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Citric vs. Sulfuric Acid For Rum Production

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I'm writing on behalf Louis Fahrasmane regarding the use of citric versus sulfuric acid for rum production in new American distilleries. The improper selection of acids to acidify a ferment could be the biggest, easiest to resolve quality setback to new rum producers. Fahrasmane is probably the last surviving rum micro biologist to survey and investigate productions at the end of the last golden era. He expressed interest in writing an article on the topic of acid options to help the American industry. I have collected many of his team's papers and have translated quite a few from French on the blog (go to the bibliography at the end):

https://www.bostonapothecary.com/thirty-years-of-rum-technology-at-inra/

This is the next great set of rum research that comes after the work of Arroyo. The INRA team even discovered the last great rum yeast that most people are using.

My understanding is that many new American rum producers acidify their ferment primarily with citric acid, but also sometimes malic, tartaric, or even lactic acid. This is done instead of sulfuric acid possibly because of safe handling concerns and the learning curves of new distillers. I think this originates in a few contemporary books aimed at home distillers and not commercial distillers, but it would be great to hear some anecdotes.

A 1985 paper from Fahrasmane (last missing page here), shows how citric acid modifies yeast metabolism and produces an abundance of ordinary congeners that have to be cut away thus shrinking the hearts fraction and forcing distillation at a higher proof. I think Fahrasmane did more earlier work on this and it was part of his PhD thesis.

Fahrasmane would love to know what people are currently practicing and where the ideas came from if they are from specific texts. It would be great to hear production anecdotes of anyone switching from citric to sulfuric acid. I'm hoping to get an informal survey so feel free to start a discussion in thread or reply privately. I'll be translating all the replies to French and sending them to Fahrasmane. If you are currently using sulfuric, but are aware of citric acid anecdotes, feel free to share what you know.

I would love help making this article by Louis Fahrasmane a success and hopefully we can encourage him to keep writing articles for the new rum industry.

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1 hour ago, bostonapothecary said:

but it would be great to hear some anecdotes

We use about 25% backset and never ever have a need for acidification. If anything, we have issues w/ our ferment bottoming out -- say in the 2.9 range.  We use CaOH to raise the pH.

 

 

 

 

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Excellent. What would the initial pH be? Does it stay steady through alcoholic fermentation then take a dive with bacterial growth to hit 2.9? I'm suspecting you have a fairly long fermentation duration.

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I had the same experience. Starting pH starting at ~5.7 then crashing early.  So, I add no acid either.  I add calcium carbonate to buffer to prevent crashing at the beginning and half way through fermentation.  If it crashes, I use calcium hydroxide to raise pH.  But I haven't had a problem with pH in the last 4 years using this procedure.

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Can we assume you are also using a high percentage of stillage/dunder? What were your very early experiences before you arrived at your current methodology?

Buffers, and their size, don't seem to be well written about, but I did just come across some nice leads in Arroyo's Circular 106 which I recently translated from Spanish. Apparently via Arroyo, and very cryptically described in the old literature, a carefully constructed buffer via plastering can help unlock rum oil.

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Thanks for commenting MDH. Is it well known from the rum literature or from general biotech literature? I have not seen it described anywhere but some brazilian cachacha literature and the work of the INRA.

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Once I started working through fischer esterification labs the merits of sulfuric acid as a pH lowering additive became very clear and I've never looked back!

Anyone doing high rye fermentations (90%+) should try sulfuric. really cool lactic esters result.

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4 hours ago, SLCDBD said:

Anyone doing high rye fermentations

Im confused. In rum??

 

 

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No silly.  rye whiskey. 

3 minutes ago, indyspirits said:

Im confused. In rum??

 

 

 

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I misunderstood because the thread topic is "Citric vs. Sulfuric Acid For Rum Production".  

 

 

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Esters form in the still, but they also break apart to reform later during maturation. I'm starting to see some spirits through birectifier analysis that have very significant volatile acid (non-acetic) in the last fractions and the birectifier stillage. This makes me think the producers figured out how to carefully ferment to justify distilling incredibly low so they could capture all that aroma. Unless esters are splitting apart in the birectifier, free fatty acids that meaningfully lower pH may be a defining part of the character of some spirit categories like mezcal or some fine tequilas.

Avoiding the negative effects of citric acid is part of the puzzle to justify distilling as low as possible.

I think Fahrasmane is concerned with helping new distillers that use no stillage and rely on citric acid because they are afraid of handling sulfuric acid and not aware of any metabolic phenomena.

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1 hour ago, bostonapothecary said:

Esters form in the still, but they also break apart to reform later during maturation. I'm starting to see some spirits through birectifier analysis that have very significant volatile acid (non-acetic) in the last fractions and the birectifier stillage. This makes me think the producers figured out how to carefully ferment to justify distilling incredibly low so they could capture all that aroma. Unless esters are splitting apart in the birectifier, free fatty acids that meaningfully lower pH may be a defining part of the character of some spirit categories like mezcal or some fine tequilas.

