The Breeding Of Saccharomyces Cerevisiae Producing Compatible Level Of SO₂ And Studies On The Flavor Stability Of Beer

SO₂ produced by yeast during metabolism was an important substance for the flavor stability of beer. Because of its less production, sulfite is often to be added into beer after fermentation. In this paper, the sulfite metabolic route of yeast was studied. A strain moderately producing SO₂ was obtained by mutation, with which beer was fermented. The genetic substance was analyzed by RAPD. The main research contents included the influence of SO₂ on the flavor stability of beer, the breeding of strain moderately producing SO₂, beer fermentation and genetic substance analysis of mutant.The influence of SO₂ on the flavor stability of beer was primary studied. It was founded that the RSV value of beer was increased by 82.6% with adding 30mg/L SO₂. Through the determination of flavor substances of beer by gas chromatographic analysis, it was founded that the flavor substances were decreased by adding of SO₂, especially the diacetyl and aldehyde. It was showed that the flavor stability of beer could be improved by adding SO₂.Six strains (Saccharomyces cerevisiae) were employed for tests. The fermentation characteristics were determined, as well as the production of SO₂ and H₂S which effect the beer flavor. Strain BY-1 with better flocculation ability, medium fermentation degree, higher fermentation velocity and higher concentration of SO₂ was selected as parent strain. After UV treatment, lead acetate media plates and different differential medium plates containing different sulfur sources were used as screening plates. After the determination of SO₂ and H₂S, mutant M24 was finally selected as the optimal strain. Compared with BY-1, SO₂ production of M24 was 29.52mg/L, increased by 15.27mg/L;the production of H₂S was 0.33mg/L, decreased by 45%.fermentation in 10L tank was done for the compare of two stains. During fermentation, the growth of two yeasts, the velocity of fermentation, the production of diacetyl, fermentation degree, flocculation ability and α-amino nitrogen assimilation rate were basic same. The influence of oxygen dissolved in wort was also studied, including the influence on the growth of two yeasts, the velocity of fermentation, the production of diacetyl, fermentation degree, flocculation ability and α- amino nitrogen assimilation rate. It was showed that oxygen demand of the M24 was high. If the M24 was used, oxygen dissolved in wort was 7～10mg/L.The genetic substances of two stains were analyzed with RAPD. The random primers were used and the genome DNA were amplified with PCR. The optimized condition were obtained by studding the factors affecting the RAPD. By the result of electrophoresis, electrophoretic bands of the genome DNA of two strains were slightly different. It showed that the genetic substances were different and the genetic substances of BY-1 was slightly changed by UV treatment.