Depending on how you were raised, food dropped onto the floor was subject to the three-second rule, the five-second rule, the one-minute rule, or the before-the-dog-gets-it rule. That homespun yarn of a law, though, may not necessarily stand up under the rigors of actual scientific research. A few years ago, Harold McGee looked at tests conducted by Clemson University researchers in which slices of bologna and bread were placed on salmonella-laced surfaces to determine how quickly each item picked up the bacteria. Suffice to say, the longer the items stayed in contact with the surface, the more bacteria they collected. More recently, researchers at Manchester Metropolitan University conducted their own experiment on behalf of household products company Vileda to test the three-second rule and found that certain foods were more likely to pick up the bacteria than others.
According to the Daily Mail, the university researchers went beyond bologna and bread for their study, choosing to test cooked pasta, ham, a biscuit, bread with jam and dried fruit because they're commonly eaten and because they contain varying levels of water, a “key factor in whether items will sustain bacterial growth in the three seconds before they are picked up from the floor.” As it turned out, that water level, and high levels of salt and sugar, made a huge difference in how much bacteria collected on each food.
Bacteria didn't adhere very much to the dry biscuit, making it safe to eat even after 10 long seconds on the floor. Similarly, the ham's preserved salt and nitrates inhibited bacteria growth, as did the sugar in the jam. Where bacteria did accumulate, however, was in the cooked pasta and the dried fruit. The pasta picked up harmful bacteria within just three seconds, and the dried fruit picked it up within five.
These are, of course, generalized study results; you probably have a little more leeway if you drop a bowl of pasta onto a newly mopped floor as opposed to one that hasn't seen a bottle of Pine-Sol in a few weeks. As McGee advised in his article, “If you drop a piece of food, pick it up quickly, take five seconds to recall that just a few bacteria can make you sick, then take a few more to think about where you dropped it and whether or not it's worth eating.” If you can remember to pause before popping that stray French fry into your mouth anyway: Old eating-off-the-floor habits die hard.