18 Dec 2023

Safe storage tips from a food scientist

From Afternoons, 2:31 pm on 18 December 2023

Dunedin food scientist Phil Bremer gives us the lowdown on how long you can safely store a range of foods - including half avocados, boiled eggs and tomato sauce.

an avocado with a pink background

Photo: Public domain

Food scientist Phil Bremer

Food scientist Phil Bremer Photo: University of Otago

Fridges - what temperature is safest?

Ideally, Between 2 degrees and -5 degrees, Bremer says.

"We know from studies that older fridges don't necessarily work that well. They may be up around six to eight [degrees]. Fridges often have temperature gauges so you can get a pretty good idea where your fridge is at."

Freezer - what temperature is safest?

"The colder the better ... generally freezers run at about minus 18 between minus 15. "

How long is frozen food okay to eat?

The general rule of thumb is about six months, Bremer says, but maybe up to a year depending on how the food is packaged and how airtight the packaging is.

"The other way that food could deteriorate in the freezer is it just simply dries out and you've probably all gone to the freezer and seeing that sort of white glaze on the food. That's what we call freezer burn. And it's just dehydrated. The food's not unsafe to eat, but it doesn't taste good.

"Food that has the highest fat content - like a fatty fish or fatty meat - the fat can oxidise. It's a chemical reaction that happens very slowly in the freezer and that makes the food just not taste as good."

With fish or seafood, either eat it or give it away while it's fresh, Bremer recommends. If you have to freeze it, keep it in an airtight container and cook it within three months.

Should tomato sauce go in the fridge?

Around the world, about 50 percent of people refrigerate their tomato sauce and 50 percent shelve it, Bremer says.

"Keep it on the fridge. That slows down the fungal growth and extends the shelf life."

How long is it safe to keep cooked meat in the fridge?

Two days is the rule of thumb for a couple of reasons, Bremer says.

After that, there's a good chance you're not going to eat it at all and will end up throwing it out.

Also some bacteria may have survived the cooking process and spending too long in the fridge will allow them to grow and produce toxins.

What is the best way to keep a block of feta fresh?

Remove it from any liquid it's packaged with and store it in an airtight container.

How long can you keep boiled eggs in the fridge?

It's, again, the two-day rule on those, Bremer says.

"It's probably more of a food quality issue than a food safety issue."

The best way to store tomatoes? On the bench till they're ripe, then into the fridge

"This will slow down the ripening process and keep them for a bit longer. Then maybe bring them out four or five hours before you eat them just to get the right taste."

Avocados? As above

"Don't put unripe avocados in the fridge - ripen them and then put them into the fridge."

How to store cut avocados

Eating half an avocado for breakfast and storing the other half in the fridge for the next day is fine, Bremer says.

"You can just bang it in there as it is you can put it in Gladwrap or you can put it into a container. It's all equally good."

If you don't use the half avocado within two or three days, he recommends scooping out the flesh and freezing it.

And don't bother with adding lemon juice to the cut half - it's worse than doing nothing.

"[NZ food charity] Love Food Hate Waste sponsored us to host a summer student one year and she tried all sorts of things with [preserving] avocado. We found that the lemon juice was a bit of an urban myth."

How to store a whole Christmas ham

Bremer stores his ham in the chilly bin and freezes two two-litre drink bottles full of water.

"Every morning I get up and I put one frozen bottle in [the chilly bin] and take the half-frozen bottle out.

"I put the ham in a ham bag or a pillowcase and make sure there's no condensation. I'll wipe out the chilly bin and make sure it looks dry.

"If the ham bag looks a bit scungy, I'll take the ham out and give the bag a quick rinse under running water, maybe throw some white vinegar on it and then put the pan back in. I keep mine to well into the New Year doing that."

How to store asparagus

The best way is in a sealed container in the fridge lined with a wet paper towel, Bremer says.

"Fridges can be quite dry and you want to avoid it drying out as it dries out ... You also want to avoid getting too much condensation, as well. So it's a trade-off, keeping it sort of moist but avoiding getting visible condensation on it."

How long should a whole lettuce last in the fridge?

"When I pick a lettuce from the garden I'll wash it all, stick it into a plastic container or a plastic bag and it will sit in the fridge for easily a week, I believe.

"In a container that's sealed, it'll keep quite happily for a week in the fridge."

Store potatoes (in a cool dark place) away from onions

"Both potatoes and onions respire so if they're in the same container they don't last as long."

Raw organic fingerling potatoes in a basket.

Raw organic fingerling potatoes in a basket. Photo: 123RF

Fruit and veges don't need to be washed, just rinsed with cold water

"Don't think you need to wash it in something add any detergent or sanitiser or anything."

How to store a bag of baby spinach

"If it's from the supermarket keep it in the original packaging in the fridge, Keep it cool. And then once it's opened, I would put it into an airtight container with a wet paper towel and a little bit of moisture."

How safe is mouldy cheese? (that's not blue)

Mould produces toxins that can migrate through the food, Bremer says, but with a really hard cheese that migration is very small.

"If you cut like a centimetre all the way around, I believe that's okay.

"If it's a softer cheese, then we're concerned about how far these toxins might migrate. Softer cheeses that've got mould on them have probably gone off anyway. But it's undesirable mould rather than the ones we ate to give you a flavour."

How safe is mouldy bread?

"If it's a whole loaf, you could perhaps get away with chopping the end off. If it's if it's sliced bread in a plastic wrapper just throw the whole thing out. you'll probably find this mould all through it and if you can't see it on one slice it's probably still there on others."

Eggs - should they go on the fridge?

You can perhaps get away with keeping them out of the fridge in cooler climates, Bremer says, but they'll last longer in the refrigerator.


Flours and grains - will they just last forever?

In airtight containers, they'll last for "a long period of time", Bremer says.

How long is canned food okay to eat?

Never eat food from a can that's dented, Bremer says.

If the can looks to be intact, it should be okay to eat the contents for about two years after purchase and possibly longer.

"If you find a can that's looking a bit old and you still want to eat it, I'd open it. Have a look at it. See it looks good and smells good. And then a taste a little bit. If it looks good, smells good and tastes good, then I'd happily eat it."