| Soybean paste has 3000 years history in our country. The traditional process mainly use soybean as raw material, using the microbes to ferment in our environment. Because of the long fermentation time and the change of quality, the traditional soybean paste is hard to meet consumer about quality and safety, although, shorter fermentation time, industrial soybean paste lack flavor, which is associated with microbial. Advanced basic research for microbial community composition, abundance and the role of microorganisms in different fermentation period of soybean paste is lack. Therefore, Based on the traditional preparation process, ultraviolet spectrophotometer, high performance liquid chromatograph, GC-MS, amino acid automatic analyzer and high throughput sequencing machine and other advanced scientific equipment were applied to research samples fermented 95d, the changes of physicochemical quality, flavor substances and microorganisms to be researched during the natural fermentation, The results were as follows:The changes of physicochemical quality were studied in the natural fermentation. The results showed that lightness value (L), red degree value (a) and yellow (b) in the process of the natural fermentation showed that 20-65d were the best period of color of soybean paste; hardness and conglutination, moisture content, amino nitrogen content, protein content, fat content, non-protein nitrogen, proteolytic index, and nitrite content were falling after rising, indicators were 0.51N,0.63N,47.04%,0.67g/100g,8.63%,5.61%,5.71%,83.12%,0.54μg/g respectively in 95d; adhesion and cohesiveness, total acid, salt content was rising, indicators were 0.81mJ,0.49,0.58%,15.62 g/100g respectively in 95d; pH value was falling, pH value was 6.13 in 95d; soluble protein content was falling after rising, keeping a period of time and then continuing to rise and remaining stable at last, content was 150.37 μg/g in 95d.The flavor component was studied in the natural fermentation. The results showed that reducing sugar content was falling after rising, content was 3.61g/100g in 95d; lactic acid was 0 before 15d, the high value was 402.27μg/g in 35d, then falling, content was 44.23μg/g in 95d; the total amino acids was falling after rising and keeping stable, nine kinds of amino acids were higher, the mean value was 2.15-4.59mg/g; five kinds of amino acids were lower, the mean value was 0-0.53mg/g; total fatty acids was falling after rising and then keeping stable, the saturated fatty acid remain between 12.49% and 15.35%, the unsaturated fatty acid remain between 61.49% and 81.77%; 56 kinds of aroma components were detected, aroma component content was falling after rising, the typical flavor compound including benzoic acid ethyl ester, n-butyl alcohol, isobutyl alcohol, phenylethyl alcohol, benzaldehyde, benzaldehyde, guaiacol, ethyl-4-phenol,4-ethyl guaiacol, malt phenol and naphthalene. The highest scores of soybean paste were fermented for 45d, the value was 2.43 and the aroma quality was best.The change of microorganism was studied in the natural fermentation. The resultes were that bacteria abundance and fungi abundance were rising before falling and the bacteria abundance was more than fungi abundance, bacteria played a main role in the early stage of the fermentation, and fungi played a main role in the last fermentation; Before 55 days, the bacteria diversity index and fungal diversity index change alternately, from 56 to 95days, the fungal diversity index was more than bacteria diversity index; on the level of phyla,15 known bacteria was found, firmicutes was the main species, the relative content was 94.473%-99.829%; three known fungi were found, ascomycota was the main species, the relative content was 81.022-99.848%; on the level of genus,220 bacteria were found, Staphylococcus and Leuconostoc were the main species, the relative content was 31.0-95.9% and 3.0-60.3%, 44 fungi were found, gibberella,paecilomyces,penicillium were the main species, the relative content was 8.9-79.5%,1.2-90.7% and 0.2-72.9%. |
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