The Effect Of Microbial Contamination During Malting On Yeast Flocculation

Malting process was infected by the microorganism which could result the loss of malt,the reduction of malt’s quality and many adverse effects in the brewing process. During thebeer brewing process, the Premature Yeast Flocculation（PYF）which caused by the infectedstuff influences the beer fermentation. This paper studied yeast flocculation and the factorswhich could influence the performance of yeast flocculation. Firstly, the methods of the yeastflocculation were optimized. Secondly, the single dominant microorganism which was foundon surface of barley was added in barley-steeping water, and then the effect of barley infectedby the microorganism on yeast’s flocculation was researched.The detection method of yeast flocculation was optimized, yeast was washed twice withsolution of250mmol/L EDTA, and then washed twice with aseptic water, the yeast was madesuspension during1.5~1.7of OD600and was oscillated with120rpm for1h, the temperaturewas28℃,3mL yeast suspension put in color plate and was reversed for40times, the freedomdegree cell was detected from30min to90min.A dominant bacterium and a dominant mold on surface of the Australia barley Gairdnerwere isolated and purified. The bacterium is gram-negative microorganism and belongs toKlebsiella (AB118219.1) by sequence analysis and identification of16S rDNA domain gene.The mold belongs to Fusarium (AY633745.1) by sequence analysis and identification of26SrDNA domain gene.The PYF factors of the malts which were infected by the screened microorganism wereextracted，It was found that the more serious barley was infected, the more complex theingredients of the PYF factors became. Along with the serious infected took place, the contentof66kDa PYF factor became increase, and78kDa,32kDa PYF factors were began to befounded；The most influent water-soluble heat-stable factor of yeast flocculation was from thebarley husk infected by bacteria. After adding a water-soluble heat-stable factor whichgenerated after barley husk was infected by bacteria, the yeast flocculation rate has an averageincrease of18.26%. The water-soluble heat-stable factor was separated from Sephadex G-25. The effect ofS24on yeast flocculation was the strongest. The SDS-PAGE electrophoresis pattern showedthat it was a single-subunit protein. Its molecular weight is66kDa. The rate of yeast floccula-tion increased in9.4%, when its concentration was2ppm.