Glorious spirits come from hot and dirty fermentations. Boring spirits come from cold and sterilized environments. But unsellable, horrible products also come from hot and dirty fermentations. Like so many things in life, knowing how hot and dirty to get is key to making fun spirits. 

Carboxylic Esters

Fruit flavors in mature spirits come from carboxylic esters, which can be made many ways during the production of spirits. They are formed when carboxylic acids are chemically bound to alcohols.

The raw acids used to make the esters in spirits come from many places. Some of the raw acids will come from the barrel. Some will come from the yeast. Some will come from bacteria, either intentionally or accidentally introduced to the fermentation. And some of the raw acids will even come from bacteria living in the walls of aging barrels.

The raw acids do not taste good. In fact, they are the very smell of rotting materials. When you open your trash bin, you are smelling carboxylic acids produced by bacteria eating your garbage. Yet, in one of nature’s most puzzling and glorious acts of chemistry, when you take those very rotten smelling molecules and chemically bind them to alcohol molecules …poof …you get the aromas of fruits and flowers.

Case in point:

Butyric Acid = aroma of vomit

Ethyl Butyrate (ethanol bound to butyric acid) = aroma of pineapple

Propionic acid = aroma of sweaty armpits

Ethyl Propionate (ethanol bound to propionic acid) = sweet tropical aroma similar to Juicy Fruit Gum

As fine spirits makers, it is our job to maximize the density of these esters and minimize the detectible acids. We tend to instinctively know to minimize the acids, because they obviously smell like rotten stuff. But all too often, we eliminate these gross smells by over sanitizing our production processes and, in doing so, we also eliminate the fruity esters, which are absolutely critical to making great spirits.

Our forefathers knew this. Jamaican rums have long exploited dunder pits to grow bacteria as a carboxylic acid feed stock to make great pungent rums. Agave spirits are partly fermented by the bacteria Zymomonas mobilis, which creates acids that are integral to flavor of Tequila and mescal. The Taylor-Hay Papers, made famous by Mike Veach, described old time bourbon sour mashing as a several-day-long acidifying fermentation, carried out prior to adding yeast. In And a Bottle of Rum, author Wayne Curtis references the gross but fairly well documented 18th century practice of adding the contents of a chamber pot to new rum fermentations.

As disgusting as these practices sound (especially the chamber pot), they actually make sense. Each of them is using bacteria to produce extra carboxylic acids in the early phases of fermentation. Those acids can then go on to be esterified into fruity aromas either by yeast biologically or chemically, as part of the barrel (or in my case, reactor) aging process.

Fermenting Hot

Yeast also produces carboxylic acids during fermentation, but cold fermentation temperatures retard the acid production of yeast. Peak production happens above 80 degrees. If the fermentation is acidified, prior to the addition of the yeast, the process is also retarded. So, if you only take one thing from this article, let it be to, once and for all, stop the practice of artificially acidifying fermentations.

The nice thing about fermenting hot (as opposed to hot and dirty) is that it is pretty hard to screw up. Yeast primarily makes non-offensive acetic acid and only trace amounts of other carboxylic acids; so, no matter what you do, you won’t overproduce the acids and ruin your product.

It is also worth noting that not all strains are created equal. Some yeast strains produce much more carboxylic acid than others. And each strain produces different ones. Experimentation is the key here. Some strains might work very well with the flavor profile you are trying to make and others not so much. I once test-fermented malt whiskeys using Belgian ale yeast, and they all tasted like bubble gum.

Fermenting Hot and Dirty
(in a 12-Step Process)

The key to making the world’s most over-the-top spirits is fermenting hot and dirty. However, unlike fermenting hot alone, this carries some risks with it. If your fermentation is too dirty, you will end up tasting the carboxylic acids along with the esters and generally ruining your product. Controlling the carboxylic acid levels produced bacterially takes practice.

Like yeast strains, each bacteria produces a different soup of carboxylic acids. However, unlike yeast, these acids (and their corresponding esters) are potent: a little dirty goes a LONG way. Most of these compounds are detectable to humans in the 2–10 ppb levels.

There are many ways to ferment dirty, but this is my preferred method:

1. Start with a bacteria. I choose mine by carefully mapping the acids produced by the bacteria from scientific papers and then looking up the corresponding ethyl esters. I then make a spreadsheet and force rank them—esters versus acids—by human aroma detection thresholds. But you don’t have to be this careful. You can just try one randomly and see what happens. Cheese-making bacteria are the safest bugs to start with.

2. Set aside a small amount of wash—3 to 5% of one fermenter’s capacity—in a food grade bucket.

3. Boil the small amount of wash to temperature-shock any native bacteria and kill any wild yeasts.

4. Add bacteria from either a cheese-making culture or a lab sample, and lots of it—the more the merrier. You don’t want to give wild bacteria any chance to grow.

5. Seal the lid and place the bucket in an incubator at around 100° F for one week.

6. Smell the bucket. It should be just slightly sharp and rancid. It doesn’t need to smell like old gym socks worn for a week straight. If it does, exercise caution and add a lot less of it in the following steps.

7. Boil a small amount of water in your fermenter, using the steam jacket. Once it is rigorously boiling, cut the steam and dump in the bucket of yuck. This should temperature-shock the bugs but not boil off the acids you worked so hard to make.

8. Add cold water and then fresh wash, and begin fermentation.

9. Let the fermentation run for 1–2 days longer than normal. The yeast will biologically convert acids to esters at the end of the fermentation, not the beginning.

10. Distill like normal.

11. Age longer than normal. The fruity esters will become the honey, leather and books found in the most valued spirits, as they become phenylated during later stages of the barrel aging process (12–20 years).

12. Drink it, win awards and prosper.

Bryan Davis of Lost Spirits
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Bryan Davis is most well known as the co-founder of Lost Spirits, and is an expert in distilled spirits chemistry. He holds numerous active patent applications related the industry and is a frequent speaker at conferences ranging from university chemical societies to TED. Davis’s 2012 book How to Make Whiskey has been on’s best-seller list. His lab has active research contracts with many of the world’s largest distilleries.