The Effects Of Cooking Time On Physicochemical Properties Of Soybean Paste

Soybean paste is a kind of Chinese traditional fermented product.The koji-making is thefirst period of soybean paste product. The quality of the koji has direct impact on the projectof fermentation and the quality of the final product. Soybean paste has pleasure aroma,taste，nutrition and function．Microorganism and the interaction of the internal componentsplay veryimportant role in the processing of soybean paste.The activity changes of four kinds key enzymes protease,cellulase, amylase andglucoamylase and the content of amino acid nitrogen, water-soluble protein, reduce sugarwere analyzed in order to determine the best time for koji-making. The results indicated thattime have significant impact on the activity of all kinds of enzymes, the activity changes offour kinds key enzymes continuously increase during the the initial stages of koji-making.During the koji incubation, the highest activity of neutral protease and acid protease are36hand48h respectively and then declineslightly.Cellulaseenzymes and amylase reach the highestactivity on the time of36h.The content of the koji decreased gradually during the incubationand the moisture content on36h is46.12%and suit the growth condition of aspergillus oryzae.The content of amino acid nitrogen, water-soluble protein, reduce sugar increase with theincubation and reach highestat36hours. Spores number reached highest at36h, thefermentedsoybean pastewould producea bitter taste if the time to long for koji-making. Byjudging all the indicators, the best time for koji-making is36h.Cooking time is the key process during the fermentation of soybean paste, the purpose is tomake starch pasting and moderate protein denaturation and different cooking time havedifferent effects on physicochemical properties during the fermentation of soybean paste.In this paper, the effects of cooking time (5,8,10,15min)on physicochemical propertiesduring the fermentation of soybean paste have been explored. The results showed that underthe condition of different cooking time, all the indicators of total nitrogen, NSI, formaldehydenitrogen, total acid, reducing sugar and color changed significantly during the first period offermentation (1~15days), thereafter with a slight change or maintain homeostasis.During the first period of fermentation, all the indicators had significant differences (p<0.05) because of the condition of different cooking time, especially for the paste mash.Cooking time had significant effect on NSI and the15minsample turned to53.15%after60days higher than the other three samples. The hardness of soybean had significant affectedunder the condition of cooking time and decreased with the longer of cooking time. Thesoybean of15min sample had been severely damaged.The variation of soybean paste flavor composition in the soybean paste fermented underfour different cooking time（5,8,10,15min）and the commercial soybean paste were analyzedby HS－SPME－GC－MS combined with sense evaluation and physical and chemicalindexes． A total of25volatile flavor compounds including esters (6), acids (2),aldehydes(5),alcohols (2), ketones(2), other nitrogen containing compounds(1), andhydrocarbons(5) were isolated and identified exactly. According to the sensory evaluation,cooking time had impact on the color, aroma, taste, body of the soybean paste. The sample ofcooking8min had higher score in color, aroma, taste and taste body.