From Free Fatty Acids to Aromatic Esters: Esterification in the Still Made Simple(r)

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[More important than esters is the mysterious congener class often referred to as rum or Cognac oil and I have unraveled its mysteries in other more recent posts. Decisions made to maximize rum oil will also maximize noble esters. Esters can be faked in contrived ways while rum oil is still divine (though that may change).

Please feel free to comment and tell me I’m wrong or ask me to clarify anything. This post will probably be edited frequently and many of the comments will precede the edits.

This is an excerpt from the book on distillation I’ve been working on forever now. I’m trying to re-frame the text in a way that explores and emphasizes three main concepts:

1. Distillation is simple or fractional. Simple distillation is the separation of the volatile from the non-volatile while fractional distillation involves the further subdividing of the volatile.

2. Distillation is non-equilbrium like a pot still or can approach equilibrium such as in a column still. Volatility is effected by more than boiling point and the relative miscibility of a substance in water or ethanol can make something with a boiling point higher than water extremely volatile.

3. Aromas are either created in the still or not depending on the presence of aroma precursors. Many fractions are recycled not only to salvage alcohol but also to allow more time under heat to turn aroma precursors into aroma compounds.

One of my goals was to give an introduction to the topic and some general advice for distillers of low levels of involvement. Many of the ideas are speculative. I was hoping to shed light on the science just below the surface of many distilling rules of thumb.

It is a complicated topic full of tangents but I kept it down to 2100 words!


Many of the most revered aroma compounds are born in the still. Under certain conditions with the right precursors to feed the various processes, significant amounts of new aroma compounds are formed. The main process of aroma creation is esterification where free fatty acids react with alcohols under heat to form esters. Esters are very volatile and their fatty acids precursors are also volatile, even though they have boiling points higher than water. Many fractions of the distillation process are recycled into subsequent runs to give more opportunity for esterification. The recycling phenomenon means that you cannot just produce one batch of product because the final distillate is actually the product of many integrated batches. Aroma creation is not always maximized and restraint is often practiced to produce an elegant spirit. Pot stills are highly regarded for their ability to create new aroma compounds but if column stills are operated with certain methods of recycling fractions, they can also produce very full flavored high ester spirits.

Many processes are responsible for aroma creation but likely the only process that can be controlled to any significant degree during still operation is esterification. Other processes like maillard reactions, hydrolysis and oxidation are definitely important but when it comes to handling the still, they are likely just byproducts of decisions made regarding esterification [there are minor problems with this claim]. Full flavored spirits in pot stills differ from column stills by how the byproduct processes happen [I’m slowly learning the chemical ins and outs of articulating this]. If a pot still and column still were operated in such a way as to end up with the same ester content they would differ greatly in compounds created by other minor processes and the pot distilled product would likely be regarded as more complex.

To get esters to form in the still, the distilling material needs free fatty acids as precursors. Some source material is higher in fatty acids than others and some fatty acids are more noble than others. For example, apple varieties differ by fatty acid content with the higher being better suited for distilling material. The same is true of grapes. Not all esters are formed in the still or during fermentation. Esters are often already present in the source material and typically the higher the better. In fruit, esters form by enzymatic processes during ripening, more form by fermentation, even more form in the still, and believe it or not, even more form in the barrel. High total acidity in the source material often correlates to both high ester contents and high fatty acid contents which is why the wines of Cognac can yield a product with a higher ester content than the acid deficient wines used for California brandies. [What I’m missing here is that at the same time esters are forming they are also breaking up due to hydrolysis but at a different rate, often they even reform. The first distillation, due to its significant total acidity, may be characterized by a positive net ester formation while the second distillation which has no significant non-volatile total acidity (to catalyze esterification) is characterized by a net loss of esters. What I need is better research papers to support this. Most research projects only look at the first distillation and none (known to me) compare and contrast the processes at work in each distillation of double distillation.]

Yeasts are big sources of fatty acids and a percentage of the lees are retained in distilling material to produce esters from those fatty acids. Most full flavored spirits do not incorporate all of the lees due to off aromas produced as a result of having too many solids in the still. If there are too many solids there is a tendency for scorching. The relatively ordinary aromas produced from the lees may also overshadow the singularity and terroir of the source material. Some traditions exist of making brandies from accumulated amounts of left over lees but these distillers likely have stills designs to handle high concentrations of solids such as the steam jacketed stills used for grappa production or “rousers” which continuously stir the distilling material. [A big thing this misses is how distillation on the lees traps certain non-desirable congeners possibly through a fixative effect of reducing their volatility.]

