Study On The Changes In Nutrition And Flavor Ingredients During Soybean Residue Cakes Fermentation

Soybean residue as main by-product of soymilk and tofu is rich in nutrients. However, more often it is used as fodder rather than food and even discarded. It causes economic loss. Few people used soybean residue as raw material for industrial production. Fermented soybean residue cake is a traditional Chinese food with a long history, and it is people's favorite in Sichuan province and Hubei province. But it lacks normative process.For the purpose of improving the traditional fermentation process of soybean residue cake, we accessed to relevant literatures and visited people who were good at making the cakes.Finally we found out the traditional method and then isolated and purified a dominant strain from the traditional fermented cakes.The strain was confirmed as Mucor racemosus Fresenius which is widely known as a safe strain applied in food fermentation industry. Suspension of the strain was added on the surface of each cake before being fermented in constant temperature incubator at different temperatures.At last, we found out a new method which was named subsection fermentation. The process is as follow:First, the soybean residue cakes were fermented at 28℃for 18 h to help the mucor grow quickly, then lowered the temperature to 24℃for 2 h-3 h as a transition stage to avoid temperature shock in order to continue the fermentation at 20℃till most of the mucor color changed to grey. The whole fermentation process took 84 h which was about 1/3 of traditional fermentation time. Comparing unfermented cake, traditional fermented cake and subsection fermented cake by determining contents of several indexes, we found that both subsection fermentation and traditional fermentation had great effects on the levels of crude protein, formaldehyde nitrogen and isoflavone, and had slight effects on the contents of crude fat and dietary fibre including soluble dietary fiber (SDF), insoluble dietary fiber (IDF) and total dietary fiber (TDF).Therefore, both fermentation methods could improve the nutritional value of soybean residue cakes which were easier to be digested and absorbed by people. Although traditional fermented cakes were a little more nutritional than subsection fermented ones, the traditional fermentation time was much longer than that of subsection fermentation. Comparing with traditional fermentation, subsection fermentation is more suitable for industrial production.For the purpose of studying the changes of soybean residue cake during subsection fermentation, some physiochemical indexes were determined, texture properties were analyzed and microstructures changes were observed. Researches showed that moisture content was gradually decreased with the increase fermentation time, and it declined most rapidly during the first 12 h of fermentation; there was significant (p<0.05) differences of total titratable acids content between unfermented and fermented cakes which reached maximum at 36 h; crude protein content continuously decreased and formaldehyde nitrogen content continuously increased during the whole subsection fermentation, respectively; there were not obvious changes in crude fat and dietary fibre during the process;total isoflavone and glucosides contents were decreased while aglycones content increased during the fermentation, and isoflavone was mainly existed as aglycones in final products; total sugar contents decreased during subsection fermentation. Texture profile analysis (TPA) showed that hardness of samples significantly (p<0.05) increased whereas springiness, cohesiveness and chewiness significantly (p<0.05) decreased with increasing fermentation time, respectively; resilience decreased first and then increased, but also there was significant difference between unfermented and subsection fermented cakes.Scanning electron microscopy (SEM) observations showed great changes of the microstructure of cake samples during subsection fermentation.For the purpose of researching the flavor of fermented soybean residue cake, comparing of flavor substances between traditional fermentation and subsection fermentation was done and flavor substances of different periods subsection fermentation were determined during the whole fermentation process.It showed that the sorts and relative amounts of both fermentation methods were decreased. There were 23 substances with high detected frequency which took accounts for above 60% of total flavor ingredients.6 of them with highest detected frequency may dominantly influence the flavor of fermented soybean residue cakes.