Culture Maintenance - The pure culture of a specific organism is to be maintained for its continuous use. The easiest method is to store them at low temperature (2-6°C) in agar or liquid medium. But the most common method is to store them frozen either at -18°C or at -80°C in freezers or at -196°C in liquid nitrogen. Frozen cultures can be stored for several years without any loss of viability.
The best method for culture is, of course the lyophilization (freeze drying). The addition of protective agents such as skimmed milk or sucrose reduces the lethality during lyophilization process. The cell suspensions are placed in small vials, which are then frozen by immersing in a mixture of dry ice and acetone or liquid nitrogen.