Distiller Terry Ballard fills a barrel of whiskey at the Willet Distillery, Bardstown, KY

How does the proof that whiskey is entered into a barrel affect the final outcome of its character and flavor? Is there an ideal proof for whiskey? These are questions frequently asked by clients or students who take my distilled spirits maturation and blending classes. My answer is often, “How far down the rabbit hole are you willing to go in order to explore those questions?” Because many of them are already whiskey distillers, or at least intend to be, they are always very curious why the Standards of Identity for Distilled Spirits, as laid out in the Code of Federal Regulations, stipulate a barrel-entry proof ceiling of 125 for bourbon, rye, wheat, malt, rye malt and corn whiskeys.

Many factors come into play, in addition to barrel-entry proof, that will determine the flavor composition of a whiskey: distillation proof and congener content of the distillate, char and toastage levels of the barrel, warehouse design, barrel placement, temperature and humidity, size of the barrel, time in wood, bottling proof, etc. Yet, I find that the clause “stored at not more than 125 degrees proof” elicits the greatest interest and speculation. Is there something particularly significant about the number 125?

Interestingly, this specific clause in the regulations has only been in force for a little more than 50 years. It certainly didn’t start out that way. Long before Prohibition, the common practice among distillers was to barrel whiskey somewhere between 100 and 104 proof. Distillation proof rarely exceeded 130, and was usually lower. It was customary at the time for distilleries to sell whole barrels to purchasers, such as bar owners, rather than selling whiskey by the bottle. And whiskey was served at bars directly from the barrel, at cask strength or lower.

Although there were a number of legal reforms—such as the Bottled in Bond Act of 1897 and the Pure Food and Drug Act of 1906—that helped ensure the quality of bourbon, it was not until after the end of Prohibition in 1933 that a codified form of standards for entry proof came into legal effect.

The Standards of Identity for Distilled Spirits were created with the passage of the Federal Alcohol Administration Act of 1935. Barrel-entry proof was set at “not less than 80 nor more than 110 proof,” thus stipulating both a floor and a ceiling limit for distillers. Also at that time, the bottle became the sole means of legal purchase. Distilled spirits could no longer be sold by the barrel to retailers, bars and consumers.

Bourbon historian Michael Veach, of the Filson Historical Society, observes that Americans’ taste for bourbon and other whiskeys changed significantly during Prohibition. Before then, bourbon was generally consumed straight from the barrel at cask strength and at a fairly low proof. Since the customary lower distillation and entry proofs provided more congeners to the whiskey, the flavors of bourbon were much bolder and richer.

During Prohibition, said Veach, very few distilleries were allowed to produce whiskey. What was produced was often known as “medicinal whiskey.” Older whiskey was most often distilled pre-Prohibition. The few existing distilleries tried to stretch out the old stock, creating blends, which were much lighter in congener content and flavor than whiskey straight from the barrel. This accustomed drinkers to lighter tasting whiskeys, and after Prohibition the trend continued. Lighter whiskeys were produced by first increasing the distillation proof, and later the barrel-entry proof.

Enter the Scientists

Mike Cameron determines the proof of a sample of a spirit using a hydrometer and thermometer.

Besides changes in consumer tastes, other forces were at work that would help drive the permitted barreling-entry-proof maximum beyond 110. In 1949, in Peoria, IL, Hiram Walker & Sons began a series of barreling-proof experiments in order to understand how entry proof affected the aging of American whiskeys.

The study looked at two bourbon recipes and one rye recipe, which were barreled at a variety of proofs. Forty-five control barrels, entered at 110 proof, were compared against 15 experimental barrels, which had entry proofs of 118, 127 and, in the case of one barrel of rye, 154. They were stored for 8 years in new charred-oak barrels in brick-and-concrete warehouses with air circulation for temperature and humidity control.

When the study was published in 1959, authors C.S. Boruff and L.A. Rittschof alluded to savings distillers could enjoy by using fewer new barrels,

“There are several obvious economic advantages in barreling whiskey at proofs higher than 110, if whiskey quality is not impaired and losses during aging are not excessive. These advantages have become increasingly pertinent recently because of the increasing high costs of new quarter-sawn white oak cooperage and warehouse aging facilities.”

The study is significant, not only because the findings would later influence a change to the Standards of Identity and thus benefit distilleries economically, it was also the first time data had been published on the chemical and sensory effects of aging whiskeys barreled above 110 proof.

After 8 years of maturation, the chemical and organoleptic quality of the bourbon barreled at 110 and 118 proofs were relatively similar, whereas the bourbon barreled at 127 proof was described as having a green wood and spice note. Boruff and Rittschof determined that because this green wood and spice flavor did not develop in the whiskey until it was nearly 8 years old, “the higher barreling proof had no commercial effect on whiskey quality at the currently common marketable ages.” Also, the congeners coming from the new charred oak, such as wood sugars, acids, esters and tannins, were more pronounced in the control whiskeys than in those barreled at higher proofs.

The rye whiskey, which was barreled at 154 proof, showed the greatest flavor differences when it was compared to its control of 110 proof. The tests showed that barreling at 154 proof resulted in slower development of mature flavor and the whiskey had a green, woody, spicy quality.

