The researchers then put the remaining 25 participants through six tasting sessions in which the concentration of caffeine in the drinks was increased. All sessions used cola from the same batch, and each consisted of 25 trials in which tasters compared two samples of cola solution labeled "A" and "B."
For the first five trials of each session, the tasters were put through their paces in a "warm-up," in which they were informed about whether they were tasting sample A or B as a means of heightening awareness of flavor differences (they were not, however, told which contained caffeine). The study subjects then downed each of two samples, with each tasting separated by a rinse with bottled spring water.
At the lowest caffeine concentration, none of the tasters could tell the difference between the two samples. At a level of caffeine similar to or slightly higher than that found in either Coke classic or Pepsi, only two of the 25 were able to discriminate between the caffeinated and decaffeinated versions. It wasn't until tasting colas with caffeine concentrations far above those allowed by the FDA that the remainder of the subjects began noticing a difference.
In its response, the NSDA asserts that 25 people is too small a test group to draw any meaningful conclusions. But, as Griffiths tells WebMD, "almost all of the taste studies that have reported taste-detection thresholds with caffeine have been done with sample sizes smaller than 25, including studies that the soft drink industry cites to support their claim [that caffeine improves the flavor of the soft drink]."
The NSDA also claims that by taking 50 samples at one sitting, the tasters might have suffered from "flavor fatigue," and that the six participants who were cigarette smokers "should have been disqualified because most smokers have desensitized taste buds."
"Too few people were tested, too little science was used in the testing and too much opinion is contained in the conclusions," Nedelman says in the written statement. "This is an exercise in scientific self-promotion, not a meaningful presentation of scientific evidence. It is yet another personal attack on the industry by a researcher who has a personal axe to grind when it comes to caffeine."