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Use of inferior yeast cultures represents one of the reasons for low fermentation efficiencies in Sri Lankan alcohol distilleries that use sugarcane molasses. The present study isolated and characterised yeast strains found in natural environments in Sri Lanka and evaluated their performance under laboratory conditions in an effort to select superior strains for industrial fermentations. Yeasts were characterised based on morphological and physiological features such as sugar fermentation and nitrate assimilation. Ethanol production, alcohol tolerance and growth rate of the most promising strains were monitored following laboratory fermentations of molasses. Over a thousand yeast cultures were collected and screened for fermentative activity and a total of 83 yeast isolates were characterised as higher ethanol producers. Most of these belonged to the genus Saccharomyces. Certain strains produced over 10% (v/v) alcohol in molasses media during 72 h laboratory fermentations. Only two strains, SL-SRI-C-102 and 111, showed an appreciable fermentation efficiency of about 90%. The latter strain produced the highest level of ethanol, 11% (v/v) within a 48 h fermentation and exhibited improved alcohol tolerance when compared with the baker's yeast strains currently used in Sri Lankan alcohol distilleries. This study highlights the benefits of exploiting indigenous yeasts for industrial fermentation processes.

This study describes bioethanol production from palmyrah (Borassus flabellifer L.) waste using yeast and optimization of fermentation conditions to increase the yield. When subjected to different pre-treatment conditions with various alkaline and acidic solutions, significantly higher reducing sugar contents were obtained from palmyrah coir dust with H 2 SO 4 and HNO 3 pre-treatments. H 2 SO 4 pre-treatment at 3%, 5% and 7% concentrations for 15, 30, and 45 minutes respectively were given to the coir dust, and a significantly higher quantity of sugar was produced with 3% H 2 SO 4 after 45 minutes of reaction. Among the acids and bases used for the pre-treatment, H 2 SO 4 is the best hydrolysing agent for coir dust. When fermentation was done with the coir dust hydrolysed solution under optimized conditions [30 °C and pH 5.0 with baker's yeast (6 × 10 6 cells mL -1 ) and incubated in peptone, yeast extract and nutrient (PYN) medium at 30 °C for 7 days] , a significantly higher amount of alcohol was produced on the 4 th day than the respective non-optimized conditions. Similarly, molasses and fruit pulp of palmyrah also produced significantly higher quantities of ethanol individually under similar optimized conditions used for coir dust on the 6 th and 4 th day of fermentation respectively, than the respective non-optimized conditions. The efficiency of alcohol fermentation by yeast with palmyrah coir dust acid hydrolysate was 19%. Therefore, coir dust could also be used as a long-term substrate for bioethanol production, considering the excess availability of this underutilized waste material.