Biotechnology Letters 21: 343–348, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 343 Characterisation of osmotolerant hybrids obtained by fusion between protoplasts of Saccharomyces cerevisiae and heat treated protoplasts of Torulaspora delbrueckii M.E. Lucca 2 , M.A. Loray 1,2 , L.I.C. de Figueroa 1,2,∗ & D.A. Callieri 1 1 PROIMI; 2 Universidad Nacional de Tucum´ an PROIMI, Av. Belgrano y Pje. Caseros, 4000 Tucum´ an, Argentina ∗ Author for correspondence (Fax: +54 81 344887; E-mail: lfigueroa@proimi.edu.ar) Received 19 November 1998; Accepted 25 February 1999 Key words: Osmotolerant hybrids, polyhydroxy alcohols, protoplasts fusion, Saccharomyces cerevisiae, Torulas- pora delbrueckii Abstract Saccharomyces cerevisiae was fused with heat-treated protoplasts of an osmotolerant yeast, Torulaspora del- brueckii, to obtain hybrids having increased tolerance to increased glucose concentrations (up to 700 gl −1 ). The production of glycerol and arabitol by the hybrids was within the range of those of the parental strains, but the production of ethanol was higher. Introduction Yeasts belonging to the genus Torulaspora are fre- quently found in fruit juice (Deak & Beuchat 1993), fermented dough (Infantes & Schmidt 1992) and alco- holic beverages (Sanni & Loenner 1993), and some of them are used for the industrial production of bread. This genus is characterised by small, round cells pro- ducing round ascospores. T. delbrueckii has been used in Japan as an osmotolerant baker’s yeast useful for raising sweet breads and pastries. Its main disadvan- tage is the small size of the cells, which makes re- covery of the biomass during production more difficult (Spencer & Spencer 1997). Living cells are able to display a molecular re- sponse when growing in adverse environmental con- ditions such as hyperosmotic medium. The degree of adaptation to osmotic stress depends on the spe- cific osmotolerance of each strain and on the nature of the factor involved in the reduction of water ac- tivity. For baker’s yeast, Saccharomyces cerevisiae, osmotolerance is one of the critical parameters related to the process improvement. For most organisms the adaptive response to high osmolality is the intracel- lular accumulation of organic compounds, which act as compatible solutes. In yeasts, compatible solutes are polyhydroxy alcohols such as glycerol, arabitol, mannitol, erythritol and xylitol (Yagi 1991). For industrial yeasts such as baker’s, distiller’s, brewer’s and other beverage yeasts, osmotolerance might be of importance. S. cerevisiae has a certain degree of osmotolerance, but grows poorly in media of very high osmotic pressure, which limits its use in the fermentation of concentrated musts for production of fermented beverages (Loray et al. 1995, Legmann & Margalith 1983). Cell size of T. delbrueckii could be increased by increasing the ploidy using protoplasts fusion and related techniques (Ohshima et al. 1987, Sasaki & Ohshima 1987). Protoplasts fusion is now a recognised technique for the improvement of industrial yeasts for baking, brewing, ethanol production and other purposes. Nu- merous strains have been constructed by intraspecific or interspecific fusions (Heluane et al. 1988, Legmann & Margalith 1988, Loray et al. 1995). In the present work an intergeneric protoplasts fusion between T. delbrueckii and S. cerevisiae is re- ported. The growth rate of the hybrids, biomass yield, ethanol production, tolerance to high osmotic pressure