“Double-duping” Research Results

February 10, 2008

A research study on “double-dipping” has gained a lot of media attention despite the fact that its methodology was dubious.

Double-dipping is the act of dunking or swiping a chip in a dip after a bite has already been taken. It has commonly been assumed that this would contaminate the dip and spread germs among snackers. Professor Paul L. Dawson, a food microbiologist, conducted the research with his undergraduate students at Clemson University. He expected to find little or no contamination, that is “microbial transfer.” He was surprised to find the opposite was true.

Well, he should not have been surprised given that his methodology was flawed. Volunteers were instructed to take a bite of a wheat cracker and then to dip the cracker for 3 seconds into a tablespoon of test dip. This process was repeated three to six double dips per tablespoon sample.

There were six test dips: sterile water with three different degrees of acidity, a commercial salsa, a cheese dip and chocolate syrup. In each case, the dip remaining at the end of the double-dipping was analyzed for aerobic bacteria. They made no attempt to count anaerobic bacteria or viruses nor did they establish whether or not the bacteria were harmful. On average, the students found that three to six double dips transferred about 10,000 bacteria from the eater’s mouth to the remaining dip.

There are a number of problems with this procedure:

1. There were no control groups. For each type of dip, there should have been a comparison made with dip that had not been touched by chips as well as dip that had only been single dipped. In other words, the real question is how much contamination does double-dipping add over and above what would be present as a result of single dipping or no dipping?

2. An obvious issue is whether the germs in the dip were harmful. Bacteria are present just about everywhere; their presence in the dip is not necessarily a health hazard.

2. A tablespoon of dip was employed. When was the last time you shared a tablespoon of dip with anyone? By limiting the dipping area, the researchers may have inflated the concentration of bacteria in the dip.

3. The repeated dipping of the chips for the extended length of time, of three seconds, employed by the researchers very likely produced results that were biased. In the real world, swiping or dipping usually takes no more than a second, if that. In this study, the combination of a small amount of dip with a long duration of dipping may have increased the likelihood of contamination far more than would occur in a naturalistic setting.

The kind of dip made a difference in the amount of contamination that resulted but that, I believe, is irrelevant given that the study itself was tainted. The study needs to be replicated with more realistic double dipping for the results to be considered reliable and valid. In 6 months, this study may be published in the Journal of Food Safety, not withstanding its questionable worth. Unfortunately, the media in its endless promotion of the sensational have given the “findings” far more attention than they deserve.


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