Studies On The Factors Influencing Product Quality And The Processing Technology Of Soy Yogurt

Soy yogurt is a product obtained by fermenting soybean with lactic acid bacteria.At present,there is no mature soy yogurt product in the domestic market,mainly due to the defects of product that lead to soy yogurt not accepted by consumers.This paper studied the effects of raw soybeans,yogurt starter cultures and formulas on the quality of soy yogurt,discussed the grinding,sterilization and fermentation processes related to the quality of soy yogurt,and provided a theoretical basis for the industrialization of high quality soy yogurt.This paper included the following:Firstly,the effects of soybean variety,yogurt starter cultures and formulas on the quality of soy yogurt were studied.In the study,normal soybean,LOX-lacking soybean and7S-lacking soybean were selected to compare their fermentation characteristics,gel properties and volatile flavor components.The results showed that the fermentation rate of lactic acid bacteria in three kinds of soymilks were close,but the gel characteristics of soy yogurt were different.Compared with the other two soybean varieties,LOX-lacking soybean could gel at a higher p H level.The gel curve of 7S-lacking soybean was different from that of normal soybean and LOX-lacking soybean.The firmness,consistency,water holding capacity and network structure of normal and 7S-lacking and LOX-lacking soy yogurt were in the order of LOX-lacking soy yogurt>normal soy yogurt>7S-lacking soy yogurt.The content of off-flavor components in LOX-lacking soy yogurt was the lowest,while the soy yogurt prepared by normal soybean was the highest.The fermentation behavior and fermentation products of five kinds of Directed Vat Set(DVS)yogurt starter cultures(A,B,C,D,E)were compared.All the strains in the soymilk enriched the low acid producing ability compared with the milk;The fermentation rate of five yogurt starter cultures were found to be remarkably different in the fermentation rate which was in the order of D>E>C>B>A.The firmness of soy yogurt products prepared by different starters were different significantly(p<0.05)and was highly correlated with the fermentation time(R~2=0.9976).In terms of volatile flavor components,the longer the time to reach p H 5.50 during fermentation was,the higher the content of carbonyl compounds in soy yogurt was.The results showed that the fermentation characteristics of A and B yogurt starter cultures were similar and produced soy yogurt with similar texture,but the volatile flavor components were different.Starter cultures D and E were characterized by rapid fermentation rate while starter culture C could produce more 2,3-butanedione.Moreover,the effects of carbon source and nitrogen source substance on the acid production of lactic acid bacteria in soymilk were studied.It was found that whey powder and soybean peptide had the strongest effect on lactic acid bacteria;the acidity of soy yogurt added with whey powder was higher than that of other carbon sources and nitrogen source about 10 ~oT.Further studies showed that the polar component of whey powder plays an important role in the production of acid by lactic acid bacteria.Secondly,the effects of soybean grinding process on the quality of soymilk and soy yogurt were investigated.Compared with the conventional grinding process,the use of oxygen-free grinding process resulted in a lower content of volatile flavor components and a higher brightness of the soymilk;at the same fermentation condition,soymilk produced by conventional grinding process and oxygen-free grinding process were fermented by lactic acid bacteria to prepare soy yogurt,both ketones and acid s volatile flavor components were produced,but the type and content of beany flavor components in the former fermentation product were still higher than the latter.It was noteworthy that,the content of hexanal was decreased sharply after fermenting with normal soymilk,while the content of hexanol was greatly increased.Finally,the effects of process conditions on the quality of soy yogurt were studied.The higher the protein concentration in the soymilk base,soy yogurt had the higher acidity,stronger gel,more color yellow;soybean dehulling had little effect on the fermentation characteristics and gel characteristics,but had a greater impact on the volatile flavor components;unexpectedly,the flavor of soy yogurt after dehulling was rather poor.With the increase of heating temperature of soymilk,the firmness,storage modulus and water holding capacity of soy yogurt obtained from different varieties of soybean all showed the trend of increasing first and then decreasing.The heating temperature of 90~95oC was the most suitable for preparation of soy yogurt.Content of AB subunit determined by non-reducing SDS-PAGE is a sensitive indicator for the appropriate heat treatment.