Geosmin degradation by acid: A mini literature review

While researching for the main article on geosmin I read that geosmin degrades in acidic dishes, either during the cooking process or afterwards. But, unfortunately, nobody cited any sources for that. Being obsessive, I looked into it and, after some reading, found something usable. I thought I’d make a little review out of it because a) I think some of you might feel the need for some evidence, too, and b) I can’t resist the urge to show that I did my homework…

Geosmin degrades in hydrochloric acid

So, indeed, geosmin has been shown to react in acidic conditions to form an odourless product. The first paper to describe geosmin (and argosmin) already mentions this reaction ¹Gerber, N N, and H A Lechevalier. ‘Geosmin, an Earthy-Smelling Substance Isolated from Actinomycetes’, n.d., 4. (see Figure): Geosmin has a hydroxyl group (-OH) that can be removed in a dehydration reaction, leaving a double bond in the product. Acids catalyse this type of reaction, so adding vinegar or lemon juice to food should do something – in theory, at least. The authors of that first paper left their geosmin sample in concentrated hydrochloric acid for four days. That’s not exactly a home cooking approach and only really tells us that this reaction does take place at all. But to be fair, improvement of food quality really wasn’t the objective there.

Household acids degrade geosmin (?)

Another group set out to test reaction conditions that come closer to the kitchen reality. The authors looked at the effect of two commonly used acids: dilute acetic acid (equivalent to vinegar) and citric acid (as present in lemon juice). They found that even low concentrations of household acids degrade at least some geosmin.²Pahila J. G., Yap E. E. S., 2013 Reduction of off-flavour compounds (geosmin and 2-methylisoborneol) using different organic acids. AACL Bioflux 6(6):511-517. However – and I really don’t understand how this can happen to full-time scientists – they didn’t mention the precise reaction conditions, i.e., how long they let the reaction take place, at what temperature and so on. So that data is somewhat useless for us too…

Household acids degrade geosmin!

There was one last paper about water treatment that focused on algae-removal compounds that I thought wasn’t worth reading at first, but when the other papers turned out to be not much help I decided to skim through this one as well. And good thing I did: In a subsection, they reported repeating the citric acid experiment from above and showed that a 1% (by weight) citric acid solution degraded 69% of geosmin over 48 hours at room temperature in water.³Hammond, David, Anthony Murri, Sergey Mastitsky, Ziming Yang, Roy Foster, and Linda Schweitzer. ‘Geosmin Reduction by Algaecide Application to Drinking Water: Field Scale Efficacy and Mechanistic Insights’. Heliyon 7, no. 8 (August 2021): e07706. https://doi.org/10.1016/j.heliyon.2021.e07706. Lemon juice contains 7-10% of citric acid by weight, so a dish containing 1% would be quite sour but not completely inedible. A tangy borsch might come close here. 48 hours, on the other hand, is way too long for most dishes, although heating should speed up the reaction significantly. Unfortunately, they did not test that, which, admittedly, is fair for a water-treatment paper… Regardless: 2+ days at room temperature in an acidic solution sounds a lot like pickling: for these, the procedure probably works like a charm.

Conclusion

Overall, it’s quite plausible that acids get rid of some of the geosmin during cooking, but it’s not clear if you would notice the effect, given how much acid and/or time you’d need for significant degradation. Heat should speed things up significantly, but that’s just a hypothesis. And besides, the experiments presented worked in ideal conditions with just geosmin and acid in water – real food is more complex than that and might make things go a little differently.

Footnotes

• 1
  Gerber, N N, and H A Lechevalier. ‘Geosmin, an Earthy-Smelling Substance Isolated from Actinomycetes’, n.d., 4.
• 2
  Pahila J. G., Yap E. E. S., 2013 Reduction of off-flavour compounds (geosmin and 2-methylisoborneol) using different organic acids. AACL Bioflux 6(6):511-517.
• 3
  Hammond, David, Anthony Murri, Sergey Mastitsky, Ziming Yang, Roy Foster, and Linda Schweitzer. ‘Geosmin Reduction by Algaecide Application to Drinking Water: Field Scale Efficacy and Mechanistic Insights’. Heliyon 7, no. 8 (August 2021): e07706. https://doi.org/10.1016/j.heliyon.2021.e07706.