriavzevii HOP-1

In the present study, milled cotton stalks were subjected to alkali pretreatment with NaOH at 1-4% (w/v) concentrations at 121 °C for time ranging from 30 to 90 min. Ozone pretreatment was performed by passing 45 mg/L of ozone gas over 2 mm cotton stalks for 150 min at a flow rate of 0.37 L/min. The residual biomass from 4% alkali pretreatment for 60 min showed 46.6% lignin degradation accompanied by 83.2% increase in glucan content, compared with the untreated biomass. Hydrolysis of 4% alkali-treated and ozone-treated cotton stalks was conducted using enzyme combination of 20 filter paper cellulase units/gram dried substrate (FPU/g-ds), 45 IU/g-ds β-glucosidase and 15 IU/g-ds pectinase. Enzymatic hydrolysis of alkali-treated and ozone-treated biomass after 48 h resulted in 42.29 g/L glucose, 6.82 g/L xylose and 24.13 g/L glucose, 8.3 g/L xylose, respectively. About 99% of glucose was consumed in 24 h by Pichia kudriavzevii HOP-1 cells resulting in 19.82 g/L of ethanol from alkali-treated cotton stalks and 10.96 g/L of ethanol from ozone-treated cotton stalks. Simultaneous saccharification and fermentation of the alkali-treated cotton stalks after 12-h pre-hydrolysis resulted in ethanol concentration, ethanol yield on dry biomass basis and ethanol productivity of 19.48 g/L, 0.21 g/g and 0.41 g/L/h, respectively which holds promise for further scale-up studies. To the best of our knowledge, this is the first study employing SSF for ethanol production from cotton stalks.