Fridge temperatures for storing samples: 2–8 °C

A home fridge in the UK should run between 0 and 5 °C (32 to 41 °F), checked with a fridge thermometer (FSA). In a laboratory, "refrigerated" has a set meaning: 2 to 8 °C. The two ranges overlap, but they are set for different reasons, and a fridge that is fine for food can still be a poor place for samples. This guide explains where the 2–8 °C range comes from, how a lab fridge differs from a home one, where the warm and cold spots are, and how to check that a fridge is holding its temperature.

It is the companion to freezer temperatures for storing samples. For storage at room temperature, see room temperature in the lab. Terms are defined in the glossary.

What temperature should a fridge be?

It depends on what is in it.

For a lab fridge, the usual set point is 5 °C, the middle of the 2–8 °C range. That leaves room for the temperature to swing a few degrees either way during normal use without leaving the range.

Fridge temperature: where 2–8 °C comes from

"Store at 2–8 °C" on a label is a convention shared by the pharmacopoeias and stability guidelines. The ICH stability guideline tests material intended for a refrigerator at 5 °C ± 3 °C (ICH Q1A(R2)). The European Medicines Agency's guidance on storage statements links that test to the label wording "Store in a refrigerator (2 °C – 8 °C)" (EMA). Laboratory suppliers and reagent labels follow the same convention.

The two ends of the range have different jobs:

• The 8 °C ceiling keeps chemical breakdown slow. Reaction rates rise steeply with temperature, so a fridge that drifts to 12 °C is not "nearly cold". By the usual rule of thumb, breakdown there can be one and a half to two and a half times as fast as at 5 °C. See the Arrhenius equation.
• The 2 °C floor stops solutions from freezing. Freezing and thawing a solution can damage what is dissolved in it and concentrate salts as ice forms. A dry powder doesn't mind being colder, but a buffer or reagent solution does.

Home fridge vs lab fridge

A home fridge is built to keep food cold on average. A laboratory fridge is built to keep everything inside it at one temperature.

Pharmacy and medical fridges are a third type, built and tested to the same tight standard as lab fridges.

Never store flammable solvents in a home fridge. See storing lab chemicals safely.

For a small stock of sealed, freeze-dried vials held for a short time, a well-run home fridge with a thermometer is workable. For solutions, standards or anything held for longer, a lab fridge's even temperature is worth the cost.

Warm and cold spots

A home fridge without a fan has a temperature map, and it is rarely the one people expect.

• The door is the warmest place. It warms every time it is opened.
• Near the back wall, especially close to the cooling plate, can be the coldest. Items touching the plate can freeze.
• Top and bottom shelves differ. Cold air sinks, so the bottom is often colder, though this depends on where the cooling plate sits.
• The salad drawer is enclosed and often a little warmer and more humid.
• The middle shelves, away from the walls and the door, are usually the most even.

So for samples: keep them on a middle shelf, away from the door and not touching the back wall. A min/max thermometer moved around for a few days shows the real pattern in a particular fridge.

Defrost cycles

Fridges collect frost or condensation on the cooling surface and get rid of it in one of two ways.

• Auto-defrost (cyclic). Most home fridges let the cooling plate at the back warm up between cooling cycles, so frost melts and runs out through a drain. The air warms a little each time.
• Frost-free. A fan blows air over a hidden cooling element, which is warmed by a heater on a timer to clear it. Temperatures swing briefly during each cycle.

For food these swings don't matter. For samples kept for weeks, they add small amounts of warming, and they can cause condensation inside loose containers. Laboratory fridges are designed to keep the contents within a narrow band during defrost. See freezer temperatures for the same issue in freezers, where it matters more.

Using a fridge well

• Keep vials sealed and in a closed box, ideally with a desiccant. Fridges are humid, and a box limits warming when the door opens.
• Open it as little as possible, and know where things are first.
• Don't overfill it or block the vents. Cold air needs to move.
• Keep food and drink out of a lab fridge, and lab materials out of a food fridge.
• Label the contents with the compound, lot number and date. See lot numbers and batch numbers.
• Let vials reach room temperature before opening them; see why cold vials collect moisture.

Fridge thermometers and data loggers: which to use

The setting on a fridge's dial is not a measurement (FSA). The numbers on a home fridge dial usually mean "colder", not degrees. A min/max fridge thermometer is enough for a home fridge or a small sample stock; a data logger suits a lab fridge, or anything where you need a record or an alarm.

Fridge thermometers

A fridge 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 fridge has warmed overnight, over a weekend or after a busy day.

Data loggers

A temperature data logger records the temperature at set intervals, for example every five minutes. The record shows every door opening and defrost cycle, and can be kept as evidence. Many loggers can send an alert when the temperature goes outside a set range, such as below 2 °C or above 8 °C. A logger with a calibration certificate traceable to national standards, for example from a UKAS-accredited lab, gives a record you can rely on.

Where to place a fridge thermometer

Air temperature jumps whenever the door opens, but the contents change much more slowly. A probe hanging in open air gives sharp spikes that don't reflect the samples. Placing the probe in a small bottle of glycol or water, or among the stored items, gives a reading closer to what the samples experience. Put it on the shelf where the samples are kept, not by the door.

If a fridge fails

• Keep the door shut. A closed fridge stays cool for a few hours.
• Check the record. A thermometer or logger shows how warm it got and for how long.
• Move the contents to another fridge or, for dry powders, to a freezer if the fault can't be fixed quickly.
• Note what happened against the affected lots.

For freeze-dried peptides, a few hours or even days at room temperature is much less of a concern than it would be for solutions. The powder tolerates the days involved in shipping 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, rather than in a fridge. A fridge at 2 to 8 °C suits short-term holding. Each product page lists the recommended storage condition for that compound, such as −20 °C for Semax 5 mg. See how to store freeze-dried peptides for storage in more detail.

Related guides

For storage below zero, see freezer temperatures for storing samples. For why temperature controls how fast a sample changes, see the Arrhenius equation and accelerated stability testing. For storage far colder than any freezer, see liquid nitrogen and cryogenic storage. Every lot has its own COA, published on the product page and on that lot's own page. See Quality.