Uncovered Raw Meat or Milk in Fridge
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.
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?
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).
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.
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.
If its a room mate issue, there are other issues of politeness and cooperation that are beyond the scope of a culinary discussion.... :-)
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.
@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.