Is Double-Dipping a Food Safety Concern?

Everyone has been in the position where they have been left with a half a chip or breadstick in your hand after dipping. You most likely have wondered if it is OK to double dip. Whether you never double dip or think anything of it, is double dipping like putting your whole mouth right in the dip? Is this habit simply bad manners or are we actually contaminating dip with bacteria by double dipping?

A team of undergraduate researchers at Clemson University decided to answer this question. They created an experiment to find out if there is bacterial transfer when you double dip. They wanted to see if double dipping caused more bacterial transfer, but they also wondered if the acidity and type of dip affect the outcome.

They first started by comparing bitten versus unbitten crackers and how much bacteria would transfer to a cup of water from the cracker. The study found that there were 1,000 more bacteria per mL of water when the cracker was bitten versus unbitten. Next they wanted to test if pH levels affected the bacteria levels. They used bitten and unbitten crackers in water solutions with pH levels of different typical food dips. They used pH levels of 4, 5, and 6 (which are all near the acidic end of the pH scale). After dipping bitten and unbitten crackers into the water and measuring the bacteria levels and again measuring the solutions after 2 hours, they found that the more acidic solutions tended to have less bacteria over time.

Now the researchers wanted to move to real food. They compared 3 types of dips: salsa, chocolate and cheese dips. All three types of dips differ in pH and viscosity. They used All Natural Tostitos Chunky Hot Salsa (pH 4), Genuine Chocolate Flavor Hershey’s Syrup (pH 5.3) and Fritos Mild Cheddar Flavor Cheese Dip (pH 6.0). They again used bitten and unbitten crackers, measuring the solutions immediately and after 2 hours of waiting in order to see how the bacteria populations were behaving. When they did not double dip (unbitten crackers) the foods had no detectable bacteria present. But once a double dipped cracker was used (bitten cracker) the salsa had about 5x more bacteria than the chocolate cheese dips. The salsa contained 1,000 bacteria/ml of dip; the chocolate had 150 bacteria/ml of dip; the cheese had 200 bacteria/ml of dip. But it is also important to note that the salsa bacterial numbers fell to about the same levels as the chocolate and cheese after two hours.

These results can be explained pretty easily. Both chocolate and cheese dips are thick and have a higher viscosity than the salsa. The lower viscosity of the salsa causes more of the dip touching the unbitten cracker to fall back into the dipping bowl. This allows for more bacteria to be present. Also since salsa is acidic and most bacteria do not like acidic environments, after 2 hours the acidity of the salsa has killed some of the bacteria lowering the levels over time.

We can conclude from this study that double dipping leads to more bacteria levels in dips. So if you see double dippers you might want to steer clear from their dip.

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