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So I was researching yeast metabolism recently and stumbled across some pretty interesting info on higher alcohol metabolism.
There is a direct relationship between amino acids in the wash and the higher alcohols a yeast produces. It is a pretty direct reaction, at least in the cases I read with very high yield. Add an amino acid, and yeast spits out an alcohol, for example in the case of leucine the yield of isoamyl alcohol is over 80%.

Amino acid ---> Fusel alcohol
Leucine ---> Isoamyl alcohol
Valine ---> Isobutanol
Isoleucine ---> Active amyl alcohol
Phenyl ---> 2-phenylethanol
Tyrosine ---> p-hydroxyphenylethanol / tyrosol
Tryptophan ---> Tryptophol
Methionine ---> Methionol

Usually the general thought most distillers have is to reduce production of these fusels. But as we all know, alcohols are half of esters, so specific amino acids could be used to manipulate the ester profile of spirits, dunders, whatever.
There has been plenty of good chatter lately about the acid side of the ester equation, mostly which bacteria produce desirable carboxyllic acids, and the general process of ester formation. So we know how to manipulate the acid half. All our fermentations produce a majority of ethanol (yay), making ethyl esters predominate. Of course all fermentation also produce some fussels too, but with this info they can be increased and targeted.
I've long had the notion of making a "rhum grand arome", a flavor bomb so strong it is only used in blending. I'm excited by the notion of targeting the esters not just by acids, but by specific alcohols too.
E.g., supplement with leucine, have acetic acid present and end up with bananas or pears (isoamyl acetate). And/or with butyric acid present you get apricot or mango (isoamyl butyrate).

I won't bother sharing any sources, apparently this is basic yeast metabolism that has been known so long it only shows up in the summaries of current research mostly bogged down in elucidating the genetic pathways.

Indeed very interesting.
I would like to see the sources, because I have seen too much misinterpretation to trust hear say.

And than again the next question: how do we feed the yeast or mash with specific amino acids? Is that more easy than to feed it with higher alcohols?

Fair enough
The Ehrlich Pathway for Fusel Alcohol Production: a Century of Research on Saccharomyces cerevisiae Metabolism▿
Lucie A. Hazelwood, Jean-Marc Daran, [...], and J. Richard Dickinson
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2293160/" onclick="window.open(this.href);return false;" rel="nofollow

Fusel alcohols are derived from amino acid catabolism via a pathway that was first proposed a century ago by Ehrlich (13). Amino acids represent the major source of the assimilable nitrogen in wort and grape must, and these amino acids are taken up by yeast in a sequential manner (23, 32). Amino acids that are assimilated by the Ehrlich pathway (valine, leucine, isoleucine, methionine, and phenylalanine) are taken up slowly throughout the fermentation time (32). After the initial transamination reaction (Fig. ​(Fig.1),1), the resulting α-keto acid cannot be redirected into central carbon metabolism. Before α-keto acids are excreted into the growth medium, yeast cells convert them into fusel alcohols or acids via the Ehrlich pathway.

It really is amazing to me I'm just learning about it now. If I ever heard about it I just didnt retain it. Felix Ehrlich figured this out in 1905, it is not new knowledge by a long shot.

Here is a GREAT overview of both esters and fusel alcohols.
http://scottjanish.com/esters-and-fusel-alcohols/" onclick="window.open(this.href);return false;" rel="nofollow
Note in these results where he says "alcohols" he is referring specifically to fusel alcohols:

Variable Possible Result
Higher fermentation temperatures Increased esters and alcohols
Trub present in the fermenter Reduced esters and increased alcohols
Adding yeast nutrient to fermenter Increased esters and alcohols
Increased oxygen during fermentation Reduced esters
Top pressure (dissolved C02 or capped ferment) Reduced esters and alcohols
Increased yeast pitching rate Increased esters (only when heavily overpitched)
Reduced wort pH (3.0) Decreased esters
Increasing fermentation temperature at climax of fermentation Decreased esters (lower than keeping constant temperature)
Decreasing fermentation temperature at climax of fermentation Increased esters
Higher gravity fermentations Increased esters and alcohols
Increasing wort gravity with maltose syrup Decreased esters (compared to other syrups)
Increased wort lipids (like oats) Decreased esters
Beer aging Decreased esters

This just a summary however and the full article gives all the nuances and contradictions and is thoroughly referenced, though unfortunately I can't seem to find most of the references for free.

You got me there!
It will take me some time to read and understand it all. But thanks nonetheless.

