Apparatus and method for storing material at and below a permissible upper temperature below the freezing point in a refrigeration chamber containing a heat exchanger and a thermostat to activate a refrigeration to supply coolant to the heat exchanger to maintain a set temperature. Periodically defrosting the heat exchanger by introducing heat, usually by an electric heating element in the refrigeration chamber, but prior to initiating defrosting, the thermostat is taken out of control of the refrigeration unit and the latter automatically activated to effect a drop in temperature in the chamber sufficient to prevent the stored material from exceeding it permissible upper temperature during subsequent defrosting.