How to tell when food has gone bad

We recently ran an article on food shelf life, and how these dates can be a useful indicator of whether your food is safe to eat. However, shelf life and expiry dates are not everything, despite what people think. It’s easy to let them control your eating habits, but often they are more a guideline to safe eating, rather than rules.

Misleading expiration dates are one of the biggest contributors to food waste worldwide, with perfectly edible food being thrown out due to food insecurity. Often, the best way to tell if your food needs to be thrown out is to examine it physically. This will involve looking at, touching and smelling your food if necessary.

Therefore, we’ve prepared this article to give you some information about how you can identify whether certain foods have expired, as well as to provide some useful tips on how to store and keep your food fresh.

What affects shelf life?

Rotten fruit and vegetables

CSIRO has an in-depth article explaining the major factors in the deterioration of food. The four biggest ones are:

  1. Microbiological changes
  2. Moisture and water vapour transfer
  3. Chemical changes
  4. Food-packaging interaction

Microbiological changes

This is likely to be the biggest factor in determining shelf life, as all foods that haven’t undergone some kind of sterilization process contain levels of microbiological growth. Some of the factors that influence this growth are:

  • Food properties, such as the presence of preservatives and acidity
  • Environmental factors like temperature and relative humidity
  • Processes designed to kill microorganism growth such as thermal processing or freezing

Moisture and water vapour transfer

Water is a critical factor in determining shelf life, as microorganism grow faster in moist conditions. Therefore, dry foods like bread and biscuits can decay faster once they are exposed to the moisture in the air, whereas others like cakes suffer due to a loss of moisture. The choice and design of packaging can also influence water levels.

Chemical changes

The most common chemical change is the oxidation of fats, which aids in the development of rancidity and off odour.

Food-packaging interaction

A lot of foods rely on their packaging to achieve their shelf life. This is why canned goods have such a long shelf life, as they are sealed tightly so no unwanted bacteria can enter.

Why off food is bad

Aside from just tasting and smelling gross, eating spoiled foods can have potentially serious health defects. The most common outcome is food poisoning, which is not very pretty in the best of cases. Symptoms include abdominal cramps, diarreha, headaches nausea, vomiting and all round weakness.

This is because expired food is host to infectious bacteria and parasites, and in severe cases people have been hospitalized due to the serious illnesses they’ve picked up from food and salmonella poisoning.  Fortunately, you can easily identify whether the food you’re about to eat is off or not, so you won’t have to blindly hope that you’re not going to get poisoned.

How to tell if your food has gone bad

To start with, we’ll go over the more obvious physical signs that your food should probably be thrown out.


mouldy cheese

Mold is probably the most distinctive feature of off food. It is extremely noticeable in dairy products in particular, and usually results in green or blue fungal growth in your food. It is also common in bread, meats and vegetables. There are some people who advocate for just cutting off the moldy parts and keep eating, but a lot of people don’t recommend this, since mold has usually spread beyond what you can see. If in doubt, chuck it out!

Rancid smell

This might even be more noticeable than mold, purely because you can sometimes notice it before you see it. If you open up the fridge and get blasted by the smell of rotting death, then there’s a good chance your chicken from a few weeks ago needs to go. If it smells different than the first time you smelled it, your food has probably gone bad. Dairy products and raw meats are the easiest to smell, but others are a bit harder.

A change in texture

aversion to milk

This is best seen in milk, which goes chunky once it goes bad. Chunky milk is a huge no-no, as are products that feel different to what they’re supposed to. Another example is fruit that has hardened when exposed, such as an apple. This doesn’t necessarily mean they should be immediately thrown away though. Check for some of the other features.

Ice crystals forming on frozen food

Frozen foods decay much slower, due to the fact that the microorganisms can’t grow as quickly in the freezing temperatures. However, that is not to say that frozen foods can’t go off or decline in quality. The best way to tell is if there are ice crystals forming on it.

This means that the water inside the food has come out and is freezing on the outside. This is an indication of declining quality, and it can change the texture and flavor. Typical villains are frozen lasagnas and other frozen ready-made meals, and to a lesser extent ice-cream.


This isn’t necessarily the case for meat products, as they can change color when exposed to light and air. But it is more common in fruit and vegetables when they have been exposed to the air. For example, your reddish brown beef is probably still ok to eat, but if your avocado has turned black then you have a problem. If the discoloration is also accompanied by a slimy or sticky film, then it is definitely off.

