Abstract: Lactic acid bacteria (LAB) starter cultures prepared by freeze-drying exhibit the advantages of high viability, activity and genetic stability. But the freeze-drying process usually has some undesirable effects. In this paper, the changes in cell membrane permeability, cell membrane fluidity, enzymes, proteins and genetic materials during freeze-drying are analyzed to elaborate systematically the injury mechanisms of freeze-dried LAB. As results, it is concluded that the mechanism by which freeze-drying causes physiological damages to LAB is mainly attributed to four aspects: the formation of ice crystals and the phase transition of phospholipids, leading to increased membrane permeability; the alteration of cell membrane fatty acid composition, influencing cell membrane fluidity; the decrease of enzyme activity and the denaturation of related proteins, affecting cell growth and metabolic rates; the structural changes of DNA-binding proteins, and the rupture of the hydrogen bonding between base pairs, resulting in the destruction of DNA supercoiling. These physiological damages may lead to decrease in the viability of lactic acid bacteria, or even cell death.

Key words: freeze-drying, lactic acid bacteria, injury mechanism, viability of lactic acid bacteria