Why shouldn't you refreeze food?

[Pragmatist]

I think most people are aware that you should not refreeze frozen food which has been thawed. My question is, why?

I was under the impression that freezing effectively stops bacterial growth. O.K. so you thaw some frozen food and the bacteria begin growing again - until you refreeze it when they presumably stop once again.

If I buy some meat which has an unfrozen lifetime of say 7 days, freeze it on day one, thaw it on day 2 and then refreeze it on day 3, then why would be any more dangerous than for example simply keeping it thawed for 3 days and then freezing on the third day? Is there something about freezing and thawing that somehow accelerates bacterial growth? Or is there some other factor I'm unaware of?

Perhaps some of the bio experts on here could throw some light on the matter?

 

[TeaBag420]

I think most people are aware that you should not refreeze frozen food which has been thawed. My question is, why?

Are most people aware of that? I'm not. Is it that someone wants to sell you more food?

 

[Donks]

I've heard of this, but so far assumed it was an old wives' tale. But then, I almost never cook. After some googling, it seems "food poisoning bacteria" grows when the food is thawed (particularly if it is thawed at room temperature, which seems to be a big no-no) and does not die when the food is frozen again. So you get all the bacterial growth of the first time you thaw and then all the bacterial growth of the second time you thaw. I do not vouch for any of this.

 

[Rob Lister]

I've heard of this, but so far assumed it was an old wives' tale. But then, I almost never cook. After some googling, it seems "food poisoning bacteria" grows when the food is thawed (particularly if it is thawed at room temperature, which seems to be a big no-no) and does not die when the food is frozen again. So you get all the bacterial growth of the first time you thaw and then all the bacterial growth of the second time you thaw. I do not vouch for any of this.

I vouch for the above. At least, that is what I learned when I took cooking classes at the local CC (non professional interest only)

 

[Benguin]

I vouch for the above. At least, that is what I learned when I took cooking classes at the local CC (non professional interest only)

Correct, and the instructions on anything you buy frozen normally point this out to ... the wording being "Do not refreeze once thawed".

My understanding is if the thing has been thoroughly heated through (effectively killing any multiplying bacteria) then it's fine for refreezing.

I'm not sure if this really applies to non-meat and non-dairy though.

The explanation I've heard is that the formation of ice crystals during the freezing process changes the food so it is far more easily attacked when defrosted. I'm not sure of the truth of that, but it should be quite easy to test with something like a few tomatoes. I would use a visual criteria though, rather than eating them!

 

[Pragmatist]

Thanks to all for the replies. This is what I was asking, in a more convoluted way: whether there was any scientific basis for the statement or whether it was simply an old wives tale.

I've seen this advice many times, it even occurs in the manual for the freezer! But I've never been able to see any logic behind it.

Although, before I dismiss it completely, I guess I ought to ask how many have actually tried it and survived?

 

[Chris Haynes]

Another reason to avoid refreezing food is that it makes it mushy and not very good quality.

When it froze the first time the water expanded, which causes cells to burst (why frozen raspberries are runny when thawed, but berries with less water like blueberries are just a bit better).

We have thawed meat the fridge, and then refrozen it (due to changing minds about use). Usually it was hamburger, so the quality was not that much of an issue. Because it was kept cold and then cooked completely -- there were no safety issues. Since it was ground beef used as an ingredient, the quality was not an issue either.

Fish filets, on the other hand, seem to quality issues when refrozen after thawing. It gets mushy, and sometimes just does not taste very good.

 

[epepke]

I don't think there's a hard-and-fast rule for this. The thing is, every time you thaw and refreeze food, you're putting it through the range of temperatures during which bacterial can flourish. Obviously, the more times you do this, the more opportunity for bacteria to grow.

 

[Carn]

I think this warning is made on the assumption, that any food that is thawed, is then parly eaten(whole while at room temperature) and is frozen again, will be rethawed with taking the same amount of time as first thawing and will then be fully eaten, again some time at room temerature.

(all numbers rough guess to illustrate my idea)
So letting bacteria grow once for 6 hours(thawing) + 2 hours till eaten is probably tested by food companies and bacteria concentration seems to be no problem.
But when you put it in normal freezer, the bacteria have aditional few hours to grow, till it gets to cold and then on thawing again, you get an additional 6 hour growth and then have some hours till eaten. So together you end up with 6 hour + 3 hours till refreezing+(maybe) 1 hour to freeze + 6 hours + 2 hours till eaten = 18 hours. Thats already more than 2 times as long as with once thawing and eating.

If someone is a bit lousy with refreezing and thaws a bit too long, he could end up with 25 hours growth time for bacteria, which could be far too long, therefore for safety, the rule of not refreezing, its just safety against stupidity and the way freezing and thawing is done normally.