A slimy film

concerned woman looking at meat

This is mostly true for meat products. The general rule is that if there is a coating of slimy film on the surface then throw it away. Don’t put it in your mouth either, because it’s definitely gone bad. Some foods like canned goods and products that live in a container can be a little trickier to identify. Luckily, there are also a few signs that the food is off based on the container or packaging they’re in.

Torn packaging and imperfect seals

While this isn’t a way to tell if food is off by itself, it can be used in conjunction with some of the other methods here. Any rip or tear is letting in air. Oxygen degrades food when it comes in contact with it. So if you already suspect that the food is off, checking for broken seals is a good way to make your mind up.

Dented and swollen cans/containers

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. You can easily test this by pressing down on the either the top or bottom of the can: if it bulges, then the contents are not safe to eat.

The same is true for containers made out of other materials such as plastic. 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.

Leaking and rusting

Rust can weaken the integrity of the can, and can even allow dangerous bacteria to enter and mix with the food. Likewise, a leaking can is not a good sign, as the integrity of the can has been breached. You should always check for these signs of damage before buying a can of food.

Too much or too little packaging

packaged meats

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.

Hissing and spraying when opened

When you open the can or puncture it, the contents inside should not spray out at you or explode as if there was mounting pressure inside. This is a sign that the food inside has been contaminated with bacteria, and may be producing gas.

5 tips to keep your food fresh

Basket With Organic Vegetables Fresh From Market

Fruit and vegetables

Fruits and veggies are quite demanding if you want to keep them fresh for as long as possible. The best advice we could give would be to only buy exactly what you need, as most of them have very short shelf lives.  However, you can also separate them based on the amount of ethylene they produce. Ethylene is a gad that prematurely ripens foods that are sensitive to it, and it is commonly found in things like bananas, avocadoes and mangos to name a few.

You may have noticed that these are all fruits that you store outside, not in cold temperatures. This is why you should always separate these fruits from apples, broccoli, carrots, leafy greens and other fruits and veggies that are best kept at cold temperatures. Real simple has a comprehensive list of the ones you should group together.

Also, it’s important that you don’t wash your vegetables until you’re ready to use them, as moisture can speed up the decaying process. Potatoes, onions and tomatoes all need to be kept in a cool, dry place. Our shelf life guide has more information for fruit and vegetable storage.

Cooked foods

After food is cooked, it should be allowed to sit out at room temperature for no more than two hours before being put in the fridge to slow down bacteria growth. You should generally try to eat these cooked meals in 3-4 days, because we all know that bacteria can still grow in there!

If you have leftovers and cooked foods in the fridge, then store them above uncooked foods like fruit and vegetables, so prevent raw food microorganisms from dripping down onto those delicious freshly cooked meals. Lastly, there is a perception that cooked foods can’t be refrozen after thawing, but this isn’t true, according to the Institute of Agriculture and Natural resources. They state that thawed foods are safe to refreeze, as long as they haven’t been outside the fridge for more than 2 hours. This may result in a loss of quality due to decreased moisture levels.

Cheese and dairy products

dairy products

The best way to prevent cheese from drying out is by wrapping it in aluminum foil instead of plastic cling wrap, which is what most people seem to use. Some dairy providers even recommend storing cheese in the vegetable compartment of your fridge, since this section is slightly warmer than the rest of it.

It is also recommended that your cheese should be finished within 2 weeks, so try to buy quantities of cheese that you are likely to finish quickly. A handy tip for getting the most out of other dairy products like milk is to pick out ones from the back of the shelf, as these ones will likely have a further away expiration date. Fresh milk, cream and some soft cheeses have only a short shelf life and lose quality rapidly, so eat and drink them quickly!

Raw meat

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 they will take a little longer. To maintain the highest quality of raw meat, the optimal temperatures you should be aiming for are 4 to 15 degrees for the fridge and -15 to -18 for the freezer.

Dry foods

The key for maintaining freshness of dry foods is to store them in a cool location that is both dark and dry. This is why most people store them in a pantry, as light causes dried food to deteriorate and lose color faster, and shelf life will decrease as temperature increases.

Above all though, keeping them in airtight packages is crucial, particularly for things like flour, sugar, rice and spices as well as snack foods like biscuits and cookies. The material of the container doesn’t matter – they can be glass or plastic – as long as they keep the air out and keep the stuff in your pantry fresh.

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