Let me preface this by saying that my opinions are from the standpoint of a microbiologist/biochemist. While I have over a decade of experience working directly with distilleries, my expertise lies more with the microbiology and biochemistry of fermentation (yeast and bacteria), rather than a chemical expert on ester biosynthesis.
What process causes the most esters to form? (Enzyme/Fischer/Other)
When looking at esters I consider which ones were produced by the yeast and bacteria in fermentation to begin with (often simple esters with fruity notes) and also what esters were formed from precursors (alcohols and carboxylic acids- ethyl alcohol plus acetic acid = ethyl ecetate, for example) during fermentation, distillation and barrel aging. Also, what do the simple esters become over time during barrel aging- do they stay the same, turn into complex esters (with more complex clove or spicey notes). I need to brush up on hard core ester chemistry and relevant literature. This is very complex and there doesn’t seem to be any simple answers.
In what stage do most esters form? (Ferment, Wash run, Low Wines, Spirit run, aging)
My guess is fermentation and aging. Our whiskies are currently aging, so every year we gain more data and learn more.
What process(es) creates the best esters for whiskey/rum?
Fermentation and aging are where it’s at. This would be dependent on several factors- yeast strain, presence or absence of bacterial contaminants, fermentation conditions (temp, duration, pH, mash recipe, etc), aging conditions (type of barrel- age of wood prior to making barrel, toasting, char level), entry proof, pH/acidity (drives ester reactions), how long in the barrel, etc. So very multi-factorial. This would vary for whiskey and rum.
How can one target specific esters? (As an example Redbreast’s rose/floral notes)
This is the billion dollar question. If you like certain notes from a particular product, then you can start with the mash recipe if you can get it. Along with the yeast strain and fermentation conditions, barrel and aging details as mentioned above. There are several different esters that produce floral notes, but most have 6+ carbons on the alcohol component and 9 carbons from the carboxylic acid. There are others such as esters derived from salicylic acid (a plant component), but it is uncertain exactly how these are formed.
Does wort clarity affect ester production, does it target specific ester production?
My guess is that any part of the fermentation process that can be changed could affect ester production, even if ever so slightly. This would also depend on the starting ingredients/grain bill, yeast strain, bacteria, etc as above.
For ester production, what process is better: Sour Mash or Muck?
This is debatable. We run a sweet mash process at our distillery and we get wonderful aromas. Our aging whiskies are also first class, so we know you can accomplish an exceptional ester profile without sour mash or muck. Having said that, either of those would contain organic acids (lactic, acetic, etc) that are precursors to esters. Also lower pH can drive ester production, so another thing to consider. If either sour mash or much contributed wild yeast or contaminating bacteria, that would be another way it would influence ester and other chemical production.
Has there been any research into putting tails in ferments?
We often recycle tails back to the beer well, but not fermentation (could be toxic to the yeast). We only make tails cuts on our pot still. Most larger distilleries, including us, use column stills, so tails are controlled by column temps and doubler conditions. Having longer (carbon) chain alcohols could influence production of more complex esters (hexyl-, benzyl-, or heptyl nonoate, for example).
For ester production, what would you consider to be a minimum ferment time? Maximum useful time?
This is debatable and would be highly dependent on the yeast strain and any contaminating bacteria, in addition to other factors like the mash bill.
Are there any ways to increase ester production in aging?
Age longer. Different barrel treatments (aged wood, toasting, char level). Position in the warehouse (temp, humidity). Barrel entry proof. pH and acidity of the distillate.
How does oxygen in the mash affect esters?
We know that oxygen can oxidize alcohols to form aldehydes and acetic acid. Acetic acid is a precursor to ethyl acetate, so that is one small example of how oxidation could influence ester production. There are likely other reactions that could similarly affect ester production. We also know that oxidation drives production of chemicals like vanillin, which is a positive influence to aged bourbon.
How does pitching rate affect yeast?
Depends on the strain, the reproductive capabilities, fermentation temperature and many other factors.
