How do you know if a product has passed its shelf life? Here are CSIRO’s top hints and tips.
What affects shelf life?
CSIRO has a detailed list of all the factors affecting the shelf life of food, including:
- Rate of growth of spoilage microorganisms (bacteria)
- pH levels
- Too much moisture or dehydration
- Packaging getting attached to food
- Oxygen spoiling of fats or oils, e.g. in nuts
- Oxygen spoiling of vitamins C or B.
Three winning storage tips
- CSIRO says the most important food safety requirement is to keep potentially hazardous foods stored below 5°C.
- Choose glass and plastic containers for storing food.
- Follow the storage instructions on the label. They are there for a reason.
What food looks like when it has gone bad
Container is swollen
If the packaging is bursting at the seams in the fridge, this means microbial action. The contents are producing gas and swelling so they no longer fit the same in the container. This happens most often if the food hasn’t been stored at a cold enough temperature.
The most common culprits are yoghurt, fruit juice, cream cheese and cooked pasta.
Severely dented cans
The contents of severely dented cans are not safe because the integrity of the separation between packaging and food has been damaged. If the walls of the container are damaged, they will be touching the food and contaminating it.
In the same way, open tins and cans will contaminate food because of how oxygen creates rust. So if you open a can and don’t use it all, put it in a plastic container straight away.
Watch out for things like tinned fruit and vegetables.
Mouldy cheese and rancid milk
It is pretty easy to tell when dairy products have gone ‘off’. Dairy products should be kept refrigerated at all times.
Frozen food with ice crystals forming
This means that water has come out of the food and is now refreezing outside the food. This can change the flavour and texture of the food.
Typical villains are frozen lasagnes and other frozen ready-made meals.
Torn packaging or imperfect seals
Any rip or tear is letting in air. Oxygen degrades food when it comes in contact with it.
Keep a special eye out for the air-tightness of packaging for stored nuts, oil, and fatty things like meat and dairy.
Too much / too little packaging
Meat should be stored unwrapped to last longer: 5 days for fresh meat and 3 weeks for cured meat at 0°C to 3°C. This is because wrapped-up meat maintains its high water content, but the water is trapped, so microorganisms grow on the surface making the meat slimy and ‘off’ after 3 days or so.
Unwrapping the meat means the surface dries out of water and fewer microorganisms can grow. It does change the colour and sometimes flavor of the meat, but this is better than having the meat go off.
By comparison, you should keep smelly things like prawns, melons and cheeses wrapped up tight, because their odour can contaminate other foods.
Tips to keep food fresh
Don’t wash your vegetables before you put them in the fridge. This is because water will ripen vegetables too fast and spoil them in the fridge. Instead, store them ‘unwashed’ and just wash them before you cook them.
The exceptions are lettuce, spinach and kale. These ones all need to get washed before storage. You should also rip off that outer layer of lettuce leaves before storage.
Take the tops off of carrots and radishes before storage to prevent rapid moisture loss.
Vegetables will last longer in storage if you wrap them in plastic, as this traps the moisture and surface-level nutrients inside.
Store cooked foods like leftovers above uncooked foods like meat and vegetables. This prevents raw food microorganisms from dripping down onto your freshly cooked foods.
Do not refreeze thawed food without first cooking it. The danger zone where bacteria grows best is between 4 – 60°C, which is exactly the temperature of thawed food.
Prevent cheese from drying out by wrapping it in aluminium foil instead of plastic cling wrap.
Thaw your meat and chicken in the fridge, below 4°C, for 24 hours before cooking it. Thawing it on the bench at room temperature encourages microorganisms to grow. If you’re in a hurry, you can thaw meat under cool, running water.
Steak and chops can be grilled straight from frozen, but it will take a little longer.