Freezer temperatures for storing samples: −18, −20 and −80 °C

A home freezer in the UK should run at around −18 °C. Laboratory freezers usually run colder: −20 °C for general storage, and −80 °C for the most sensitive materials. The differences matter more for samples than for food, because samples are kept for longer and small changes add up. This guide explains the common freezer temperatures, how defrost cycles affect samples, and how to check that a freezer is holding its temperature.

Terms are defined in the glossary.

Common storage temperatures

−18 °C is roughly 0 °F. In practice, most domestic freezers can be set colder than that.

Is −18 °C close enough to −20 °C?

For freeze-dried powder, a well-run home freezer holding steady at −18 to −20 °C is close to a laboratory −20 °C freezer. The bigger risks are not the 2 °C difference, but:

• Temperature swings, from defrost cycles or opening the door often.
• Moisture, from frost and condensation reaching the vials.
• Not knowing the real temperature, because nobody measured it.

Chemical breakdown slows as temperature falls, so colder is generally better for long-term storage. See shelf life and stability. But a steady, dry −20 °C is better than a −25 °C that rises and falls.

Frost-free and manual-defrost freezers

Every freezer slowly collects frost from moisture in the air. Freezers deal with it in two ways.

• Manual-defrost (static) freezers let frost build up. You switch them off and defrost them by hand every few months, moving the contents to another freezer while you do. Between defrosts, the temperature stays steady.
• Frost-free (auto-defrost) freezers switch on a heating element on a timer, typically once or more a day, to melt the frost. The air in the compartment warms during each cycle, then cools again.

For food, a frost-free cycle makes no practical difference. For samples, repeated small warmings add up over months, and items near the walls or the top can warm more than those in the middle. Laboratory auto-defrost freezers are designed to keep the samples themselves within a narrow range during the cycle. A manufacturer's own testing of one lab model reported sample temperatures rising by no more than 2 °C (Helmer Scientific). Domestic frost-free freezers aren't designed or tested for that.

The usual choice for a small stock of freeze-dried vials is a manual-defrost chest freezer, or a laboratory freezer. Chest freezers also lose less cold when opened, because cold air sinks and stays in the box.

Using a freezer well

• Keep vials sealed and in a closed container, ideally with a desiccant. This protects them from frost and from moisture when the door opens.
• Store them away from the door and the walls, where temperatures swing most. In an upright freezer, avoid the door shelves.
• Don't overfill or block the vents. Air needs to circulate. A reasonably full freezer holds its temperature better than an empty one, though.
• Open it as little as possible, and know where things are before you open it.
• Label and list the contents with the compound, lot number and date, so you can find what you need quickly. See lot numbers and batch numbers.
• Let vials warm up before opening them. A cold vial opened in room air collects condensation. See hygroscopic powders.

Checking the temperature

The setting on a freezer's dial is not a measurement (FSA). Two kinds of device tell you what is actually happening.

Thermometers

A freezer thermometer shows the current temperature. A min/max thermometer also records the highest and lowest temperatures since it was last reset. Checking and resetting it daily shows whether the freezer has warmed up overnight or over a weekend.

Data loggers

A temperature data logger records the temperature at set intervals, for example every five minutes. You can download the record, see every defrost cycle and door opening, and keep it as evidence. Many can send an alert by text, email or app when the temperature goes outside a set range. A logger with a calibration certificate traceable to national standards (for example from a UKAS-accredited lab) gives a record you can rely on.

Placing the probe

The air in a freezer changes temperature quickly whenever the door opens, but the contents change much more slowly. A probe in open air gives alarming-looking spikes that don't reflect the samples. Placing the probe in a small bottle of glycol or glass beads, or among the stored items, gives a reading closer to what the samples experience. Put it in the middle of the storage area, not by the door.

If a freezer fails

• Keep the door shut. A closed, full chest freezer can stay below freezing for many hours.
• Check the record. A data logger shows how warm it got and for how long.
• Move the contents to another freezer if the fault can't be fixed quickly.
• Note what happened against the affected lots.

For freeze-dried peptides, a short rise in temperature is much less of a concern than it would be for solutions. The powder is stable for days at room temperature, which is why it can be posted without cooling. See why lyophilised peptides ship at room temperature.

How we store our stock

We store freeze-dried peptides at −20 °C, sealed and protected from light. Each product page lists the recommended storage condition for that compound. See how to store lyophilised peptides for storage in more detail, and Quality for how we document our stock.