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.

Large scale consumption of fossil fuel to meet the increasing demand causes adverse effects on the environment due to the emission of harmful greenhouse gases. The production of bioethanol from diverse fruit juices that are underutilized because of poor taste quality could be one of the alternative fuels to overcome the issues. The objective of the study was to produce bioethanol from sour Citrus aurantifolia fruit juice using palmyrah toddy yeast and to optimize the conditions to increase the bioethanol yield. The sour Citrus fruit juice was inoculated with palmyrah toddy yeast (Saccharomyces cerevisiae - 2g/L) in the fermentation media (100ml, sour Citrus fruit juice: distilled water = 1:3) composed of 10 g/L yeast extract, 10 g/L KH2PO4, 2 g/L (NH4)2SO4, 2 g/L peptone and 0.5 g/L MgSO4·7H2O and allowed for fermentation for 24 hours at room temperature. The amount of ethanol produced from the Citrus juice was 0.8% (V/V) after 24 hours of fermentation. In order to optimize the fermentation process for Citrus aurantifolia, a variety of experimental parameters were studied, including the type of nitrogen source (ammonium sulphate, ammonium nitrate, ammonium chloride, and urea), inoculum size (0.4 to 1.0 g/100 mL), temperature (20 to 40 °C), rotation speed (50 to 250 rpm), concentration of raw fruit juice (5 to 100%), amount of urea (0.1 to 2.0 g/100 mL), carbon source (glucose, sucrose, maltose, and dextrose), amount of sucrose (1 to 20 g), pH of the medium (3.0 to 8.0), and incubation period (24 to 96 h). After conducting the experiments, it was found that using Citrus aurantifolia at 100% concentration resulted in significantly higher ethanol yields of 11.50%, which was 14.37 times higher than the non-optimized conditions. The highest yield was achieved when the fermentation was carried out at 35 °C for 24 h with an inoculum concentration of 0.5 g/100 mL, a rotation speed of 150 rpm, a pH of 4.0, 0.1 g/100 mL urea as the nitrogen source, and 10 g/100 mL sucrose as the carbon source. Large scale fermentation study using bioreactor should be done to determine whether this finding could be commercialized. Key words: Bioethanol, Fermentation, Palmyrah toddy yeast, Citrus fruit juice, Urea

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 °C), 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.