Make it a zero-second rule

July 20, 2010

Oops. You've just finished grilling a nice juicy steak and dropped it on the ground. According to the five-second rule, it's perfectly okay to eat it if you pick it up right away and dust it off, right?

Well, not so fast.

Paul Dawson, a professor of food science at Clemson University in South Carolina, studies all manner of icky bacterial transfers, including double-dipping and drinking milk straight from the carton. And, having tested the five-second rule, you won't find him eating food that has been on the floor. Dr. Dawson explains:

What happens within that five seconds of your food hitting the ground?

Upon contact, bacteria will transfer to another surface almost immediately. Some of the variables that might affect that are the wetness of the surfaces that are coming in contact, but even with a dry surface, we found with dry, white bread, there was transfer.

If I'm at a party and there's a dip, I probably won't participate in it. Paul Dawson, a professor of food science at Clemson University

How long can bacteria survive on a dry floor?

We left [salmonella]in a room-temperature environment, trying to simulate what would happen at home or a restaurant or whatever. And they survived several weeks, probably in a dormant state, but when they're in contact with a food surface where the conditions become good for growth, then they're reactivated. They were surviving up to three or four weeks in our study.

The other ones we've worked with are E-coli, listeria.

So these are bacteria that are commonly found on kitchen floors?

Well, most ones that would be found on the floor probably wouldn't be pathogens unless there are reasons for them to be there, whether people have walked around in their shoes and had stepped into something like feces somewhere or they'd had raw food there on the surface, like a counter surface.

The odds are that they won't be there, but eating food that you've dropped is kind of like not wearing a seatbelt in a car because you can drive your whole life without wearing a seatbelt and if you don't have an accident, then you're not going to have a problem. But if you do have an accident, the outcome's probably not going to be so good.

It's a similar thing with food. If there's pathogens there, the outcome probably isn't going to be so good either.

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From the research you've done, are some types of floors riskier to eat food off of than others?

We did a statistical analysis of it and from that standpoint, yes. But from a practical standpoint, probably not so much. Carpet was lower [in bacteria transfer]than tile, but there was still significant enough transfer to make someone sick. Also, carpet would allow bacteria to survive longer than would the tile. Carpet would hold more moisture.

Do you follow a zero-second rule then yourself?

Yeah, pretty much. If [pathogens are]there, one second, any contact, it doesn't matter. With bacteria, it's not like they walk or anything. They're just present and if a surface hits them, they're going to be picked up.

Between eating something that's been dropped on the floor, double-dipping and drinking milk straight from the carton, what's worse?

I'd say probably double-dipping because your chances of getting in contact with someone else who's sick are probably greater if you're at a party and there's multiple people double-dipping. There are probably going to be fewer people drinking from the same carton. But either one is going to transfer bacteria; it's just the number of people you're going to be crossing paths with, so to speak.

What other similar studies have you conducted?

We're doing one now - but haven't quite completed it - on blowing out candles on a birthday cake, ... again counting the bacteria and to see if there's transfer. There's a lot of variation, as you might expect. One person might be slightly different from someone else, so there might be one [instance] where there's no measurable transfer and one that has high levels of transfer. But at the end of the day, there's a good chance that there's transfer.

Another one we did, a student did an extensive study on eating out of a common bowl, like rice. We used chopsticks and spoons and whatnot, where you eat right out of a common bowl.

And people share food too. Like I've got a soup and you want to taste my soup when I've already eaten out of it. We did a lot of different scenarios: if you stir the soup, if you use a chopstick versus a spoon, or some cultures, they use their hand. If you eat out of the bowl, put the food in your mouth and go back and eat again, or get another bit of rice, we found significant transfer in that also, much like maybe sharing popcorn or things like that.

So never share food then?

Well, with someone you don't know. With someone you know, you're probably all right.

Has all this research changed the way you eat?

Well, a little bit. If I'm at a party and there's a dip, I probably won't participate in it.

This interview has been condensed and edited.

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