Stress markers as indicators of fermentative ability of a Saccharomyces cerevisiae brewery strain

[摘要] ENGLISH ABSTRACT: In the brewing industry yeast cells are re-used in successive fermentations. Consequently,the state of the cells at the end of each successive fermentation could impact on thequality of the subsequent fermentations. The use of markers to evaluate the fermentativeability of yeast to resist stress enables brewers to select populations of yeast for brewing.Yeasts are typically exposed to osmotic-, ethanol- and cold-stress during the high-gravitybrewing process. In this study the vitality of the yeast cells was monitored during andafter each successive high-gravity brewing fermentation. This was done by measuring thecell metabolites, which included glycerol, trehalose and glycogen. Others markers thatwere evaluated for yeast viability were the number of budding scars, the levels of activityof the enzymes neutral trehalase and esterase and the expression level of the heat shockprotein Hsp12p. Coupled to these evaluations, the growth of the yeast and the utilisationof the sugars glucose, fructose, maltose and maltotriose were monitored during thefermentations. The experiments were conducted in 2-litre E.B.C. tubes at either 14 oC orat 18oC using standard techniques.Comparable growth patterns were obtained for different re-pitching fermentations, withfermentation 1 at 18ºC and 5 and 6 at 14°C being the most active fermentations. Thehigher temperature encouraged more rapid growth and a greater numbers of cells. Thewort attenuation was more rapid at 18°C than at 14°C. Glucose and fructose in wort wereutilised prior to maltose and maltotriose. At 18°C the yeast consumed the sugars faster,with mean utilisation values of 97.3% glucose, 100% fructose, 59.9% maltose and 65.6%maltotriose. At the lower temperature of 14°C high concentrations of residual sugarsremained at the end of the fermentation. All re-pitching fermentations revealed lowerviabilities at 18°C in comparison to the 14°C fermentations.Simultaneously, a number of other markers were evaluated. The intracellular trehaloseconcentration per cell varied considerably with each fermentation. Trehalose levels at18°C gradually increased in concentration from 48h until the end of the stationary phase. Much lower trehalose concentrations were observed in fermentations conducted at 14°C.Higher and more consistent glycerol concentrations were found in fermentations at 14°Cwith mean concentrations of 12 mg/g dry weight at pitching. The expression of the heatshock protein Hsp12p level increased during the fermentation but no sharp increase wasdetected in any particular fermentation. No increase in yeast budding scar number wasobserved during re-pitching fermentations. Neutral trehalase and esterase activities infermentations at 18°C were especially high at pitching. Neutral trehalase activities at14°C were all generally lower than in the case of fermentations at 18°C.The fermentation ability of flocculated yeast in slurry and yeast suspended in beer wasinvestigated after exposure to various stresses. The aged yeast present in the slurry wasgenerally found to be more resistant to stress, in particularly to osmotic stress, throughoutthe serial re-pitching process. The fermentation rates of both yeast types were especiallysensitive to prior exposure to ethanol stress.

[发布机构] Stellenbosch University