Exploring The Mechanism Of Malt Flavors Formation And Microbes Effects On The Flavors During Malting Process

In this dissertation, the mechanism of microbe effection on the flavors of malt, thecompostions and changing patterns of the flavors during malting process were investigated,with the aids of volatile compounds analysis techniques. Ecological analysis wasimplemented to identifiy the microbes which affected the flavors in malting process and todetermine the major microbes during different stage in malting process. In addition, thecompositions of aroma-active compounds originated from different microbes and theirinfluences on green-malt based flavors in malting process were also studied. The major resultsof this dissertation were summarized as below:(1) The techniques of SPME combined with GC-MS were used to fastly and accurately toidentify the compositions of volatile compounds in barley. A total of41volatile compoundswere identified from10barley cultivars. The volatile compounds were constituted byaldehydes, alcohols, ketones, organic acids, and tiny amounts of aromatics and funancompounds. The compositions of the flavors in various barley with different origins weresimilar, with some discrepancies on certain individual compound.(2)16volatile compounds were identified as the key aroma compounds of brewing barley, bycalculating the odor active valves (OAV) of each compound. Among these compounds,8aldehydes (acetaldehyde,2-methyl-propanal,3-methyl-butanal,2-methyl-butanal, hexanal,heptanal, octanal and nonanal),2alcohols (3-methyl-1-butanol and cyclopentanol),2ketones(2,3-butanedione and2-heptanone),1organic acid (acitic acid) and3other compounds (ethylacetate,2-pentyl-furan, benzeneacetaldehyde) were found.(3) Principal component analysis technique showed that the aroma compositions of domesticbarley cultivars were different from those of the imported ones. The major volatilecompounds contributing the flavors of imported barley cultivars were acetaldehyde,2-methyl-propanal,3-methyl-butanal,2-methyl-butanal, hexanal, heptanal, octanal,2-methyl-1-propanol,1-pentanol,2,3-pentanedione,2-heptanone, hexanoic acid, heptanoicacid, ethyl acetate,1-hexanol, and2-pentyl-furan, while those of domestic barley cultivarswere nonanal, cyclopentanol,3-methyl-1-butanol,2,3-butanedione, hexanoic acid andbenzeneacetaldehyde.(4) A total of48volatile compounds were identified in an industrial malting process, and theywere also constituted by the substances of aldehydes, alcohols, ketones, organic acids, funanand aromatic compounds. By classifying these compounds into six catagories, the appearancetime and changing patterns of each volatile compound during malting process could be clearlyindicated. These results also showed that, among the entire malting process, the kilning androasting process had a significant effection on volatile compounds concentration.(5) The microbe diversity for industrial malting process was investigated using PCR-DGGE technique. The results indicated that the microbe group constitutions differed largely indifferent malting stage, but they kept higher similarity at different instant within one particularmalting stage. The dominant stains were Sphingomonas spp. and Enterobacter spp. in rawbarley and barley subject to steeping operation. In germinating stage, the microbe floraincreased rapidly, and the dominant species were Enterobacter spp., Neisseria spp., Erwiniaspp. and Klebsiella spp. The mibrobe flora in malt was the least and the dominant strain wasEnterobacter spp. During malting process, Enterobacter spp. and Erwinia spp. were the majorflora, while Sphingomonas spp, Enterobacter spp. Sandarakinorhabdus spp. and Erwinia spp.were microbes existed at the surface of barley, and all the other mibrobes were from theaerated air during operation.(6) The volatile compounds at different instant during cultivations of Fusarium poae,Penicillium oxalicum, Escherichia coli and Klebsiella peneunomiae was collected andanalyzed using the techniques of SPME combined with GC-MS. The results showed that3-methyl-1-butanol,1-octen-3-ol,3-octanone and3-octanol were the major volatilemetabolites produced by the four microbes. It was also found that the volatile compoundsproduced were different at different cultivation stage: during the early stages, the majorvolatile metabolites were alcohols and ketones with short-chains; while in the later stage, themain volatile compounds turned to be lipids and terpeness. Fusarium poae was veryimportant for the entire malting process, as its volatile compounds almost covered most ofthose identified in each individual barley. Fusarium poae greatly affected barley storage,malting, and malt flavors.(7) During malting process, microbes could affect the flavors by varying volatile compoundscontents or concentrations, but they could not produce new volatile metabolites. The majormicrobes had certain inhibitory effects on the formation of volatile compounds, and theinhibitory mechanism lied on the follwing facts: the volatile metabolites were originated fromlipid oxidation and hydrolysis reactions in malting. When microbe infection occurred, volatilecompounds contents were determined by the coordinate actions of microbes and green-maltmetabolisms. During malting process, proteases secreted from microbes deterioated lipidoxidation and hydrolysis enzymatic system in green-malt, leading to a lower enzymaticactivity; while at the same time, the infected microbes utilized the precurosors of volatilecompounds for their growth and metabolism.