With regard to evaporative loss, there was a slight tendency toward lower percentage losses at barreling proofs higher than 110, although the researchers concluded that there were not enough barrels in the experiment to make this statistically significant.

The Hiram Walker study concluded, “the current U.S. legal maximum limit of 110 proof on barreling whiskeys could be increased to about 125 without affecting aging losses, quality, or filterability.” By the conclusion of the study, industry-wide experimentation on barreling proof was underway.

Enter the Government

The results and recommendations of this study were most likely instrumental in the formation of United States Treasury Decision 6597, in 1962, which effectively amended the labeling regulations in 27 CFR Part 5. Maximum barrel-entry proof of whiskey increased from 110 to 125 proof. The minimum barreling proof disappeared.

Bourbon historians Michael Veach and Chuck Cowdery both note that most distilleries did not start taking advantage of the new maximum barreling proof until nearly 18 years later, during the early years of the Reagan administration, when the Federal Excise Tax (FET) increased. They speculate that the reason distilleries waited until the early 1980s was most likely due to the culture of traditionalism found among the older generation of distillers. It is likely that a combination of factors, economical and otherwise, led to the eventual change in entry proof practices. An increase in the FET could easily have prompted distillers to be more mindful of elevated costs associated with lower barreling proofs.

Another important study that examined the chemical mechanisms and physical conditions related to congener formation that occur during whiskey maturation coincided with the change in industry practice of using higher barrel entry proofs. This study, written in 1981 by George H. Reazin, of Joseph E. Seagram and Sons, examined the effect of alcohol concentration on congener formation in whiskeys ranging from 110 to 155 proof after 6 years of maturation. The research results showed that the levels of solids, wood sugars, color, tannins, and volatile acids appear to decrease as the proof increases, while the esters and aldehydes remain fairly stable regardless of proof. Drops in these congener levels appear to occur when the entry proof is somewhere between 120 and 125. Reazin observed that:

“It may well be that the barrel reactions initially involve a breakdown of the polymeric material present, which requires the presence of water. Thus, as the water concentration decreases, and as alcohol levels increase, hydrolytic processes in the wood proceed at slower rates, causing a lower level of congeners in the aged product.”

These and other studies indicate that lower alcohol strengths help to extract the water-soluble wood components such as glycerol, sugars, and hydrolysable tannins, while higher alcohol strengths extract more alcohol-soluble elements from the wood, such as coconut-flavored lactones, aromatic aldehydes like vanillin, and terpenols that give pine or resin notes. Producing whiskey at lower alcohol strengths means that more barrels and storage space will be needed. Higher filling proof means there will be more proof gallons per barrel, thus saving money on both barrels and storage. So it all comes down to a battle between sweeter, more luscious flavor and richness in the whiskey versus lower production costs… or does it?

Enter the Buffalo

The story gets even more complex and the answers not so simple when one examines the results of Buffalo Trace Distillery’s experiments in the 2000s with the effects of entry proof. Two studies were conducted: one rye-recipe bourbon and one wheat-recipe bourbon, both of which were distilled at 130 proof. Both recipes were entered into barrels at four different proofs, from 90 to 125, and matured somewhere between 11 and 12 years. Bottling proof was set at 90.

The Buffalo Trace research team’s sensory panel found that they preferred the taste of the rye recipe bourbon entered at 125 proof, while the favorite for the wheat recipe bourbon had an entry proof of 115. It is worth noting that both of these entry proofs are very close to the actual entry proofs used by Buffalo Trace in their normal range of similar products. Also, the higher entry proofs also meant higher evaporation rates, contradicting one of the findings of the 1959 Hiram Walker & Sons study. Given the fact that the Buffalo Trace experiment used many more barrels, its findings might be more accurate.

In Conclusion, or Inconclusion

What are we to believe about the ideal entry proof for whiskey? Does this variable change depending upon the mash bill used, as the Buffalo Trace experiments seem to suggest? If this recent data is correct, is the cost of greater loss found in higher entry proof whiskeys worth the savings in fewer barrels and storage space? And while lower entry proofs might guarantee that more wood sugars and richer flavors will be in a whiskey, is this necessarily desirable over the greater amount of vanillin and coconut lactones in whiskeys entered at higher proofs?

In addition to traditional bourbon distilleries such as Wild Turkey, which enters their whiskeys into the barrel at lower proofs, several craft distillers have strived to overcome the tyranny of 125 by reverting back to techniques of bygone days, such as lower proofs off the still and barrel-entry proofs that are somewhere between 100 and 110. Most notable among these craft distillers is Leopold Brothers, in Denver, who enter their whiskey into the barrel much closer to the pre-Prohibition style at a very low 98 proof.

The theory behind this practice is that the grain notes become more prominent, while the oak assumes a more reticent role and aids in sweetness. If entry proof is set lower, it also means that less water has to be used when whiskey is reduced from barrel to bottling proof, thus keeping the aromatic integrity intact. Alternatively, the whiskey can be released at its natural barrel strength.

So, in circling back to answer the initial two questions, we can say definitively that entry proof does indeed affect a whiskey’s character and flavor. However, as to an ideal entry proof, this ultimately comes down to a matter of taste.  n