| Daqu is rich in a variety of microflora,enzymes and flavor substances,which is the main starter in the production of Chinese traditional Baijiu,and its quality directly affects the quality of Baijiu.At present,based on the quality difference of Daqu,there are few studies on the relationship between physical and chemical indexes,microbial community and volatile compounds in Daqu,and there are few studies on the function of thermophilic fungi in high-temperature Daqu.In this study,starting from the sensory indexes,superior high-temperature Daqu(SHD)and common high-temperature Daqu(CHD)were compared and analyzed in order to explore the relationship between physical and chemical indexes,microbial community and volatile compounds of different grades of high-temperature Daqu and the common characteristics of superior high-temperature Daqu for Nongxiang-Jiangxiang Baijiu.The dominant thermophilic fungi in SHD were isolated and identified,and their enzyme production characteristics and volatile products under solid-state fermentation were detected and analyzed.Paecilomyces variotii PV3 strain was used to produce pure solid functional bacterial agent,which was added to prepare fortified Daqu according to different proportions in the process of Daqu production,and the contribution and influence of the strain in the process of enhanced Daqu fermentation were analyzed.The major results of this paper are as follows:1.Through the detection and analysis of physical and chemical indexes,microbial community and volatile substances of superior high-temperature Daqu and common high-temperature Daqu for Nongxiang-Jiangxiang Baijiu.The acidity,ammonia nitrogen content,fermentation power and esterification power of SHD were higher than that of CHD.The water content,liquefaction power and saccharifying power of SHD were lower than that of CHD.The number of culturable bacteria and moulds in CHD was higher than those in SHD,and the difference was significant.Diversity of bacterial community in SHD was more abundant,and Bacillus and Saccharopolyspora were the dominant bacteria.Species of fungal community of the two kinds of Daqu were the same,the proportion of Aspergillus in CHD was 18.24%,and that in SHD was 0.57% and the proportion of thermophilic fungi reached 98.89%.A total of 23 volatile compounds were detected in high-temperature Daqu.The contents of isovaleric acid,ethyl hexanoate,furfural and tetramethylpyrazine in SHD were higher than those in CHD,while the contents of isobutyraldehyde and isovaleraldehyde in SHD were lower than those in CHD.2.Through the isolation and identification of the dominant thermophilic fungi in Nongxiang-Jiangxiang Baijiu distillery,7 strains of three kinds of thermophilic fungi were obtained,which were 3 strains of Paecilomyces variotii,2 strains of Thermoascus crustaceus and 2 strains of Rhizomucor pusillus.The results showed that R.pusillus RP-1 had strong amylase activity(1434.05 U/g),P.variotii PV-3 had strong glucoamylase activity(2856.06 U/g),and T.crustaceus TC-1 had high protease activity.There was little difference in esterase activity among different strains,there was no significant correlation between cellulase activity and fungal species,and there was a great difference in cellulase activity among different strains of the same thermophilic fungi.3.The metabolites of three kinds of thermophilic fungi with strong enzyme activity were detected and analyzed under liquid and solid state fermentation.The results showed that each strain could produce three or more organic acids under liquid fermentation.Among them,the yield of P.variotii PV-3 succinic acid was the highest,which was 287.63 mg.The yield of lactic acid and tartaric acid was the highest,which was 240.53 mg/L and 53.26 mg/L,respectively.The solid-state fermentation products of three thermophilic fungi were detected and analyzed by HS-SPME-GC-MS.It was found that the type and total content of volatile compounds produced by strain P.variotii PV-3 were higher than those of T.crustaceus TC-1 and R.pusillus RP-1,and strain P.variotii PV-3 had strong ability to produce esters and aldehydes.The main esters were ethyl propionate and ethyl 2-methyl butyrate,and the main aldehydes were isobutyraldehyde.At the same time,the yield of tetramethylpyrazine and acetoin in strain P.variotii PV-3 was also the highest among the three thermophilic fungi.4.The pure solid functional bacterial agent was made from the isolated P.variotii PV3 strain,and the fortified Daqu was prepared by adding different proportions in the production process of Daqu.The results showed that the water content,acidity and ammonia nitrogen content of fortified Daqu increased,and the saccharifying power of Daqu increased significantly and positively correlated with the addition of bacterial agent,and the highest 140 U/g was 1.58 times higher than that before strengthening.In terms of volatile substances,the contents of ethyl propionate and ethyl 2-methyl butyrate in fortified Daqu increased significantly,with a maximum increase of1027.42 μg/kg and 69.43 μg/kg,but the contents of isobutyric acid and n-hexanol increased to a certain extent.The total amount of aldehydes increased,in which the contents of isovaleraldehyde,benzaldehyde and isobutyraldehyde increased significantly,while the content of furfural decreased,which was negatively correlated with the addition of bacterial agents,and the content of tetramethylpyrazine increased in varying degrees,with a maximum increase of 666.78 μg/kg. |
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