High total acidity in distilling material may be important to catalyze esterification during the first distillation phase of double distillation in a pot still. There are many approaches to having a high total acidity as seen in high ester spirits like Cognac, Bourbon, and Jamaican rum. The wines used for Cognac are naturally high in total acidity which might be why Cognac distillers have always favored slow double distillation in a pot still as opposed to a column still. California brandies may have opted for a column still because they never had enough total acidity to justify the extra time and fuel expense of a pot still.

Distillers of Bourbon increase the total acidity of their distilling material with the sour mash process where a portion of the non-volatile fraction of the first distillation phase, often called backset, is recycled into the next fermentation. The sour mash process has the three fold effect of increasing micro-biological stability, recycling fatty acids, and inducing acid-catalyzed esterification in the still.

The use of dunder to make high ester Jamaican rums is very similar to the sour mash process. Dunder is the non-volatile fraction of a sugar cane fermentation after distillation and characterized by high acidity. Molasses is so concentrated a sugar source that significant amounts of dunder can be used to bring the sugar content down to a reasonable level. Dunder can represent as much as 50% of a molasses based wash for a high ester style of rum. Sugar cane is deficient in acids so Dunder is further acidified with sulphuric acid. The high total acidity creates slow, inefficient fermentations where secondary bacteria produce numerous extra aroma compounds [I have recently learned many of the finer points of this and attached the evolution of dunder’s use to some first name and specific dates). In the 19th century, high ester styles of Jamaican rum were produced as concentrates for exportation which would be diluted with relatively neutral spirits before bottling. The distilling material used for high ester Jamaican rums may have had 2.5 times the total acidity of the wines used for Cognac. Such high acidity would likely shorten the lifespan of a copper boiler but it is unknown (to me) whether Jamaican distillers used copper or wooden boilers [wooden boilers and I’ve found more specifics on that!].

Another idea for increasing the total acidity of distilling material to catalyze esterification is with freeze concentration also known as jacking. Freeze concentration, which is sometimes called freeze distillation or fractional freezing, separates compounds by melting points. Distilling material that is freeze concentrated ends up higher in alcohols, aroma compounds, and total acidity.

Ciders are known to have been freeze concentrated and the process may have given early American cider brandies the name Applejack. There is no clear historical references to ciders being freeze concentrated before distillation, but it seems likely that it was practiced if inadvertently. Ciders were probably distilled later in the winter when there were less farm chores to be done and by that time casks of cider stored in the barn would start to freeze. Ice crystals would form on the top which could be separated and a zingingly tart, higher alcohol beverage would result. Supposedly, concentrations as high as 30% alcohol can be achieved. The elevated alcohol content may have even negated the need for a second distillation. Freeze concentration before distillation was rumored by the mid 20th century flavor chemist Joseph Merory to create the highest quality fruit eau-de-vies, but no explanation of the underlying science accompanied his claim and it is not known if any commercial producers use the technique. The increased total acidity that results from freeze concentration likely increases aroma creation in the still through acid-catalyzed esterification.

Each phase of double distillation in a pot still is carried out slowly to maximize esterification [this may not be true if the second phase can be characterized by a net loss due to hydrolysis]. Applying too much energy can result in super heated hot spots in the boiler. Hot spots may break down yeast cells and reducing sugars generating favorable aroma compounds, but too many may also create off aromas by scorching. A pot still is thought to operate at non-equilibrium because there is no reflux beyond what naturally condenses on the walls of the still, but running the still too fast by increasing the energy applied to the boiler has been observed to change the distribution of the fractions. The observation may be the result of super heated hotspots volatilizing components in unpredictable ways [This is incorrect and the real reason is that reflux, due to the shape of a pot still, is more significant than you’d think. Run the still too fast and you challenge that natural reflux thus altering the fractions.]. Avoiding hot spots with a slow distillation may increase the predictability of fractions and therefore also increase product consistency. [Not challenging the natural reflux of the pot still in the second distillation will lead to a higher proof product and thus less spread out congeners and therefore opportunity for a larger hearts section.]

Not all fatty acids form esters in the first run. Often more time under heat is required so many of the fractions that are separated are recycled in the next batch. When you consider the linkage of batches, double distillation in a pot still ends up employing more fractions than people think and distillers often use various names confusingly. To understand double distillation it is useful to consider where every fraction ends up and why.