Avoiding the negative effects of citric acid is part of the puzzle to justify distilling as low as possible.

I think Fahrasmane is concerned with helping new distillers that use no stillage and rely on citric acid because they are afraid of handling sulfuric acid and not aware of any metabolic phenomena.

As someone that is primarily concerned with process efficiency (and especially process simplicity) here's why I was always turned off by arroyo's patent method:

It basically plays with fire. When you are basing your method on using fractions 2, 3,4, 6, and 8 (that's probably wrong, just used as an illustrative example) you open up your product to variation via human error. As we all know volatilities of organics change depending on the relative composition of the complex mixture. It's well and good to say "well, Raffie Arroyo did it, so can you all" but expecting to not make cut point errors in such a complex cutting scenario is unrealistic at a production scale. I'm sure the birectifier provides insight into that, but I have to ask... is it valid information if your production scale equipment doesn't have exactly the same capabilities?

Maybe Arroyo had the infrastructural resources to conduct his method flawlessly. Maybe he didn't and the patent was a panacea to enfranchise his company's lawyers to sue competitors that had a similar flavor profile to theirs. Who knows.

All I know is that understanding the process of how esters are formed in the still from constituent products that result from fermentative biology (and conversely how they break apart via hydrolysys when the concentration of alcohol is reduced during proofing) is essential to understanding how the flavors in great rums are created. Strong acids, alcohols interacting with carboxylic acids and long durations of reflux are how these esters are created... and whether or not you have a birectifier doesn't really enable you nor does it prevent you from making these flavors present in your finished product. I must say that I'm very motivated by @bostonapothecary's body of work and it made a huge impact on my development as a distiller. Thank you sir!

I'm reminded of 'lost spirits' and how they said they could make 30 year rum in 30 minutes, but couldn't present a rum under 120 proof that had a heavy ester load. That's because they were trying to sell 'accelerated aging' licenses to their so-called 'proprietary technology' which was really just a combination of a hot-process oak product extraction vessel and a large lab still that didn't do a great job of fischer esterification. They were banking on people that didn't take an o-chem class being willing to spend a lot of money on some magical way to print money. Speaking from experience... anyone interested in these topics should download some lab procedures for fischer esterification and do the work. You'll learn a lot.

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I think the Arroyo idea you are talking about is the Simultaneous production of light and heavy rums (from the same beer...). I would not confuse that with birectifier analysis though it was clearly inspired by it. No one knows anything practical about the simultaneous production idea and it may never have been practiced. It may finally be viable now that we have accessible automation ideas like iStills.

The birectifier and distilling slowly at very high reflux just sorts things and in doing that gives a magnified look that allows you to make a useful assessments organoleptically. The significance is how delineated the fractions are. It doesn't matter if you distill a tequila slow and low at 55% or a rum higher at say 75%. If you want to translate any of the fractions to evolving your own production you have to extrapolate and it is not hard to do, but don't confuse it with the above linked Arroyo idea which not much is known about.

The way this relates to citric acid and ferments is that we don't know enough about what makes a spirit worth distilling incredibly low versus high. We also don't know enough about comparisons across spirit categories. Tequila and Mezcal producers seem to have a far easier time distilling low than rum producers. Longer chain esters and rum oil may even allow a spirit to contain more of other basic congeners while still being harmonious, but that is just an emerging theory after looking at lots of role models across categories.

My other rough theory after looking at tequila and mezcal is that it may not matter that you got the complete ester formed in the still and into the spirit. It may just be enough that you got the fatty acid into the spirit by being able to justify distilling very low. During maturation it may form an ester or it may be valuable by itself.

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Just curious, going through that study again, what specifically in the study is leading you to believe that the choice of citric as an organic acid is somehow problematic?

The only yeast in the study that's currently relevant, I believe, is 493 (aka EDV493 or Distillamax RM).  It's a fairly common rum yeast strain, I actually use it quite a bit, it's absolutely a fuller-flavored yeast strain (higher congeners).  However, there aren't really any solid benchmarks, since the rest of the yeasts in the study are not in common use.  I can't seem to find any current reference to the INRA 390, which may have been renamed/numbered.

Not to mention, the statistical relevance is kind of thin for many of the columns in the two tables I think you are focused on (Tables 4 and 5) - the data seem relatively noisy as well.

Looking at the 493 rows as the most relevant, Oura Synthetic Medium + Citric was resulting in a slightly lower congener volume compared to the synthetic reference alone, higher alcohols were slightly higher and short chain fatty acids were slightly lower, but the net difference was marginal at best.

Acetic acid production is very tightly correlated to heads cut percentage (ethyl acetate concentration), and 493+Citric was among the lowest total acetic acid production values (Table 5).

