How Frozen Is Too Frozen?

There’s a suggestion that raising the temperature of your food deep freeze by 3 degrees C could save a huge amount (the article suggests 17 megatons) of CO2 annually. Setting the thermostat from -18C to -15C, it’s suggested, will help the climate crisis. And look – that is NOT the main point of the research paper that the article is based on, but I’ll address the Anthropocene piece in my article.


The implied thing in the piece is that we’re wrong for wanting our food frozen to the world-wide accepted standard for safe food handling. And they have a point – the 0F/-18C point may well have been randomly chosen.

On the other hand, raising the thermostat would need (just as proving that the current setpoint is unnecessary would require) years – or even decades – of research and studies. You can’t say what ten years at -15C would do to every food as compared to ten years at -18C, unless you actually spend ten years researching what happens to each food over the course of those years. And if you wanted to be completely fair, you should also go to -21C and -12C and test a range of foods at those temperatures for ten years, too.

I’ll save you some trouble – we have decades of food handling research that has been done at -18C and also a few decades of experience in applying much colder temperatures than that. Supermarkets are greedy and won’t spend a cent they don’t have to, and if they use colder freezers than most households, there’s some compelling reason for that.

Not much will convince me to make my freezers (yes, plural) warmer. I’ve lived in isolated regions and farms and kinda relied on my freezers and freezer techniques for so long that I can tell this recommendation is a bare minimal standard and as soon as anywhere obtains a source of cheap plentiful energy they should lower temperatures as appropriate for each food.

Puttin’ It On Ice

One of the reasons is that no refrigeration is ever perfect. You change the temperature of the freezer when you open it to access the food within, it’s impossible to avoid that happening in the real world. You change the temperature more if your freezer has a vertical entry door (walk-in freezer, upright freezer, freezer compartment of your refrigerator, etc) than if the door opens upwards as in a chest freezer or ship’s hold.

The most common household freezers and freezers in commercial food have vertical doors. Cold air falls down just like water does. Imagine the cold air in the freezer compartment of your fridge when you open the door was water. You open the door and – a few litres of cold air flows out, and all the food in the compartment starts gaining heat from the inrushing warm air, and starts thawing. (This is the reason our parents yelled at us to not stand in front of the open refrigerator while choosing something to eat or drink as kids. Take note next time you open your fridge for more than a minute, how soon afterwards it has to start up again to pump out the heat you let in. That’s heat that’s warming up your carefully frozen food and allowing pathogens into the food.)

The other type of deep freezer quite a few households have is the “chest freezer,” where the door opens upwards, and the cold air inside pools inside just like a pool of water would. These were the gold standard for farms and homesteads, and homes in isolated towns, because they only gain external heat slowly if you open them. A drawback was that things had to be piled atop each other, meaning you had to take items out to get at items lower down. So those items that had to be taken out gained heat while you were digging around that the freezer, heat that then had to be removed by the freezer heat pump when you put those items back.

The point is – we do expend a lot of energy to pump the heat out of frozen foods, and every time the door to the refrigerating/freezing compartment is opened, heat will get in and raise the temperature in the compartment. Pathogens will get in along with the heat and air.

Taking It Further

I’m firmly of the mind that if the food is barely at the threshold temperature for bacterial activity to suspend, that’s going to result in more spoilage than if the food is six to ten degrees lower. That extra few degrees means that even if my foray into the freezer lasts two minutes and raises the temperature in the compartment by 5C, no surface of any of the food in there will have had its temperature raised above -12C.

Another thing to consider is the bacteria you introduce into the compartment, in that inrushing air and on your hands. Here are reasons why that’s a worry:

You’d think any bacteria you introduce would just die when it got cold, wouldn’t you? But the plain fact is that they don’t die. The key part of Allouche’s interview is the highlighted bit in this direct quote: “Food safety is not an issue from the moment food is stored below -12°C, as all microbial growth is inactivated.” The cold just pauses them. And when the temperature comes up, they don’t take the time to yawn, stretch, and brush their teeth. As soon as they warm up, they start eating and excreting again, spoiling the food.

