Improvement of sucrose tolerance of bakers yeast, Saccharomyces cerevisiae by mating

Abstract

ABSTRACT

Bakers yeast, Saccharomyces cerevisiae, used as a starter in fermentation during a baking process is the main determinant of bread quality. S. cerevisiae SPC-SNU 70-1, isolated from Nuruk has excellent fermentation characteristics such as good leavening ability, generating volatile organic acids which affect a mild flavor and delaying the aging of bread. S. cerevisiae SPC-SNU 70-1 was able to show a good fermentability in lean (0% sucrose) and regular dough (8% sucrose), though, CO2 production decreased significantly in sweet dough containing 20 % sucrose. It is necessary to improve traits of the SPC-SNU 70-1 strain in high sucrose concentrations for various applications, while maintaining the inherent fermentation characteristics. Eight novel strains to be mated with SPC-SNU 70-1 were isolated from Nuruk and grape. Those strains have high sucrose tolerance but poor fermentation activity in lean dough. Among 8 strains isolated, N1, N2, N5 and N6 strains with excellent leavening ability in sweet dough were selected for mating, but the N1 strain was excluded because it was impossible to form spores which is the first step of mating. The selected strains underwent sporulation, tetrad dissection and analysis. The isolated spores maintained haploids, thus the genetic sequences of the HO gene (HOmothallic switching endonuclease gene, encoding HO endonuclease that changes the mating type) were analyzed. All selected strains were found to be heterothallism. The haploid strains for mating were selected by the MAT locus PCR, barcode PCR and leavening ability. After mating, the same selection process was repeated to select mated strains. Total 17 strains were selected and checked for the amount of CO2 production in lean, regular and sweet doughs. The strains with high industrial capability are designated as S-HW2, S-HW8, S-HW9, S-HW11 and S-HW16, S-HW17. Among them, the S-HW16 was selected based the fermentation ability in three different doughs. The amount of CO2 production of the S-HW16 strain increases 1.1-folds in lean dough, 1.3-folds in regular dough and 1.8-folds in sweet dough compared with that of SPC-SNU 70-1. The amount of fragrance components also increases. In sweet dough, the amount of esters which give a sweet flavor increases 10.6-folds and the amount of acetoin which give a buttery flavor increases 2.1-folds. This study demonstrated that mating was successfully used to develop new yeast strains with desired characteristics for baking.