World Journal of Microbiology & Biotechnology 12, 357-360 Ethanol production by fermentation of sweet-stem sorghum juice using various yeast strains B. Bulawayo, J.M. Bvochora, M.I. Muzondo and R. Zvauya* Ethanol tolerance, osmotolerance and sugar conversion efficiency were used to screen yeasts for potential ethanol production from sweet-stem sorghum juice. Of the ten strains of Saccharomyces sp. that produced ethanol from the sorghum juice or from yeast extract/phosphate/sucrose (YEPS) media, the best sugar conversion efficiencies were greater than 85% for the strains Vin7, SBg, N96 and GSL Vin7 and SB9 had higher sugar conversion efficiencies for sweet-stem sorghum juice, while strains N96 and GSL gave higher conversions in YEPS. Key words: Ethanol production, fermentation, sweet-stem sorghum. Ethanol production for fuel has been industrialized in a number of developing countries. Most of these industries use sugar cane molasses as a raw material. Although Saccha- romyces spp. have been used to ferment molasses for ethanol production, tolerance of these strains to high alcohol levels and sugar concentrations limits the concentration of alcohol that can be obtained from such processes (Ezeogu & Okolo 1994). Zimbabwe has an ethanol plant that has been running for over a decade (Rosenchein & Hall 1991). Over the past few years the sugar cane crop has been affected by drought resulting in the temporary closure of the plant. Efforts are therefore under way to find alternative raw materials for ethanol production. Sweet-stem sorghum is a potentially good alternative crop to sugar cane. Some of the advantages of sweet-stem sorghum over sugar cane include its ability to withstand dry conditions, a lower requirement for fertilizer, a rapid growth rate, ease of planting and the lower cost of total fermentable sugars from sweet-stem sorghum than from sugar cane (Lu Nan & Hianxue Ma 1989). Despite these advantages there are relatively few studies on ethanol production from sweet-stem sorghum (Mohite & Hephzibar 1984; Gibbons & Westby 1989; Smith 1993). This study was therefore aimed at screening yeasts from various sources for their potential to convert sweet-stem sorghum The authors are with the Food and Fermentation Laboratory, Department of Biochemistry, University of Zimbabwe, M.P.167, Mount Pleasant, Harare, Zimbabwe; fax: 263 4 333407. * Corresponding author. @ 1996 Rapid Science Publishers juice to ethanol on the basis of osmotolerance, ethanol tolerance and invertase activity. The strains which showed good tolerance values were then assessed for their potential to convert sweet-stem sorghum juice to ethanol on the basis of their sugar conversion efficiencies. Materials and Methods Microorganisms The yeast strains used in this study were all Saccharomyces. Three strains designated SBg, Vin7 and N96 were wine yeasts while strains Z7 and Zlt were isolated from the ethanol plant in Triangle Estates, Chiredzi, Zimbabwe by Dr Mpandi-Khosa. Strains EA21, Z12, and Z15 were isolated from a sorghum brew. A reference strain, Saccharomyces uvarum, ATCC26602, was ob- tained from the American Type Culture Collection. The strains were kept on Yeast/Peptone/Dextrose (YPD) (Difco) agar plates at 4°C and subcu[tured every four weeks. Media Stalks of sweet-stem sorghum cultivar Vl1152 were obtained from fields in Triangle Estates, Lowveld, Zimbabwe. The mature crop was harvested, defoliated and the stalks cut at the nodes. The stalks were immediately transferred to the laboratory where scale blenders were used to extract the juice. Two parts water to one part juice were added during the extraction of the sugars. The extracted juice was immediately frozen at -20°C until required for use. The juice used for the fermentations was supplemented to give the following concentrations (g/l): sucrose, 150; urea, 1.6; (NH4)2HPO4, 1.2. The synthetic media used was YEPS containing World Journal of Microbiology & Biotechnology. Val 12, 199o 3 5 7