Synopsis: Super Bowl party snacks are prime territory for contamination via cross contamination and being dropped on the floor. A scientist who has studied both phenomena discusses the truth (or lack of truth) in two old myths.

Host: Nancy Benson. Guest: Dr. Paul Dawson, Professor of Food, Nutrition and Packaging Science, Clemson University

Links for more information:

Stay in the loop! Follow us on Twitter and like us on Facebook!

Double Dipping/5-Second Rule

Nancy Benson:  The Super Bowl is the nation’s number one reason to throw a party, with millions of us digging into the drinks and snacks, the chips and the dip. But there’s an etiquette to eating those foods, built on our conceptions of sanitation. For example, what do you do when you’ve dipped your chip, taken a bite, and still have half a chip left? Do you dip it again and perhaps contaminate the dip? Do you turn the chip around and dip the other end? Or is double dipping really not that big a deal?

Dawson:  The recommendation is certainly during cold and flu season from the CDC is you cover your mouth when you sneeze. The fact of the matter is that’s how colds and flu spread through the population, is you’re touching something and someone sneezes and touches a doorknob. It’s not a mystery how it gets from one person to another. That’s primarily through the bacteria in the oral cavity. So this would be just one other way that it might be transferred.

Benson: That’s Dr. Paul Dawson, professor of food, nutrition and packaging science at Clemson University. And yes, he’s done controlled studies on just how much bacteria can be spread by double dipping.

Dawson:  An analogy we use, it’s kind of a crude way to say it, but if someone’s double dipping, you’re actually kissing each person in the room. They’re pretty much spreading their saliva to the dip. So people kiss each other and they get along fine, but it is a way that germs are spread.

Benson: Dawson and his team originally didn’t think that double dipping would spread very many germs. After all, most of the chip or cracker that’s contacted by your mouth stays in your mouth. So they initially ran an experiment using sterile water as a dip.

Dawson:  We found that a thousand bacteria in the solution that had been exposed to chips that had been bit, versus the low detection on the ones that had not been bit. So the thousand to one there are about a thousand bacteria per milliliter, and again, to give a ball park figure what a milliliter would be, if you estimate that if you pulled out a chip of salsa and on the chip you probably have 20 or 30 milliliters of salsa, so about 20-30 times that, so potentially there could be quite a lot. We did a controlled study of 20 mills of water, dipping a chip in that and then measuring the amount of bacteria transferred after three times. We found about a thousand bacteria in that first study. We were surprised actually.

Benson: Dawson’s team then considered different kinds of dip and whether that makes a difference. Salsa dips, for example, are more acidic than cheese dips, and bacteria don’t like acid. Once again, the researchers initially simulated double dipping under controlled conditions.

Dawson:  So we took sterile water again and adjusted the PH levels to 4,5, and 6 and compared those different PH levels, thinking that maybe the lower PH, the more acidic, would have an inhibitory affect on the bacteria that was transferred, maybe get rid of it right away. Actually we found that it was an effect. We actually held the solution there two hours simulating what might happen at a party. It dropped the level down, still pretty high in the hundreds per milliliter, but there was an effect in the lowest PH, still very high levels.

Benson: Those results proved to Dawson that double dipping can, indeed, be trouble. So it was time to move on to real food, to see if the results held.

Dawson:  We found that there were again about 500,000 more bacteria per milliliter initially than when you didn’t bite. The big thing was though that salsa, we didn’t expect it so we just used water before, the salsa had a higher transfer rate to the dip than did the chocolate and the cheese initially. Thinking about it, kind of obvious that salsa is much thinner, a lower viscosity, so when you dip it in the dipping solution more of that salsa falls back into the common bowl or remains there than would be with the cheese, which would adhere greater to the chip. Numbers were about five times higher in the salsa than they were in the cheese.

