Double dipping: the new challenge for health and safety

BMJ 2008; 336 doi: (Published 07 February 2008) Cite this as: BMJ 2008;336:297
  1. Fred Kavalier
  1. 1London

    The pervasive influence of evidence based medicine is about to invade tapas bars and cocktail parties.

    Scientists from Clemson University in South Carolina have discovered that double dipping, the practice of dipping a single tortilla chip into the guacamole more than once, is a good way to transfer bacteria from one person to another.

    Double dipping first came to prominence in the 1990s in the US television series Seinfeld. A character in the series is confronted with the accusation, “Did you just double dip that chip?” after he was spotted dipping the same chip twice. Timmy, the accuser, then says, “That’s like putting your whole mouth right in the dip.”

    Paul Dawson, a food microbiologist at Clemson University, decided to test the hypothesis that double dipping might be a true microbiological or health hazard. He encouraged a group of students to design an experiment to see whether bacteria were transferred from mouth to dip during the cracker’s second visit to the dip. The results have been accepted for publication in the Journal of Food Safety.

    The students used wheat crackers with six different test dips. Three of the dips were sterile water with varying degrees of acidity. The other dips were salsa, cheese dip, and chocolate syrup.

    The participants took a bite of the cracker and then dipped it for three seconds into a tablespoon of test dip. This was repeated up to six times, each time with a new cracker.

    The results were definitive: three to six immersions of the cracker managed to transfer about 10 000 bacteria from the mouth to the dip. The researchers estimate that sporadic double dipping at a real life party would, on average, transfer 50 to 100 bacteria from person to person per bite, depending on the size of the bowls and the consistency of the dips.

    Some dips seem to be riskier than others. Salsa allowed more bacteria to be transferred than the cheese dip or chocolate sauce.

    Peter Mehlman, the Seinfeld writer responsible for the original double dipping episode, told the New York Times writer Harold McGee ( that he had some sympathy with the double dipper. “We get exposed to germs in a thousand different ways,” he said. “Besides, I thought the dip was enough to kill anything. It was probably one of those ’60s style dips with artificial dried onion soup.”

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