My lab mate gave me a 87% glycerol stock solution that had been autoclaved a year ago. I made a 60% glycerol solution from it and autoclaved the 60% glycerol solution. I am going to use it to make bacterial glycerol stocks, but I am worried that autoclaving glycerol solution twice would damage the glycerol and thus can't be used as cell protectant in -80 oC.