Oh, and pure amino acids are widely available and pretty cheap.
A typical BCAA supplement provides L-Leucine, L-Isoleucine and L-Valine; which should become Isoamyl alcohol, Isobutanol and active amyl alcohol respectively (and their esters):
https://www.amazon.com/Branched-Chain-E ... GV6G0?th=1" onclick="window.open(this.href);return false;" rel="nofollow
Individual amino acids are also available. Maybe even more crude sources for amino acids could be used?
Almost every hippie or health nut's kitchen I've ever been in had a bottle of this stuff:
https://www.amazon.com/Bragg-Liquid-Ami ... B0006Z7NNQ" onclick="window.open(this.href);return false;" rel="nofollow
Or hell, just toss in a goat head, right?

And a quote from the Scott janish page above summarizing
Fermentation Kinetics and the Production of Volatiles During Alcoholic Fermentation. (1995). Journal of the American Society of Brewing Chemists ASBCJ, 53. doi:10.1094/asbcj-53-0072

The addition of amino acids of valine, leucine, and isoleucine (total nitrogen added = 95 mg/L) was found to strongly increase the production of fusel alcohols (isobutanol, isoamyl alcohol, and amyl alcohol). Specifically, 60-70% of the added leucine and isoleucine were transformed into isoamyl alcohol and amyl alcohol, and the all the valine was transformed into isobutanol.

AG worts are so chock a bock full of free amino nitrogen, how would you get the yeasties to preferentially consume the specific amino acids you want? The only way I can think of... outside of genetic manipulation, would be a massive overabundance of the specific AA creating a more probable consumption.... I'd imagine this would create some deleterious effects in flavor though.

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That may well be true. Or not, I'd love to get that last article in its entirety, since it's from a brewing journal it may well have been tested on all grain wort.

But honestly I'm more excited about targeting these alcohols to produce non ethyl esters for an ester bomb rum. I don't know for sure, but expect there are fewer competing amino acids in cane products.

Dude you're on a roll! This is a good addition to all the other stuff we've been working on. And some great research to read!

Looking through the bibliography of that article I found a couple more than could be helpful:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1204256/" onclick="window.open(this.href);return false;" rel="nofollow - The mechanism of the formation of higher alcohols from amino acids by Saccharomyces cerevisiae

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC106145/" onclick="window.open(this.href);return false;" rel="nofollow - Pyruvate Decarboxylase Catalyzes Decarboxylation of Branched-Chain 2-Oxo Acids but Is Not Essential for Fusel Alcohol Production by Saccharomyces cerevisiae

http://www.jbc.org/content/278/10/8028.long" onclick="window.open(this.href);return false;" rel="nofollow - The Catabolism of Amino Acids to Long Chain and Complex Alcohols in Saccharomyces cerevisiae
I found this very interesting:

In S. cerevisiae catabolism of the three branched-chain amino acids (leucine, valine, and isoleucine) and the two aromatic amino acids (phenylalanine and tryptophan) proceeds essentially as Ehrlich proposed nearly 100 years ago (28). However, it is much more complex than Ehrlich ever imagined, because yeast uses at least three aminotransferases, five decarboxylases, and six alcohol dehydrogenases. The precise combination of enzymes used at a particular time depends upon the amino acid, the carbon source, and the stage of growth of the culture.

I am a bit sceptical. Two arguments:

1. If a high amount of those amino acids would lead to more fusel alcohols, this would mean, that at least for clean and nice spirits we shouldn't use natural yeast nutrients, because they are full of those amino acids. Look at nutrition facts about yeast (natural yeast nutrient contains mainly dead yeast). Dry yeast contains for example around 4% leucin and 3% isoleucine. But IMO in reality many nutrients lead to a better and cleaner ferment. This is contradicting. Try it out, two batches, one with and one without dead yeast. Or instead of yeast try eggwhite (BTW, if I ever want to invent a new neutral wash recipe, powdered eggwhite would be one of the ingredients, especially because of the amino acid arginine). I don't think, the one with more amino acids will smell more like fusels. At least the distillate! The wash? I am not sure. Those fusel alcohols have high boiling points (isoamyl alc 131°C, isobutanol 108°C). But some esters also have high boiling points (ethyl butyrate 121°C), but easily come over... perhaps again this graph is interesting:
http://homedistiller.org/forum/download ... &mode=view
Looks like that you need a reflux still to keep isoamyl and isobutanol in the boiler.