That means that if you keep the freezer open for a few minutes while searching for something, the surfaces of some food will warm up and get condensation on it, to which bacteria and other pathogens will stick. If that surface gets warmed a bit next time, the bacteria will just start doing their thing.

Also, of course, only some bacteria cease being active at temperature -12C, some rare extremophile bacteria keep right on eating and pooping (albeit really slowly) under a layer of permafrost. You won’t find any extremophiles, but don’t bet that all bacteria have heard about how they’re supposed to cease and desist all activities when the thermometer shows a certain temperature. Most species of food-borne bacteria hit the pause button at -12C, but some may not.

The Wonderful World Of Parasites

So bacteria are just one thing that inhabits your food. Even the best-grown meat and vegetables can have parasites in them. I’m not sure what internal parasites a stick of celery might suffer from but I’m sure that there are no niches in the food chain that some organism hasn’t taken the opportunity to take root in and adapt itself to exploit.

I’m also aware that as the climate changes, so some organisms are going to die while others take over their niches. As permafrost melts, as glaciers melt, so older organisms are going to warm up, and not all of them will have died from the cold. A species of campion has survived in seed form for 32,000 years and a plant cloned from one of the seeds. If the squirrels of the time hadn’t deliberately gnawed out the vital centre to stop the seeds sprouting underground, I’ve no doubt at least several of them would have just sprouted.

I know – a campion is a lovely little flowering plant, not a parasite. But if fragile complicated things like seeds have survived for tens of thousands of years under permafrost, I’m sure parasite cysts would have no trouble.

An anthrax outbreak in Siberia has been blamed on a reindeer carcass being exposed by melting permafrost. And I know they think that it was that carcass from about 75 years ago but a) it may also have come from something else in that layer that has obviously been covered and exposed a few times.

That a reindeer 75 years ago got frozen-in indicates that the permafrost is variable, and that region may have been exposed and re-frozen hundreds of times in 10,000 years, and may have trapped a lot of passing biology each time.

One very important reason we freeze some meats is because the cold kills parasites that live in animal muscles. Note the temperature needed to kill a fairly common pork parasite – it takes 20 days at -15C at least to kill the Trichinella worm. Trichinosis (Trichinellosis) is a roundworm infection that can be severely debilitating.

Some kinds of parasites lay eggs (cysts) in their host animal, and cysts also may be resistant to quite extreme cold, and certainly -12C wouldn’t kill them, only preserve them. Most sites I’ve investigated are more interested in killing parasites and cysts with heat than with cold, but many parasites are tough little buggers.

And as I mentioned, some bacteria are not “inactivated” by -12C. Nor even by -15C. These bacteria may just be enormously slowed down, and so perhaps many studies have lumped them in with the “inactivated” bacteria, but the fact of the matter is that some still live at their frozen slow rate, still eat the products they’re in, and still excrete toxins into their environment.


When a food freezes, most commonly water or some other fluid freezes and seals the pores in the food with a solid ice layer. But some liquids in some foods don’t become hard at 0C as water does. We’ve all seen stuff like gelatin that’s still like gel to touch when it’s in the freezer. The bacteria that drift in on the air as you open the door and handle the food can easily embed themselves in gel or in a temporarily thawed water film that forms on the outside of a food or condenses out of the air onto the stuff in the freezer. Next time you open the compartment, they come right back to life and keep spoiling whatever they’re on.

When you put something into the freezer, the heat from it flows into all the adjoining food, and raises the temperature of the whole compartment until the heat pump takes that excess heat out. Even though you’ve closed the door again, it’s still introducing a lot of heat into the compartment.

Air inside the foods and their packaging expands slightly as it warms, and forces out through pores and gaps in the food or its packaging. As things cool again, some fresh outside air you let in (and its load of whatever was floating around at the time) gets into the food and food packaging.

