Study Of Adaption To Environmental Stress Of Lager Yeast Regulated By Key Amino Acids During High Gravity Fermentation

The main factors influencing the fermentation performance of yeast are high osmotic pressure at the early stage and high ethanol toxicity at the later stage of fermentation in the process of high gravity beer brewing.The lack of nitrogen is a common problem in the beer high gravity brewing,which could lead to the decrease of yeast cell viability and fermentation performance.The main objectives of this study are as follows: firstly,the physiological characteristics and fermentation performance of lager yeast under different wort gravity and different ethanol concentration were studied respectively.Then,the key amino acids improving the cell viability and fermentation performance were found out by correlation analysis.Finally,the optimal fermentation conditions were determined by key amino acids supplementation according to the ratio of key amino acids in the initial wort.The results would provide the theoretical basis and method instruction to the selection of high quality nitrogen source in the industrial production of beer.The main contents and results are as follows:(1)Effects of different gravities all-malt wort on the yeast fermentation performance,amino acids assimilation and beer flavor were studied.The results showed that the increase of wort gravity significantly increased the yield of ethanol and the production of higher alcohols,decreased yeast growth and cell viability.Compared with the control group(12 °P),the ethanol yield of 16 °P,20 °P and 24 °P wort fermentation were increased by 32%,43% and116%,at the later stage of fermentation the cell viability were 93.8%,88.50% and 76.44%,higher alcohol production as increased by 9%,13%,39%,flavor ester production was decreased by 42%,55%,50%,respectively.The correlation between assimilation amount of free amino acid and fermentation performance was analyzed under high osmotic stress conditions,the results suggested that Ser,Met,Phe,Trp,His could improve the adaption of yeast to high osmotic stresses.(2)Effects of different ethanol concentrations in normal gravity wort on yeast fermentation performance,amino acid assimilation and beer flavor were studied.The results showed that the fermentability was significantly decreased with the increase of ethanol concentration.Compared with the control group,adding 2%,4%,6%,8%,10% ethanol inwort after fermentation,net ethanol yield was decreased by 17%,18%,34%,65%,99%respectively,cell viability were 91.18%,89.85%,84.83%,72.36%,50% at the later stage of fermentation,respectively.In addition,the maximum number of suspended cells was reduced by 34%,48%,50%,70%,98% respectively,indicating that the addition of 10% ethanol could lead to stuck fermentation.Moreover,the accumulation of ethanol in the late fermentation was the most important factor caused the decay of yeast cell fermentation performance.Assimilation of amino acids was significant influencedby different ethanol concentrations.The correlation between assimilation amount of free amino acid and fermentation performance was analyzed under ethanol stress,the results suggested that Asp,Ser,Gly,Arg,Tyr,Val,Met,Lys,Ile,Leu could improve the adaption of yeast to high ethanol toxicity.(3)Effects of different amino acids supplementations(0.5 times,1 times,2 times)on the fermentation performance of yeast during high gravity brewing(24 °P)were evaluated.The results showed that 10 kinds of key amino acid supplementation improved the fermentability and ethanol yield,promoted yeast cell growth significantly,as well as yeast cell viability,and the beer color value.High gravity wort supplemented with 1 times of key amino acids mixture exhibited better fermentation performance,compared with the control group,fermentability,ethanol yield,maximum number of suspended cells and cell viability at the later stage was increased by 6%,17%,11% and 10%,respectively.It was confirmed further that Ser,Met,Phe,Trp,Arg,His,Ile,Leu,Lys and Tyr were key amino acids to regulate yeast cells to adapt to high osmotic pressure and high ethanol toxicity.Furthermore,the diluted beer color was still bright,and 1 times of key amino acids supplementation made the final beer minimum ?E value,the color was the closest to Qingdao pure draft beer.The results provide a reliable theoretical basis for the selection of high quality nitrogen sources in industrial high gravity beer brewing.