Could sharing ketchup over french fries be an 'indirect kiss?'

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Sharing french fries with a friend can sometimes lead to a familiar moment: You take a bite, then plunge the same fry back into the communal ketchup.

It may seem harmless, particularly among close friends. Children and teenagers sometimes jokingly call sharing food this way an "indirect kiss." As it turns out, the joke may have a little science behind it.

Bacteria from the mouth can indeed make their way into a shared sauce when someone takes a bite and dips the food again, a practice known as "double dipping."

But how many double dips would it take to transfer roughly as many bacteria as a 10-second kiss?

The answer, based on a playful comparison of findings from separate studies, is around 800,000.

Double dipping can transfer oral bacteria

Researchers at Clemson University in the United States examined how much bacteria could be transferred when a person bites into food and then dips it back into a shared sauce. Their findings were published in the Journal of Food Safety.

The experiment confirmed that bacteria from the mouth can enter the sauce after double dipping. The researchers estimated that under conditions involving people actually sharing food, a single double dip could transfer roughly 50 to 100 bacteria between people.

That figure pales in comparison with what can happen during a kiss.

A separate study published in the journal Microbiome estimated that an intimate 10-second kiss can transfer an average of about 80 million bacteria between partners.

If one assumes that each double dip transfers 100 bacteria, a simple calculation suggests it would take about 800,000 double dips to transfer the same number of bacteria as a 10-second kiss.

The comparison is intended more as a fun illustration than a direct scientific equivalence. The studies examined different circumstances, and the types of bacteria, methods of transmission and experimental conditions were not the same.

Still, the research demonstrates that simply dipping partially eaten food back into a communal sauce can transfer oral bacteria to something other people may subsequently eat.

Salsa, cheese or chocolate: Which picks up the most bacteria?

The amount of bacterial contamination also differed depending on the sauce.

Immediately after double dipping, researchers detected around 1,000 colony-forming units per milliliter, or CFU/mL, in salsa. Cheese dip contained around 200 CFU/mL, while chocolate dip had about 150 CFU/mL.

CFU is a measure used to estimate the number of viable microorganisms capable of multiplying and forming colonies.

The researchers said differences in characteristics such as viscosity may help explain the results. A relatively thin sauce such as salsa can more readily drip off a cracker and back into the communal bowl, potentially carrying more saliva and oral bacteria with it.

However, bacterial levels in the salsa declined over time. The researchers said salsa's relatively high acidity may have made it more difficult for some bacteria to survive.

In other words, a large number of bacteria may initially enter the sauce without necessarily remaining there at the same levels.

What about sharing bottles, chopsticks and spoons?

Drinking directly from the same bottle can also leave substantial amounts of bacteria behind.

In a study published in the International Journal of Environmental Research and Public Health, 12 healthy adults drank unsweetened tea directly from plastic bottles. Immediately afterward, researchers detected an average of about 18,000 CFU/mL of bacteria around the bottle opening and about 8,000 CFU/mL in the remaining tea.

Chopsticks and spoons that have been placed in someone's mouth can similarly pick up oral bacteria.

A Clemson University study found that about 100,000 CFU of bacteria could be transferred to spoons or chopsticks after they were placed in the mouth. Researchers also confirmed that bacteria could subsequently move from the used utensils to food.

But a larger number of transferred bacteria does not necessarily mean a proportionally greater health risk.

What matters is not simply how many microorganisms are transferred, but which microorganisms are involved. If someone has symptoms of an infectious illness such as a cold or the flu, it is best to avoid double dipping into communal sauces or sharing bottles and eating utensils.

This article from Kormedi.com, Korea’s top health care and medical portal, was translated by a generative AI system and edited by The Korea Times.