What is Flavor – A lesson in Orthonasal Olfaction

Flavor is a Jedi trick of the mind, a combination of the gustatory sense of taste and the olfactory sense of smell that your brain fuses into a new sensation. To give you an idea of just how clever your brain is about flavor, consider this: your brain detects odors differently based on whether you are breathing in or out. This is crazy! It’s like saying swiping your hand left to right on a cold countertop causes you to feel temperatures differently than swiping right to left. Our brains are wired to process smell signals in two different ways; flavor uses the second way.

Some definitions will make this easier to discuss. Orthonasal olfaction is defined as what your nose detects from sniffing something that exists in the world. Sniffing a rose, unless you’re also chewing it, uses the orthonasal route for smell. Retronasal olfaction is what your nose detects in the foods you eat when air is taken in from the mouth and circulated up to your nasal cavity. Even if you don’t notice it happening, it is! Try chewing food with your nose pinched: cut off the airflow, and poof, the flavor sensation’s gone.

To unravel this trick of the brain, a researcher, Paul Rozin, gave subjects unfamiliar fruit juices and soups via the orthonasal route—“Here, sniff this; remember this odor”—and then gave the foods to the subjects again via the retronasal route (through a plastic tube), asking them to identify the previously remembered odor. They did horribly. Same compound, same sensory apparatus, completely different experience. As I promised, smell is simple in the abstract but complicated in the details, so it follows that flavor is no different.

From a practical perspective, which flavors you’ll like or dislike is a matter of exposure and preference. Rozin started studying the orthonasal and retronasal issue when stumped by stinky cheeses—how is it that we have a different experience of flavor for something that smells disgusting? There’s a lot that psychologists and physiologists are still exploring. Fortunately, you needn’t be one to cook a good meal. When working with food, keep in mind that flavor is a specific combination of the two senses of taste and smell, but not a straightforward summation of the two. Taste the food to adjust its flavor before serving it! Smelling alone isn’t enough.

Here are some tips for great flavor when cooking:

  • Chew! Admittedly an odd suggestion for good flavor, chewing food crushes, mixes, and kicks up a bunch of compounds for your olfactory system to detect, adding smells that fold into flavor sensation. Remember, for a compound to activate an odor receptor, it has to be present at the point of detection. This raises the question: does chewing food with your mouth open lead to a different flavor experience? (If animals always chew with their mouths open…)
  • Use fresh herbs. Most dried herbs have weaker flavor because the volatile oils that are responsible for the aromas oxidize and break down, meaning that the dry herbs are a pale substitute. Dried herbs have their place, though; it makes sense to use them in the dead of winter when annual plants like basil aren’t in season. Store dry herbs in a cool, dark place (not above the stove!) to limit their exposure to heat and light, which contribute to the breakdown of organic compounds in spices. Grind your own spices. Don’t used preground black pepper; it loses much of its flavor over time as many of the volatile compounds change. Fresh-grated nutmeg is also much stronger than preground nutmeg. The aromatics in a preground spice will have had time to either hydrate or oxidize and disperse, resulting in flavor changes. Most dried spices also benefit from being bloomed—cooked in oil or a dry skillet under moderate but not scorching heat—as a way of releasing their volatile chemicals without breaking them down.
  • Don’t discount frozen ingredients. Commercially frozen vegetables and fruits are convenient and work fine in some dishes. Freezing produce right when it is harvested has advantages: nutritional breakdown is halted, and the frozen item is from the peak of the season with maximal flavor (whereas the fresh version in your store may have been harvested early or late). Frozen produce is especially useful if you’re cooking for just yourself: you can pull out a single portion as needed. Want to freeze your own crop or a surplus from a CSA (community-supported agriculture) food share? See page 365 of my cookbook (you can buy it here) for how to use dry ice. (Freezing in your home freezer takes too long and leads to mushy veggies.)
  • Use alcohol in cooking. My favorite restaurant in San Francisco uses kirschwasser in its fruit soufflés, and adding a splash of wine in sauces or to deglaze a pan to make a quick sauce is standard practice. Using alcohol changes flavors because of its chemistry: it takes the place of water molecules normally attached to compounds, resulting in lighter molecules that are more likely to evaporate, and with higher evaporation rates there are more volatiles for your nose to detect.

*See Also: Taste Aversions