The first phase of double distillation in a pot still, often called a stripping run yields a foreshots (this term has different meanings historically) fraction that is discarded, a heads and tails fraction that are both recycled, a hearts fraction that is passed on to the second distillation phase and a non-volatile fraction that is often employed to recycle fatty acids that do not volatilize and to increase the total acidity of the next fermentation.

Under non-equilibrium distillation, what appears in the very beginning of the run can change significantly depending on the alcohol content. Besides above recognition threshold amounts of ethyl-acetate and acetaldehyde (that smell like nail polish remover) as well as methanol, the discarded foreshots of the first phase of double distillation may contain a lot of fusel oils that eventually move to the tails of the second distillation where the starting alcohol content is significantly higher [The foreshots may even contain very high boiling point fatty acids that physically clung to the condenser, but are dissolved by the high ethanol content of the beginning of the next run. This residue become significant when a still is shared across different product categories.). Fusel oils do not benefit from being recycled and are often only desired in low concentrations. Fractioning is used to separate fusel oils but they are also limited by manipulating fermentation variables to minimize their production. Fusel oils are the byproduct of stressing yeasts so distilling material is typically fermented to much lower alcohol levels than yeasts are known to produce in theory.

Not all distillers make a heads and tails cut during the stripping run. Some opt to pass everything along to the second distillation which might be due to a lack of involvement. The first phase of double distillation sees much more time under heat than the second phase as well as contains significant acidity to catalyze esterification so separating fractions high in aroma precursors and recycling them to the next batch’s first phase may result in the best opportunities for aroma creation. Some distillers only opt to return a portion of these heads and tails fractions to the next batch. These distillers are not trying to create the fullest flavored spirit possible but rather something elegant and refined.

During the second phase of double distillation, the alcohol content of the distilling material is much higher and all the non-volatile acids have been separated leaving nothing to catalyze esterification. Esterification continues to happen to fatty acids that have made it over into the second phase [Esterification continues, but so does hydrolysis which breaks up the esters and there may actually be a net loss of esters but no literature states a rule of thumb for what happens to ordinary esters versus more desirable extraordinary esters]. The increased alcohol content increases the rate of esterification in a way that may partially make up for the lack of significantly total acidity. The second phase is separated into five fractions. A foreshots fraction is discarded while a heads and tails fraction is taken and a percentage recycled during the next batch’s second phase. The hearts fraction becomes the final distillate while the non-volatilized fraction is discarded. The non-volatilized fraction of the second phase which is mostly water also likely has a lot of congeners in common with the foreshots of the first phase.

When cuts are made and it is decided how much of the heads and tails fractions will be recycled or discarded, it is important to note that not all fatty acids and their esters are created equal. They are definitely not all desirable. The esters of longer chain fatty acids are typically more desirable than the esters of shorter chain fatty acids like acetic, lactic, and butyric. Cutting a spirit will often involve separating undesirable shorter chain fatty acids and their esters, but like the higher alcohols, many of these components are better controlled and minimized by manipulating fermentation variables.

A column still also has many options to generate aromas in the still by the continuous recycling of small fractions that are high in aroma precursors. Once the still comes close to equilibrium a fraction that is high in volatile fatty acids is isolated and recycled through the column to increase its time under heat. A column still can increase the time under heat of small fractions while not investing in heating the rest of the volatile fractions which saves significant amounts of energy. The juggling of small fractions can produce large amounts of esters but the process becomes detached from other secondary aroma creation processes that typically happen alongside esterification during a slow double distillation in a pot still. [I don’t know how to explain this relative to the simultaneous esterification/hydrolysis phenomenon. I suspect the the fractions are relocated to a part of the column where alcohol is particularly high favoring ester formation over ester break up. Where alcohol is particularly high there is little water to break up the esters.]

Believe it or not, fatty acids have the potential of escaping esterification and making it all the way to the barrel. Barrel aging presents yet another opportunity for esterification (or even the breakdown of esters and other aroma compounds), but some distillers choose to remove many fatty acids by filtering through activated charcoal or using the chill filtration process. Untreated spirits have the potential to become cloudy upon dilution which many consumers are thought to object to. Particularly high fatty acid contents can also cause the distillate to appear cloudy after either phase of double distillation in a pot still.