This is one reason why stable temperature is more important than the actual temperature. Variations “pump” new contaminants into the food and food packaging. And variations are going to happen every time you open the compartment. Having the food at a lower temperature reduces the risk that some parts of the surface of the food thaws and lets in some of that pumped contaminant.

Also, your freezer will work far more efficiently and economically if it’s always full of frozen mass. The reason is thermal inertia – several frozen objects scattered around will gain heat faster than if you push them all tightly together, because their total surface area when spread out is a lot larger than the outside of the whole group when they’re all pushed together, since a lot of their individual surface areas will be touching another object inside the group instead of being exposed to the air.

That is to say, if you have a freezer packed tight with food, it holds its cool for longer than a freezer with less ice-cold mass in it, is less affected by opening and closing the door, and uses less energy to maintain its cold. To make the freezer work at its best, it needs to be filled as much as possible with material that’s as cold as possible when it goes in.

So to make the best use of your home deep freezer, pre-cool anything you bring home in the fridge freezer compartment before transferring it to your deep freezer. Don’t store anything in the fridge freezer for the long term because you’ll be altering its temperature to “pre-freeze” stuff going into the main freezer. Keep the main freezer as full as possible, even freezing bottles of water in the fridge and then putting those into the freezer to maintain the thermal mass.

Taken To Extremes

And how do we know that “colder is better and lasts longer?”

Well, supermarkets and food distribution. Supermarkets have always been at the forefront of saving a buck. And one of the ways they’ve done that is with some really deep, deep freezing. There’s a point where meat can pretty much become a rock.

If you just did this by turning your freezer down to -30C really quickly (yep, that won’t happen unless you have a really REALLY big heatpump in it, and the electric supply to run it) the meat would pretty much turn to mush when you thawed it out due to the extreme and rapid freezing causing muscle cells to rupture. But supermarkets found that they can stage the freezing process through several freezers each colder than the last, and then reverse the procedure when they need it back – and you couldn’t tell if the meat had been frozen for three years – or thirty.

And believe me, if this didn’t offer some financial advantage, they wouldn’t do it.

Sending It

When a bulk store has (for example – and to take a food needing the most refrigeration –) frozen meat destined for another country then it’s important for it not to repeatedly warm up and cool down to a different temperature and then up and back down to the original temperature and so forth, for the reasons given above. Many use “reefers,” refrigerated shipping containers, and they have processes to match the temperatures end-to-end and to minimise temperature variations en route. The better they do this, the less product they have to throw out due to spoilage. They love not having to throw spoiled food out…

By the time a kilo of mince has come into your possession it’s been up and down in temperature several times, has been exposed to possible pathogens while not frozen or in process of thawing, and it’s only because supermarkets really HATE losing profits to spoilage that it arrives to you in as good a condition as it generally is. But you have to imagine that you’ve had it from fresh and had to ship it to yourself, making you painfully aware of all these processes. And then you might understand why the -18C so-called “random” temperature is really needed…

Mission: Freeze It.

But to reiterate: A freezer set to -15C may very well hold some foods safe for an extended period – but remember that we use the freezing process to extend the usable life of foods in a great many scenarios that have different requirements.

The type of mission is important here.

Urban home Scenario/Only have a freezer atop the fridge.

You just want to stash the ice cream for the kids, a spare jug of milk, some bags of frozen veg, a few frozen meals, and maybe a roast for the weekend. The kids will open it and stand for three minutes trying to will a tub of chocolate ice cream to appear, or you may need to dig around to find that Massaman Curry & Rice frozen meal. -15C may appear to be good enough but if two people open the door in a row, the safety of all the food can be compromised and you don’t really want that. -18C is a much safer temperature.

