Cooling rates of tissue samples during freezing with liquid nitrogen.

The time-course of temperature changes undergone by tissue samples during freezing in liquid nitrogen has been studied with small thermocouple probes. The specific heat of liver, muscle, adipose tissue and blood has been measured with a calorimeter. Most tissues have lower specific heat values than water. The lower thermal conductance of adipose tissue and fat muscle result in much longer freezing times. It may take an inordinate amount of time for tissue samples to freeze when immersed in liquid nitrogen; in addition, not all tissues behave in the same way, so it takes longer for a muscle sample to freeze than for a piece of liver. These differences between tissues are considerable and may affect the outcome of experiments if not taken into account.