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Jun 08, 2022 View:

Evaporation In Barrels

Does anyone out there have any idea of proof loss over time in bourbon barrels? I am just wondering what proof I should begin with, assuming I dont want to possibly undershoot my final proof. I assume the big boys put in their product on the high side (and up to the limit of what the law allows, at least for bourbon) so they may then maximize the storage space in the barrels. Since we are a small craft distillery, I was thinking it might be better not to store so high and actually age closer to the final proof. The main reason being not to alter the final color and flavor of the product by adding a bunch of water when it finally comes time to bottle. I realize this might be a hard thing to nail down, but even a ballpark number would help. Thanks.


Loss of alcohol is normally around 2-3 percent annually per barrel due to evaporation through the wood. The evaporation rate is going to vary depending on your humidity level; (low humidity level -> more water evaporation; High humidity level -> More alcohol evaporation)



Loss of LIQUID is 2-3% per annum normally, up to 10% in the tropics. But WHICH part of the liquid is the question. In some areas more water evaporates, in others more alcohol. It all depends upon the local humidity and heat. Most of the time, proportionally more water evaporates than alcohol, and the abv. in the barrels INCREASES over time.

If I remember correctly, when the humidity is lower, the abv. increases. If the humidity is higher, the abv. decreases.

Read this short article.

Different aging strengths bring out different flavors from the wood. There is a reason why the optimum abv. for barrel aging is 55-78%, depending upon the spirit.

As mentioned in the article: Bourbon: 55% , Malt Whisky: 63.5 – 68 %, Grain Whisky*: 68 % - 74%, Canadian Flavor: 57%, Tequila: 56%, Rum: 78%

* 68 % for grain spirit is Scotland; 74% for Canadian grain whisky. The strengths are approximate and may differ from company to company. The Tequila strength refers to 100% Agave made in pot stills.


A small correction to the article. Most American whiskey, including most bourbon, is entered at the legal maximum of 62.5% ABV, or very near to it (e.g., 60%), but rarely as low as 55%. The big difference between American whiskey and most other whiskey is that the barrel for American whiskey is always new and charred.


Also, the article states that high humidity usually results in strength gain over time. I thought the opposite was true. If there is a lot of water in the surrounding atmosphere (high humidity) then water inside the barrel would be less likely to leave and alcohol would leave. Vice versa for low humidity.

The author also claims that low humidity is associated with better flavor. I wish there was more explanation there. Some of the best spirit in the world is aged in highly humid conditions (Islay malts and most Calvados). Certainly great spirits are made in low humidity aging conditions too.

I'd love to read something that really laid out the differences in impact on flavor profile between high and low humidity aging conditions.


Following on the wisdom of Chuck and Jonathan, and our own experience, if we go in at higher proof on our bourbon, we tend to end up at 113 proof in any case. So we now choose to barrel at 110-115 proof.


Here is a pretty heady blog post for the cerebral types: