There’s a lot of overwrought marketing copy and high-caliber BS around cask-fill strength. Instead of piling on, let’s critically examine this variable in a way that’s more useful to distillers.
When you’re building a world-class spirit, there are numerous angles to consider. Selecting raw materials for virtually any spirit category can be a labyrinthine task—and processing and fermenting those materials, even more so. Then we get into the heady world of distillation, with all the techniques contained therein.
However, step into the realm of barrels, and you’re in for a Carrollian Wonderland in which many people are never quite sure which direction to take next, and most things are often not what they seem. Some decisions may be laid out for you—even dictated—seemingly removing your input and control from the equation entirely. The U.S. regulatory insistence on the use of charred new oak barrels for bourbon, for example, immediately comes to mind.
However, the choice of entry strength of the spirit into the cask is one in which—even here in the United States, where the legal maximum entry proof is 125 proof (62.5 percent ABV)—we have great latitude. So, let’s take control when and where we can.
[PAYWALL]
Higher Proof Makes Cents
For whiskey, the predominant barrel-fill strength used in the United States is 62.5 percent ABV, the legal maximum. In Britain, for single-malt whiskies, the predominant strength is often around 63.5 percent ABV—that’s based on tradition, but there is no legal maximum fill strength.
Here in the United States, you can see the logistical and financial benefits of going into the cask at 125 proof. Most notably, it means you can fit more alcohol into the barrel, which in turns means you need fewer barrels. That’s a big money saver, but it goes further: Fewer barrels means less warehouse space to hold all that booze and, subsequently, less labor needed to take care of everything.
I’m sure the larger distillers have done their financial due diligence by performing cost-saving analyses on the various entry proofs versus 125. While I haven’t seen those numbers myself, I think we can safely assume the savings would amount to a boatload of rainy-day piggy banks.
So, there you have it: Higher entry proof saves cash. End. Of. Story.
Or is it?
Of course not. We haven’t even touched on the other angle dangling well within our sights: the character of the spirit. The concentration of alcohol that you put into the cask has an enormous impact on the maturation that ensues.
Solutions for Flavor
Ethanol and water are both solvents—at some point in your life, you may have heard that water is the “universal solvent.” And while many things do easily and often quickly dissolve in water, the title of “universal solvent” is a little undeserved.
Solution chemistry is complex stuff, and we can dissolve only so much of it into this article. But we can sum this up: Some things have more of an affinity for dissolving in water, while other things dissolve more easily in ethanol.
It’s important to understand that when it comes to spirits and maturation, there are very few absolutes. Among all the compounds that have some kind of sensory impact on the final spirit, virtually none of them swing entirely one way or the other when it comes to their preference for dissolving in ethanol or water. And “preference” really is the operative word here: The constituents in our spirit-barrel matrix merely tend to skew only somewhat in either direction—either water or ethanol dissolution.
In short: All those compounds that affect the spirit’s character in the barrel will effectively dissolve in both pure ethanol and water—but they do so at different rates.
Many in the industry, including myself, have long assumed that the facts were quite simple: Wood sugars, color compounds, tannins, and the like extract more quickly at lower ethanol concentrations—read: more water in the matrix—while wood lactones extract more quickly at higher ethanol concentrations. The truth is that there is very little research to back up those assumptions. However, as it often happens, word passes through the industry chain links, and before we know it, assumptions become hardened gospel.
Fortunately, things are starting to move in the world of cask science, and researchers are taking an interest in the fill-strength conundrum.
A Word from the Science
Jennifer Riffkin is currently finishing up her doctoral work at Abertay University in Scotland under the esteemed Graeme Walker, who was kind enough to send me a copy of her thesis. While we can’t say too much yet about the results because it’s still unpublished, we can say that her research found some interesting surprises that contradicted the prevailing narrative.
