UNSW microbiologist shares tips on freezing and thawing food safely to prevent foodborne illnesses.
Whether you're meal-prepping or dealing with leftovers, freezing meals can save time and money while reducing food waste.
But contrary to popular belief, freezing food doesn't always kill bacteria - it can reduce their numbers, or it may simply put them on pause. And if frozen food isn't thawed correctly, bacteria can start multiplying again once the temperature rises, and grow faster in thawed food than in fresh food.
Food microbiologist and food safety expert Associate Professor Julian Cox from UNSW Chemical Engineering says freezing doesn't undo what has already happened to food.
"If bacteria were present before you put food in the freezer, they'll still likely be there when you thaw it," he says.
He shares some practical tips to help households freeze and thaw food safely while maintaining quality.
Hot meals
If you know your meal is destined for the freezer, A/Prof. Cox recommends cooling it down in the fridge as soon as possible after cooking.
"Avoid putting steaming hot food straight into the freezer because you risk partially thawing the food around it and potentially giving bacteria a chance to start growing in those foods," he says.
"Fridges handle the heat better than freezers and reduce the chance of bacteria growing on the food compared to allowing it to cool at room temperature.
"If needed, hold food on the bench only until it stops steaming, then pop it into the fridge. Once it's cold, transfer to the freezer.
"You want to establish the cold chain as quickly as possible.
"And sometimes meal plans don't go according to…plan. Food that you thought you'd eat soon, stored in the fridge for a day or two, can still safely be frozen."
Ice crystals and food quality
Freezing can affect the overall quality of the food due to the formation of ice crystals.
Foods are frozen commercially using blast-freezing in industrial equipment, which freezes food very quickly and creates much smaller ice crystals.
"Domestic freezers can't freeze food as quickly as industrial freezers," A/Prof. Cox says.
"That means ice crystals are larger, and when you defrost the food, the food structure is more disrupted, and more liquid and nutrients are released.
"If food is thawed and refrozen, this damage increases and overall quality drops significantly.
"It also means that thawed food can support greater microbial growth than the equivalent fresh food."
While freezing may slow or halt bacterial activity, it doesn't kill all pathogens.
"Some bacteria, including Salmonella, can survive for long periods in frozen food, especially if the food is protective, for example, containing high fat and protein," says A/Prof. Cox.
"We shouldn't consider freezing as a typical means of destroying bad bugs - there will be some loss, but there's also survival. That said, for certain pathogens, such as parasites, freezing has proven effective as a kill step in fish."
When freezing food, avoid freezing large quantities at once, as this slows the freezing process, giving bacteria more time to multiply and potentially causing greater loss of quality.
A/Prof. Cox recommends dividing food into smaller portions and storing each in airtight containers.
"Smaller portions freeze and defrost faster, which is safer and more convenient. Think about portions that suit your household in terms of a single meal serving," he says.
A/Prof. Cox says packaging also matters.
"Freezer bags aren't always airtight, so moisture can move in and out more easily," he says.
"Sealed (airtight) containers filled close to the brim that leave little or no headspace for air, help food last longer and maintain quality."
Ready to eat?
When you're ready to eat your frozen meal, thaw cooked or ready-to-eat foods in the fridge or microwave - not on the bench-top.
When food is thawed at room temperature, there is initially a balance in temperature between the surface and the core of the food.
As food approaches complete thawing, its surface begins to warm. When left at room temperature, it risks sitting in the temperature "danger zone" - between 5 and 60 degrees Celsius - where bacteria can grow.
Thawing in the fridge ensures it will be maintained at a safe, chilled temperature. Alternatively, microwaving provides a quick thaw before further cooking, but it may also begin to cook the food, which might be undesirable.
"Some people thaw in static water, but this carries its own risks as the outer layers of the food can remain in the danger zone for too long, which allows bacteria to multiply," says A/Prof. Julian Cox.
"While thawing under running water is an acceptable option, it certainly isn't ideal as it requires large volumes of water, which is wasteful.
"The safest option, and that most recommended by authorities, is slow thawing in the fridge, because even after a day or two, the highest temperature the food reaches is around five degrees Celsius, or whatever temperature your fridge is set to."
Frozen versus fresh?
People often worry about the quality of frozen foods, especially their nutritional value. However, some foods, such as peas, are frozen almost immediately after harvest, which helps lock in and maximise their quality, including nutrition.
In contrast, the same food making its way through the supply chain and sold as fresh, may actually have lower quality than the frozen version.
"By the time fresh peas are picked, sent to the retailer and make their way into a customer's kitchen, it's already been in the supply chain for many days," says A/Prof. Cox.
"When you're talking about frozen products, they're typically moved very quickly from the field into a frozen state so the quality is sometimes higher compared to buying fresh - and often cheaper too!"
Be Berry Aware
Some frozen foods might need a warning label, having been associated with outbreaks of foodborne illness.
In 2017, frozen berries were linked to three cases of hepatitis A in Australia. The berries, which were imported and packaged locally, had passed company testing before sale.
A/Prof. Julian Cox says poor agricultural practices are often to blame.
"Luckily in Australia we have good agricultural practices and food safety standards, so the risk is much lower for locally grown produce," he says.
"Personally, at present, if frozen berries aren't grown in Australia, I'd avoid eating them straight from the packet.
"I still use imported frozen berries in cooking, in a range of dishes, but first I heat them up to at least 80 degrees Celsius to significantly reduce the risk of any harmful bugs surviving.
"The problem we face is that it's very hard to detect viruses in any food, including frozen berries. So, assuming risk and preparing and cooking foods in ways to reduce or eliminate that risk is the way to go."
