Alcohol (ethanol) distillation: mystery congener

Question

I regularly home-distil small amounts of potable alcohol (ethanol), mainly from fermented sugar(s) solutions (the 'wash').

My still is a box standard, $4.5\mathrm{L}$ electrically heated (safety!), air-cooled pot still, which produces exactly $1$ single stage distillation ('one Theoretical Plate') Starting from a wash of about $17\text{ %}$ ABV, I can distil that to (initial ABV) about $65\text{ %}$ ABV.

About $1.0\mathrm{L}$ is distilled off from $1$ gallon that way and the distillate then diluted with water to about $37\text{ %}$ ABV.

Distillers (home, pro, moonshiners) use a system to get the most and the best out of their distilled washes and it goes as follows:

1. The "Heads": the first part ($5 - 10\text{ %}$) of the distillate is often discarded because it contains non-ethanol low boiling congeners like methanol, ethanal, acetone and low boiling esters (it's a commonly held but erroneous belief that methanol and ethanol can be separated by means of a single distillation - see the $\text{MeOH/EtOH}$ VLE). These congeners give the distillate a 'synthetic' and undesired taste profile.
2. The "Heart": this is the good tasting part of the distillate, that is kept as an alcoholic beverage.
3. The 'Tails' (or 'faints'): the last part of the distillate contains very little alcohol, as well as higher boiling compounds like higher alcohols and $\text{C8 - C12}$-based esters. The high boiling congeners are also referred to as "fusel oils".

Apologies for the lengthy preamble.

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Due to the pandemic and restrictions on the hospitality sector I obtained several gallon of out-of-date $4.5\text{ %}$ ABV apple cider of a very well known brand, from a friendly publican.

I first consumed a couple of liters as such and it tasted fine and caused no adverse effects. The product was indistinguishable from 'in date' virgin cider. But my purpose was always to distill the ethanol of out it.

So, using my standard procedure I distilled several gallon of the out of date cider, obtaining about $2\mathrm{L}$ of distillate at about $35\text{ %}$ ABV. Heads were discarded and the distillate was run over activated carbon (a practice highly recommended by home distillers and moonshiners, even though I'm not sure it serves a real purpose)

And here's the rub: the 'heart' smelled and tasted awful. The taste is hard to describe but keywords would be 'spicy', 'hot', 'sharp', 'solvent-like'.

I then took the step of redistilling the $2\mathrm{L}$ of product, discarding the first $300\mathrm{ml}$ and keeping a 'heart' of about $1.1\mathrm{L}$. Alas it changed very little, if anything: the redistilled alcohol was still basically undrinkable!

The contrast between the redistilled alcohol and cheap commercial gin is quite stark.

I measured the paper $\text{pH}$ of both and they were both essentially neutral.

That redistilling didn't remove the offending compound(s?) suggests that its BP is close to that of ethanol or that it forms an azeotrope with it.

The smell and taste of the mystery congener(s?) doesn't really comply with what is usually suspected to be in the 'heads':

• $\text{MeOH}$ smells and tastes very much like alcohol (and in any case should only be present in very small amounts)
• acetone and ethanal probably are too low boiling to remain partly in the 'heart'. Ethanal has a pleasant odour anyway.
• all the low boiling esters have pleasant, fruity flavours.

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My questions are:

1. What could be the problem congener{s)?
2. How to further investigate to find out what might be the problem congener{s)? I don't have access to modern analytical techniques like IR, GC, GC-MS etc.

I do have access to $\text{TLC}$ and a $1\mathrm{L}$ lab still with an $800\mathrm{mm}$ Vigreux column.

Answer 1

People turn apple cider all the way to apple cider vinegar, so during the process, there will be a time when the amount and ratio of ethanol and acetic acid are optimally balanced for making ethyl acetate - but it doesn't seem to happen. However, given enough time (and oxygen and bacteria), maybe different things happen. Interesting that the expired cider didn't have the nasty taste, but the concentrate did. I wonder if you actually made the ethyl acetate by your heating and distillation procedure.

Ethyl acetate presence gives nail polish remover/ solvent flavor/aroma. This is not related to volatile acidity. It's known to be produced by Pichia (and probably other microbes) in the presence of oxygen.

_{Source: https://www.homebrewtalk.com/threads/need-a-little-help-with-cider-flaws.666728/}

The CRC Handbook shows a ternary azeotrope of water (9.0%), ethanol (8.4%) and ethyl acetate (82.6%) that boils at 70.23 °C vs the water-ethanol azeotrope that boils at 78.17 °C (96% ethanol). The boiling points are sufficiently close that a one-plate column might not do a significant separation, but the heads still should taste worse.

Suggestion: if the apple cider is acidic from a little acetic acid, neutralize to pH 7 or 8 before distillation ($\ce{NaHCO3}$). This might reduce the possibility of ethyl acetate formation. Another experiment would be to take fresh apple cider and see if you can get good tasting alcohol from it.

TLC of volatile solvents is difficult, but you might be able to detect different spreading patterns for the heads, the nasty heart, and some other heart (which tastes OK).

Answer 2

Like 'Jamie Gaidis' I was convinced at least some of the suspected $\ce{EtAc}$ is formed during the distillation step but presently I no longer believe that.

Instead I believe that compound is formed during fermentation and then concentrated into the distillate as the ternary azeotrope that 'Jamie Gaidis' suggests.

My evidence:

Firstly, this page of Home Distiller is awash with 'chemistry salad', half-truths, lies and misconceptions re. chemistry but still allows a chemist to read between the lines and conclude that:

$\ce{EtAc}$ is formed during fermentation and can be removed from the fermented 'wash' (or wine or beer) by means of bicar ($\ce{NaHCO3}$), washing soda ($\ce{Na2CO3}$) or lye ($\ce{NaOH}$), in an irreversible alkaline de-esterification (saponification).

Secondly, this empirical study shows convincingly that low levels of $\ce{EtAc}$ can be saponified by low concentration $\ce{NaOH}$, at near-RT temperatures and in short reaction times. The conditions described in that paper are probably not a million miles removed to what happens in my factor-space.

Empirical:

The $\mathrm{pH}$ of the cider used below was about $3 - 4$ (still carbonated)

1. Evaluating $\ce{NaHCO3}$:

To $2\mathrm{L}$ of virgin apple cider, about $8\mathrm{g}$ of $\ce{NaHCO3}$ (baking powder) was added and stirred in with a high speed stick mixer, which minutes before had been used to decarbonate the cider.

Without further ado, the cider was then distilled, with the first distillate (the 'heads') coming over after $30 - 40\text{ minutes}$.

The result was quite astonishing: even the 'heads' were much less 'aromatic', 'solventy' than those obtained without the bicar treatment and were in fact drinkable.

2. Evaluating $\ce{NaOH}$:

To $3\mathrm{L}$ of virgin apple cider, about $12\mathrm{g}$ of $\ce{NaOH}$ (drainage unblocker, small pellets) was added and stirred in with a high speed stick mixer, which minutes before had been used to decarbonate the cider.

Again, this concoction was distilled immediately after mixing.

Here too a distinct improvement was noted but subjectively I'd say the 'heads' of run $\text{#1}$ were marginally better but the 'heart' of run $\text{#2}$ was very good.

There was one marked difference: the liquid left in the pot with the $\ce{NaOH}$ run was much darker than its $\ce{NaHCO3}$ counterpart.

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I now need to test the use of these alkalis on my own $15 - 17\text{ %}$ washes.