Brewing Science - The Smell of Soles

The Honey Smells Like the Soles of Feet

I received some honey from a customer recently. I took a whiff and was surprised.The honey actually smelled like the soles of feet.

Around the same time, an incident occurred at the office where the air smelled like feet—or perhaps like natto (fermented soybeans).
Finally, it was determined that the scent was coming from hops scattered around the office.

Having cast away the slight anxiety that maybe it was my own feet that smelled, I decided to investigate whether there might be some connection between these two scents.

Please go ahead and take a sniff of your own feet before reading on to find out what causes the scent of foot soles and why honey might smell that way.

1. Causes of the Scent of Foot Soles

2. Hops and Isovaleric Acid

3. Why Does Honey Smell Like the Soles of Feet?

4. Conclusion

Causes of the Scent of Foot Soles

The scent of feet that arrives with the humid season.
Have you ever smelled it? You have, haven’t you? I actually like it.

The true identity of this scent is actually a substance called "isovaleric acid."
Isovaleric acid is a type of substance known as a fatty acid, which has weak acidic properties.

There are swarms of bacteria called Staphylococcus epidermidis on the soles of our feet, and it is through these bacteria feeding on sebum and dead skin cells that isovaleric acid is metabolized.

You might want to ask these bacteria what they think they're doing, but they are actually very good little helpers; by keeping the skin weakly acidic, they prevent the growth of highly pathogenic Staphylococcus aureus and protect the skin by producing glycerin.

As such, isovaleric acid is also present in foods like cheese and natto due to microbial metabolism (it is broken down into fatty acids from the amino acid BCAA through enzymatic reactions).

And when it comes to brewing, speaking of isovaleric acid... to be continued

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Hops and Isovaleric Acid

Actually, the cause of the smelly hops at the beginning was also isovaleric acid.

Hops contain a yellow resinous component called lupulin, which holds the bitter components alpha-acid and beta-acid. These alpha and beta acids oxidize more and more over time.

And through oxidation, isovaleric acid is generated from alpha-acid, known as humulone. It seems that leaving them at room temperature accelerates this oxidation, which is why the whole room ended up smelling like feet as in the example.

Also, like Staphylococcus epidermidis, yeast seems to metabolize isovaleric acid, but it is said that in beer, isovaleric acid is often an off-flavor caused by hops.

If you discover any "foot-smelling" alcohol, please contact us immediately.

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Why Does Honey Smell Like the Soles of Feet?

The identity of the foot scent was a substance called isovaleric acid.

Actually, this isovaleric acid is a natural substance that exists normally in the natural world, such as in plant essential oils.

Furthermore, I discovered a study that provides a valuable hint to this problem.
It is a study comparing the scent components contained in rapeseed honey with those found in rapeseed flowers.

This is very interesting: there seem to be scent components that exist in honey but not in flowers, and vice versa.
In particular, a scent component called damascenone, found in wine and shochu, was detected as a very strong scent in honey, whereas it was not detected in the flowers at all.

What I focused on in this study is that this rapeseed honey contains isovaleric acid, which was transferred from the rapeseed flowers by the honeybees.

"Aha! So the nectar source was the cause of the foot smell!" is how it goes.

One doubt I had here was that I have never noticed that kind of scent from plants.
It could be the case that even if the nectar source plant doesn't smell that way, the honey does.

One possibility is that in the plant, the scent component exists in a state bound to other substances, and that this bond is broken by the bee's enzymes or microorganisms.
Alternatively, it could be that the components are concentrated as the honey becomes lower in moisture content than it was when it was in the plant's nectar.

In other words, it has been determined that the honey that smelled like foot soles likely came from some nectar source plant containing isovaleric acid!

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Conclusion

This time, due to a miracle of sorts, I investigated the relationship between the scent of foot soles and honey.

What I found:

The cause of the scent is "isovaleric acid."
It also occurs when hops oxidize.
If honey smells like feet, it is highly likely that the nectar source contained isovaleric acid.

That was the result!

Next, I want to verify what happens to the scent if I turn this smelly honey into mead. Buckwheat mead retains the same scent as the honey, so perhaps a "foot-smelling mead" can be made.