Changes occurring during refrigeration of fermented soymilk

Soymilk and soymilk enriched with vegetable oil which had been pasteurized for 30min at 80°C were fermented jointly with Lactobacillus bulgaricus, Streptococcus thermophilus and Bifidobacterium longum at 37°C for 20hr. The resulting fermented soymilks, as well as the plain a yogurt available on the market, were refrigerated for 10 days at 5°C, and changes in their bacterial cell counts and wheying-off were measured every 48hr in addition to the determination of changes in pH and acidity. In the two fermented soymilks, the similar changes were observed in the decrease of pH and the increase of acidity during the storage period. Furthermore, there was a similar tendency for the viable cell counts of lactic acid bacteria to drop gradually, but a count equal to or more than 107 still was maintained after the storage period. When considering the difficulty in maintaining the desired number of bifidobacteria in yogurt made from cow's milk, the finding that the viable cell counts of Bifidobacterium longum in the higher than 107 ml for the storage period seems to be of special interest from a point of view supplying Bifidobacterium longum to human. Furthermore, the volume of whey formed on the surface of the fermented soymilk increased slightly with the elapse of storage time after manufacturing, but the increase was less than that of commercial yogurt. Next, the soymilk and the soymilk with added vegetable oil which were heated for 30min at 80°C, 90°C and 100°C, and for 15min at 121°C, were fermented with the same starters above described, and the physico-chemical changes of the products during the storage were examined. As a result, on day zero, the pH of the fermented soymilk made from the soymilk heated for 30 min at 90°C was the highest, and the acidity was the lowest. Furthermore, the changes in the levels of pH and acidity during the storage were greatest in the fermented soymilk made from soymilk heated at 90°C for 30min. In spite of the difference in heating temperature used for soymilk, the viable cell counts of lactic acid bacteria showed a similar tendency to decrease gradually during the storage period, but the counts were always maintained at equal to or more than 107ml for the storage period. The volume of wheying-off formed in the two fermented soymilks increased as the heating temperature for soymilk is decreased. However, it was less than that of commercial yogurt. On the otherhand, the effect of the vegetable oil added to soymilk on the preventi