The Science of Double-Dipping: A Health Risk or Just Gross?
Double-dipping a chip sure sounds gross, but can enough germs really be transferred to make you sick? What does science have to say about double dipping?
Sabrina Stierwalt, PhD
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The Science of Double-Dipping: A Health Risk or Just Gross?
Around the holidays we tend to like to gather…for holiday parties, family get-togethers, and end-of-the-year work functions. These gatherings often mean delicious food, especially gooey bowls of dip like salsa, ranch, cheese, or my favorite, spinach artichoke. All of this togetherness and shared food, unfortunately, coincides with the middle of flu season. And with every bite of tasty dip, you run the risk of having been preceded by the infamous double-dipper.
The double-dipper is the person who will re-dip their chip (or piece of bread or vegetable stick) back into the communal bowl of dip after having bitten into it, thus transferring their saliva, and any germs that come with it, also back into the bowl. Swapping saliva with a dozen other people at the party who share the same taste in dip as you sure sounds gross, but can enough germs really be transferred via double-dipping to cause someone to get sick? What does science have to say about double-dipping?
Does Double-Dipping Spread Bacteria?
The most thorough, and often quoted, study on double-dipping in the laboratory was conducted by researchers at Clemson University and published in 2009 under the title “Effect of Biting Before Dipping (Double-Dipping) Chips on the Bacterial Population of the Dipping Solution” in the Journal of Food Safety. The experiment was done in three parts.
First, researchers asked participants to repeatedly dip crackers or chips into sterilized water both before and after biting the chip. They found significantly higher levels of bacteria (1,000 more per milliliter) in the water after double-dipping had occurred. Next, the researchers again used sterilized water as a “dip” but this time they varied the pH or acidity levels of the water. Again, they found more bacteria in the water dip after a chip had been double-dipped. They also found that the more acidic watery dip had lower levels of bacteria—both immediately after double-dipping took place and two hours later, compared to the other, less acidic watery dips.
Finally, participants no longer had to eat chips dipped in water but instead tested salsa (All Natural Tostitos Chunky Hot Salsa, to be precise), chocolate sauce (Genuine Chocolate Flavor Hershey’s Syrup), and cheese (Fritos Mild Cheddar Flavor Cheese Dip). In these cases, researchers found that double-dipping transferred five times more bacteria to the salsa than the other two dips. However, after two hours at room temperature, the bacteria levels in the salsa were back down to levels lower than those in the other dips.
Their experiment thus proves that double-dipping increases the amount of bacteria in the communal dip bowl. How much bacteria depends on the acidity of the dip and how long the dip sits out at the party, with more acidic dips like salsa being more effective at killing off the bacteria more quickly.
Does Double-Dipping Spread Disease?
While the results are clear on the transfer of bacteria through double-dipping, the question of whether double-dipping actually spreads disease still remains. First, there are plenty of nasty illnesses you can get at a party that are viral in nature and not bacterial, like, for example, the flu. (That’s part of what makes it so challenging to get the most effective flu vaccine each season.) You’re likely to be more at risk from a sneezing or coughing coworker who is clearly already sick than a potential germ transfer via some rich and creamy ranch dressing.
Second, we are covered in bacteria pretty much all of the time so unless you didn’t touch a door knob, shake anyone’s hand, or use your phone before you picked up that chip in the first place, it doesn’t matter if a double-dipper got there first. You’ve already transferred more bacteria to that chip than the dip ever could.
The Most Infamous Double-Dipper
The most infamous suspected double dipper may very well be Mary Mallon, more familiarly known as Typhoid Mary. She was an Irish American chef and also the first person in the U.S. known to be an asymptomatic carrier of typhoid fever. (In other words, she could pass on typhoid fever while displaying no symptoms herself.) She is suspected of having infected more than 50 people with typhoid fever in the late 1800s and early 1900s via her cooking. It has not been confirmed that she tasted her own dishes, but it is highly likely that she was able to spread typhoid fever in part due to double-dipping.
Mary was known for her lack of cooperation, including using aliases and refusing to admit to her possible role in the infections. She ultimately spent almost three decades of her life in forced isolation and we now use the term “Typhoid Mary” colloquially to describe anyone who unwittingly passes along diseases.
So while you may want to avoid that shared dip if you suspect double dipping has occurred, your best protection against the flu is getting your flu shot. And if you’re the one who is sick, or may have been exposed to a sick person recently, don’t be the “Typhoid Mary” of your holiday party and stick to single dipping that chip.
Until next time, this is Sabrina Stierwalt with Ask Science’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Ask Science on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.comcreate new email.
