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About 8 months ago we had a beef processed a meat processing plant. When we picked up the meat it was all pre-packaged and frozen. We immediately put it all in the freezer when we got home to be sure it stayed frozen. Now I am finding packages of ground beef that has blue and green mold on the meat while it is frozen. How does this happen? We have had to throw away at least 15 pounds of ground beef.

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Was it frozen at -18°C or lower? –  Anpan Dec 21 '13 at 13:47
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Is it possible that you you had a freezer malfunction at some point in that 8 months? –  sourd'oh Dec 21 '13 at 15:00
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Frozen food does not necessarily stay edible forever. There are some microbes that love (even extremely) cold climate. Such microbes are called cryophiles and can survive or even need temperatures as low as -15°C. There are microbes that show activity at temperatures as low as ~-200°C (http://microbewiki.kenyon.edu/index.php/Colwellia_psychrerythraea).

However, if you store your food at at least -18°C, most microbes will go dormant and you can store it for a long time without problems. Above that, however, there are several microbes that can and will grow and spoil your food, though I can't tell you which ones in particular. That's the reason frozen food bought in a store (in germany at least) has a label how cold it has to be stored for the minimum shelf-life to apply (often includes a table for minimum shelf-life at various temperatures as well).

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According to your link, c. psychrerythraea is not pathogenic. Are there known cryophylic pathogens? Food also degrades in quality faster at warmer temperatures, which may be the real motivation for shelf life recommendations. It seems vanishingly unlikely that cryophyilic bacteria are at the root of visible moldy fruiting bodies. –  SAJ14SAJ Dec 21 '13 at 15:55
    
I personally do not know of pathogenic cryophilic microbes. My point was that there are cryophilic ones and that it therefore makes sense that there may be molds that can grow even slightly below 0°C, which is why I asked him in the comments at what temp he froze the meat. Of course, his freezer may have malfunctioned or there was simply a prolonged power outage in his absense. I was simply giving the best answer I could think of with the given information. –  Anpan Dec 23 '13 at 16:34
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