I have two questions related to food safety, both stemming from disagreements with my mother-in-law.
Why is it safe to eat some foods that are cooked and left on the counter at room temperature for hours, or even days (cookies or cake for example), and why are we compelled to throw away other foods that are left unrefridgerated for a couple of hours (pizza for example)? I know that the issue is the suitability of food as a bacterial culture, but what are the guidelines that allow you to know which foods are good cultures (and therefore bad risks) and which foods are not?
Stemming from this argument, we disagreed about whether eating raw or undercooked eggs is “more dangerous” today than it used to be. I thought it likely that the danger was always present, but we underestimated it until recently. She thought that the food handling processes of today are less sanitary than they used to be, and that eating raw eggs in the past was not dangerous. It seemed to me very unlikely that today’s food handling conditions are less sanitary than those of the past. So, are raw eggs more dangerous today than they used to be, and if so, why?
Food handling is vastly safer today than it was; it is, however, a much larger enterprise than it once was, and there’s more opportunity for contamination to “sneak through.”
As for why some foods seem safe to leave out, I think it’s because most pathogens are anaerobic, and only flourish inside of food items. A pizza is not inherently unsafe after lying around for two days, but if some bacterial endospores survived the cooking of the sausage, they will multiply frantically if not refrigeration. This won’t happen inside of baked goods; they’re cooked through. Both can get moldy, though.
The more likely thing to happen to unrefrigerated cooked meats and cheese is rancidity, which is a chemical process.
More likely - or at least more dangerous, is that any surviving bacteria (and there are always some) will be allowed to gleefully multiply. If you refrigerate it right away, you stop the multiplication and division. (They have to rely upon subtraction and addition. :))
There are always some bacteria even after cooking, but your body can deal with those small amounts. Left alone to their own devices, these bacteria will become… well, more bacteria.