Stabilizing esters in finished spirit

I've been experimenting with creating a super funky high-ester rum. However, I've been noticing that the esters degrade over time when it's barrel aged. I also made a Navy strength high ester rum and noticed that the esters degraded significantly after sitting in the bottle for a few months to the point where it was unrecognizable. Is there a way to stabilize the esters in finished spirit? My best guess is to add a little bit of acid (citric?) to the spirit but I'm not sure.

The lower the ABV the more esters you're going to lose. Keep the ABV as high as possible for as long as possible. Water breaks esters apart. Acidity probably helps reform the esters. https://www.jove.com/science-education/11213/hydrolysis-of-an-ester

You also can have transesterification - it's where higher polarity molecules replace the acid side of the equation and it becomes a different ester. https://en.wikipedia.org/wiki/Transesterification

Noticed the same thing - unique ester profiles are seemingly obliterated after 1-2 years aging.  We really noticed this with our 100% rye - which had a very fruity profile at the 1 year mark, and nothing at all by the 2 year mark.  So much so, that regular customers actually asked us to bottle the younger rye.

One of these days I'm going to run a batch of rum, take no heads cut at all, and put it all into a barrel just to see what happens.

I second maintaining higher proof, barrel and bottle.

Check this one out.  This is the longest study I've ever seen - tracking changes over 10 year span of product.

Baijiu_Esters_Long_Term.pdf

That's a good find.

Oxidation of ethyl alcohol: https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Alcohols/Reactivity_of_Alcohols/The_Oxidation_of_Alcohols

Oxidation of aldehydes: https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Chemistry_(Smith)/Chapter_20%3A_Introduction_to_Carbonyl_Chemistry_Organometallic_Reagents_Oxidation_and_Reduction/20.08_Oxidation_of_Aldehydes

Another thing I saw in those pages is that higher alcohols oxidize to ketones - some of which have interesting aromatics/flavors.

On 5/20/2021 at 1:03 PM, Foreshot said:

The lower the ABV the more esters you're going to lose. Keep the ABV as high as possible for as long as possible. Water breaks esters apart. Acidity probably helps reform the esters. https://www.jove.com/science-education/11213/hydrolysis-of-an-ester

You also can have transesterification - it's where higher polarity molecules replace the acid side of the equation and it becomes a different ester. https://en.wikipedia.org/wiki/Transesterification

Thanks for the info. On Friday I actually tried an experiment related to this. I added a bit of dunder to a spirit run - great results, ended up with desirably funky distillate. On the next spirit run, I added dunder and citric acid. My theory was that citric acid would aid in Fischer esterification and magnify the flavors I achieved during the previous run. Nope. I found it destroyed the good esters I was trying to make and instead I got relatively neutral hearts and ended up with tails that tasted bitter and horrible. It tastes like some sort of salicylate (menthol/wintergreen) ester was formed as well.

On 5/22/2021 at 12:53 PM, Silk City Distillers said:

Noticed the same thing - unique ester profiles are seemingly obliterated after 1-2 years aging.  We really noticed this with our 100% rye - which had a very fruity profile at the 1 year mark, and nothing at all by the 2 year mark.  So much so, that regular customers actually asked us to bottle the younger rye.

One of these days I'm going to run a batch of rum, take no heads cut at all, and put it all into a barrel just to see what happens.

I second maintaining higher proof, barrel and bottle.

Yep, I've noticed it in my whiskey too. Isoamyl acetate (banana) in particular - which I don't necessarily want a ton of in my whiskey - but I want it in my rum and I can't get it to stick around!

I used to make very conservative cuts. Now if I'm making spirit that I plan on ageing, I'm going really deep into the tails. I'm starting to find going deep on the heads yields good results as well - as long as there isn't too much ethyl acetate.

Sulfuric acid is what’s typically used as a catalyst.  It needs to be a mineral acid.

2 hours ago, CalwiseSpirits said:

I added dunder and citric acid. 

