Most of us know that in the warmer months it's a good idea to turn the temperature on your fridge down a bit to compensate for the warmer air coming in, and vice versa in winter. However, there's a lot more involved in getting the temperature just right in your fridge and freezer.
The cold, hard numbers
You don't want to be the reason for your friends getting sick at your next party, so you'll want to make sure your fresh meat doesn't spoil before you get around to cooking and serving it. But you also don't want your fresh vegetables to freeze and be unusable either. The fridge temperatures below are a good guide to balancing both your health and bank account by keeping food safe – and your guests from suing!
- Fresh food: between 0°C to 4°C.
- Freezer: close to -18°C.
- Chiller compartment: close to 0°C.
The Australian Standard for fridges uses a fresh food compartment average temperature of 3°C. It's a good target to aim for because it means not freezing foods yet still keeping them below 4°C.
There are always exceptions to the rule – some fruit and vegetables keep much better at temperatures higher than 5°C. People who grow fruit or store it for wholesale might need to know these details, but the average consumer doesn't really need to know all the variations for each piece of fruit or vegetable. However, there are some special requirements and fridge characteristics that are easy to learn. Some tropical fruits can deteriorate in the fresh food compartment of a normal fridge – 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 fridge door and some maintain temperatures around 8°C. This keeps butter and foods like soft cheeses ready to serve. These can also be kept at colder temperatures so it is possible to move them out in favour of foods such as green beans, which also like a warmer temperature.
Should I change the fridge temperature...
...when I've put in fresh groceries?
Once you've got your fridge temperatures right, you won't need to fiddle with the settings to cope with a load of warm groceries. If a fridge does its job properly, turning the temperature control to colder when you put in a large load won't speed up the cooling process – it'll just mean your food ends up colder.
If your fridge doesn't seem to react to a warm load by starting the compressor to cool it down, you can try setting it colder for a while, but don't let the food get too cold before turning it back. Alternatively, have it checked by a service person – or, if it's old, consider buying a new fridge.
...from summer to winter and vice versa?
You may find your freezer gets warmer – not colder as you might expect – during the winter months. It's a good idea to keep an eye on temperatures using a fridge/freezer thermometer or two, particularly in the height of winter and summer.
Why do foods go off?
Foods spoil if they undergo a chemical or physical change. Chemical changes include ripening, maturing or oxidising, also known as going rancid. Physical changes include:
- Mechanical stress – poor packaging and handling.
- Cooling – freezing or changes in phase.
- Heating – cooking and migration of moisture.
Microbial micro-organisms, or bacteria and moulds, spoil food by eating the cell structure and then increasing in number. Think of how sliced ham gets slimy when it's been in the fridge too long. Some micro-organisms are bad not only because they spoil the smell, texture and nutritional value of food, but also because they're poisonous! While most pathogenic micro-organisms can be easily killed by thorough cooking, some produce toxins that aren't destroyed that simply.
Raw foods can be especially risky. Some bacteria can be fatal to certain people, particularly infants, those with a serious illness, and the elderly. For the rest of us they mean (at the very least) a few days on the toilet.
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.
The low temperature in your fridge slows down the growth of most bacteria and mould. They aren't destroyed; they just slow down till it warms up again – that means if you put in some already rotten food it isn't going to magically be okay when it's cooled down. Not all bacteria stop growing below 4°C, e.g. Listeria monocytogenes. Some foods are considered risky and/or highly perishable. These foods are often stored just above freezing temperature, in a chiller.
Some foods deteriorate suddenly when they get below certain temperatures. This often occurs when the food freezes. Other foods undergo subtler changes when they get too cold, such as some tropical fruits and vegetables. Foods like lettuce are mostly water, so they freeze easily when in temperatures below 0°C and when they thaw you can see 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 you'll want to freeze some foods, but even if a food remains frozen, fluctuations in temperature can be very detrimental. For example, if ice-cream is warmed above -15°C on a regular basis then you'll see physical changes. The ingredients separate and crystals form, spoiling the texture and taste.
Another example is frozen peas; these lose moisture, which you'll see as loose ice crystals that form within the packet.
Freezer burn occurs when water moves out of the surface of a frozen food. When air reaches the frozen food, it causes dehydration and oxidation. It's generally caused by non-airtight packaging, and it happens more when temperature fluctuations are high. When the food thaws you'll see the texture has spoiled.
Oxidisation and other chemical changes are also slowed right down at -18°C. Fatty foods tend to have a shorter storage life in the freezer as the fats become rancid faster than some of the other ageing effects.