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The yeast Torulaspora pretoriensis YK-1 possesses both high potential leavening ability and freeze-thaw resistance, but its leavening ability in dough, without the addition of sugar, is much less than commercial baking strains (Oda and Tonomura 1993) Institute for Fermentation, Osaka, Japan (IFO) 0022, another strain of T. pretoriensis, previously found to leaven dough efficiently without the addition of sugar, showed higher activities of both maltose permease and α-glucosidase and fermented maltose in an aqueous medium faster than YK-1. No significant difference was observed between the affinities of maltose permease and α-glucosidase from IFO 0022 and those from YK-1. MAL61 and MAL62 genes of Saccharomyces cerevisiae were expressed in T. pretoriensis YK-1 and elevated the activities of maltose permease and α-glucosidase, respectively, when these genes were introduced on YCp-type plasmids under control of the glyceraldehyde 3-phosphate dehydrogenase promoter. Maltose fermentation by cells harbouring the plasmids carrying either MAL61 or MAL62 genes was slightly stimulated. Introduction of both genes further enhanced the maltose fermentation rate but not leavening ability in dough without the addition of sugar. These results suggest that maltose permease and α-glucosidase determine the overall velocity of maltose fermentation. |
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