The Science of Smoke - What The Phenol?!

Whether it’s the comforting smell of a bonfire on our clothes, or the putrid smell of creosote from asphalt newly laid on the highway, we’ve all experienced the “smoke chemicals” we’re about to dive into here. Sometimes it’s comforting, but more often than not it’s a warning sign indicating damage. So what’s actually happening to cause these reactions? 

What we would call “smoky” is the presence of detectable levels of molecules from a family of phenols, in this case a degradation product formed from wood lignins that were set on fire.  Lignins are the things that provide wood its structure- its like plant glue, if you will. The smell and make-up of smoke depends on the lignins and the way they are burned.  In general, these molecules are relatively robust and fairly large as far as volatile compounds go. The smallest of them are still have a hard time staying in the air, and so deposit on surfaces fairly easily (Figure 1 shows a smattering of these compounds).1 

These phenolic compounds are only a small slice of the identifiable chemicals in smoke, but remember guiacol -- it's the big one.. As you’d expect, the types of compounds also depend on the type of smoke, and hence the type of fuel. In fact, it also depends on the temperature the fire is burning at- so really intense wildfires can have different smoke profiles than a less intense one, even with the same fuel source.2 These volatile compounds aren’t super long lasting in the air (anywhere from hours to a little over a week), and so the distance the smoke travelled, the humidity and the age of the smoke are all factors in how much and what type of these compounds are deposited.3  

This is a good thing for growers -- otherwise they may have been dealing with smoke taint that could persist from year to year, even when there were no fires in proximity. Fortunately, this isn’t the case and smoke effects in fields do not carry over from year to year.4

Once plants are exposed to these phenols a lot of chemistry can occur. The results are dependent on both the phenol and the reacting compound, typically the sugars present in the fruit. Harmful flavors can occur when phenols bond with these sugars found in grapes and other fruiting crops, and may even affect grains and hops.5 Depending on the plant, climate and duration of exposure, it can alter the taste and potentially ruin an entire year’s harvest.  However, the same phenols combined with different sugars can result in flavors we love.  

Let’s nerd out on this -- there is some really cool chemistry here! While at Agrology we help growers avoid the negative impacts of smoke exposure on their crops, we should be certain to also celebrate the positive aspects of this process. The chemistry that we’ll talk about here applies to both smoke-taint and the delicious flavors used across kitchens everywhere!  

A great example is how guaiacol (remember that one from Figure 1?) undergoes a series of chemical reactions that lead to vanillin, the vanilla flavor you may have used for baking or enjoyed while eating ice cream. (Figure 2).6 

So guaiacol that originated from lignins in trees can be converted to a delicious flavor like vanilla via a relatively simple synthetic route! Vanilla pods are expensive, so this was pretty exciting at the time it was first identified. While these steps are probably not the exact way it happens during cooking, they are exactly what is used in some industrial syntheses. In fact, in 2006 this industrial process was improved upon by taking a step back and starting from lignin again, rather than starting from guaiacol (which comes from lignin in smoke). An Ig Nobel prize was awarded discovering this lignin-to-vanilla approach since it dramatically lowered the cost of vanillin production (no expensive vanilla pods needed). And this process started from cow dung as the source of lignin! As a happy side effect, the leftover ligning-deprived cow dung that led to the vanilla flavoring can be used as a fertilizer. Ah... modern science. 

As a more pointed example, grapes are grown in large part for their sugars, and their sugars are susceptible to this same chemistry. In wine grapes, guaiacol and other phenols can undergo an additional process with the sugars present in the fruit, and yield non-volatile glycoconjugates or bound guaiacol. 7

While that sounds like a mouth full, it definitely isn’t: these compounds are like little ticking time bombs -- you can’t detect them through smell, and they’re difficult to even taste.  That doesn’t mean they can’t be identified in a lab, though. Since you can’t really smell them, and can’t really taste them, if you don’t have a laboratory perform a few chemical steps, you may never notice this bound glycoside time-bomb. Unfortunately, due to the surge in wildfires last year there was not enough testing capacity and vineyards were left without knowing if their grapes were impacted.8 This is what makes smoke taint impacts in wine grapes and hops so complicated.  Once in the fermenter, acid formation and other processes can unlock the flavor of guiacol by cleaving it from the sugar again. And this can happen years after a grape has been bottled, and without any indication it was there when it came out of the press.  

There is a lot we can do to avoid this outcome, though. What if we didn’t have to rely solely on laboratory testing? What if we could predict what type of smoke was present, and had a better understanding of how our actions once the smoke arrived could impact our produce? How would that knowledge impact our management of the field both during and after exposure? Those questions (and the answers to other questions that come after) are all part of the smoke prediction and monitoring process.

References

1. Alu’datt, Muhammad H.; Rababah, Taha; Alhamad, Mohammad N.; Al-Mahasneh, Majdi A.; Almajwal, Ali; Gammoh, Sana; Ereifej, Khalil; Johargy, Ayman; Alli, Inteaz; A review of phenolic compounds in oil-bearing plants: Distribution, identification and occurrence of phenolic compounds. Food Chemistry, 2017, 218(), 99–106.

2. Urbanski, Shawn P. Developments in Environmental Science Wildland Fires and Air Pollution Volume 8, 2008, 79–107.

3. Krstic, M. P., Johnson, D. L., & Herderich, M. J. Review of smoke taint in wine: smoke-derived volatile phenols and their glycosidic metabolites in grapes and vines as biomarkers for smoke exposure and their role in the sensory perception of smoke taint. Australian Journal of Grape and Wine Research, 2015, 21, 537–553.

4. Kennison, K. R.; Wilkinson, K. L.; Pollnitz, A. P.; Williams, H. G.; Gibberd, M. R. Effect of Timing and Duration of Grapevine Exposure to Smoke on the Composition and Sensory Properties of Wine. Aust. J. Grape Wine Res. 2009, 15 (3), 228–237.

5. “Studies underway on impact of wildfire smoke on hemp and hops.” https://www.kptv.com/news/studies-underway-on-impact-of-wildfire-smoke-on-hemp-and-hops/article_ab06e434-045d-11eb-8fc7-7bf73e718833.html accessed on 03/18/2021.

   and 

   “Hop Queries 4.10: Smoke-tainted hops; public breeding program reborn” https://tinyletter.com/fortheloveofhops/letters/hop-queries-4-10-smoke-tainted-hops-public-breeding-program-reborn  accessed on 03/18/2021

6. Kalikar, Rajendra G.; Deshpande, Ramesh S.; Chandalia, Sampatraj B.  Synthesis of vanillin and 4-hydroxybenzaldehyde by a reaction scheme involving condensation of phenols with glyoxylic acid. Journal of Chemical Technology & Biotechnology,  2007 36(1), 38–46.         doi:10.1002/jctb.280360107    

7. Hayasaka, Y., Baldock, G. A., Pardon, K. H., Jeffery, D. W., & Herderich, M. J. Investigation into the Formation of Guaiacol Conjugates in Berries and Leaves of Grapevine Vitis vinifera L. Cv. Cabernet Sauvignon Using Stable Isotope Tracers Combined with HPLC-MS and MS/MS Analysis. Journal of Agricultural and Food Chemistry, 2010, 58(4), 2076–2081.

8. “West Coast wineries face additional issue: smoke taint testing delays” https://news.cornell.edu/media-relations/tip-sheets/west-coast-wineries-face-additional-issue-smoke-taint-testing-delays accessed on 03/18/2021.