For enzymes and kit reagents, the defrost system matters more than the size of the freezer. Manual defrost is the steadier choice, at the cost of defrosting it yourself.
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A frost-free freezer melts its own frost by switching on a heater around the cooling coils. GE Appliances' guide to defrost systems says timer-based models defrost every 8 to 15 hours, and adaptive models usually once or twice a day, for about 25 to 45 minutes each time.
During that cycle, GE's automatic defrost page says, the compressor and fans stop while the heater runs. That is designed not to thaw food, and for frozen peas it doesn't matter.
Illumina's answer on frost-free freezers is direct: they go through cycles of light thawing to prevent frost, and "Because of this temperature variation, frost-free freezers are not recommended for reagent storage." It also suggests a freezer that logs its temperature, to confirm reagents stayed in range.
GE adds that the heater cycles in an automatic-defrost freezer can dry food out slightly. Small volumes in tubes are the things you least want drying out, so close every tube and box tightly.
| Manual defrost | Frost-free | |
|---|---|---|
| Heater cycles | None | Every 8 to 15 hours (timer) or about once or twice a day (adaptive), 25 to 45 minutes each |
| Upkeep | Defrost by hand when frost reaches 1/4 to 1/2 inch | None |
| For enzymes and kit reagents | The usual choice | Not recommended by Illumina |
The Whynter CUF-112SS is manual defrost, 1.1 cubic feet and under 20 inches tall, so it fits beneath a bench. Its dial sets between -11 and 3 °F, and the door locks with a key.
Many enzymes and kit reagents are labeled for -20 °C, which is -4 °F. Set the dial toward its cold end, put a freezer thermometer inside, and check it holds the label's temperature before you trust it with stocks.
1.1 cu ft manual-defrost upright freezer that sets between -11 and 3 °F, with a removable wire shelf and a cylinder lock with two keys.
GE says to defrost when the frost is 1/4 to 1/2 inch thick. Thicker frost makes the freezer work harder.
Put the reagents in a cooler with ice packs. Keep enzymes upright and closed.
Unplug the freezer and leave the door open. GE suggests lifting out loose pieces of frost and setting a pan of hot water inside to speed it up. Do not chip at the ice with anything sharp.
Wipe out the water, plug it back in, and wait until the thermometer shows the target temperature before moving the reagents back.