Following the path of aroma precursors and their reactions can help distillers make more sense of the various fractions and where they end up. The extent to which fractions are recycled illustrates how integrated production runs are. Understanding the various options for operating a still when new aromas can be formed provides a basis for the experimentation required to sculpt distillates. This look at aroma creation is probably oversimplified by focusing on fatty acids and esterification as there are so many other reactions.  Esterificaiton is just a starting point and eventually as involvement deepens distillers will eventually build an understanding of the other aroma creation processes.

Creating beautiful spirits is more than just operating a still.  Aromas born in the still, whether a fault or a feature, have precursors that can be traced back to fermentation variables or the source material. Reasonable guidelines for operating a still will free the distiller to take a deeper look back at fermentation and then eventually a further investigation of the source material, be it grains or grapes. Extraordinary aroma born in the still comes from extraordinary precursors. So many variables may be daunting, but that is why spirits are so revered and so special. Exploring the potential for aroma creation from selection of the source material all the way to bottling may help small distilleries create new classes of full flavored spirits which allow them to differentiate themselves from larger distilleries making more restrained styles of spirit. Awareness of all the considerations that go into producing a full flavored spirit will certainly increase appreciation for the work of any deeply involved producer.

[edited to add: One things this post doesn’t do is differentiate the common esters. In another post I spend a little bit of time musing about the most generic of esters, ethyl-acetate which is not widely understood. I connect this congener to the threshold idea where for some reason we want as much of it in a spirit as we can without recognizing it. This can be said as we want it above the absolute threshold but below the threshold of recognition. Understanding these thresholds might teach us about the distillation of highly aromatic spirit concentrates that eventually get blended down. Certain terroirs of sugar cane or grapes or even certain apple varieties from a diverse orchard might be treated differently during production to maximize aroma. These concentrates cannot stand on their own but have to be blended down to keep certain key generic congeners in check. I haven’t found any great literature that specifically explains the concentrate idea so I’m just musing about it in theory.

Another ester issue of note that can differentiate the pot still from the column still is that because higher alcohols tend to stratify and bunch up in a column, more esters of higher alcohols have the opportunity to form such as amyl esters which are undesirable and considered flaws.]

11 thoughts on “From Free Fatty Acids to Aromatic Esters: Esterification in the Still Made Simple(r)

  1. I liked your article, I think it explains a lot. I have two questions. 1) What about copper and how it can catalyze certain reactions? 2) do you have actual evidence for both fatty acid conversion to esters and how acidity effects hydrolysis? As a distiller I like this explanation, but as a scientist, I am skeptical of chemical reaction descriptions without proof.

  2. hi there wino darko.

    this is a very accessible recent paper that gives a comparative look at a pear eau-de-vie’s distilled in copper or glass. the author also does a great job of explaining esters. the longer chain esters are far more desirable than shorter chain esters. besides reactions involving sulphur, copper reactions are a hard thing to pin down.

    the first look at esters comes from Ordinneau in the late 19th century.
    the paper i have is unfortunately in french but it is summarized well by Amerine in his 1941 Commercial production of brandies: ( Amerine gives an in depth explanation of esters and explains the chemical reactions but eludes that there hasn’t been a lot of systematic experimentation and at that time not a lot was known.

    Ordinneau takes a spent wash, adds more ethanol, then distills it again and finds he produces more esters. Peter Valaer of the IRS and Amerine exhaustively collected everything on the subject but it doesn’t seem like there was a lot at the time.

    some where, i think in his paper on rums (, Peter Valaer hints at the idea of acid-catalyzed esterification.

    in Food Flavorings ( the flavor chemist guru Joseph Merory claims that the best fruit eau-de-vies are freeze concentrated before distillation. he provides no explanation but i suspect the increased acidity is a catalyst for aroma creation.

    i found a great paper describing an undergrad chemistry experiment which explores esterification and evaluates the results solely by the nose. using numerous different fatty acids, the experiment varied ester formation by changing the alcohol content and by adding an acid catalyst. all the esters produced were esters found in food flavorings.

    my ideas definitely need some revision and some help from some other interested people. i like the quote from spirits researcher Vernon Singleton about proposing speculative ideas: “Some may prove incorrect or at least incomplete, but advancement will be served if new research is stimulated.”

    i want to re-frame everything again and make it seem more speculative as well as try to explain that there are desired and undesired esters. and see if i can give more attention to aldehydes, furfural and the like.

    thanks for taking a look and if you come across any good reading please send it my way.

    cheers! -stephen

  3. I always viewed esters as the enemy when it comes to distillation. I just shifted from using beer yeast to a champagne yeast in my distillate, so as to increase yield and make a smoother whiskey,the result remains to be seen.
    According to Wikipedia, the reaction of alcohol and acids to create esters is actually reversible. I prefer to think of this as volatility. I believe there must be an equilibrium factor that comes into play, involving % alcohol, % acidity and initial ester concentration.
    It seems there are now some experiments with these three variables that now must be done!

