Physical and temporal factors involved in the death of yeast at subzero temperatures.

The survival of yeast cells after exposure to subzero temperatures was affected by the cooling and warming velocity, temperature itself, and the physical state of the water surrounding the cells. The cells were injured only when the external medium was frozen and then only when the temperature was -10 degrees or below. Survival dropped abruptly between -10 degrees and -30 degrees regardless of whether the cells were suspended in water or 0.1 M solutions of KH(2)PO(4), NaC1, or CaC1(2). The critical temperature range of -10 degrees to -30 degrees was unrelated to the temperatures at which the suspending fluids completely solidified, these temperatures being -0.3 degrees , -11 degrees , -30 degrees , and -71 degrees for the four liquids, respectively. Survival at -30 degrees or below was greatly affected by the rate at which the cells were cooled or warmed, with higher survivals being obtained with slow cooling and with rapid warming. Length of exposure at -30 degrees was not a factor; injury was inflicted within 1 minute. The results are interpreted as indicating that death is a result of intracellular ice formation. Internal freezing is believed to occur when external ice crystals grow through aqueous channels in the cell wall or membrane and seed supercooled water in the cell interior.