Rural/homestead/remote home Scenario

You’re storing food to cover a possible shortfall of a month or two if there’s severe weather and roads get washed out. You may also have locally sourced foods that you need to store seasonally, and seasonal foods sourced from the nearest town’s supermarkets and stores. Generally, only one person manages the freezer and they know fairly well where everything is, how long its been there, and so forth. Quite often there’s a smaller freezer built into the fridge that stuff gets transferred into and out of the “big freezer.” Most often that’s a chest style freezer but sometimes it’s a walk-in type. Your chest freezer needs to be at -18C – -20C and your walk-in at -20C to -22C.

Consider yourself in this category if you live in a larger town or a city but have separate deep freezer and like to buy things on special and store them for future meals. The best advice is to use the fridge freezer as described in the previous “Physics” section, LABEL EVERYTHING with what it is and a date it went into storage, and use the oldest first.

Store/supermarket Scenario

Stores have display / floor chillers and displays, and usually some storage out the back for day to day shelf re-stocking. There are legal requirements on stores to maintain legally-set limits on temperatures, but think of it as the reverse of the homestead process above – stuff arives “shipping cold” and gets put into their storage freezers out the back, then transferred to the shop floor on demand.

Bulk Store Scenario

Many supermarket chains and large food distributors will have bulk storage facilities to be able to smoothe out demand peaks and troughs. Their foods are stored much colder than the supermarket freezers and coolrooms, and in some case as mentioned above, may undergo staged cooling to much lower temperatures more suited to long-term storage.

Shipping Scenario

Shipping large quantities of frozen and chilled foods will need temperatures to be as cold as bulk stores, and indeed may go to bulk stores on another continent or in another country. The constraint to prevent as many changes of temperature as possible means almost all of them are using self-powered refrigerated containers with logging thermostats to allow close monitoring, to ensure that there have been no large temperature swings.

The Cold Wrap

We use refrigeration for a good many different purposes. The 17 million tons of CO2 mentioned in that linked article is a figure that they admit in the original paper that “…estimates for energy and emissions are at this initial report stage approximate…” and I’ll take that a step further and say that I feel that it’s already outdated as sustainables take over base load production. I also think that it’s not the most important driver of climate change nor the major one.

Yes it would be great to reduce that figure but alongside the cost of transporting food (3 GIGAtonnes!!! of CO2 according to that article, which is paywalled – one of the nice things that your donations help me with, paying subscriptions to get to those figures…) 17megatonnes is actually very little, and covers far more than just tranpostation.

It’s important to note that the original paper says that every day 25,000 people die from – hunger. Almost a billion live with hunger daily, another two billion aren’t food-secure, food-borne illnesses cause 600,000,000 cases of illness every year and more than 400,000,000 to die.

Refrigeration at the recommended -18C is a small price to pay to reduce those numbers, but the sad fact is that the unsafe delivery of food isn’t down to corporations adhering to existing standards but rather their already ignoring food refrigeration safety in the name of shareholder profits.

If you give those corporations a lower target to maintain and they’ll just lower their unofficial standards even further.

We need to spend many times as much as that paper cost to promote more sustainable energy and reduction in use of fossil fuels, and raise the standards for food transportation, refrigeration, and handling, with rebates for saving food waste and transporting saved food to areas where there is a shortage.

I applaud that the study has been done; and that it has raised some important facts to public visibility. But the reporter at Antrhropocene magazine has taken and amplified quite the wrong message

One Closing Thought

Imagine a restaurant with a walk-in freezer and coolroom, being exhorted to keep everything warmer because CO2 emissions, when in fact the adoption of cleaner energy sources would result in fewer emissions across all sectors. Would you be confident that ALL their meals would be safe to eat? Or would you find a restaurant that decided that patrons not getting food-poisoning was more important, and followed the approved rules for food handling?

Okay, maybe some will be okay with that despite the carbon cost is orders of magnitude less than the cost of transporting their food. But what about hospital kitchens? Aged care kitchens?

I know what I’d rather do with my food. Bring on the end of fossil fuels and sustainable energy!
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