01.Keeping your cool
Ideal settings in your fridge
- Your freezer compartment should be set to a temperature close to -18°C.
- The fresh food compartment should be set between temperatures of 0 to 4°C.
- The chiller compartment should be set close to 0°C.
The fridge standard uses a fresh food compartment average temperature of 3°C. This is also a good target to aim for as it provides a good chance of not freezing foods and keeping them below 4°C (considering the normal lack of uniformity in temperatures across the compartment and the fluctuations in temperature that occur with compressor cycling). These temperatures are based on scientific reasoning and historical convention.
Less than ideal settings reduce the storage life and quality of foods. Inappropriate storage (food stored too long or too warm) is a key contributing factor of food borne illness in Australia.
Please note: this information was current as of March 2009 but is still a useful guide to today's market.
Some fruit and vegetables keep much better at temperatures higher than 5°C. In industry these can be met, however, householders generally don’t know these special requirements and their fridge well enough to take advantage of them. There are some special requirements and fridge characteristics that are easy to learn. Some tropical fruits can deteriorate in normal fridge fresh food compartment temperatures: they store better in slightly warmer environments.
Most fridges have at least one warmer area; the most obvious of these is the dairy compartment. Dairy compartments are usually located at the top of the fresh food compartment door and some maintain temperatures around 8°C. This keeps old-fashioned butter and foods like soft cheeses ready to serve. Butter and soft cheeses can also be kept at colder temperatures so it is possible to move them out in favour of common foods such as green beans.
Why do foods spoil?
To understand why these temperatures are considered optimal it helps to know what it is that causes foods to spoil. Foods change markedly if they suffer a chemical or physical change. Chemical changes include ripening, maturing or oxidising, also known as becoming rancid. Physical changes include:
Mechanical stress – particularly poor packaging and handling.
Cooling – particularly freezing or changes in phase.
Heating – particularly cooking and migration of moisture.
Microbial micro-organisms, also known as bacteria and moulds, spoil food by eating the cell structure. In the process they increase in number. Some micro-organisms are bad not only because they spoil the sensory and nutritional value of food but also because they can be poisonous. While most pathogenic micro-organisms can be easily killed by thorough cooking, some produce toxins that are not destroyed by cooking.
Many foods are eaten raw and some of these can be risky. Some bacteria can cause food poisoning and other illnesses which can be fatal to vulnerable segments of the population, particularly immunocompromised persons including foetuses, infants, those with a serious illness, and the frail aged.
Blanching can have a positive effect on preserving vegetables. A sudden increase in temperature and cooling deactivates the enzymes that cause some foods to ripen and go off.
Most microbial micro-organism growth slows to a very low level at temperatures below 4 or 5°C. They are not necessarily destroyed; they simply go into a dormant or near dormant state and wait to warm up. If there are few bacteria to start with then any growth will result in small total numbers by the time the food is to be eaten. Not all bacteria stop growing below 4°C and can become harmful, e.g. Listeria monocytogenes . Some foods are known to be likely to be more readily degraded and they are considered risky and/or highly perishable. These foods are often stored just above freezing temperature – in the chiller.
Some foods deteriorate suddenly when they get below certain temperatures. This often occurs when the food freezes (change phase from liquid to solid). That said other foods undergo subtler changes when they get too cold for example some tropical fruits and vegetables. Foods low in thermal mass and dissolved solids freeze easily below 0°C. The most common example of this is lettuce but celery, spinach and Asian greens are also prone to freezing easily.
On thawing it is easy to see that the cell structure has been destroyed. They turn into a jelly-like mess. Other foods don’t freeze until much colder. Ice-cream has some components that freeze around -15°C; others freeze at much colder temperatures. For longer term storage it is desirable to freeze some foods. Even if a food remains frozen, fluctuations in temperature can be very detrimental. If ice-cream swings through its freezing point regularly then there can be physical changes. The gums separate, water migrates and crystals form, spoiling the texture and taste of the ice-cream. Frozen peas lose moisture, which is evident as loose ice crystals that form within the packet.
Freezer burn occurs when water migrates out of the surface of a frozen food. When air reaches the frozen food, it causes dehydration and oxidation. It is generally induced by substandard (non-airtight) packaging. It is more prevalent when temperature fluctuations are considerable. When the food is thawed it is easy to see that the texture is spoiled.
Oxidisation and other chemical changes are also slowed right down at -18°C. Fatty foods tend to have shorter storage life in the freezer as the fats become rancid sooner than other ageing effects.
Convention and history
-18°Celsius is the SI equivalent to 0°Fahrenheit. 0°Fahrenheit was defined in 1724 probably because it was the temperature of a reliable reference point created with a water/salt mixture. Therefore, -18°Celsius is convention based on scientific history. Even so, it is a sensible long term storage temperature that is accepted in most societies.
In Australia, frozen food manufacturers recommend -18°C and state the storage period based on this temperature. The Australian refrigeration standard requires freezers to maintain only -15°C but other standards including ISO use -18°C. At -18°C bacterial growth is almost non-existent and other physical and chemical changes slow down significantly. The storage life of foods is extended significantly.
For more information on fridges, see our latest fridge reviews.