Study On The Application Of Immobilized Yeast In Beer Fermentaion

For centuries, brewers have produced beer through batch fermentationby by freely suspended yeast. But rapid continuous fermentation by immobilized yeast may be the most effective method of improving brewery productivity. One yeast strain with strong fermentation capacity and low content of diacetyl was selected from ten yeast strains which storaged in our lab by comparing to their limit fermentation, fermentation capacity and performance. The optimal conditions of entraped yeast by polyvinyl alcohol-sodium alginate and absorbing yeast by macroporous resin were studied.The effects of fermentation parameters (temperature, resident time and wort concentration) on the physical and chemical indexes of beer produced by the immobilized yeast were studied. The best fermentation process was achieved by the orthogonal experiment of filler content, wort concentration, fermentation temperature and resident time. Under two-month steady-state continuous fermentation, the effects of fermentation temperature and adjuncts proportion on the formation of flavor compounds of beer were studied. The indexes and contents of flavor compounds of beers produced by continuous fermentation and batch fermentation were compared. The main results were as follows:Four yeast strains were selected through the 80% apparent limit degree fermentation of 12°P wort and 68% real limit degree fermentation and the 1-2 d weight losses(g/d)>2.6 fermented with 300 mL wort. And then they were fermented with 3 L wort. The No. 21 strain was selected for its efficiency of assimilating free amino nitrogen.The conditions of entraping yeast by polyvinyl alcohol-sodium alginate were studied. The optimal conditions were 8% polyvinyl alcohol, 2% sodium alginate, 50%(w/v) NaNO₃ and 2% CaCl2 The immobilized yeast had the highest mechanical strength. Compared to free yeast, fermentation with immobilized yeast was slower in initial stage, but was faster after 24 h. In ten times of batch experiments, the real degree fermentation was almost unchanged, and it showed good operational stability.The absorbtion of yeast by macroporous resin was studied. The macroporous polarity resin ZGCAD45 was selected from 7 kinds of resins. The highest cell loading content was under the conditions of 14℃, the ratio of resin and yeast suspension 1:4, the concentration of yeast suspension 3.05×10⁸ cell/mL, and the absorbtion time 16 h.The optimized conditions of continuous fermentation were as follows: Filler content was 50%, Wort concentration was 10°P, Fermentation temperature was 12℃and Resident time was 12 h. Under two-month continuous fermentation, alcohol content was basically maintained at 3.26%(w/w), the real extract kept in 3.38°P, the real degree of fermentation remained 67%, diacetyl content was kept in 0.127 mg/L, and the number of yeast was (4⁵)×10⁶ cell/mL.The fermentation temperature and adjunct proportion which affect the flavor compounds formation during continuous fermentation were studied. When fermentation temperature increased from 10℃to 15℃, the higher alcohol contents increased slowly. At 15℃isoamyl alcohol content was higher than its taste threshold. The contents of ethyl acetate and isoamyl acetate decreased, but the content of isoamyl acetate exceeded its threshold in above temperatures. When adjunct proportion increased from 25% to 55%, the content of diacetyl increased, while contents of ethyl acetate and isoamyl acetate decreased. The lower content of higher alcohols was achieved with 30% adjunct proportion.Comparing with batch fermentation, the contents of diacetyl and volatile esters were higher, while the contents of higher alcohols were lower by continuous fermentation with immobilized yeast. There were few differences in alcohol content, real extract and real degree fermentation between these two kinds of fermentation.