چكيده به لاتين
Abstract
Due to the decline in fossil fuel resources and their associated environmental pollution, researchers have recently focused on alternate renewable clean energies such as biofuels, obtainable from low cost primary resources. Bioethanol is an important biofuel formed by fermentation of glucose and xylose, sugars released after hydrolysis of agricultural residues (lignocellulosic). While Zymomonas mobilis is the best glucose fermenting bacteria, it is unable to ferment xylose. In this study, co-culture of Zymomonas mobilis and Pichia stipites, a xylose fermenting yeast, was used to enhance ethanol production. Firstly, growth and ethanol production by bacteria under anaerobic and aerobic conditions using glucose were investigated. Results showed that ethanol production yield was twofold at anaerobic compared to aerobic condition due to the lack of cellular respiration. Bioethanol production from xylose by yeast in flasks at three volumetric medium/flask ratios of 2:5, 3:5 and 4:5 showed the ratio of 2:5 as the best ratio with the fastest sugar consumption rate and ethanol production yield of 0.37 (g ethanol/g sugar). Using a mineral salt medium containing both sugars, ethanol production by co-cultures as well as pure cultures was studied and compared. Anaerobic culture of bacteria in 400 ml bioreactor followed by cell removal after glucose depletion and inoculation of yeast at micro-aerobic condition was found as the superior strategy with ethanol yield of 0.41 (g ethanol/g sugar) and volumetric productivity of 0.26 g ethanol/ (l-h). These results show that cost-effective ethanol production from hydrolysate of lignocellulosic material requires an appropriate co-culture of bacteria and yeast.
Keywords: Bioethanol, Agricultural residues, Lignocellulose, Co-culture, Zymomonas mobilis, Pichia stipitis
