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Whiskey and water have a complex and symbiotic relationship. You may have heard people mention how water helps "open up" a whiskey's flavor. Similarly, it is common for distillers to purposefully dilute their whiskey twice during the standard production process, once from still proof/distillate proof to barrel entry proof and then from barrel proof to bottling proof. Dilution from still proof is typically done if they are distilling to more than 125 proof, the maximum proof for barreling bourbon, so they must add water to bring it down for barreling. Bottle proof on the other hand is may be driven by product design, extensive testing, economics, or tradition. For a whiskey like Jack Daniel's which is distilled to 140 proof and bottled at 80, that's more than 40% dilution of the original alcohol. With the craft whiskey boom of the 2010s, smaller producers started to ask if there is a way to do this down proofing while retaining more of a whiskey's flavor. Enter slow-proofing.
Slow-Proofing Defined
Slow-proofing is the process of gradually adding water to whiskey while it is still in the barrel a few liters at a time, typically over months, to reduce the proof of the whiskey for bottling. Due to its whiskey origins in the whiskey movement, you may hear slow-proofing referred to by a number of names like "Slow-Dilution", "Barrel Down-Proofing", "Slow Cutting", or "Slow Water Reduction", as many producers have tried the idea independently. It is worth noting that the practice is much more common and historically applied by Cognac and Armagnac makers.
Advantages to Slow-Proofing
It comes back to whiskey chemistry; the basic fact that different chemicals are better soluble in water and alcohol. Water dissolves ionic and polar compounds well while alcohol better dissolves non-polar compounds and things like esters, hydrocarbons, oils, fats, and lipids. The theory is that the aged whiskey has already extracted most of the alcohol soluble chemicals from the barrel, so by adding water slowly over time, the producer can help coax out more water soluble chemicals. Notably, wood sugars (i.e. zylose, rhamnose, arabinose, etc.) and aldehydes (like lignin which turns to vanillin when charged) are better extracted by water, so its addition helps to soften the overall harshness of a whiskey. This is particularly key in hut or arid environments as there is a tendency for the alcohol to over extract smokey phenols and tannins (high heat expands the alcohol far into the barrel due to its slower specific and more water is lost to the dry air on balance). It can also help to prevent a process called saponification, a chemical reaction in which fatty acids break down, that produces an off-putting soapy flavor. Whiskies have significantly less of these fatty acids than brandies (often distilled with dead yeast) but can still benefit similarly.
Why isn't Slow-Proofing used more widely?
Visits to the barrel month over month are a labor intensive process. Each time, the bung must be removed, and the water must be measured exactly and tailored to each individual barrel (barrels will have different proofs due to differences in angel's share evaporation). I am not aware of any producers using special hardware, but it seems to me that swapping the bung for a valve once slow-proofing begins may help expedite each trip significantly. Labor = cost, so heritage producers who are likely competing directly with each other are unlikely to bear the additional expense, resulting in the practice being more prevalent with craft producers, benefitting from local customers and connoisseurs' higher willingness to pay.
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| TTB filing per Owensboro Bourbonite |
Producers Known to Slow-Proof their Whiskey
The following producers are known to use slow-proofing for at least some of their lineup (not an exhaustive list).
- Iron Fish - Michigan, Mad Angler Bottled-in-Bond
- Ironroot Republic - Texas, various
- J. Henry & Sons - Wisconsin, Small Batch 92 Proof and Bellefontaine Reserve
- Lost Lantern - Vermont, Used for first edition American Vatted Malt, reduced from 60% down to 52.5% abv over three months
- Old Elk - Colorado, Slow proofs eye, bourbon, and wheat whiskey over 1.5 weeks, notably shorter than others
- Sonoma County Distilling Co. - California, various
- Sidewinder Spirits Co. - California, various
- Still Austin - Texas, used for all in iterations
- Virginia Distillery Co - Virginia, Courage and Conviction Single Malt is down proofed over at least three months to get to the final bottling strength of 46% abv
- Wyoming Whiskey - Wyoming, various