Reprinted by permission of McGill University Office for Science and Society.
Although the study isn’t new, its message needs to be reiterated — taking a bite out of something that has already been bitten once and then dipping it into something is by no means a sanitary practice. A group of undergraduate students at Clemson University in South Carolina investigated whether dipping a piece of food into several different dips would transfer bacteria into the dip, and to what effect the type of dip had on the extent of bacterial contamination. They examined three different dips: a chocolate sauce, a salsa, and a cheese dip. The salsa was the most fluid of the dips, and also the most acidic.
To start, the investigators (eight students) dipped crackers into sterile water before and after biting them, and found that the bitten crackers indeed transferred more bacteria than the whole crackers (the crackers themselves were not sterilized). They further tested the effect of acidity (pH) on bacterial growth, and found that the water with the lowest pH (the most acidic) both initially and after two hours post-dipping had the fewest bacteria.
Finally, they tested the real sauces. And they found that although more bacteria were initially transferred to the salsa — probably because it was more liquid than the others — two hours later, when looked at again, there were fewer bacteria. Perhaps because it was also the most acidic of the dips.
Thus, you can, indeed, transfer bacteria from bitten crackers to dips. And, although it wasn’t tested in this study, it’s unlikely that simply turning the chip around so that the unbitten portion enters the dip is not a good fix. Although mouth bacteria wouldn’t transfer, hand bacteria assuredly would. So one scenario might be the following: a party-goer doesn’t wash his or her hands after using the restroom, then uses the dip, turn and dip again technique. Not so great.
Perhaps the best advice is to put your favourite dip on a plate and eat from that — sparing both yourself and others from the dangers of unsanitary double-dipping!
The original McGill University OSS post can be found here.