In Too Afraid to Ask, we’re answering food-related questions that may or may not give you goosebumps. Today: Does double-dipping spread germs?
It was a 1993 episode of Seinfeld that inspired Paul Dawson, PhD, to study double-dipping. In “The Implant,” George double-dips a chip at a wake and is razzed by Timmy, who tells him that it’s like “putting your whole mouth right in the dip.” Dawson, a food scientist at South Carolina’s Clemson University, wanted a real-world research example so that he could to teach his students how to conduct studies, and Timmy’s line gave him “a hypothesis to test,” he tells me.
Dunking a carrot into a bowl of hummus, taking a bite, then dunking again, seems harmless enough when you’re by yourself. It’s your carrot; it’s your hummus. (Who hasn’t double-dipped from the comfort of their own couch?) But in social settings, double dipping has long-been the subject of debate.
Nobody cares what you do at home, says Nick Leighton, an etiquette expert and host of the Were You Raised by Wolves podcast. “If you’re eating guacamole alone, then have at it,” he says. “But once other people are involved, the rules of etiquette apply.” The way we behave in public sends signals to those around us, says Leighton, and double-dipping makes people nervous. “I don’t want your mouth on the food I’m about to eat,” he says. To him, double-dipping creates trust issues. “The mind wanders,” he says. “What else are you capable of doing?”
Scientifically, though, was Timmy right in scolding George for double-dipping at the wake? “Absolutely,” says Dawson, who conducted various experiments with his students to study the germ trail left behind by double-dippers, like measuring the bacteria left behind after dipping bitten crackers in various dips. “There were between 100- and 1,000-times the number of bacteria transferred to the dip when a chip was bitten before dipping compared with when the chip was dipped without biting first.”
Here’s what you need to know about double-dipping—and how you can politely prevent Uncle Bill from contaminating the queso.
What are the health risks associated with double-dipping?
The primary concern with double-dipping revolves around the potential transfer of oral bacteria and saliva from an individual’s mouth back into the communal dip, via some sort of dunking vessel.
“The mouth is home to a number of bacteria” that are perfectly normal, says Harjot Singh, MD, an infectious disease specialist at Weill Cornell Medicine. That said, not everything lurking between your molars is quite as benign. Streptococcus, depending on the species, is common and can cause cavities. There are also oral bacteria and viruses in the mouth that could spread via saliva, Dawson says, citing meningitis, pneumonia, influenza, and more. COVID falls into this category too.
In his 2009 research on the microbial consequences of double-dipping, which remains the most widely referenced study on the practice, Dawson found that the practice does transfer bacteria from mouth to dip. Initially, students assessed the bacterial transfer from a cracker to a cup of plain water. They found that the chomped cracker dipped into the water yielded approximately 1,000 more bacteria per milliliter than an unbitten cracker.
Then, the researchers submerged crackers in water with pH levels mirroring those typical of a dip at a party spread (which lean toward the acidic side of the pH spectrum). They examined the bacterial content of the water immediately after the crackers were dipped, then after a two-hour interval. There were fewer bacteria during the second measurement: The acid in the water had worked “to kill some of the bacteria that contaminated it,” says Donald Schaffner, PhD, a food scientist at New Jersey’s Rutgers University who wasn’t involved in the study.
Lastly, the focus shifted to various dips: salsa, chocolate syrup, and jarred cheese dip, each differing in pH and viscosity. The researchers found that when “clean” crackers were used, the dips harbored negligible additional bacteria. However, after being exposed to a double-dipped cracker, the amount of germs harbored in each dip also differed: Salsa accumulated approximately five times more bacteria than the chocolate and cheese dips. And two hours later, the bacterial counts in the salsa reduced to levels similar to the other dips.
This can be explained by a couple of factors: A thin dip like salsa is more likely to drip from your bitten chip back into the communal dip, dragging your lurgies along with it. “In a thicker food the bacteria are going to be more immobilized and not spread throughout the dip,” Schaffner says. But salsa is also more acidic than the others, obliterating some of the bacteria over time. At the end of the day, it’s the thickness of the dip and its acidity that really determines how many germs end up partying in the hummus bowl. But that doesn’t mean you should double-dip the salsa; the differences in risk are somewhat marginal and it’s still feral behavior, okay?
What other factors might influence germ spread?
People with higher bacterial loads in their mouths are more likely to introduce a larger number of germs if they double-dip—and the dip’s temperature can also impact their growth. Food safety experts refer to temperatures that fall between 40°F and 140°F as “the danger zone,” which offers a more favorable environment for bacteria to multiply; room temperature in the US is “often set somewhere in the range of 68 to 74 degrees Fahrenheit,” per Epicurious. Storing a dip at temperatures below or above this range “would mitigate the growth of the bacteria but would not necessarily eliminate contamination by a double-dipper,” says Dawson, nor would they “kill the bacteria quickly enough to prevent it from being picked up by another dipper.”
So, should I be worried about double-dipping?
“There is no definitive science to prove that double-dipping is unsafe or harmless,” says Singh, the infectious disease doctor. “But the magnitude [of risk] is not enough to be a public health concern.”
Contextually speaking, the numbers of bacteria that end up in a communal dip are low. We all have about 80 billion bacteria in our mouths, “and about 1 million of them are transferred with a single kiss,” says Singh. Even if you’re at a double-dippers convention, it’s unlikely the guac would be harboring anywhere near the number of bacteria in our own mouths (or those of our crushes’).
The risk depends on if any double-dippers are carrying a potentially contagious bacteria or virus and on the strength of the immune system of those that dip afterward, says Dawson. Assuming the bacteria transferred to a dip are benign, it’s not a big deal. “If I have harmless bacteria in my mouth, does it really matter if they end up in your mouth?” asks Schaffner, the Rutgers professor. “That is certainly gross but not risky.”
Still, to completely avoid the potential uninvited bacteria from double-dippers, Singh says prevention is simple: Encourage “single-dipping.”
How can you stop people from double-dipping?
“If you feel like your guest list is full of people that are capable of such a monstrous act, then let’s not even tempt them to double dip,” says Leighton. Providing serving spoons or small plates and ramekins can discourage direct mouth-to-dip contact, by implying guests should, ahem, use the dishes and utensils provided. “You get to set the rules and it doesn’t take much to guide people in a direction that you would like them to go,” says etiquette expert Elaine Swann.
Leighton’s on the same page, adding that guests should always transfer communal foods onto their individual plates—rather than dunking directly from the shared resource. “Ideally, there’s a spoon in the dip, and we’re going to scoop what we want from the communal dish onto our personal dish,” he says, “and then harmony is restored to the universe.”