2 hours ago, Silk City Distillers said:

It needs to be a mineral acid.

So to explain this a little (for everyone aside from SCD) - citric acid is a carboxylic acid where sulfuric isn't. Citric acid will take part in esterification reactions, sulfuric won't. So when you added Citric acid you changed what esters were being formed. If you used sulfuric it would have enhanced what esters were previously being made. You wouldn't change the flavor as much as enhance it. The flavor might change though as the catalyst will enhance all esterification reactions, not just the "good" ones.

On 5/25/2021 at 10:52 AM, Silk City Distillers said:

Sulfuric acid is what’s typically used as a catalyst.  It needs to be a mineral acid.

22 hours ago, Foreshot said:

 

So to explain this a little (for everyone aside from SCD) - citric acid is a carboxylic acid where sulfuric isn't. Citric acid will take part in esterification reactions, sulfuric won't. So when you added Citric acid you changed what esters were being formed. If you used sulfuric it would have enhanced what esters were previously being made. You wouldn't change the flavor as much as enhance it. The flavor might change though as the catalyst will enhance all esterification reactions, not just the "good" ones.

Thank you for explaining that. It makes sense. I should have known.

Is sulfuric acid ok to use in distillation? Won't it decompose and form some nasty byproducts? 

On 5/26/2021 at 2:39 PM, CalwiseSpirits said:

Is sulfuric acid ok to use in distillation? Won't it decompose and form some nasty byproducts? 

As far as I know it's ok to use. It's been used in the rum world for a long time and I've never heard anything bad about it.

 Stumbled onto this thread a little late.
Adding sulfuric acid does help esterification, through it still depends on the water content and heat conditions. Even a relatively high gravity wash (14-17%) will still have too much water to allow for good esterification. Meanwhile if you add sulfuric acid to the wash, the acid will start interacting and create sulfites and sulfides.

The sulfides (rotten egg smell and the likes) can be absorbed and filtered out by a reaction with copper, though the sulfites (burned matches) do not react with copper. So too much, or too long on the sulfuric acid during fermentation or in the wash at any point can be an issue.
Adding sulfuric to low/high wines will work better to promote esterification. Though I wouldn’t drink that product, I’d redistill it.

Using sulfuric acid as a stabilizer in a final product sounds a bit suspect to me. It’s a very strong acid and I would avoid drinking it at any concentration.

Far more likely that the reactions are taking place in higher alcohol environment of the lower trays, with the newly formed esters riding the escalator up and out of the column, or as vapor phase reactions in the headspace of the pot or the headspace of the trays (also lower-water environments compared to the pot itself).  Low alcohol washes still do contain esters, and ester precursors, but yeah, the equilibrium point is not going to be favoring esters, as you move into the vapor phase, or up the trays, the concentration of alcohol increases, as well the equilibrium point to favor esters.

Would be really curious to know if the sulfur compounds on the copper could potentially be acting as catalysts in the reactions.

If this is the case, dirty copper might yield a superior spirit vs. clean copper.  Plenty of old references indicating an aversion to bright copper.

The copper state is an interesting one. It could be a catch 22. Not neutralizing the sulfuric acid is an upside. Though not neutralizing the sulfides is going to be a problem. The sulfites are not a major issue as they are very volatile and exposing the distillate to air for a while should get rid of them. Though the rotten egg sulfides are quite heavy. 
The heat and alcohol content on the plates should provide a good platform for esterification. Though the time parameter could be a challenge. Even in favorable conditions, most documentation recommends a minimum 24 hours for esterification. You can push the plate process with a full dephleg for a while. 
thumpers with a valve bypass can be super useful here. You can pre charge the thumper for esterification. Then run heads through the bypass then engage the thumper. 

@Silk City Distillers, have you had any luck with esterification on a played setup? I’ve hit some walls trying that. Wonder if there's somrething I'm missing in the proccess.