| Beer is a kind of product which is fermented by Saccharomyces cerevisiae. Its main raw materials are malt,water,hop,Saccharomyces cerevisiae.Beer has been regarded as a safe beverage due to the presence of ethanol (0.5%-10%,w/v),hop bitter compounds(approx 17-55ppm of iso-α-acids),the high content of carbon dioxide(approx 0.5%w/v),the low pH(3.8-4.7),the extremely reduced content of oxygen(<0.1ppm). However,a few microorganisms still survived in beer as a result of the wrong operation and poor sanitation.These microorganisms are called as beer spoilage bacteria.Lactic acid bacteria are the most polluted and harmful bacteria.18 strains were isolated from breweries in JiNan, QingDao and BeiJing. 15 strains were determined to be lactic acid bacteria by the Gram staining, catalase test, morphological examination and so on. The test for acid–producing in wort showed that 6 strains from15 strains could make the acidity of wort increase 1.5°T.Beer spoilage lactic acid bacteria can be divided into lactobacilli and pediococci according to morphology. The composition of lactobacilli and pediococci'selective medium and their conditions for incubation were optimized by single factor experiment and orthogonal experiment.The optimal carbon source and nitrogen source were glucose, typtone, soybean peptone.Their concentrations were 4%, 2%, 0.5% respectively according to orthogonal experiment; MgSO4·7H2O,MnSO4·4H2O,K2HPO4 were used as the mineral salts of the medium and their concentrations were 0.02%, 0.005%, 0.4% respectively. The experiment of growth factors for L1, L3, P1, P2 showed that VB5 and calcium pantothenate could have an incentive effect on strains'growth. Some natural materials could substitute for these Vitamine.Carrot juice,tomato juice,apple juice could be lactobacilli'natural growth factors and their concentrations were 3%,25%,3%;yeast powder,bean juice could substitute for pediococci'natural growth factors and their concentrations were 0.3% and 5%.The original pH of the medium was determined at 6.0-6.2 and the temperature for incubation was at 25℃。Saccharomyces cerevisiae existed in many processes of brewing.The presence of Saccharomyces cerevisiae disturbed the determination of beer spoilage bacteria during the incubation on plate.Natamycin is a kind of new antibiotics and can be used in the determination of beer spoilage bacteria. It examined the inhibition of Natamycin on Saccharomyces cerevisiae 206#,S. cerevisiae 308#,S. cerevisiae 303#,S. cerevisiae 354# by use of round filter paper. The result showed that the diamters of antimicrobialzone made by 5ppm Natamycin were 14mm,16mm,16mm,18mm respectively.The test for minimal inhibition concentration showed that the MIC on 206#,308#,303#,354#were 3ppm,4ppm,3ppm,3ppm.Temperature,acid,base have an negative effect on Natamycin. The result showed that 10ppm Natamycin lose inhibition effect after the thermal treatment at 121℃for 30min. The inhibition activist of Natamycin was the highest when the pH of solution at 5.0-7.0.The solution of Natamycin in the water was improved when the pH was lower than 5.0 or higher than 7.0, but the activist was lowed at the same time.Lactic acid bacteria growed in the media,as a result, they produced lactic acid and the pH of the medium was changed. Single indicator was deficited in colour change and sensitivity, so optimized the mixed indicator. Mixed chlorophenol red with bromocresol purple, bromocresol green, bromophenol blue, bromothymol blue respectively according to different proportion .The mixed indicator which changed colour significantly was Chlorophenol red with Bromocresol purple.When mixed Chlorophenol red with Bromocresol purple according to the proportion of 1:3(v/v),the colour was changed from red to yellow and the pH of mixed indicator was determined from 6.45 to 5.93 by pH meter. The best concentration of this mixed indicator used in the medium was 1.0%(v/v) according to single factor experiment. |
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