Environmental-friendly pretreatment and process optimization of macroalgal biomass for effective ethanol production as an alternative fuel using Saccharomyces cerevisiae

Abstract

Fossil fuel depletion and gas limitations require tremendous efforts to the production of renewable energy from environmental-friendly and sustainable sources. The third-generation bioethanol from non-lignocellulosic biomass of macroalgae is an effective candidate for renewable and sustainable fuel–making that mainly involves pretreatment, enzyme hydrolysis, and microbial fermentation. Therefore, the purpose of this research was to develop a consolidated process for the effective conversion of macroalgal biomass into third-generation bioethanol. The pretreatment of two macroalgae biomasses (namely Ulva rigida and Ulva intestinalis) was optimized using a steam explosion, compared with ultrasonication, to maximize the fermentable reducing sugars yield. A condition of pressure at 25 psi and pretreatment time of 15 min by steam explosion pretreatment of U. intestinalis was the best condition for maximizing fermentable reducing sugars up to 5.41 g/L, generating 86.6 kJ/L of energy. In subsequent enzyme hydrolysis of pretreated biomass, using commercial cellulose enhanced the fermentable reducing sugars up to 6.18 g/L. Fermentation of the hydrolysate using yeast Saccharomyces cerevisiae produced 0.12 g–ethanol/g–fermentable reducing sugar in a short time with ethanol productivity of 2.54 mg–ethanol/g–fermentable reducing sugar/h. Interestingly, the produced ethanol can be converted into energy up to 21.91 kJ/L. This study demonstrates the macroalgal biomass is a suitable feedstock of non-lignocellulosic biomass for third-generation bioethanol production and the developed process confirmed the practical, low-cost, and non-polluting conversion of macroalgal biomass into bioethanol based renewable energy.