EFFECTS OF QUICK-FREEZING AND SLOW-FREEZING ON ACTIVITY OF LACTIC ACID BACTERIA IN FROZEN SOURDOUGH

Sourdough is a starter culture of traditional flour products,but it is difficult to realize commercial production due to its complicated components.Food freezing technology is an effective method to control quality stability of sourdough,however,low temperature will affect the activity of lactic acid bacteria.In order to find a freezing preservation method for sourdough,the activity of lactic acid bacteria in frozen sourdough which was treated with quick-freezing (freezing for 18 min at-80℃) and slow-freezing (freezing for 1 h at-20℃) during storage (stored for 28 d at-20℃) was investigated.The results showed that the survival rate and acid-producing capacity of lactic acid bacteria decreased significantly with storage time;the survival rate of lactic acid bacteria in quick-freezing group was higher than that in slow-freezing group at the same storage period;the survival rate of lactic acid bacteria in quick-freezing group was 1.2 times that in the slow-freezing group after freezing storage for 28 days;the acid-producing capacity of lactic acid bacteria in the two groups had no significant differences at the beginning of storage (0 to 7 days);and the pH of sourdough in quick-freezing group was significantly lower than that in slow-freezing group at the middle and later stage of storage (14 to 28 days),but the total acid value of sourdough in quick-freezing group was significantly higher than that in the slow-freezing group,which indicated that the acid-producing capacity of lactic acid bacteria in quick-freezing group was higher than that in slow-freezing group at the middle and later stage of storage.In general,quick-freezing treatment is superior to the slow-freezing treatment in keeping the activity of lactic acid bacteria in sourdough during freezing storage.The results of this experiment would provide technical supports for the commercial production of sourdough.