Studies On The Effect Of Enzymatic Catalysis On Soy Yogurt Quality

In this article, the acidity, water holding capacity, rheological property, microstructure and sensory property of soy yogurt, which was made from soybean treated by protease and amylase, were studied. The soy yogurt was fermented by starter cultures of XPL-1, which contains Lactococcus lactis subsp. cermoris, Lactococcus lactis subsp. lactis, Leuconostoc mesenteorides subsp. cremoris, Lactococcus lactis subsp. lactisbiovar diacetylactis, Streptococcus thermophile. And then, for improving the quality of soy yogurt, the ways of enzymatic treatment and the conditions of fermentation were optimized.The effects of protease and carbon sources on the characteristics of soy yogurt were evaluated. Before fermentation, the raw material, soybean, was mildly hydrolyzed by Alcalase protease during the soybean soaked stage. At the same time, the soymilk was added lactose and glucose that fermented soy yogurt. Results showed that the mild hydrolysis of soybean protein was helpful to improve the water holding capacity, viscoelastic, apparent viscosity and reduce consistency coefficient of soy yogurt. Other than that, lactose was beneficial to the strains to produce acids, and glucose can significantly improve the flavor of the product. The enzymatic action of protease and addition of glucose were helpful for improving the taste of soy yogurt.The characteristics of soy yogurt treated by different ways of enzymatic catalysis of protease were studied. Different amounts of Alcalase protease were added in the soybean soaked stage and soymilk stage. SDS-PAGE picture displayed that the degree of degradation of β-conglycinin(7S) and glycinin(11S) were higher, which treated by protease in soymilk stage. At the meantime, the content of free amino acid(FAA) increased from 62.1 μg/100 m L to 70.5 μg/100 m L and 88.5 μg/100 m L in soymilk treated by different amounts of protease. The water holding capacity of soy yogurt treated by protease was improved, especially enzymatic catalysis in soymilk stage. At the same time, the acidity of soy yogurt treated by protease in soymilk stage had increased 21.35 percent. This might due to free amino acids could promote the growth of strains to produce acids. With the increase of the degree of hydrolysis of soybean protein, the yield stress, consistency coefficient and apparent viscosity of soy yogurt, which hydrolyzed by protease in soymilk stage, showed a decreasing trend. After the process of enzymatic hydrolysis, the microstructure of soy yogurt became more smooth and uniform, with diameter of interspace reduced from 10-15 μm to 3-5 μm. The sensory evaluation showed that the texture of soy yogurt improved. Soy yogurt treated by higher amount of protease in soymilk had the highest acceptability score.The influences of protease and amylase on the characteristics of fermented products were assayed. On the basis of application of protease in soymilk stage, α-amylase was applied in soybean crushed stage and then gluco-amylase in soymilk stage respectively. Results showed the contents of total sugar in soymilk were up 3.49 percent. The content of reducing sugar in soymilk treated by gluco-amylase was up 14.71 percent. This means that gluco-amylase and the starter cultures of XPL-1 were capable of promoting the degradation of starch or oligosaccharide to reducing sugars. The acidity, water holding capacity and consistency coefficient of soy yogurt treated by protease and two kinds of amylase were improved. The soy yogurt had the highest acceptability score, with pleasant sweet taste.The effects of fermentation condition on soy yogurt treated by enzymatic catalysis were assayed. By the way, the metabolism of strains was evaluated. Results showed that, with the reduction of fermentation temperature and the extension of fermentation time, the acidity and the water holding capacity of soy yogurt were improved. In addition, low fermentation temperature and long fermentation time was helpful to improve the viscoelastic of soy yogurt. It was also suitable for symbiosis between five strains in the starter of XPL-1 and the acceptability score was highest. Sucrose and glucose was utilized firstly, when fermented by the starter of XPL-1. In the process of storage, sucrose was continuely used by the starter of XPL-1.All results showed that, the texture, flavor and taste of soy yogurt, which was treated by α-amylase in soybean crushed stage and then Alcalase protease and gluco-amylase was applied in soymilk stage, with suitable temperature, were improved significantly, with a broad prospects.