Taste without Smell
Introduction: Taste, Smell, and Flavor
The ancient Greeks coined the words “taste” and “smell” to mean the sensations arising from the mouth and nose, respectively. But it is a little-known fact (to either the ancient Greeks or to people today) that the perception of food taste is determined to a large degree by olfaction, not gustation. In other words, you actually “taste” foods with both your mouth and your nose.
The figure at left shows how this works. The process of chewing and swallowing releases some food molecules into the air inside your mouth. Since your oral cavity is connected to your nasal cavity by a retronasal passage, many of these airborne food molecules waft up into the nose, where they make contact with the olfactory epithelium and are subsequently smelled. You may have noticed that foods taste rather bland when you have a cold—that’s because when your nose is stuffed, food particles have a hard time wafting up the retronasal passage to your olfactory epithelium.
To avoid further confusion, we’ll restrict our use of the word “taste” to the sensations arising from gustatory receptor cells in your mouth. We’ll use the word “flavor” for the cross-modal perceptual gestalt that arises when olfactory (as well as somatosensory and visual) responses to foods are added to the gustatory sensations.
In this activity, you’ll do a little self-experimentation to gain first-hand knowledge of the distinction between the taste (in our narrowly-defined sense) and the flavor of food. But before you start, you’ll need to make a quick trip to the grocery store and recruit a friend to help you.
Get some baby food and a helper and follow the instructions in the next section of the activity to experience the difference between taste and flavor.
Experiment Setup and Trials
To conduct this experiment, you first need to go to your local grocery store and spend about $5.00 on small jars of the following flavors of baby food (if you can, try to get “Stage 1” baby food, which is pureed as finely as possible):
The baby food will be the stimuli for your experiment. You will also need a helper (H) to play the part of the experimenter, and a small spoon.
Have H open all four jars of baby food and prepare the spoon.
You must then prepare yourself in two ways: close your eyes and hold your nose.
H should then click the START TRIAL button at left. He or she will then see the text “Have the subject taste: XXX,” and under that, “Then click the subject’s response below,” where XXX will be filled in with the name of one of our stimuli (apple, pear, banana, or carrot). H should then give you a small spoonful of the indicated food.
Your job is to try to determine what the food is. Tell H what you think it is (if you have no idea, just take a wild guess) and then he or she will click the picture at right that corresponds to your response. A NEXT TRIAL button will then appear. H should click this button and start the process over again.
For the duration of the trials, try not to open your eyes and do not release your nose. If you must release your nose, drink some water and wait at least a minute before starting the trials again. Also, H should not give you feedback, at least at the start.
Try to go through at least 12–24 trials—go as long as you can stand keeping your eyes closed and nose held shut. Then view your results (you can always come back and do more trials after you’ve looked at your results for the first time).
The table at left shows the results of your experiment. Look first at the bottom row, which shows the percentage correct for guessing each food. If you guessed randomly every time, you would be correct on 25% of the trials, on average. If you kept your nose and eyes completely closed during the trials, your percentages may not be much higher than this chance rate.
As you’ll learn in this chapter, there are only four basic tastes: sweet, salty, sour, and bitter. Without retronasal olfaction, every food’s flavor is just some combination of these four taste qualities. (Holding your nose doesn’t block the retronasal pathway directly, but stopping the airflow out of the nose will effectively stop most food molecules from moving up into the nasal passage.) Furthermore, the baby foods that you are tasting should all be low in salt and should contain few sour- or bitter-tasting molecules (baby food is designed to taste bland).
All that was left to taste during the experimental trials was sweetness. You should have detected this quality in all four foods, and if you taste each food now with your eyes open (but your nose still closed), you should find that there is a definite ordering in the sweetness of the four foods: the banana should be sweetest, followed by the apple and pear, with the carrot tasting the least sweet.
This is why you had to keep your eyes closed and not get feedback during the experimental trials. With feedback, you could have learned to associate the sweetest taste with banana and the least sweet taste with carrot. You also could have learned to identify the foods on the basis of their texture (detected by somatosensory receptors in your mouth). Actually, if you did enough trials and thought about it enough, you may have made guesses about relative sweetness and texture that allowed you to do better than chance.
Now, hold your nose one more time and put a spoonful of the banana in your mouth. Keep it there for a few seconds, experiencing the pure sweetness of the food. Then stop holding your nose. Odorant molecules will instantly travel up through the retronasal pathway to your olfactory epithelium, and the true “banana-ness” of the food will be revealed!