| Soybean, which is mainly used for the processing of tofu, soy milk, natto and other traditional bean products, contains high quality vegetable protein. The antioxidive components of soy’s, such as isoflavones were retained in the soymilk yogurt fermented by lactic acid bacteria combined with proteolytic enzymolysis, and antioxidive peptides were produced at the same time. The produce of soymilk yogurt was not only expanded ways of soybean processing, but also extended the chain of soybean industry.Currently, the soymilk yogurt was mainly obtained by direct fermentation of lactic acid bacteria, with low fermentation acidity, poor tissue state and poor fermentation flavor.In this paper, using soybean as the main raw materials, a functional soymilk yogurt was prepared through fermentation by lactic acid bacteria combined with proteolytic enzymolysis. It was mainly researched that the screening and domestication of the production strains, processing of soybmilk yogurt, and effect of enzyme hydrolysis and high pressure treatment on aroma of soymilk yogurt. The optimum process paraeters of soymilk yogurt were obtained, and the antioxidant activity was evaluated.The detailed conclusions are showed as follows:(1) By the analysis of sensory evaluation and SPME-GC-MS, results showed that:the acid yield of strains formed by S.t and L.b at the ratio of 1:1 is high, and the soymilk yogurt has rich sweet and cream flavor. While the aroma of soymilk yogurt, fermented by strains formed by S.t and L.h at the ratio of 1:1 and the strains formed by S.t and L.b at the ratio of 1:2, is faint and the sweet incense is weak. Therefore, strains formed by S.t and L.b at the ratio of 1:1 is the preferred combination of soy milk fermentation.(2) S.t and L.b were domesticated by raised the ratio of soymilk in fermentation broth gradually. Comparing to the undomesticated strains, the fermentation acidities of the domesticated S.t and L.b in the pure soymilk are increased by 39.9% and 44.7%, and the clotting time are shortened by 1.6h and 1.7h respectively.(3) The fermentation acidity and viable count of the soymilk yogurt can be increased through proteolytic enzymolysis.(4) Through four factors and three level orthogonal design experiments, which using acidity and sensory evaluation score as index, optimum conditions are obtained: the ratio of beans and water is 1:8, inoculum is 4%, enzyme dosage is 1% and fermentation time is 5h.(5) According to the Guadagni theory,12 kinds of characteristics aroma compounds were established, such as:2,3-butanedione,3-Cyclohexanone, 2-heptanone,2-nonyl ketone, isoamyl aldehyde, acetaldehyde,1-octene-3-ol, hexanol, ethyl butyrate, hexanoic acid, furfural and 2- pentyl furan.(6) Comparing to soymilk yogurt which was not treated by high-pressure, the type and content of ketones, acids, alcohols, aldehydes and esters, which composing the main aromas of soymilk yogurt, were influenced by the treatment of UHP. The characteristic aroma components in soymilk yogurt, such as 2,3-butanedione, 2-butanone, acetaldehyde, acetic acid, and etc., are increased; while the contents of 1-octene-3-alcohol, hexyl alcohol and furans are decreased.(7) The scavenging ability of DPPH free radical is as follows:synchronized proteolytic enzymolysis and fermentation soymilk yogurt>single fermentation soymilk yogurt> soy milk>soy milk enzyme hydrolysis liquid; the sort of reducing power and hydroxyl radical scavenging ability is the same:synchronized proteolytic enzymolysis and fermentation soymilk yogurt>single fermentation soymilk yogurt> soy milk enzyme hydrolysis liquid>soy milk.(8) Using Vc as the control, the DPPH free radical scavenging ability of soymilk yogurt diluted 19 times is the same with the 0.05 mg/mL solution of Vc; the reducing power of a soymilk yogurt diluted 27 times is the same with 2.49 mg/mL solution of Vc; the hydroxyl radical scavenging ability of soymilk yogurt diluted 9.27 times is the same with 8.48 mg/mL solution of Vc. |
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