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This article about testing for Alzheimer's, via changes in the ability to smell, said:

She thought of peanut butter because, she said, it is a “pure odorant” that is only detected by the olfactory nerve and is easy to access.

I'm not clear on what that means. Assuming a person had their mouth closed, then wouldn't any odor be detected only by the olfactory nerve? Wouldn't a lemon, say, work exactly the same?

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I could not find a concrete definition after searching several sources, but after reading a few papers I think the term Pure Odorant refers to its Olfactory Bulb profile. Peanut Butter will stimulate specific Olfactory nerves, and only that set of nerves, whereas impure Odorants may stimulate part of the profile and also other parts of the Bulb depending upon the impurity.

Basically, Peanut Butter has a unique smell that you can't replicate, and thus cannot mistake in either flavor or scent when compared to similar compounds. You may not even be able to tell it was Peanut Butter at all if not for the scent.

A Lemon would not work the same way, as the major Odorant in a Lemon is Limonene - which is present in all citrus fruits. You would activate the same neuronal pathways for Lemons that you would with Oranges, Limes, and Grapefruits.

That's my best assessment.

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  • $\begingroup$ So you're saying that peanut butter stimulates only a small section (of the Bulb, whatever that is), whereas other scents will trigger multiple nerve sections, and so can be faked with a different set of odorants? $\endgroup$
    – John C
    Commented Oct 13, 2013 at 17:33
  • $\begingroup$ @JohnC - More or less. Peanut Butter has a unique scent and is readily available. Using purified Limonene would work, as I'm sure there's a nerve profile for Limonene - but the purified product isn't readily available. The benefit of Peanut Butter is specificity. It's a known scent (to people in countries that like peanut butter), and a unique scent that's instantly identifiable. You could confuse oranges with limes or lemons by scent, but a healthy person will always recognize peanut butter as peanut butter. It tests sense of smell and cognitive function simultaneously. $\endgroup$
    – MCM
    Commented Oct 13, 2013 at 22:07
  • $\begingroup$ Not sure I really understand why a "pure odorant" was needed for the test, then. All they were doing, was measuring the distance at which the odor could be detected, not what it was. While I can see that peanut butter is useful, for non-odor reasons (sticks to a spoon, etc), I still don't see why a lemon (or lemon juice on a Q-tip) wouldn't have worked just as well. Possibly that line was just intended to sound more scientific than, "it sticks to the spoon." :) $\endgroup$
    – John C
    Commented Oct 13, 2013 at 22:55
  • $\begingroup$ @JohnC - There may not have been a very good reason. They wanted something with a unique smell that was cheap, and Peanut Butter was probably a convenient option. More important than the smell is the caveat that the lack of smelling abilities found in the study can be attributed to more than just Alzheimer's. It's a quick and dirty early signal, but it also has a pretty high false-positive rate. $\endgroup$
    – MCM
    Commented Oct 14, 2013 at 0:04
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To answer this question, you need some background info first.

There are three "systems" used for the sense of smell. The first is the olfactory system. This system allows us to recognize what a smell is. You can tell the difference between gardenias and grass because of your olfactory system. The second system is the trigeminal system. This system tells you whether an odor is irritating or pleasing. It gets its name from the trigeminal nerve--the nerve that gives feeling to your entire face. Your sense of smell is all tangled up with the trigeminal nerve, meaning that some odors are painful to smell and some are not. When you smell ammonia and it stings your nose, that is the trigeminal system at work. The third is the vomeronasal system. It's thought to be use for detecting pheromones. However, it isn't important to this explanation.

So. What is meant by a "pure" odorant? It means that it only smells. It doesn't create a reaction in the trigeminal system. It only activates the olfactory system. Those who have lost their sense of smell are still able to distinguish many smells because of the reaction by the trigeminal system. One study I've read tested 47 different smells for people who have lost function in their olfactory system. Out of those 47, 45 of them were identified by the people with no sense of smell. The two that were not identified (decanoic acid and vanillin) didn't stimulate the trigeminal system and are thus considered to be "pure" odorants. In other words, pure odorants are odors that only smell and don't feel like anything. There are very few "pure" odorants, and peanut butter is one of them.

So, when someone says Alzheimer's changes the ability to smell, they have to make sure they're only testing smell and not touch too. That's why they need to use a "pure" odorant. Otherwise, the Alzheimer's patient might still be able to identify the scent and the test would be useless.

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  • $\begingroup$ Can you add a reference/review to the description of the three "smell systems", and the reference to the "One study I've read..." section? $\endgroup$
    – fileunderwater
    Commented Dec 3, 2013 at 16:02
  • $\begingroup$ One of the downsides to asking questions on this site, (unlike tech-sites), I'm not really qualified to decide if an answer is correct. :) But looking at it, it does answer the issue of "pure-ness" in an odorant, so giving a checkmark. $\endgroup$
    – John C
    Commented Apr 6, 2020 at 15:53

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