You've been there: a bowl of salsa, guacamole, or some other type of dip sits on a party table right next to crackers and tortilla chips. You want some, but can't help wondering: have the other people at the party been double dipping?
Double dipping—practice of dipping a chip (or cracker, carrot, or anything other food that can be dipped) into a dip, then biting onto it, and dipping it again with the bitten side down (dipping a non-bitten corner of a previously bitten food does not qualify as double dipping)—is widely frowned upon and considered, aside from plain gross, unhygienic. But how bad is it, really?
Paul Dawson, a professor of food safety at Clemson University has an answer—which is: it depends.
In 2009, Dawson and his team conducted an empirical observation of the amount of bacteria transferred to a dip through double dipping. The study was published in the Journal of Food Safety (paywall), and will be part of Dawson's upcoming book on popular rules on food safety, such as the "five second rule."
Crackers and water
To understand what level of contamination double dipping could cause, Dawson explained, they did a first experiment with water. Dipped crackers in water, bit them, and dipped them in water again. Then, they observed the amount of bacteria transferred to the water. They then compared it to the amount of bacteria in water without any contact with a cracker, or with water after contact with a unbitten cracker.
They found that there was a significant transfer of bacteria in the water: about 5,000/ml. The most common bacteria were Streptococcus, Prevotella, and Veillonella, which are found in healthy individual's mouths but can also cause infections. By comparison, water that was not subject to double dipping had levels of bacteria that were often below detection, or anyway under 10/ml.
Crackers and acidic water
But, Dawson, explains, in real life a dip would hardly behave like water—for instance, it would likely be more acidic. So the second experiment was to make the pH of water between four and six–which is the level of most common dips. "We tried to simulate a party scenario," Dawson said, "we measured immediately and after two hours," since a dip is likely to sit outside for a long time.
Once again, the transfer was significant, though the pH had a significant effect, and the more acidic the solution, the less bacteria was found in it: 3,800/ml at pH 6, 3,500 at pH 5, 2,500 at pH 4. Two hours later, the acidity took the levels of bacteria down, but only or pH 4 (1,900/ml) and pH5 (3.350/ml)—with no difference for pH 6.
Crackers and real life dips
The third phase of the experiment looked at three of the most commonly used dips: salsa, cheese dip, and chocolate dip. Here, other than the different acidity, another factor was in play—viscosity. "We found that there was more transfer to the salsa than cheese and chocolate," Dawson says, because "the salsa is very thin, with cheese you're bringing more to your mouth." The weight of the dip left in the plate confirmed this, too.
In other words, the saliva and all the bacteria it contains are more likely to stick to the cracker and the second helping of dip, than to mix back into the bowl with the rest of the dip. For chocolate and cheese dip, the bacteria was only about 200/ml. Salsa was higher (about 1,000/ml), but went down to a similar level after two hours, since it's more acidic. "This is still significantly more bacteria than there should be," Dawson says, and "though the odds are pretty low that it's going to be dangerous, that is how common colds are spread."
So, how bad it is?
Double dipping is kind of bad—worse than shaking hands with a stranger (which is already pretty gross, in case you didn't know), or sneezing, Dawson estimates because you are directly bringing the bacteria into your mouth.
You should avoid doing it—and diving into that bowl of unsupervised spinach and artichoke dip at a friend of a friend of a friend's house party may not be the most hygienic way to spend an evening.
But, if you must, the research results show that you're better off with some really thick guacamole than some watery salsa, which are "only" 20 times more likely to give you an infection than a "clean" dip, rather than 100 times more.