Physiological And Autolysis Performance Of Lager Yeasts During Serial Re-pitching

In order to reduce the economic cost,yeast cells will be reused for several times in beer brewing industry.During serial re-pitching,lager yeasts are always coping with various stresses,which will cause the decline of the quality of yeasts.Cells begin to self-degrade when yeast cells are unable to resist the pressures,resulting in the autolysis of yeast.Yeast autolysis will bring unfavorable impact on beer flavor and quality.Therefore,it is of great significance to understand the physiological changes and autolysis performances of lager yeast during serial re-pitching,and the results will benefit the breeding of brewer’s yeasts.In this study,the physiological and autolysis properties of lager yeasts during serial re-pitching were analyzed by studying the cell morphology,autolysis performance,cell viability and stress related gene expression levels.The main results of this paper are as follows:1.Cell morphology analysis: Lager yeast cells with round and full surface and plump shape became wrinkled during serial re-pitching,and even withered out because of the severe hydrolysis of the inner structure of cells.β-glucan contents of yeast cell wall decreased gradually during serial re-pitching,Pilsner and 5-2 decreased by 41.2% and 22.6% after 6 generations’ s culture,respectively.The contents of mannan did not change very much,and the content of chitin fluctuated irregularly.As chitin takes a very small fraction in the total polysaccharides of yeast cell wall,the content of total polysaccharide in yeast cell wall showed a decreasing trend during serial re-pitching.As a result,the cell wall of lager yeasts gradually became thinner during serial re-pitching,which affected the morphology of the cells.2.Analysis of autolysis performance: The anti-autolytic abilities of the strains gradually decreased with the increase of re-pitching times.And the degrees of autolysis and mortality of the Pilsner were significantly higher than those of 5-2.In addition,the extracellular protease A secretion of 5-2 was lower than that of Pilsner,indicating that the anti-autolytic ability of 5-2 was better than Pilsner.3.Analysis of cell viability and vitality: cell mortality reflects cell viability,and intracellular ATP content reflects cell vitality.In the process of serial re-pitching and autolysis,the viability of the two strains decreased slowly,while the vitality of the two strains decreased rapidly.The intracellular ATP levels of both strains decreased drastically by 70% when the cell viability only decreased by 5%.Lager yeast 5-2 with better anti-autolytic ability showed higher vitality,better stress resistance,lower ROS accumulation,and less DNA damage.4.Analysis of stress related gene expression levels: According to the transcriptome information of both strains collected from first batch of beer fermentation,it was found that the regulation patterns of about half of the genes were similar,while 1375 genes showed opposite regulation patterns.Genes related to aging,stress response and DNA damage repair were analyzed with qRT-PCR during serial re-pitching It was found that the genes related to aging and stress response were up-regulated to some extent during serial re-pitching,while the genes associated with DNA damage repair were down-regulated in both strains.Among the genes related to stress responses,most up-regulated genes in 5-2 were found to be involved in mitochondria functions.