Bioconversion process of rice straw by thermotolerant cellulolytic Streptomyces viridiochromogenes under solid-state fermentation conditions for bioethanol production

Abstract

Enzymatic hydrolysis of the cellulose fraction of rice straw to glucose using solid-state fermentation for bioethanol production is a focus of current attention. A total of 10 actinomycetes isolates were isolated from soils and decayed rice straw. All these isolates were purified and screened for their cellulolytic activity; one strain was selected for further study and identified as Streptomyces viridiochromogenes. Optimization of fermentation conditions showed highest cellulolytic enzymes production on the 5th day at pH 6.5 and at 40°C. The production of enzymes reached its maximal value at 4.0 g of rice straw/250 ml flask. Avicelase and total cellulase productivity were highly increased by the addition of NH₄Cl as Nsource, while maximum activity of CMCase was recorded by the addition of peptone as N-source to the fermentation medium. The influence of various physico-chemical factors on enzyme activity was also investigated. The half life time of avicelase, total cellulase and CMCase at 60°C was 39.4, 50.0 and 78.58 min, respectively. A maximum of ethanol production 1.428±0.074% (v/v) by Saccharomyces cereviseae using dilute acid pretreated rice straw hydrolysate with initial soluble sugar 2.340±0.072%  was recorded after 2 days of fermentation.

Key words: Bioethanol, cellulolytic enzymes, rice straw, solid-state fermentation, Streptomycesviridiochromogenes.