| Set soybean yogurt (set sogurt) is a nutrient-rich new product with unique flavor, which was made from fresh soymilk by fermentation of lactobacillus. Whey separation and lack of fine texture are the mean defects of set sogurt. This paper was tried to increase the quality of set sogurt through selecting starter culture and improving texture, and its storage stability was also studied.According to the acid-producing velocity, viable count, sensory evaluation, water holding capacity, syneresis, apparent viscosity, texture and storage stability, this research chose TY 973 as starter culture for fermenting set sogurt from four commercial DVS starters. These four commercial DVS starters were YF-331, TY 973, YO-MIX and CY-346.The effect of total solids content of soymilk on the quality of set sogurt was investigated. The results showed that water holding capacity, anti-syneresis, colour, apparent viscosity and texture were all heightened as total solids content increased. When the content of solids was higher than 14% the sensory quality would be seriously deteriorated, so in the experimental the best content of solids was 14%. Gelatin, high methoxyl pectin (HMP), modified agar and modified starch, these common thickeners all could improve the quality of set sogurt. Based on the single-factor experiments, combination of thickeners' research were carried out. Through response surface analysis the thickeners formulation were optimized as follows:0.11% modified agar,0.08% HMP,0.22% modified starch and 0.28% gelatin.The developed set sogurt contained 3.28% crude protein and 0.96% crude fat, and its total free amino acids increased about 50% compared with soymilk. Postacidification was occured during storage, as pH and viable count dropped, acidity increased. The set sogurt's water holding capacity, anti-syneresis, apparent viscosity, firmness and consistency were all enhanced during 1～14 days storage. On the twenty-first day, water holding capacity began to decrease and on the twenty-eighth day all the qualities of set sogurt began to drop. The quality of set sogurt kept improving during 1～14 days storage, so it was the best period to drink it. In the experimental, the product's shelf-life could attained to 28 days.Milk which obtained pH<4.5 was nearly one hour shorter than soymilk at the same fermentation conditions, was more suitable for the growth of lactobacillus. Set yogurt had fine and smooth texture, which viable count and titratable acidity were significantly higher than that of set sogurt, but its water holding capacity, anti-syneresis, apparent viscosity and texture were all marked lower than that of set sogurt. The research turned out that 0.1% HMP could improve qualities both of set yogurt and set sogurt.The differences of particle size distribution, isoelectric point, and relaxation time (T2) between milk and soymilk were studied. The results showed that the particle size distribution of milk was tinier than soymilk, but the isoelectric point of casein and soy protein were almost the same. Milk and soymilk had the same relaxation time, but after fermented the relaxation time of yogurt was significantly raised meanwhile sogurt's was just increased by 26.6 ms. These implied that fermentation had greater impact in water mobility of yogurt than that of sogurt. The microstructure network images which got from SEM showed that set sogurt's network which had less and smaller pores was dramatic denser than set yogurt's. Added 0.1% HMP could provok great changes in milk and set yogurt, but it made no obvious change in soymilk and set sogurt. The changes of milk and set yogurt caused by 0.1% HMP were as follows:1) The particle size of milk was significantly grew; 2) The isoelectric point of casein was greatly decreased; 3) The water mobility of yogurt was dropped, that meant HMP could increase its water hold capacity; 4) The microstructure network of set yogurt became denser and had less holes. |
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