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In my experience, most foods smell like they taste. Fragrant soaps, however, taste quite awful in comparison to their fragrances. How can some things smell and taste identical while others are so vastly different? I have a feeling this may have something to do with the vapor pressures of the molecules that make up the "thing," but I cannot find anything to confirm my hypothesis exactly.

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The food that you eat regularly is something that you are used to. You know exactly which smell combines with which taste ... but that does not imply that the chemicals you smell are the same that you taste.

If you had been eating your whole life long fragrant soap with apple smell, you would wonder about the taste of real apples as you wonder right now about the taste of apple soaps.

There are a lot of different substances that are responsible for the special taste of everything you eat. You can find more about flavor or aroma compounds on wikipedia.

In contrast to real fruits, fragrant soaps and other industrial products with a certain smell only contain those substances that are responsible for the characteristic smell of e.g. fruits (often only the main one or two aroma substances or even only synthetic aroma substances that smell similar to the natural products).

But as not all products that shall smell like certain thing also need to taste like those things you only smell the "apple" but you won't taste it.

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You have about 400 smell receptors which only sense volatile compounds that can travel to the olfactory bulb. Taste (pure taste) on the other hand is limited to salt, sweet, bitter, sour, pungent, astringent, savory, fat, smokey and only a few more. Taste can tell between an apple and a pear alone, you need smell to sense the various ester compounds that give them distinct taste/smell. You can test all of this by eating while holding your nose$-$there will be a difference in tastes.

Soap is no exception. Soap is loaded with essential oils that smell great, but it also has sodium based organic salts namely sodium dodecyl sulfate and cocamide diethanolamine, and a few others. These compounds are basic and because of that are bitter. But the sodium salts are not volatile therefore you do not smell them but do taste them. That is why soap smell good and tastes bad.

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