Bioethanol production from Chara globularis using yeast and yield improvement by optimization of conditions

Abstract

The rising population, depletion of petroleum-based fossil fuel and atmospheric contaminations by combustion of fossil fuel have opened avenues for alternative, eco-friendly and renewable energy sources. Bioethanol is an alternative and renewable source that has drawn attention due environmental concerns and energy security with non-renewable sources. This study was aimed at determining the potential bioethanol producing freshwater flora that are abundantly available in the Northern Province of Sri Lanka using Saccharomyces cerevisiae and to optimize the fermentation conditions to enhance the ethanol yield from Chara globularis. Freshwater flora such as C. globularis, Cabomba caroliniana, Spirodela polyrhiza, Salvinia minima, Salvinia natans, Wolffia arrhiza and Wolffia globosa were hydrolysed with 1M sulfuric acid solution to determine the reducing sugar and bioethanol yields. C. globularis produced a higher amount of reducing sugar and bioethanol than other species tested. When C. globularis was pre-treated with 1 M acid solutions (sulfuric acid, nitric acid, and hydrochloric acid) and alkaline solutions (sodium hydroxide and potassium hydroxide), a higher reducing sugar and bioethanol yields were obtained with sulfuric acid. When bioethanol was produced from C. globularis using S. cerevisiae following three different hydrolysis methods viz., acid hydrolysis (1 M sulfuric acid), enzymatic hydrolysis (1% alphaamylase) and combination of chemical and enzymatic hydrolysis (1 M sulfuric acid and 1% alpha-amylase), the combination of chemical and enzymatic hydrolysis gave a higher yield, thus was selected. The conditions for fermentation of C. globularis substrate using S. cerevisiae were optimized sequentially by changing one factor at a time while keeping the other variables constant. After the optimization of fermentation time (24 hours), operating temperature (35 oC), rotation speed (200 rpm) and sulfuric acid concentration for combined pre-treatment (0.75 M) with an inoculum size of 100 g l-1, bioethanol yield was increased by 2 times compared with the non-optimized condition.