Improvement of Bioethanol Production using Saccharomyces cerevisiae

Ethanol, when mixed with gasoline, is an economical and environmentally friendly liquid fuel. Yeast cells under anaerobic conditions can ferment glucose to ethanol. However, glucose is expensive for industrial applications and starch is an economical alternative. Simultaneous cold starch hydrolysis and fermentation was investigated for ethanol production from wheat starch particles. With a view to minimize process costs while maintaining a satisfactory ethanol yield, both a recombinant yeast cell and an inexpensive medium were tested for their fermentation abilities. Initially, NRRL Y132 strain was compared to Muntons yeast for their abilities to produce ethanol from glucose. Both the wild-type and the recombinant NRRL Y132 strains were cultured on soluble starch to determine if the plasmid bearing strain could produce ethanol without the addition of α-amylase. Finally, Muntons yeast was cultured on starch particles using both expensive and inexpensive media. Sequential hydrolysis and fermentation runs were performed using the inexpensive medium, with hydrolysis carried out at 30°C, 37.5°C, 45°C and 52.5°C. The wild-type, NRRL Y132 strain grew faster and produced more ethanol than Muntons yeast when cultured on glucose. Compared to the wild-type strain, the recombinant NRRL Y132 strain did not show enhanced ethanol production from soluble starch. The results of the simultaneous hydrolysis and fermentation runs showed that the ethanol yields for runs performed in expensive medium (0.41, 0.38 and 0.42 g ethanol / g glucose) were slightly lower than those for runs performed in the inexpensive medium (0.46, 0.44 and 0.43 g ethanol / g glucose). The growth rates for the expensive and inexpensive media runs were comparable. Hence, it was concluded that the inexpensive medium can be used for ethanol production from starch particles with good ethanol productivities. For the sequential hydrolysis and fermentation runs, it was observed that the growth rates (0.11, 0.10, 0.10 and 0.11 h-1) as well as the ethanol yields (0.44, 0.37, 0.44 and 0.39 g ethanol / g glucose) were similar in spite of the four different hydrolysis temperatures. Therefore, it was concluded that increasing the temperature above 30°C for enhancing starch particle hydrolysis does not increase fermentation productivity significantly.