Advanced Fractioning and Volatility Basics

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One issue that needs some attention is an explanation of how 8 fraction Micko distillation with the birectifier sorts congeners (with consistency) so distillers can gain insights. It is all about sorting.

The volatility of aromatic congeners is really complex and is effected by boiling point, relative solubility in ethanol and water and even liquid entrainment within the column which varies with throughput (distillation rate). Surfaces and various zones can create zones of slight entrainment that trap vapors slowing their journey through the still.

I created a simplified ascii art representation of how congeners change as distillation proof changes:


The top row represents orderly, repeatable 25ml / 15 minute optimal throughput birectifier fractioning (extremely well sorted!). Notice that the groups of numbers go across in orderly sequence. I used eight groups because Micko distillation typically has eight even fractions (each row is 99 characters across so I hope it scales to mobile viewing).

The bottom represents distillation at a lower proof, such as what a pot still may use to create a full flavored spirit. You’ll notice the numbers slowly bleed across each other. In actuality, the bleeding is probably more dramatic and definitely not so symmetrical.

Each number, which represents a fraction in Micko distillation, described in all my role model case studies, also represents multiple congener categories within. Arroyo gives us an incredibly concise primer if you need one.

It will be found that the first fraction contains most of the esters and aldehydes of low boiling point and low molecular weight, besides a large amount of ethyl alcohol. The second fraction has a similar composition to the first, but the amount of esters and aldehydes is much diminished. The third fraction is practically made up of ethanol, with traces of aldehydes, esters, and higher alcohols. The fourth fraction could be called “the higher alcohols fraction,” as from 70 to 75% of all the higher alcohols present in the sample will make their appearance in this fraction. The fifth fraction may be said to regulate the quality of the sample as a beverage, for here we find the major part of the esters and aldehydes of high boiling point and high molecular weight which play such an important role in rum taste and aroma; what is still more important, we find here also the major part of the invaluable rum oil. Fractions sixth to eighth will be made up mostly of water with a little ethyl alcohol and very minute amounts of very high molecular weight, high boiling point, esters and aldehydes.

It can get mind boggling complex if you try to track every single congener. The take away is that you cannot easily apply organoleptic analysis to the bottom row:

Fraction 5 showing a terpene emulsion.



If you look at the eight groupings one by one it is madness and you will face the sensory distraction of one congener while you try to assess another. Pre-sorting may also have a lot of value when you apply titration.

To assess a spirit, you have a better shot like this:


1111111111 <—– Ethyl acetate and acetaldehyde
4444444444 <—— 75% of the fusel oil
5555555555 <—– Rum oil and big esters!

This could help you change your heads cuts by a a certain amount of absolute alcohol to reduce your final ethyl acetate numbers in the next batch. This cut is typically not made by temperature or proof because those metrics vary so little across the beginning of the run.

It could also help you change your tails cut which you will make by temperature or hydrometer parrot.

This, sorted via the birectifier:


helps you make reasonable decisions to apply to the chaos your still:


There is one more lesson that the birectifier helps illustrate and it relates to the liquid entrainment concept and the idiosyncrasies of distilling non-ideal mixtures (ethanol, water, and congeners!). It is all about the rate of distillation (throughput). The birectifier is operated quite slowly at 25ml / 15 minutes for every fraction which allows consistency and repeatability so comparisons can be made across distilling runs.

If you operated at 25ml / 10 minutes (higher throughput!) you would get significantly different results even though the alcohol content would appear to change negligibly. Congeners need time to bounce around and escape entrainment zones. More time means less impact of liquid entrainment and more sorting. By degrees, you lose faithfulness when you try to compare a birectifier fraction 1 collected at say 10 minutes with another fraction 1 properly collected at 15. Part of the significance of the birectifier design is that with only one variable, boiler output, you can hit repeatable targets for fraction collection to make faithful comparisons.

The time concept gets more complicated when translated to a larger still because faster distillation (higher throughput) can easily challenge a condenser beyond its abilities (and cause plate flooding) and slower distillation can challenge your labor and energy bill. Aroma is also created by time under heat which is extra opportunity for chemical reactions that are meaningful to full flavored spirits productions.

The reflux of the birectifier is incredible and it easily distills very close to the azeotrope. Traditionally 100 ml of absolute alcohol is inputted into the birectifier and it is exhausted at the very beginning of the 5th fraction (with automation and data logging, we will better know how repeatable this point is!).

One more thing to consider is that if we hold a bunch of things constant and distill a spirit again but with more ethyl acetate (or acetaldehyde), that will not only mean more of that congener in fraction 1, it will mean more in 1,2 and 3. To keep a constant level in the spirit, the cut will have to be moved which will sacrifice aroma beneficial congeners in fractions 2 and 3.

This could be described again from the rear for fusel oil. To keep a constant level of fusel oil, the cut would have to be moved reducing incredibly valuable long chain esters and rum oil. Increases in ordinary congeners force the removal of extraordinary congeners. A lot of Arroyo’s ideas aren’t just about creating extraordinary congeners, they are about limiting ordinary congeners. The birectifier is a key to assessing and pursuing all of that.

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