What yeast produce the most esters?
In beer production, ale yeast produce more esters than lager yeast for example, but they are generally fermented at highly varying temperature. But that isn’t the only way esters get into the beer. One must also consider the precursors produced by yeast and bacteria (organic acids).
Is there a way to encourage yeast to produce esters?
My belief is to start with a simple process that can be repeated and if it works it takes less effort to maintain great consistency. There are all kinds of different opinions and methods for “improving” esters, but at the end of the day if you do something (sweet mash vs sour mash, this yeast vs that yeast, level of bacterial contamination, grain bill, etc, etc,) to make a distillate, if the end result is favorable and has a distinguishable bouquet, then you don’t even need to know anything about ester chemistry to be successful. Many distillers that talk about ester profiles, may just be referring to aromatic qualities in general without elucidating if specific notes are from esters, alcohols, aldehydes, acids, oils, etc.
Does clarity of the wort affect esters?
How does floculation affect ester production?
This probably matters more in beer production, because grain based alcohols have such high solids in fermentation the yeast don’t settle out like they do in a clear wort. Rum production would be more like beer (less solid and a clear mash), but I don’t know how flocculation would affect ester production.
How does open/closed fermenting affect esters?
Open top could allow for more bacteria and wild yeast contamination. Closed top may cause backpressure or additional temperatures, so I’d say there are good and bad with each type. Closed top would also likely be more “anaerobic” due to less ability to vent CO2. This could affect oxidative reactions and is known to affect yeast metabolism as it relates to unsaturated fatty acid biosynthesis and sterol production. Not sure how that would fit in with ester production, but the point is the level of oxygen can affect yeast metabolism, which is bound to affect ester or ester precursor production.
What are we missing with esters?
I would like to know more about the following:
Examples of simple esters (fruity) and how complex esters (floral, spicy, clove, etc) are formed and examples for each. Also what drives these reactions in fermentation, distillation, and barrel aging.
Examples of esters that can be tasted at levels below detection.
Does breaking down proteins to amino acids (the protein rest) in unmalted grains increase ester
It sure could as liberated amino acids lead to all kinds of different chemical reactions and serve as precursors to many metabolic byproducts. We cook our small grains (wheat, rye and malted barley) at lower temps than corn to reduce breaking down proteins. We are mainly concerned about liberating arginine, a precursor to ethyl carbamate (a carcinogen).
Does Fischer esterification require a second type of acid or is one type of acid enough? (example: an
alcohol plus a lactic acid plus another lactic acid work, or would it require a different acid like
Sulfuric can serve as a co-factor and drives the reaction, but I don’t think it incorporates with an alcohol like an organic acid (acetic, etc). This gets a little out of my area of expertise, although I could read up on it.
What’s the best way to reduce ester production? (for neutrals)
That is hard to say.
What order do you see contributors to flavor profiles?
Grain bill and grain quality, barrel quality and aging, yeast strain, method of fermentation, method of cooking the grains (from most likely contributor to least).
Aside from esters, what do do you think homedistillers should focus on for flavor development?
Running a process that is consistent and can be repeated while producing an excellent spirit. I think errors or just plain bad ideas are what lead to bad juice. If you stick to tradition, develop a nice prototype and change it slightly from there by making incremental changes you can formulate a unique process and great juice.
Processes, materials, etc.
What’s the hardest thing to master in developing a great flavor?
Consistency between batches.
How to keep Muck pits from become ammonia?
This would depend on several factors (muck chemistry, microbes present, oxygen potential, etc).
For conversion, is Alpha required or can it be done completely by Beta?
My understanding is that you need alpha. After all it is alpha 1,4 and 1,6 linkages you are trying to break. Beta can help reduce viscosity, but nothing that alpha won’t do.
What books you would recommend for aspiring distillers?
All of the articles I sent to you that I wrote. As well as the old Seagrams books and manuscripts. Also come to one of our training sessions and we’ll show you how it’s done.