Freezing and Storage of Tissue Culture - Freezing may be either slow (cooling rate 0.5-4°C min-I from 0°C to –l00°C and then transfer to liquid nitrogen, LN) or rapid (> l000°C min-I), or it may be initially slow, e.g., 5°C min-I up to - 30 to - 50°C, held at this temperature for about 30 min and then cooled rapidly by plunging the vials into LN.

A partial dehydration of the material (in vacuum or by treating them with a specially formulated concentrated solution called plant vitrification solution, PVS) before freezing has been found to increase the survival of cells tissues. The cells and tissues may also be encapsulated in matrices like alginate before freezing. A cryoprotectant, "£='.g., DMSO (5_8%), glycerol or proline ( 10%), must be added to the culture medium to protect the cells from injury.