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My freezer recently warmed to -6 degrees Centigrade during a long power outage from its usual -18C. It's now on its super freeze cycle and cooling back down again. Is the food inside still safe to eat? -6 is obviously less than zero, the freezing point of water, but I've heard that food in a freezer can go bad even below 0. Is this true?

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    how long was it at that temperature? Sep 15 '14 at 14:53
  • @KateGregory: Not for very long. It's kept inside our utility room which is at about +15C therefore it would have kept on warming had the power not come back on then [i.e. -6C was not close to ambient temperature outside of the freezer]. I would therefore hazard a guess at under half an hour at that temperature. Thanks for the edit and help by the way :)
    – niemiro
    Sep 15 '14 at 14:58
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    -6C is still freezing, you should be good.
    – GdD
    Sep 15 '14 at 15:45
  • "I've heard that food in a freezer can go bad even below 0" Yes, bacteria are still multiplying, just slower. At -18, it's deemed safe indefinitely (source: EU and USA government websites) with a tolerance of 3°C (source: EU legislation). Above that, it will spoil, just much much slower than at room temperature. According to the star rating system, -6°C keeps "three or four days" and -12°C "fifteen to twenty days". Why -18°C is said to be three months is beyond me though, so these probably err on the side of caution.
    – Luc
    Sep 22 '20 at 1:30
  • Note that these are cumulative times. If your food spent 3 days at -6°C (where 4 days at -6°C is considered safe), then storing it at -12 will only be good for another ¼ of those "fifteen to twenty days" i.e. up to 5 more days (if those values are to be taken literally, which I doubt; it's about the concept). As an example, let's say 20 bacteria is the allowance and at -6°C they double every day (2× per day) whereas at -12 they multiply by 1.25× per day, then you can count out at which point 1 bacterium turns into 20.
    – Luc
    Sep 22 '20 at 1:33
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In your question, does "go bad" mean food quality or food safety?

As for safety, you were nowhere near the "danger zone" either temperature-wise or time-wise.

Initially safe, properly stored, frozen food that warms to -6C (or 22 F) for 30 minutes will not render it unsafe to eat.

In terms of food quality, freezers (most of which these days are self-defrosting), go through temperature cycles regularly. The problem with this is not food safety, but the accumulation of ice crystals in foods, which degrades quality. Freezer burn is the extreme example. Of course, attention to packaging (vacuum sealing, for example) helps.

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    I was talking very much about food safety. In this house we live by "Will it kill me?" rather than best-before dates :) I'm glad to know that it should still be good. Thank you all very much.
    – niemiro
    Sep 15 '14 at 16:42
  • "freezers (most of which these days are self-defrosting), go through temperature cycles regularly" Just to note: if items in your freezer ever reach -10°C or higher under normal conditions (not when you just filled the freezer with lots of new stuff or opened the door for a long time, for example), this is definitely not a normal defrost cycle. It should not heat up the air by more than 1-2°C, if that, due to the air circulation fan turning off while the evaporator coils are being heated. (Source: mechanic that came over and confirmed our freezer is broken within warranty.)
    – Luc
    Sep 22 '20 at 1:23

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