In her study, Riffkin looked at three different fill strengths: 50 percent, 63.5 percent, and 70 percent ABV. The data suggest that the spirit extracts tannins, furfural, and phenolic aldehydes at higher rates when the fill strength is 63.5 percent ABV, when compared to the other two fill strengths. That flies a little in the face of the conventional, anecdotal wisdom often shared around the distillery watercooler—many would expect the spirit to extract those compounds better at lower fill strengths.
Based on Riffkin’s work—which largely supports theories on fill strength that have been floating around Scotland for decades—it would appear that the larger U.S. distilleries have it right with higher entry proofs and flavor development. If that’s the case, however, then why are so many of us obsessed with going into the barrel at lower proofs?
There has been some research done on this in the past, in the context of American whiskey. In 1974, Sidney Baldwin and Arthur Andreasen published a study for which they looked at the evolution of congeners and cask extractives of six entry proofs, starting at 109 (54.5 percent ABV) and moving all the way up to 155 (77.5 percent ABV). Interestingly, the researchers found that color, solids (sugars), tannins, and so on all seemed to increase at higher rates in the lower entry-proof barrels.
That’s obviously a different finding from Riffkin’s, but the conditions for each experiment also were different. Clearly, more research is needed.
However, at least stateside, Baldwin and Andreasen’s work in the 1970s—coupled with anecdotal data from industry veterans—is likely where many distillers get their ideas on using a lower entry proof.
Past and Current Practices
Michter’s is no slouch when it comes to producing delicious high-end whiskey, and they famously go into the barrel at 103 proof. Kentucky Peerless hits the cask at 107 proof. In Colorado, Todd Leopold and the Leopold Bros crew go in at around 100. My own distillery—Iron City, in Pittsburgh—does everything at 101.
We all have our reasons. And much of it is rooted in history.
In 1962, the government raised the legal maximum entry proof for most American whiskey to 125. Before that, it was 110 proof. If you’re a cynic on such things—as I most definitely am—then it would be easy to see this as the results of lobbying by distilleries led by bottom line–oriented accountants and investors trying to save a few cents.
When the law changed, many distilleries followed suit and went to the maximum; it allowed them to get more product from fewer casks. Only a few distilleries, most notably Maker’s Mark, stuck with the lower-proof practices of the old days.
Before Prohibition, as we know from documents from that time, entry proofs in the United States were even lower in many distilling regions, often hovering around 101 proof. In the second half of the 19th century, grocers and taverns often bought whiskey by the barrel and served it directly from the wood. If you were a thirsty, booze-loving American, you’d take whatever container your family normally used for such things down to the local purveyor and have them fill it to the brim with whatever hooch had your fancy.
Knowing that so much of their whiskey was likely going to be served from the barrel, many distillers of the time filled their casks at a lower—and subsequently more drinkable—proof. (Of course, serving from the barrel also allowed too many unscrupulous grocery and bar owners to adulterate the spirit and stretch it through the addition of myriad substances, benign and otherwise—but that’s a story for another day.)
But did the lower entry proofs of the day make a better whiskey? The answer to that is more complicated. If you ask me, it simply made a stylistically different whiskey—not better in any meaningfully quantifiable way. Just different. Yet if some distilleries claim qualitative victory in the entry-proof debate—pointing to their own successful products, of course, as the irrefutable evidence—then this is could be where some of the half-truths enter the conversation.
A More Sophisticated Approach
Over the past few years, I’ve read and heard a large number of the low-entry-proof proselytizers, and one of their most common refrains is that lower entry proof matures the spirit “faster.” There is not enough breath in my lungs to release the elongated sigh that my soul feels when I hear such nonsense.
“Maturity” in distilled spirits is difficult to define—it’s very much in the eye of the beholder. What may taste perfectly mature to one imbiber may come off as young and brash to another and overdone to someone else. For this piece, let’s roll with the idea that maturity is simply the steady reduction in immature, new-make spirit character coupled with the gradual rise in wood character.