If you thawed something, then drop it into liquid helium, then "thaw" it to room temerature with a 2000 w microwave oven, there shouldn't be any bacteria problem.

Carn

 

[Rolfe]

Donks is basically right. Get Prester John here, he's the expert.

You can actually work out what's safe, logically. You can thaw fresh food, for example, then cook it (killing the bacteria in the cooking process), and then freeze the cooked food.

Oh, and remember all the stuff about not letting cooked meat touch raw meat!

I'm inclined to think that if you thaw something very quickly, using a microwave set at defrost, and it never gets above a chilled temperature, you can refreeze it again within a few minutes with little risk of harm as there hasn't been time for the bacteria to grow significantly. I find it handy if I want to separate frozen portions that have stuck together, or divide a frozen pie or something like that into portions. I probably wouldn't recommend doing it too often though!

Rolfe.

 

[alfaniner]

Isn't bacteria growth exponential? I.E. If thaw time is 4x normal, wouldn't the bacteria be something like 16x normal?

 

[Benguin]

I'm inclined to think that if you thaw something very quickly, using a microwave set at defrost, and it never gets above a chilled temperature, you can refreeze it again within a few minutes with little risk of harm as there hasn't been time for the bacteria to grow significantly. I find it handy if I want to separate frozen portions that have stuck together, or divide a frozen pie or something like that into portions. I probably wouldn't recommend doing it too often though!

Rolfe.

I know some suppliers (especially seafood ones) make great play of using high speed commercial freezers that will do the food ten to twenty times quicker than shoving it in the home freezer.

Presumably the same logic of opportunity for bacteria growth would apply at that end too.

 

[steenkh]

It has always been my understanding that it is only some things that should be refrozen. Fish, and some fruits and berries are among them, but I do not know the correct list. I am sure that you can refreeze bread as much as you like, but I believe meat should be frozen as little as possible, and fish only once.

It is probably right what one said, that those kinds of food where the cells walls are destroyed by the freezing, they are the ones that should not be refrozen. I think this is not just for quality reasons, but also because the bacteria go wild when the nutrients are without their "wrapping". With meat the rationale is presumably along the same lines as these calculations we have seen on how long the bacterias are allowed to grow.

Lastly, I would think that different foods tend to have different bacteria, and some might tend to have bacteria that produce poison, and these should have less time to grow than food types that have more benign bacteria. This could also be one reason why some foods should not be frozen more than once.

 

[Pragmatist]

It is probably right what one said, that those kinds of food where the cells walls are destroyed by the freezing, they are the ones that should not be refrozen. I think this is not just for quality reasons, but also because the bacteria go wild when the nutrients are without their "wrapping".

Thanks Steen, that's a good answer, I hadn't considered that. I am particularly interested in the "why's" and that seems like a plausible possibility.

 

[Rolfe]

Um, I think it's got more to do with the fact that meat products come from things with guts, and guts are the home of a lot of bacteria you really don't want to get too familiar with. The chances of meat (and especially poultry) being contaminated with stuff like Salmonella and Campylobacter and even real evil stuff like E. coli O157 are enormously greater than something like bread which is vegetable to start with and baked on top of that.

Rolfe.

 

[American]

After some googling, it seems "food poisoning bacteria" grows when the food is thawed (particularly if it is thawed at room temperature, which seems to be a big no-no) and does not die when the food is frozen again. So you get all the bacterial growth of the first time you thaw and then all the bacterial growth of the second time you thaw. I do not vouch for any of this.

Not only that, but some bacteria produce chemical toxins (I think that's what Salmonella does). Those compounds remain even if you kill all the bacteria by cooking (unless you cook it to some crazy temperature that the compund burns off or reacts....).

As for food quality, the point of cooking is to partially break the chemical bonds so your gut-works can absorb the most out of them. Every time you do this (freeze-thaw), more of those bonds are broken (bond break = energy release) that would otherwise eventually be captured and used deep in each cell to produce energy in the form of ATP (not to mention grow and repair tissue on some level. That's an interesting question- how much of malnourishment is due to deficiency of vitamins and the ability to carry out cellular housekeeping, and how much is due to the lack of structural building blocks assuming they can't always be manufactured from molecular crud...).


(edit- you don't "partially" break a bond of course. I was referring to some abstract QUANTITY of bonds, probably of a particular kind that escapes my current knowledge.)

 

[Just thinking]

I think most people are aware that you should not refreeze frozen food which has been thawed. My question is, why?

As for me, any food that has been frozen twice tastes like cardboard, and that's reason enough for me. Perhaps that's my body's way of saying -- "Don't freeze that stuff twice, you idiot!"