| As a traditional condiment,soybean paste has the characteristics of rich odor and rich nutritional value.In China,the fermentation strains used in factory-made sauces are generally commercialized.Although safe and controllable,the product tastes less than natural fermented soybean paste.Traditional soybean paste produces a variety of microorganisms under the influence of air and production environment through natural fermentation,and some of the fungi produce various enzymes as fermenting beneficial bacteria,giving the soybean paste a unique flavor.Therefore,it is of great value to isolate and screen good fungi from natural fermented soybean paste.In this experiment,68 suspected fungi were isolated and screened from 50 pieces of naturally fermented soybean paste.18 protein-producing strains and 17 starch-producing strains were screened by transparent circle method,and the enzyme production activities of the strains were determined respectively.Growth tests under different salt concentration,p H value and nitrite conditions were performed to further screen the enzyme-producing strains,and the strains were identified by ITS sequence.Finally,HS-SPME and GC-MS were used to determine the volatile substances and amino acid components of fermented sauce with pure mildew strain for further verification.1.After screening,four strains with good protein production ability were FX3-3,LZ22-2,XDD4-2,and XDD5-2.Six strains with good amylase production were J-1,XDD4-2,XDD5-1,XDD5-2,XDD2-3 and FX3-3,respectively.The enzyme activity under different temperature and p H conditions were analyzed.The results showed that strain FX3-3had the strongest enzyme production ability at 50 °C and p H 7.The remaining strains have the best protease production ability at 40?and p H 7.From the perspective of protease production,strain FX3-3>XDD5-2>XDD4-2>LZ22-2;Strain XDD2-3 has the strongest ability to produce amylase at 30?,p H 3.6.From the perspective of starch production,the result is strain XDD2-3>J-1>FX3-3>XDD5-2>XDDD5-1>XDD4-2.2.The enzyme producing strains were further screened with different salt concentration,p H value and nitrite conditions as indicators.The results showed that the p H value and the salinity of the growth environment were stable to the growth of strain XDD4-2;The strain LZ22-2 was poor in acidity and salt tolerance;The remaining strains can grow normally under the conditions of p H 5-7 and salt concentration 8%.The results of ITS sequence identification indicated that the strains FX3-3,XDD5-2,XDD4-2,XDD2-3,J-1,XDD5-1,LZ22-2 are Mucor circinelloides,Monascus rutilus,Penicillium cordubense,Aspergillus fumigatus,Uncultured Mucor,Penicillium cordubense and Uncultured Ascomycetes.3.Analysis of volatile substances in soy fermented soy sauce by HS-SPME and GC-MS,26 volatile substances were identified,the highest content of which was alcohol substance.The free amino acid in the sauce was determined.The results showed that the free amino acid content of the fermented soybean XDD5-2 strain was much higher than that of other strains,especially the fresh Glutamic acid.Finally,the strains XDD5-2?XDD5-1 and FX3-3 were well screened,which were Monascus,Penicillium cordubense and Mucor.Analysis of them has a big relation with the ability of decomposition of protein in soybean paste.This is valuable for the use of purebred fermented soybean products in the future. |
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