Some people hate when their friends do it. Some people don't mind. The debate on whether you should or should not "double-dip" your chips, or any food item, into a sauce or dip twice, has raged ever since one particular episode of Seinfeld, where George Costanza is accused of double-dipping the same chip.
"Did you just double dip that chip? You dipped the chip, you took a bite, and you dipped again," the accuser says in the episode. "That's like putting your whole mouth right in the dip."
It turns out that the accuser of the "double-dip" in the episode might not be completely wrong. A group of students from Clemson University in South Carolina put together a series of experiments to test for the transferal of bacteria. The undergraduates found that the practice of double dipping actually does transfer bacteria to another person.
The experiment used wheat crackers and three different dips: salsa, cheese dip, and chocolate syrup. The students also measured how much bacteria could transfer from the wheat crackers to a cup of water.
"We found that in the absence of double-dipping, our foods had no detectable bacteria present," Paul Dawson, a food microbiologist at Clemson University, wrote in The Scientific American. "Once subjected to double-dipping, the salsa took on about five times more bacteria (1,000 bacteria/ml of dip) from the bitten chip when compared to chocolate and cheese dips (150-200 bacteria/ml of dip)."
But double-dippers don't need to worry too much. Thousands of bacteria and viruses already live in the the oral cavity, and the majority of them pose no immediate health threat. Double-dipping probably poses no major threat to public or global health, but there are some diseases that can be spread through saliva, like the influenza virus or strep throat.
"We get exposed to germs in a thousand different ways," Peter Mehlman, who wrote the Seinfeld double-dipping episode, told the New York Times in 2008, after recalling his own experience with someone admonishing him for double-dipping. "Besides, I thought the dip was enough to kill anything. It was probably one of those '60s style dips with artificial dried onion soup."
If you want to keep your mouth free of bacteria from others, the best bet might be to just eat from your own separate bowl.