Storing Lactic Acid Bacteria: Current Methodologies and Physiological Implications

Abstract

The high viability of lactic acid bacteria (LAB) during storage is greatly important for starter cultures used for the direct inoculation to food matrices and for the development of probiotic products. The established methods for preservation are freezing and freeze-drying, in which cells are maintained in frozen and dried forms. The frozen cells should be kept at a low storage temperature, such as −80°C, and rapid thawing is recommended for cells frozen with liquid nitrogen. The dried cells should have a low moisture content (<4%). They should be stored at a low relative humidity and temperature and rehydrated in a warm rehydration medium. In addition to these established methodologies, dried cells can be prepared by alternative drying processes such as spray-, fluidized bed-, and vacuum-drying. The viability of frozen and dried cells can be improved by the addition of protectants such as skim milk and sugars. The physiological state of LAB plays a crucial role, and an increased viability can be obtained by the sublethal stress treatment of cells. Exposing LAB cells to a mild stress triggers cells’ protective mechanisms to subsequent stresses occurring during the preservation processes. These stresses are, for example, the entry of cells to the stationary phase; osmotic, heat, cold, and acid shock; as well as genetic modification of genes related to those stresses.