When purchasing a refrigerator or freezer for clinical, research, or laboratory use, most people do not take into consideration the type of defrost cycle the unit offers. What they do not realize is that storing temperature sensitive samples (especially vaccines) in the wrong defrost cycle can damage them costing time and money.
Freezers will obviously form frost and ice, but refrigerators are often thought of as a unit that does not go below freezing temperatures. So why worry about the defrost cycle within a refrigerator? Even though the interior of the unit may not fall below freezing, the cooling evaporator tubes, coils, or plates the refrigerator uses for temperature typically do. Frost and ice will eventually form and buildup if some type of defrost does not occur and the type of defrost cycle utilized can dramatically impact interior cabinet temperatures.
For refrigerators, there are two different defrost methods to chose from; cycle defrost or auto-defrost. Cycle defrost occurs during the actual cycling (the regular on/off cycle) of the compressor, hence the name. This process occurs routinely in the refrigerator. Temperature fluctuation does not occur any more than it normally would, which helps to keep your samples safe and is an effective defrost method for refrigerators.
In an auto-defrost, or frost free refrigerator, defrost cycles occur in a quicker process for frost and ice removal and can cause a higher increase in temperature for a short period of time. The process is accelerated with the assistance of a heater and a timer that allows the defrost process to occur 2-3 times a day. The timer is typically set for every 8 hours to turn off the power to the compressor while a heater will kick on to aid in the removal of the frost. The spike in temperature can be harmful to temperature sensitive samples.
When it comes to freezer defrost cycles, there are also two different methods; Auto defrost (frost-free) and Manual defrost. The auto-defrost freezers are similar to the refrigerators, incorporating a timer and usually a heater which typically cycle 2-3 times within a 24 hour period. The designs for the auto-defrost units can differ which varies the cycle duration and the interior temperature. This can lead to some cycles raising the temperature above 0°C which can potentially damage temperature sensitive samples within the unit.
Manual defrost freezers require more work having to physically turn the freezer off or unplug the unit. This also requires transferring items quickly from freezer to freezer so you can clean up after the ice has melted. The key advantage of the manual defrost method is not having to worry about temperature spikes found in an auto-defrost freezer that can potentially damage medical and scientific products specifically biological samples such as enzymes.