Study On The Physiological Injury Of Lactic Acid Bacteria By Freeze-drying

The freeze-dried lactic acid bacteria (LAB) starter culture had been a hot researching topic in dairy industry in recent years. In China, the research on physiological damage mechanism was less than picking an efficient protective agent.To search for the physiological damage level of LAB in process of freeze-drying, the article researched the change of cell membrane permeability. Cell membrane was a basic of keeping normal physiological stage, it was important to cell integrity and physical energy metabolism. The key enzyme in metabolic process, ion gradient for changing ions inside and outside the membrane andâ–³pH needed a integrate cell membrane structure.The LAB cell membrane permeability changed by testing theâ–³pH inside and outside the cell using fluorescent probe BCECF, which maintained the normal physiological metabolism.Fluo-3/AM (Ca²⁺ fluorescent probe) was used to detecting the intracellular content of free Ca²⁺. Compared with Ca²⁺ in normal cell, the intracellular Ca²⁺ was much more by testing the membrane permeability change. The Ca²⁺ in normal cell was less as much as 22.4% to the latter. It was proved that trehalose had a significant role in reducing the damage to cell membrane which had a positive effect on keeping the LAB cell permeability.The protein and nucleic acid were leaked during the freeze-drying by testing the alteration of supernatant content before and after freeze-drying. The protein and nucleic content of normal cells in supernatant was much more than the cells adding trehalose.The important enzyme of LAB was influenced because of freeze-drying. Theβ-galactosidase and LDH were leaked during freeze-drying. Trehalose had a significant role in reducing the leaking. The activity of intracellular enzyme was also lost during freeze-drying.The cell condition before freeze-drying had a significant influence to physiological damage, survival rate and cell activity. It was efficient to improve the survival rate and cell activity by incubating cells for 1h in pH6.0 condition. 10℃cold stress for 60min had a same positive effect.The pre-freezing time had influence on cell survival rate and activity. In the fires 24h, the cell survival rate and activity protected by trehalose would not be lost. On the contrary, they both reduced when the pre-freezing time was more than 24h. If adding sucrose as a protective agent, the survival rate and cell activity reduced after the process of 12h pre-freezing.