  4. one of my experimental distillates was overholt rye re-distilled with marmite brand yeast extract.

    i’m hoping to redo some of these experiments but manipulate acidity in the still using a winemakers acid blend. as well as varying the time under heat and seeing if i can produce easily noticeable esters from the marmite.

    i was dehydrating other volumes of overholt to a powder (non-volatile barrel essence) and reconstituting it with the distillate to mimic aging. marmite-rye sazeracs, delicious!

  5. i’m starting to think i am wrong in my approach to the subject. over emphasizing esters is probably incorrect.

    if i want to give a (over) simplified look at aroma creation and how it relates to how a still is operated i should probably describe the five basic aroma groups: fatty acids, esters, aldehydes, furfural, and fusel oil.

    once a still is operated in a consistent way, slow to maximize time under heat and avoid hot spots, then with an understanding of why fractions are recycled, distillers can learn to manipulate precursors at other stages of production.

    briefly describing the groups of aroma compounds will also help new distillers read and interpret old research papers like the stuff the IRS put out in the beginning of the 20th century.

    i understand furfural and fusel oil but i need a better understanding the aldehydes other than acetaldehyde.

  6. I’m so glad I encountered your post. I’m actually selecting varieties for a new orchard which will grow apples for producing apple brandy.
    Until I read this, I did not understand why acidic varieties of fruits might be preferable for distilling. I had read that highly acidic apples lead to a harsh, thin distillate and so were of little interest in producing apple brandy.
    I want to create an aged apple brandy with a very full flavor. I understand that this is a very complex process with a lot of factors. What I’m wondering is, if I want a distillate with as much “apple” aroma and flavor as possible, should I select more acidic varieties or will that lead to a distillate which is dominated by aroma created in the distilling process rather than in the fruit itself?

  7. hi there. it is hard to do this really involved topic any justice.

    varieties known for their aroma when distilling tend to be higher in acid. also, stable aroma compounds and their precursors are correlated to higher acidity.

    sometimes with fruit a large determinant isn’t the varietal but when it is harvested. grapes harvested later when they have a higher potential alcohol and less acidity do not age well as wines. they also make for poor distilling material. hang time on the vine destroys aroma compounds and their precursors that would be stable in the bottle as a wine or after distillation.

    I think the one compound to really track down in the process is fatty acids and their esters described in the post. fruits either come with esters produced during ripening or they have unique longer chain fatty acids that can become esters in the still. I’ve seen some papers on distillation that differentiate apple varieties by their fatty acid and ester contents.

    I’m due to get some last & forgotten mid 20th century distillation papers from an Australian agricultural college that looks at some of these topics specifically. these issues represent big holes in the distillation literature. I found the papers in the index of their archives. I have very high hopes. stay tuned.

    cheers! -Stephen

  8. Any luck with those Australian papers?

    The standard practice with producing cidre seems to be to let apples become as ripe as possible–even overripe. You seem to be suggesting that this practice reduces the chance of varietal character carrying over into the distillate–and you are partly basing this assessment on what occurs with grapes. Am I getting it?

  9. hi Nat.

    I just got the Australian papers the other day! really fun stuff to read and pretty accessible. Hopefully I’ll be uploading them all soon (next two weeks hopefully).

    Letting apples get over ripe will compromise varietal character and sensory quality. Most all the literature out there thinks only of economy and ethanol yield. Practices become different when the goal is maximizing sensory quality. I think pretty much all practices that maximize sensory quality in grapes will also maximize sensory quality in apples.

    I have a great reference from the 1970’s that covers apple brandy that I need to take a look at. If there is any unique practices for making apple brandies I bet it would be there. I’m really curious about the idea you mentioned on the home distiller forum where the ciders are put in barrels to precipitate tastants which the final distillates will later re-dissolve.

    cheers! -Stephen

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