The hypothesis is proven to be false: participants tasted the bitter chocolates more acutely than the other foods. The tongue is the most sensitive to bitterness.
Tongue sensitivity is not the same for everybody. It depends on the number of taste receptors a person has and the number of taste buds in his/her tongue. Taste, however, is not confined to the sensation of the tongue. The smell, texture, color and temperature of the food also contribute its flavor and taste.
Use different types of foods.
Heat up the foods. Will the test results change?
Taste - http://en.wikipedia.org/wiki/Taste
How temperature influences our taste - http://www.medicalnewstoday.com/articles/35142.php
Basic safety requirements