Taste and texture

Taste is a notoriously difficult sense to study. My son Jim can't stand baked potatoes, but I can't get enough of them. I don't like watermelon, but the rest of my family gobbles it up. Even more perplexingly, I do like watermelon candy. With all the individual differences in taste, how can scientists learn anything specific about how the sense works?

The difficulties in taste study are compounded by the fact that taste is intimately associated with the sense of smell. Every kid knows to plug his nose when trying a food he or she doesn't like. Researchers must be constantly aware that differences in taste may also be due to smell. So when they want to answer a question like "how does the texture of a food impact its taste?", they know it will not be a simple matter to explore.

A group of scientists led by David Cook wanted to examine just that question. Initially, the problem seems simple enough: just cook up a bunch of food with different textures but the same amount of flavoring, and see how people perceive the taste. But then the problems come in: how do you vary texture? Will the texturing agents themselves change the food's flavor? What about individual differences between tasters? Do foods with different textures smell different?

The team started by trying two different flavors: sugar and iso-amyl acetate—a banana flavoring. They diluted each flavor in water, and then progressively varied the viscosity of each solution by mixing with three different tasteless thickening agents: guar gum, carageenan, and hydroxypropylmethyl cellulose (HPMC). (Now you know what about half the ingredients listed on your Hostess Twinkie are for!)

First they controlled for smell having taster smell and taste each sample and then place their nostrils on a device that physically measured the concentration of odorant that remained in their breath. In this way, they could determine how much odorant was present while the samples were being consumed, instead of relying on the tasters' perception of smell. They found no systematic difference in odor across the range of samples.

Next, a set of tasters who were trained and had two or more years' experience tasting food tried each sample, cleansing their palate with crackers and water between tastings, and rating them separately for "banana flavor" and "sweetness." As previous research had indicated, perception of flavor decreased as the solutions thickened. But why? Earlier studies had suggested that flavor diminished in proportion to the concentration of the thickening agent. While Cook et al.'s data confirmed this, they obtained different results for each thickener:

As the concentration of thickener increased, the perceived sweetness of solutions thickened with carrageenan decreased less than the other solutions. Perhaps concentration of thickener was not the best way model the impact of a thickener. So the team turned to the model of Jozef Kokini, who in the 1970s and 1980s developed a mathematical representation of how the mouth determines the viscosity of a substance. There's no reason to believe humans have detectors for carrageenan concentration in their mouths, but we do have nerves that can detect the sensation of touch. Perhaps we approximate viscosity by pressing food against the roof of our mouths with our tongues. Kokini developed a complex mathematical formula to model this method of determining "oral shear stress," or the amount of force it takes us to compress a thick liquid in our mouths. Cook et al. then applied this formula to their solutions and compared it to perceived sweetness:

Now, the results for each thickener follow a nearly identical path. Similar results were obtained for the banana flavoring.

Cook et al. conclude that we not only consider information from our taste buds and sensory organs in our noses, but also the feeling of the food in our mouths to determine flavor. While this research doesn't explain why Jim doesn't like potatoes, it does get us closer to understanding the many processes involved in the sensation of taste.

Cook, D. J., Hollowood, T. A., Linford, R., & Taylor, A. J. (2003). Oral shear stress predicts flavour perception in viscous solutions. Chemical Senses, 28, 11-23.