Benson: However, the acid present in salsa dip kills more bacteria over time than in a cheese dip… So within about two hours, the dips end up about equal in bacteria content. Dawson says there’s also a little less opportunity for double dipping with cheese dip… because it’s thicker. That means people eat more of it per chip.

Dawson:  We also put the dip on a scale in a bowl and dip three or four times and measure the average amount of dip that is carried by each cracker or chip. That’s when we found out that a lot more is carried back out of the bowl to the mouth with the cheese it adheres better and is thicker than the salsa. So, yes, I would assume that given the same amount of dip and the same amount of dippers, the cheese dip would be used up first.

Benson: But double dipping is only one of the important snack food contamination questions Dawson and his team have investigated. They’ve also probed the validity of the well-known “five-second rule,” which says any food dropped on the floor is still clean enough to eat if you pick it up within five seconds. Reportedly, 70 percent of women and nearly 60 percent of men adhere to the five-second rule. But where did the rule come from in the first place? Dawson says the most likely place may be an extremely popular TV cooking show.

Dawson:  Julia Child made a comment I think on one show I think it was misquoted, but in general she made a comment that if she dropped something in the kitchen and picked it up if nobody sees it it’s going to be okay. I think that’s where it may have started, not really sure.

Benson: Dawson investigated the five-second rule, again with controlled studies, counting bacteria transferred to food after five seconds, 30 seconds and one minute of contact with the floor.

Dawson:  So we inoculated three different surfaces – tile, wood laminate floor, and a berber carpet – and found in all cases that the bacteria was picked up immediately. Bacteria’s faster than the human hand I guess is the answer to that. Like the double dipping, we found interesting differences in numbers and amount of bacteria picked up through the surfaces. Carpet actually was much lower based on what we had inoculated than were tile and wood. Common sense will tell you that if you have a bacteria on a surface that’s kind of absorbent like a carpet it’s not going to be as much of the surface area exposed to the bacteria. One the other hand, the bacteria would probably live longer on a carpet than it would on a tile or survive better because it’s kind of protected, there’s probably moisture there.

Benson: Dawson’s group also came up with another scary finding. Once a floor is contaminated, it stays that way for a long, long time.

Dawson:  Even salmonella, things that are just bacterial cells survive very long on surfaces. We inoculated tile and left it at room condition – temperature as well as humidity. The tile feels completely dry, there’s no appearance, it’s not dirty. We were recovering bacteria from that tile 28 days later. That was quite surprising. You’d think once it dries out the bacteria will die. So that was kind of an eye opening reminder that cleaning surfaces after you’ve had something there like raw food, chicken or whatever is very important.

Benson: However, cleaning in the kitchen isn’t foolproof, even when you think it is. Labels on disinfectant cleaners can be misleading.

Dawson:  You always hear the use of percent with bacteria, like this kills 99.9% of the bacteria, or something of that nature, and that is somewhat of a misleading terminology, because bacteria is in such high concentration. If you have a highly contaminated surface, like preparing raw food, or you’re in a hospital, killing 99.9% is not going to put you in the safety zone. Now, it’s not better than anything, but that’s a thing, we deal with bacterial contamination that those percent, until you get out to 99.9999, about four or five nines out there, you get the highly contaminated surface then you’re still– it’s misleading sometimes, I guess is what I’m trying to say.

Benson: So if you drop that chip on the floor, contamination depends a lot more on what’s on the floor than how long your chip was there.

Dawson:  Most surfaces are not going to have pathogens on them, so 999 times out of a thousand there’s probably nothing there that would hurt you, but I make the analogy of kind of like wearing a seat belt or not. You could drive your whole life and not have an accident and the seat belt has no effect. But if there’s an accident, or you have bacteria there, the seat belt or not eating the food is probably going to be a good idea.

Benson: It’s almost enough to make you avoid the snacks completely.

You can find out more about all our guests on our website,

Our production director is Sean Waldron.

I’m Nancy Benson.


Join the discussion

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.