| Soybean paste is one of the most typical kinds of traditional fermented food in China. It wasused as condiment in daily life. Traditional soybean paste is naturally incubated and fermentedabout 3-4months. Microorganism and their relative microbiological enzymes play an important rolein koji incubation and the paste fermentation. The microorganisms found in koji almost alwaysbelong to fungi species, Aspergillus oryzae and Asp. sojia. The koji mold has the ability to utilizestarch, oligosaccharides, simple sugars, organic acids and alcohols, etc as carbon sources, andprotein amino acids and urea, etc as nitrogen sources. At the same time the koji mold secrete manykinds' enzymes such as protease, lipase, amylase and others. In the fermentation of the paste, themicrobiological enzymes decompose the protein, starch and fat into small molecules compounds,such as amino acids, organic acids, and small molecules organic compounds. These compoundscontribute the particular flavor and the abundant nutrition of soybean paste. Halophilic yeast andlactic acid bacterial also play an important role in developing flavour during fermentation ofsoybean paste.It is a very complex process that the interaction between microbiological enzymes andmacromolecular during the koji incubation and paste fermentation. The formation of the flavor,nutrition and texture involved several microbiological enzymes (protease, Lipase, polyphenoloxidase, amylase, ADH, and etc,). In this paper, based on a typical traditional natural fermentedsoybean paste we focus on the forming and changes of microbiological enzymes and their relativelydecomposed and metabolized compounds during the koji incubation and the paste fermentation.During the entire incubation of the koji, the pH and the moisture content of the koji weresignificant difference (p＜0.05). The pH of the outside koji was decreased from 6.88 to 5.24 in 1～20days, and then the pH was increased from 5.24 to 7.04 in 20～45 day. The pH of the inside koji wasdecreased from 6.87 to 5.76 in 1～25 days, and then the pH was increased from 5.76 to 6.48 in25～45 day. The pH of the paste was significantly decreased from 6.91 to 5.49 in 1～15 days(p＜0.05), and the pH of the paste was not changed. The moisture content of the outside koji wasdecreased from 38.59% to 21.09%, and the moisture content of the inside koji was decreased from38.80% to 30.46%.During the incubation of the koji, the protease activity was gradually increased and the highestactivity was obtained at 30th days, and neutral protease was predominant protease and it was significantly increased during the incubation of the koji (p＜0.05). However the protease activitywas significantly decreased during the fermentation of the paste (p＜0.05), but the neutral proteasestill was predominant protease in entire fermentation. Concerning the proteolysis, NSI, NPN,aminophenol nitrogen and peptides concentration were gradually increased and they were 69.07％,42.07％, 3.73％and 38.34％respectively at the end of fermentation of the paste.The activities ofα-amylase, glucoamylase and cellulase have different changing periods duringentire incubation of the koji. Theα-amylase activity of the outside koji was significantly increasedfrom 1～20 days (p＜0.05), and the activity of the inside koji was significantly increased from 1～25days (p＜0.05). The glucoamylase activity of the outside koji was significantly increased from 1～25days (p＜0.05), and the activity of the inside koji was significantly increased from 1～30 days(p＜0.05). The cellulase activity of the outside koji also was significantly increased from1～15days (p＜0.05), and the activity of the inside koji was significantly increased from 1～25 days(p＜0.05). Then all three enzymes activities were gradually decreased at later koji incubation.However, the activities of the three enzymes were significantly decreased during the fermentationof the paste (p＜0.05). During entire incubation of the koji and fermentation of paste, the celluloseand total sugar were gradually decreased, and the reducing sugar was gradually increased. In thefermented soybean paste, the cellulose, total sugar and reducing sugar were 3.63％, 14.3％and4.84％respectively.The lipase activity of outside koji was significantly increased from 1～25 days (p＜0.05), andthen the activity was gradually decreased in later koji incubation. However, the lipase activity ofinside koji was continuously increased in entire koji incubation. The lipase activity wassignificantly decreased during the fermentation of the paste (p＜0.05). The lipolytic compoundscorresponded the lipase were increased during the entire incubation of the koji and fermentation ofthe paste. In the fermented soybean paste, the free fatty acids content had arrived 5.26％(total fat).The polyphenol oxidase activity of the outside koji was significantly increased from 1～10 days(p＜0.05), but its activity gradually decreased from 15～45 days. And the activity of the inside kojiwas gradually increased from 1～10 day, then continuously decreased in entire koji incubation. Thepolyphenol oxidase activity was significantly decreased during the fermentation of the paste(p＜0.05). The dark reddish brown was mainly formed during the koji incubation. At the first 10days, polyphenol oxidase played important role in dark reddish brown, Mailland reaction playedkey role in the following time, finally the soybean paste browning had arrived 0.97(OD420nm).The ADH and LDH activity were continuously increased in the koji incubation. The ADHactivity of the paste was gradually increased from 1～10 days, and then significantly decreased from15～35days (p＜0.05). The LDH activity of the paste was gradually increased from 1～10 days, andthen significantly decreased from 15～30days (p＜0.05), but was increased at the later 5days. Thevolatility compounds was determined by SPME-GC-MS. The results showed the alcohol contentincreased when the ADH activity risen, and lactic acid content was varied when the LDH activitychanged, and acetic acid was created posterior to them. In the fermented soybean paste, the alcohol, lactic acid and acetic acid content had arrived 1.73(mg/g), 1.57(mg/g) and 0.70(mg/g) respectively.The esterase activity was always higher in the koji and paste. Its activity was significantlyincreased during the koji incubation (p＜0.05). And then the activity was significantly decreasedduring the fermentation of the paste (p＜0.05). At the end of the fermentation, the esterase activitywas 4.38 U/g (initial dry substrate), therefore the continuative fermentation can increase the totalester contents, and improve fiavour as well. In the fermented soybean paste, the total ester contenthad arrived 0.24％.
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