So, if your definition of mature spirit from the cask is liquid that’s taken up a lot of wood sugars, tannin, and color, then yes—we can say that a lower entry proof speeds things along quite nicely. However, this is a bit like the discussion around the surface area of smaller casks; I’d argue that viewing maturity as nothing more than higher levels of dissolved solids in the spirit matrix is overly simple. It plainly misses some of the subtler chemical reactions that improve spirit character. For many of these oxygen-related reactions, there is no substitute for time.
Alas, the story gets more complicated still. Because proof changes in the cask over time, some casks see an increase in alcoholic concentration while others see a decrease. Take a barrel from the upper reaches of any old wooden warehouse dotting Kentucky’s bourbon-soaked fields, and you’ll more than likely find that the proof is a bit higher than whatever the spirit’s strength was when filled. Indeed, it’s not uncommon to see some barrels work their way up to a “hazmat” level above 70 percent ABV.
However, in cooler climates such as Scotland, the opposite tends to happen: Alcoholic strength often goes down with time, dropping from 63.5 percent ABV to the mid to low 50s over many years. (There are even stories of whiskeys falling below 40 percent ABV after many decades, which means the liquid can no longer be considered “whisky” in Scotland. Ouch.)
In either instance, you can see how the changing proof inside the cask would slowly and subtly affect the preferential extraction rate of wood compounds.
Then there’s the issue of forecasting future product needs and the importance of age statements. Some distillers use a lower entry proof so they can have product ready for sale a bit sooner. (As I’ve already mentioned, the underlying truth is more nuanced.) A few years go by, and they get some product out the door. The sales orders keep coming in, and the distillery expands production to meet demand while also trying to think about the future. It’s at this point that some distillers consider adding products with older age statements, for a variety of reasons. With those longer-aged products in mind, they may revisit their initial fill-strength policy.
As my friend Ryan Venrick, cask manager at Watershed Distillery in Columbus, Ohio, recently told me, “As our stock gets older, there may be a need to adjust the fill strength again, but for now, 115 [proof] works great for our spirits. I think having some varied fill strengths is a big advantage when it comes to creating blends. It gives me more brushes to paint with, so to speak.”
Putting Entry Proof to the Test
So, should you start using lower entry proofs in your own program? That depends on what you want to get out of it.
I think Ryan is spot on: Having a few different cask-fill strengths to work with affords you some flexibility when it comes to product development and blending. Personally, I prefer the character I get from whiskeys placed into the cask at lower proofs. But I also like a lot of spirits matured at higher entry proofs. Obviously, entry proof is just one factor out of countless others that impact the final quality of spirits we produce.
The truth is, we just don’t have much in the way of modern data to say exactly what happens one way or the other. A number of the larger U.S. distilling firms have most certainly done their own internal research into the matter, but to the best of my knowledge, no data have been published publicly.
With that in mind, my team and I recently set ourselves the task of looking at this subject more closely within our own program.
On February 25, 2025, Iron City Distilling laid down 25 barrels, with these five entry proofs each going into five barrels: 125, 115, 105, 95, and 85. The spirit going into these 53-gallon casks came from the same tank batch, and we saved several liters of new-make spirit for a baseline analysis. The barrels themselves are all char No. 4, also produced in the same batch by West Virginia Great Barrel Company. We put all 25 barrels on the same rick in our steam-heated warehouse.
We’ll pull samples for analysis every few months to determine amounts of the usual suspects, including esters, higher alcohols, alcohol concentration, tannins, sugars, color, and so on.
We’re excited to see what happens. And don’t worry: We’ll share what we find.
Want to Dive Deeper into Proof and Maturation?
Here’s some further reading:
Baldwin and Andreasen, “Congener Development in Bourbon Whisky Matured at Various Proofs for Twelve Years,” Journal of Association of Official Analytical Chemists, Volume 57, Issue 4, 1 July 1974, pages 940–950.
Crampton and Tolman, “A Study of the Changes Taking Place in Whiskey Stored in Wood,” Journal of the American Chemical Society, Volume 30, Issue 1, 1 January 1908, pages 98–136.