It's absolutely not so clear cut.

Especially in the context of these two excerpts:

 

Quote

 

It is also noteworthy that citric acid induced the production of three other fatty acids normally absent from the fermentation medium: propionic, valeric, and isocaproic acids.


 

Propionic Acid production is probably the most important factor in high ester/heavy rums.  The propionate esters are a huge factor in the characteristic of rum, with Isobutyl Propionate being identifiable as "rum" by most people.  Citric inducing propionic acid production in 493 is a hugely positive factor.

The last factor which is material here, is the fact that molasses and cane juice will already contain these acids:

 

Quote

 

Cane juice and molasses are particularly rich in cis- aconitic acid and citric acid, sometimes they also contain malic acid (Matsui, 1976; Meade, 1977).


 

The addition of sulfuric acid as a mineral acid catalyst in Fischer esterification is a very, very different thing.

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It is not the clearest in that paper, and that is the zoomed in perspective a micro biologist and not a distiller concerned with beauty, but I think it is better described in his thesis which I've only seen a couple pages of. I think the problem is that even if those extraordinary congeners are produced, they are inaccessible because of the surplus of ordinary stuff that needs cut away.

In the Brazilian papers, citric acid is used as a yeast starter at the beginning of the season because of how it accelerates the creation of yeast biomass.

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On Home Distiller they usually try to avoid the rum ferment pH from crashing so they don't add too much acid at the beginning.  Maybe just a little to make the initial yeast happy, but the pH is going to be dropping anyway. Even if only from dissolved CO2. Usually they're putting in a buffer, calcium carbonate or just oyster shells.  They're not "pros" but they do have a lot of experience and the ability to do a lot of low cost experimentation.

This guy used a slab of marble with humorous results:

https://homedistiller.org/forum/viewtopic.php?f=3&t=65703&p=7478337#p7478320

 

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It’s counterintuitive but acidification of the non-sterile rum wash can prevent bacteria from taking hold in the ferment, actually preventing the pH from crashing.  This can also yield a cleaner rum as a result.

Pure sugar washes also have nearly non-existent buffering capacity, so they crash like no other.

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Interesting, I guess that makes sense.  I had imagined the pH crash was mostly from CO2 production, but had no evidence.

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16 hours ago, Silk City Distillers said:

It’s counterintuitive but acidification of the non-sterile rum wash can prevent bacteria from taking hold in the ferment, actually preventing the pH from crashing.  This can also yield a cleaner rum as a result.

Pure sugar washes also have nearly non-existent buffering capacity, so they crash like no other.

When many of us started investigating rum from a drinkers perspective with little chemistry and biology experience, it was thought for a while that low initial pH ferments were for full flavored rums which just isn't the case at all. Stressfully low pH can make an S. pombe yeast dominant, but not exactly produce a full flavored rum.

I suspect the thing to learn about is the size of the buffer and its relationship to quality.

Since there is little interest in citric acid, we should probably figure out what else people are curious about that we can get Fahrasmane to answer. 

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The possible citric acid mediated gene expression of 493 to produce propionic acid is probably the most important part of that paper for rum producers.

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26 minutes ago, Silk City Distillers said:

The possible citric acid mediated gene expression of 493 to produce propionic acid is probably the most important part of that paper for rum producers.

I'll ask him to clarify that so we know whether it is accessible or not because of other congeners produced that will need cut away. It is interesting to examine the relationship between micro biologists and the practical work of the distillery.

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Regarding the cut away of undesirable congeners - acetic acid and ethyl acetate.

This accounts for something like 90-95% of carboxylic acids and corresponding esters produced in a typical ferment.

 

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16 hours ago, Silk City Distillers said:

Regarding the cut away of undesirable congeners - acetic acid and ethyl acetate.

This accounts for something like 90-95% of carboxylic acids and corresponding esters produced in a typical ferment.

 

True, but as they accumulate, they still bleed further into run and shrink your hearts fractions because you can only justify having so many. Increases in ordinary congeners decrease the ability to capture extraordinary congeners. Even at extremely high distillation proof, such as with birectifier distillation, if a spirit is ladden with ethyl acetate you don't exactly get a massive accumulation in fraction 1, where the vast majority is collected, you get a bleeding into fraction 2 and 3. If the distillation proof is much lower, such as with a normal production, you can just imagine how far across the run they spread. The same is true of fusel oil on the other end and desirable aromas such as long chain esters and bizarro terpenes are less volatile than fusel oil so walking that line becomes critical.

What I've been observing through analysis is that new distillers don't nail their cuts ideally and are often pretty far off of role models. This all gets complicated by Maturation. Analysis to be better execute cuts would probably improve new American spirits a lot. You'll be able to distill at the maximum potential of your ferments.

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