text: why are glycerol used for preparation of bacterial stock in -70 c or - 20 c ? why are glycerol used for preparation of bacterial stock in -70 c or - 20 c ? glycerol is used as a cryoprotectant at concentrations of between 5 and 20% for a number of reasons. firstly, however, you need to know why a cryoprotectant is needed at all... cells would otherwise be in some type of buffer and media, which if frozen, would cause ice formation inside and outside the cells, causing their death due to various reasnons including the freezing and non-freezing of solutes causing local increases in concentration, and ice crystals can cause rupture of membranes, as well as some other effects. glycerol is therefore used as a cryoprotectant which allows for vitrification to occur, which is basically the lowering of the freezing temperature of the solutin as well as the increasing of its viscosity. this process prevents crystallization (ie. the ice crystals which are harmful to the cells). obviously there are a number of cryoprotectants, but glycerol is cheap, fairly non-toxic to the cells, and since it is a polyol, provides some nutrient value to the cells.