Distiller’s Workbook exercise 5 of 15

This is the umpteenth draft of the fifth lesson in my Distiller’s Workbook. I started it as a book project with the idea of generating interest in distillation by showing a simplified form of it based on the re-distillation of tax paid commercial products.

Over time, the recipes have been elevated from merely low involvement cocktail-centric creations into being a workbook of exercises for new distillers to learn big concepts in distillation on small scale equipment with affordable batch sizes. Hopefully new distillers will be able to learn most all the what-if scenarios of operating a still so they can instead deepen their involvement with the sourcing & processing of raw materials, fermentation, and then the maturing of spirit.

A big focus of the workbook is to expose new distillers to the giant body of research concerning the subject via referencing it. I started by collecting every book on the distillation I could find and that still left a lot of questions. I eventually started collecting forgotten and seldom seen journal articles.  These were newly digitized or trapped behind pay walls and I have read hundreds in the last few years.  Most professional distillers do not even know this massive body of work exists so I hope to weave it into the content and introduce it to people.

Hershey’s Chocolate Bourbon

Cocoa aromatized Bourbon started simply as an attempt to get extraordinary aroma from an affordable source and ended up illustrating important concepts of aroma creation in the still. Cocoa powder, which will be added to bourbon and re-distilled, is high in butyric acid, a volatile fatty acid, which has the potential to react with alcohol and form an ester. Esterification is one of the most important aroma creation processes in distillation from fermented material. Ester creation influences many decisions when operating the still such as how time under heat should be varied, how fractions should be recycled, and how distilling material should be acidified to catalyze the process. Sophisticated distilleries count esters and their precursors with analysis tools like titration, chromatography or mass spectroscopy but this exercise is setup so that all changes to the spirit will be readily apparent through organoleptic analysis which is just simply by smelling.

Not all cocoa powders are the same. Each exhibit different tonal effects and some have more distinct rancio aromas than others. The rancio descriptor refers to both the olfactory-umami aromas which are a result of the roasting process as well as specifically volatile butyrates which are common cocoa powder additives. Butyrates are commonly found in milk products such as butter and Parmesan cheese. Butyrates on their own and in high concentrations can smell noxious and reminiscent of vomit, but in small quantities and in the presence of other sources of attentional tension, they can add exquisite complexity which is why they are so widely used as an additive. The boiling point of Butyric acid is 163.5°C, which is well above that of water, but its relative miscibility in water and ethanol makes it volatile during beverage distillation. The ethyl ester, ethyl butanoate, is even more volatile.

Feel free to execute this exercise multiple times and be patient. The final distillates can take months to mature, illustrating the slow but dramatic changes that happen to new make spirit post distillation. Also feel free to perform the exercises with variation so you can explore the influence of a specific variable. For example, run the still slowly one time to maximize time under heat while the next time run the still fast. Use the recommended acid catalyst or not. These are lessons that most distillers never get to learn first hand until they run a big rig and even then there isn’t always time and money to explore every what-if scenario.

When the exercise was first performed (distilled fast with no catalyst), the results were considered an inharmonious failure, but over time the aroma changed significantly in the bottle. At first the aroma was vomit-like due to above recognition threshold amounts of butyric acid, but over time butyric ester was formed pushing the aroma of the distillate back to something wonderful and recognizable as chocolate.

Esters are both formed in the still and broken up leaving a net amount. The best explanation of esterification at the molecular level, which helps explain the role of an acid catalyst, is from Peter Atkins in his chemistry primer for the layman, Reactions. Accessible explorations of esterification have also been done in the form of student projects at Roseworthy agricultural college in the mid 20th century and the early Scotch researcher S. H. Hastie tackled parts of the subject even earlier on. Each researcher studied double distillation in pot stills but from different (and not complete) angles. Roseworthy looked at the first distillation and saw a net accumulation of esters while Hastie began by working backwards, looking only at the second distillation and saw a loss of esters. The first distillation may see a net accumulation of esters due to high total acidity in the distilling material while the second distillation may see a net loss, but no papers seem to explain the whole story conclusively.

As the exercise illustrates, fatty acids that survive to the final distillate can even form esters post distillation. Certain equilibriums of esters exist as a function of the pH of the spirit and barrel aging, which lowers the pH, changes the equilibriums. A way to explore aspects of the pH impact on post distillation esterification is to use a novel fake aging technique. Samples of commercially produced and sufficiently barrel aged spirit can be put into a food dehydrator to remove the volatile fractions at fairly low temperatures. What remains is barrel essence which is essentially the perfect soup of non-volatile acids and tannins. The newly distilled spirit can be used to reconstitute the barrel essence and thus quickly take on the pH of an aged spirit. The spirit produced in the exercise can be divided in two with one portion being fake aged to lower the pH and one half left alone as a control. If done, the new distiller will have a small scale, slow motion, post distillation esterification kinetics experiment with a distinct fatty acid. The spirits should be repeatedly evaluated over time to observe the changes.


500 mL Bourbon whiskey (we used Evan Williams)
35 g cocoa powder (we used Hershey’s brand)
8 g tartaric, malic, or citric acid (optional acid catalyst)
250 ml water

Mix and re-distill together on low reflux until the thermometer on the still reads 93.33°C. Going past 93.33°C may result in a cloudy distillate. The extra water is added to reduce the chances of cocoa powder solids scorching on the bottom of the boiler.

Using your hydrometer re-cut the distillate to your desired proof (we recommend 90-100).

Optionally, to synthesize the pH and non-volatile characteristics of an aged spirit, de-hydrate a volume of aged Bourbon proportional to the amount you want to fake age then reconstitute the resulting barrel essence.

Try this exercise multiple times changing variables so as to produce comparisons and you will be rewarded.


cocoa aromatized Bourbon Sazerac
2 oz. cocoa aromatized Bourbon
4 g. non aromatic white sugar
4 dashes Peychaud’s bitters
rinse of Absinthe


cocoa aromatized Manhattan
2 oz. cocoa aromatized Bourbon
1 oz. sweet vermouth
2 dashes Angostura bitters

inverse Brandy Alexander
.75 oz. cocoa aromatized Bourbon
1.5 oz. Pineau des Charentes
.75 oz. cream (we enjoy heavy cream)

1 thought on “Distiller’s Workbook exercise 5 of 15

  1. I find that a 50% addition of chocolate malt in a standard whiskey mash yields a terrific chocolate quality to the resulting spirit.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Discover more from Boston Apothecary

Subscribe now to keep reading and get access to the full archive.

Continue reading

search previous next tag category expand menu location phone mail time cart zoom edit close