Abstract
Although the commercial lactic acid bacterium is often preserved in a freezing state, the viable rate of the lactic acid bacterium is reduced largely after thawing. In this study, to understand the physical damages caused by ice crystallization during freezing for the lactic acid bacterium suspensions, the suspensions were frozen with three different freezing rate to be observed by the scanning electron microscope (SEM). Furthermore, it is considered that some freezing damages of the lactic acid bacterium would be caused by osmotic pressure alteration due to freezing concentration. So, to prove the presume, the effect of osmotic pressure alteration at room temperature for the damage of the lactic acid bacterium was observed by direct observation with the SEM. As the result, aggregated and compressed lactic acid bacterium between extracellular large ice crystals were observed in the samples which were frozen at slow and medium freezing rate. On the other hand, they could not be observed in the frozen suspension that was prepared at rapid freezing rate. Furthermore, a large step change in osmotic pressure of the medium solution for bacterium suspension, which is considered as a factor of freezing damage, caused shrinking of the configurations of the lactic acid bacterium.
