Bourbon Science: Heat Cycling and Barrel Entry Proof

Maker’s Mark temperature controlled aging cave, courtesy Maker’s Mark

We all know that a bourbon’s time in the barrel gives it many of the characteristics we equate with its bourbon-ness. In my last article about what’s happening in the barrel we learned about esterification and other chemical processes as well as the effect weather has on open warehouses. But even those factors are just some of the few that influence the end product. There are so many ways outside influences can affect whiskey maturation that we may not even be able to know them all. Two additional factors to examine are barrel entry proof and the differences that come from heat cycling warehouses - they may make a much bigger difference than you think.

Barrel entry proof makes a bigger difference than you might think

Prior to 1962 barrel entry proof was typically around 107. The reason, according to bourbon historian Michael Veach, is because prior to the Bottled-In Bond Act of 1897, bourbon was often sold by the barrel and you would have to take your jug down to the tavern to fill it up, so the whiskey would need to come out at a reasonable proof. This is why you see so many bourbons bottled at 107, such as Weller (Old Weller Antique 107) and Old Grand-Dad. Barrel proof bottlings used to mean what it was barreled at, not what it was dumped at. In 1962, many distillers began to barrel their whiskey at 125 proof for many reasons, including to save on barrels.

Andrea Wilson of Michter’s, photo via Maggie Kimberl

“Barrel entry proof also plays a role in flavor development,” says Michter’s Whiskey’s Master of Maturation Andrea Wilson. “At Michter’s we choose to enter our whiskey into wood at 51.5% alcohol by volume or 103 proof. This is extremely low by industry standard and results in less yield per barrel and requires more financial outlay due to more barrels and more barrel spaces required - so why do it? The proportion of water in the barrel has a significant effect on the speed of some chemical reactions and the formation of some compounds. For example, the much desired wood sugars are more soluble faster, delivering increased maturation quality sooner in the aging process. Additionally, barrels are rich in oak wood polyphenols (tannins). These can often have bitter and astringent properties. When a barrel is toasted it softens the tannins in the wood. Lowering the entry proof of the spirit and adding additional water allows for the phenolic compounds to partially dissolve into solution. Then as the barrel breathes in oxygen it will oxidize the dissolved compounds creating a much smoother whiskey due to the chemical changes of the astringent compounds.”

Heat cycling may offer advantages in certain parts of the maturation process

Another factor that can be adjusted to affect the final product is whether the barrels are subject to regular weather cycles or if some sort of climate control is utilized. The vast majority of distilleries in Kentucky choose to allow their barrels to age in the regular temperature swings of the climate - those signature hot summers and cold winters. But distilleries like Woodford Reserve and Michter’s choose heat cycling, while Buffalo Trace and Maker’s Mark (which uses a modern cellar built out of the natural rock face on its campus to store barrels for its Private Select and Maker's 46)) choose cooler warehouses for some of their products.

“The maturation of whiskey is a complicated series of chemical processes influenced by a number of contributing factors – first and foremost the chemical makeup of the distillate profile inclusive of flavor compounds developed during fermentation, but then wood selection, wood preparation, toasting and charring levels, proof of entry, temperature and humidity fluctuations during storage, overall storage conditions and environmental factors,” says Wilson.

“Typically there are three types of reactions happening inside the barrel:

• Oxidation of original organic compounds.
• Extraction of wood compounds by the liquid.
• Reaction of organic compounds in the liquid to create new compounds over time.”

“Now, we typically measure things in terms of maturing quality focusing both on the chemical and physical changes in the liquid over time,” continues Wilson. “We utilize gas chromatography – mass spectrometry and high performance liquid chromatography to analyze for the specific compounds we are looking to be present at various concentrations to define our Michter’s profile. We also use organoleptic methods to ensure we have both a qualitative as well as quantitative methods in measuring the quality of our product.”

And while these quality control measures are pretty standard industry-wide, the use of heat cycled warehouses is a clear departure from what a good many of the bourbon producers use in Kentucky.

temperature controlled warehouse at Woodford, photo by Maggie Kimberl

“In Kentucky, we typically see anywhere from 4-6 cycles a year in a traditional Kentucky style rack (wood) house,” Wilson says. “At Michter’s we heat cycle during winter months to simulate additional cycles further contributing to the overall maturation quality of the liquid. This is an expensive process but we know by organoleptic and analytical means that it absolutely can increase the quality of the liquid. We typically see about an additional 6 months of maturing quality for every year that we heat cycle so if we said a product had been in barrel for 6 years, we would typically view the quality analysis as similar to a product in a traditional aging warehouse of somewhere between 8-9 years old. Now, while we see increased quality, there is a downside which is increased angel share but our focus is making a great American whiskey.”

Many factors contribute to the aging process of bourbon

It takes a lot to end up as the beautiful amber liquid known as bourbon. Does the distillery enter the distillate into the barrel at 125 or 103? Does it use heat cycles or open warehouses? What level of char or toast and char combination is used on the barrel? There's a lot to consider. It's worth taste testing various examples from the whiskeys mentioned to detect the differences - they definitely show through in the final product.