But probably I am wrong:
http://scottjanish.com/esters-and-fusel-alcohols/" onclick="window.open(this.href);return false;" rel="nofollow
Interesting detail:
"the levels of esters in the beer fermented with no trub were higher than the beers fermented with the trub added."
"On the other hand, the formation of fusel alcohols slightly increased in beer fermented with high levels of trub."
trub = proteins = amino acids. More amino acids -> more fusel alcohols. But less esters? Strange, because later they write, that yeast nutrient (dead yeast) increases both, fusel and esters:
"The addition of amino acids of valine, leucine, and isoleucine (total nitrogen added = 95 mg/L) was found to strongly increase the production of fusel alcohols (isobutanol, isoamyl alcohol, and amyl alcohol). Specifically, 60-70% of the added leucine and isoleucine were transformed into isoamyl alcohol and amyl alcohol, and the all the valine was transformed into isobutanol."
What the hell is the difference between amino acids of trub and of dead yeast?


2. A typical high ester Rum has still 200 times more ethanol than esters. This means there is only 1g carboxylic acid and 1g esterification-alcohol in one liter 40% Rum. This means only 0.25% of all alcohols (ethanol and fusel alcohols) will build an ester. I know, this calculation has a few flaws, but I think it's good enough to have an idea of the amounts. And now the problem: Even if you manage that your yeast forms many fusel alcohols, only 0.25% of them will turn to an ester. 99.75% will remain stinky fusel alcohols (which come over or not?). Or will the acid prefer the fusel alcohols over the ethanol? I don't know. But to get a nice fruity and not stinky result, the esterification rate of fusel alcohols should be much higher than the rate of ethanol. But actually I am a bit unsure here: I don't know how bad in what amounts each of the fusel alcohols smells. I have propanol here. Of course it doesn't smell yummy, but it's not intense. Probably the flavor of the ester is much stronger than the smell of the fusel alcohols. Like with ethanol and ethanol-esters (ethanol doesn't have a strong smell, but its esters have). Perhaps 0.25% or less esterification is enough to mask the fusel smell with fruity flavors?
But still, for me the chance to get an ethanol-ester looks much higher than the chance to get a fusel-ester. If your esters are 98% ethanol-esters and 2% fusel-esters, will it smell really different than if you have 100% ethanol-esters?
http://scottjanish.com/esters-and-fusel-alcohols/" onclick="window.open(this.href);return false;" rel="nofollow
Yes, it looks like 2% non-ethanol esters makes a difference. Taste thresholds for isoamyl acetate is 20 times lower than for ethyl acetate. Unbelievable, when I think about the intense smell of ethyl acetate. I can't imagine something smelling 20 times more. But the threshold of isoamyl acetate is 40 times lower than of isoamyl alcohol. This means 0.25% esterification is not enough to cover the fusel alcohol smell. It would need 2.5% to notice both equally.


If I am right, the highest chance to get fusel-esters in a dominating amount is in the dunder pit. Because only here it's possible to have more fusel alcohols than ethanol (because fresh dunder contains almost zero ethanol, but probably many fusel alcohols). But the (fusel-) alcohol strength in dunder is very low for a good esterification rate. So when we use the dunder, it will contain much more acids than esters. And the acids will come in contact to the massive amount of ethanol sooner or later and will build ethanol-esters. Or how much could sulphuric acid help here? Perhaps adding sulphuric acid to dunder before adding the dunder to the low wines or wash gives the fusel-alcohols a head start? I don't know how effective the catalyst works under such bad circumstances (low (fusel-) alcohol strength). When trying this out, the dunder should be stored sealed after the sulphuric acid treatment or you will loose esters to the angels.


This is why I am not sure at the moment, if there is a for us useable method for fusel-esters dominating ethanol-esters.
But are the fusel-esters really needed? How many of you have ever tasted a high ester rum? This stuff is expensive. I only one (but often, I have a whole bottle, Habitation Verlier Hampden LROK). For me it smells possible, that almost only ethanol-esters come from the wash and the dunder, mainly ethyl acetate and ethyl butyrate. And then happens much with the oak of course. And probably longer ester chains are formed with time. Perhaps unfortunately it's like always with spirits, you simply have to wait a long time, and then perhaps you don't get exactly what you wanted to get at the beginning, but the quality rises continuously.

Good to see you der wo.

And although I do not understand it, it is good to see higher technical discussion about our hobby. It benefits all of us in the long run.

Geoff

{Sorry to hijack, but Der Wo your PM and e-mail are disabled and I just had to welcome you back somehow!}

I don't mind a good hijack at all, in fact I had wanted to pm him sometime ago and found I couldn't either. Might as well have an informal der wo party while we're here!

Hey buddy, good to see you back. Hope all has been well for you and I'm glad to see you haven't lost your touch. ha ha
Your knowledge is powerful to the hobbyists and now we have a couple more with some powerful knowledge as well.