Taste + Smell = Crazy Delicious!

Synesthesia -- the ability to experience a sensation like vision in another mode, like hearing -- is thought to be quite rare. Yet all of us have the ability to combine sensory modes, and we do it every day. The modes we combine just happen to be ones we don't think about as often: taste and smell.

While vision gets the lion's share of attention in perception research, research on olfaction and taste has begun to be more prominent. However, though we know that the senses of taste and smell interact, few studies have explored the interaction between the two sensory modes. The problem is that chemical reactions can occur within the nose and mouth which actually affect the smell and taste of the substance being consumed, so it's unclear how to measure the impact of aroma versus taste on overall flavor.

A team led by Johann C. Pfeiffer has perfected a method to study the interaction of smell and taste while controlling for any chemical changes in the substance being tested: they measure the actual chemicals present within the taster's nose. If a chemical reaction occurs during tasting, the researchers can take this into account in their analysis.

The human mouth is sensitive to only a few different tastes, including sweetness, acidity, and saltiness. The nose, however, can detect a large number of subtle variations. Pfeiffer's group took a look at how sweetness and acidity affected the perception of a strawberry flavored drink. To simplify matters, the researchers kept the strawberry aroma constant throughout the study, but varied the sweetness and acidity of the beverage.

They used 11 trained food tasters for the experiment: these individuals had been trained at least four years to identify and distinguish between specific aspects of food taste and aroma. Five different mixtures of were used. In every case 0.18% of the commercial strawberry aroma was used, but it was mixed in water with different amounts of sucrose and acid. The tasters drank each sample, then reported on the perceived overall level of strawberry flavor (the combined perception of aroma and taste). Here are the results:

When the strawberry aroma was presented without sucrose or acid, almost no strawberry flavor was detected. Adding sucrose or acid alone increased the strawberry flavor intensity, even though no additional aroma had been added. The combined effect of acid and sucrose caused a significantly more intense strawberry flavor perception.

The researchers used atmospheric pressure chemical ionization-mass spectrometry (APcI-MS) to measure the concentration of chemicals within the nose during tasting (I expect this was rather awkward for the tasters, because it required them to exhale into a tube after each taste). They found no difference in the concentration of chemicals known to be responsible for strawberry aroma, regardless of the level of acid and sucrose in the samples. Thus, the researchers argue, there must be a cross-modal effect: sucrose and acid levels combine with strawberry aroma to create a more intense strawberry flavor.

An interesting aside: three of the tasters weren't sensitive to sucrose levels. Though they were perfectly capable of tasting sucrose, only the acid level affected their perception of strawberry flavor. So, like synesthetes, different tasters appear to have different cross-modal effects. What's less clear is whether these differences among tasters reflect real physical differences or learned associations between tastes and flavors.

Pfeiffer, J.C., Hort, J., Hollowodd, T.A., & Taylor, A.J. (2006). Taste-aroma interactions in a ternary system: A model of fruitiness perception in sucrose/acid solutions. Perception & Psychophysics, 68(2), 216-227.