Is Double-dipping a Food Safety Problem or Just a Nasty Habit?

Paul Dawson, Clemson University

What do you do when you are left with half a chip in your hand after dipping? Admit it, you’ve wondered whether it’s OK to double dip the chip.

Maybe you’re the sort who dips their chip only once. Maybe you look around the room before loading your half-eaten chip with a bit more dip, hoping that no one will notice.

If you’ve seen that classic episode of Seinfeld, “The Implant,” where George Costanza double-dips a chip at a wake, maybe you’ve wondered if double-dipping is really like “putting your whole mouth right in the dip!”


‘You doubled-dipped the chip.’

But is it, really? Can the bacteria in your mouth make it onto the chip then into the dip? Is this habit simply bad manners, or are you actively contaminating communal snacks with your particular germs?

This question intrigued our undergraduate research team at Clemson University, so we designed a series of experiments to find out just what happens when you double-dip. Testing to see if there is bacterial transfer seems straightforward, but there are more subtle questions to be answered. How does the acidity of the dip affect bacteria, and do different dips affect the outcome? Members of the no-double-dipping enforcement squad, prepare to have your worst, most repulsive suspicions confirmed.

Start with a cracker

Presumably some of your mouth’s bacteria transfer to a food when you take a bite. But the question of the day is whether that happens, and if so, how much bacteria makes it from mouth to dip. Students started by comparing bitten versus unbitten crackers, measuring how much bacteria could transfer from the cracker to a cup of water.

We found about 1,000 more bacteria per milliliter of water when crackers were bitten before dipping than solutions where unbitten crackers were dipped.

In a second experiment, students tested bitten and unbitten crackers in water solutions with pH levels typical of food dips (pH levels of 4, 5 and 6, which are all toward the more acidic end of the pH scale). They tested for bacteria right after the bitten and unbitten crackers were dipped, then measured the solutions again two hours later. More acidic solutions tended to lower the bacterial numbers over time.

The time had come to turn our attention to real food.

But what about the dip?

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