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I believe the harm in leaving raw meat uncovered in the fridge is that more bacteria will enter, more oxygen will enter (causing faster bacteria multiplication), and more water will escape (causing dryness which probably slows bacteria multiplication, but toughens the meat, so let's consider dryness to be bad; I'm not making beef jerky). Please ignore the stinky odors that escape; just consider the quality of that one uncovered piece of meat after we eventually cook it.

How much faster will the food spoil (percentage, compared to the same meat wrapped in plastic wrap)? I imagine that meat with skin, like a whole chicken, is not effected as much as meat without skin, like a bare chicken breast, and that ground meat would be effected the most. I wish I had some data to convince my roommate to cover his meat; is there any research on this? Feel free to answer this for milk instead if there is more research for it.

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Isn't raw meat covered when you buy it in the first place? What's your roommate doing, opening/unwrapping it and then refrigerating it? –  Aaronut Aug 10 '13 at 12:37
    
This time he prepared a turkey but didn't have the stuffing, so its been sitting out in the fridge for 2 days while he procrastinates. Or, last time he bought a pack of two steaks, opened the package to cook one, and then cooked the final one a day later (so the final one was exposed without wrapping for one day in the fridge). Even for me, it could happen that I run out of plastic wrap, so I want to leave something exposed for a few hours, so I think it's a fair question to get an estimate on how much time is ok (I'm surprised that someone down-voted it). –  bobuhito Aug 10 '13 at 13:18

3 Answers 3

up vote 2 down vote accepted

There is no practical difference in spoilage time for wrapped versus unwrapped. Spoilage is a factor pretty much of temperature, since in practice, all foods have pathogens present which can breed.

Assuming your refrigerator is free of insects, dogs and similar macro-fauna, wrapping is to prevent odors from going from one food to another, drying, or cross-contamination of one food by another through drips or splashing. Now, outside of a refrigerator, where ants, flies, and so on are abundant, wrapping also provides a physical barrier to prevent infestation, but the body of the refrigerator already does that.

In fact, in some rare occasions, you may wish to specifically refrigerate your meat unwrapped: for example, after brining a turkey, refrigerating it for 24 hours unwrapped will permit the skin to dry out, and permit a more crispy result.

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Surprising, but all sounds reasonable. Then, I guess my best argument to my roommate is that it will dry out his meat (still ignoring the sanitary reasons everyone mentions). Dryness seems significant after just half a day...but maybe he likes the chewiness. –  bobuhito Aug 13 '13 at 17:35
    
If its a room mate issue, there are other issues of politeness and cooperation that are beyond the scope of a culinary discussion.... :-) –  SAJ14SAJ Aug 13 '13 at 17:35
    
Spoilage certainly depends on factors such as exposure to oxygen. Oxygen exchange is limited by tight wrapping, so one expects that the conditions for chemical change and bacterial activity are different for a tightly wrapped piece of meat than they are for an unwrapped piece of meat. An extreme case is that vaccuum sealed meats tend to last longer than thoe that are wrapped by hand. Wrapping will also effect moisture exchange. Certainly wrapped cheese generally grows mold less rapidly than unwrapped cheese. –  Dan Fox Aug 26 '13 at 9:06
    
@DanFox Interesting point, and certainly true at the limiting cases, but I don't think they apply in the spirit of the question. Vacuum sealing aside, most containers have sufficient oxygen inside for spoilage, and there are anaerobic pathogens as well, some of which are very dangerous. –  SAJ14SAJ Aug 26 '13 at 11:47

Only a half answer, but particularly with meat, you're really risking contaminating other things in your fridge. It's dangerous for that reason alone, so I wouldn't even bother worrying about whether the meat itself spoils faster, since I'd be worried about everything else in the fridge.

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I see your point, but it seems that if you are careful to place the uncovered meat on the bottom of the fridge and don't have any drips from it, other food shouldn't get spoiled. I'd rather give my roommate a reason that works even if he's totally careful. –  bobuhito Aug 10 '13 at 5:09
    
@bobuhito Must be a pretty empty fridge if you can have total confidence it will never touch anything, and drips do sometimes happen despite your best effort, and will probably happen more if the meat's not covered, making it even easier to accidentally contaminate the fridge... –  Jefromi Aug 10 '13 at 5:19

Your initial assessment sounds pretty dead on. OFCOURSE sealing foods will slow spoiling. Your concern about bacteria and such needing oxygen to breed is certainly correct - for for all micro-organisms, but most.

As far as the argunent made that the bacteria is always present internally in meat is only part true. first, it makes a world of difference whether we're talking about cooked or raw meats.

in the case of cooked meat, assume that after you properly cooked it there is no living bacteria and it. That means that for it to spoil, it needs to be exposed to bacteria from the external environment. A lot of sources exist to do this such as, handling it, breathing on it, siliva exposure to contaminants in the air in general, exposure to other food products, etc.

so it's pretty clear that proper storage that's not open and exposed will slow spoiling, especially when you add in the oxygenation effect. that said,if you wash food before sealing it you certainly want to try it off since moisture is quite the catalyst for breading micro-organisims.

in the case of raw meat that potentially contains live bacteria and parasites in it, the type of meat makes a pretty big difference. that's why we thoroughly cook poultry and pork but not necessarily beef.

rather than believe me, since others seem to disagree with what I just said, here's a pretty good link with professional advice on the matter.

http://www.foodsafetysite.com/educators/competencies/general/spoilage/spg1.html

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I just realized you also mentioned milk and not just meets. milk is the same principle. Pasteurized milk contains very little living bacteria, hence if it's sealed properly it will last a long time compare to if it was not pasteurized, since pasteurization kills of the internal bacteria. That said some bacteria survive pasteurization, therefore the milk will still eventually spoil. However, ultra pasteurized milk which is heated much hotter than regular pasteurization will last quite a bit longer since the additional he kills remaining bacteria internal in the milk. –  Lee Birnbaum Nov 24 '13 at 18:42
    
I totally agree but am still looking for data/numbers showing how much time things can really be left uncovered... –  bobuhito Nov 24 '13 at 23:12

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