The alcoholic strength of whiskey at the point it enters a cask plays a pivotal yet often overlooked role in the maturation journey.

written by

Jacopo Mazzeo

published

October 14, 2024

Experts in the whiskey industry generally concur that the most significant factor affecting a whiskey’s profile is the duration it remains in wooden barrels. During this maturation phase, the whiskey interacts with the wood, absorbing different compounds and engaging in intricate chemical processes, all of which gradually mold and enhance its flavor, aroma, texture, and visual appeal.

Among all factors that impact this process, the influence of cask filling strength remains one of the least understood elements. Many distilleries adhere to practices shaped by economic and historical influences. For instance, in Scotland, freshly distilled spirit meant for single malt whiskey is generally barreled at 127 proof. In contrast, other whiskey-producing regions are often governed by regulations; for example, in the U.S., the maximum allowable proof for newly distilled spirits entering barrels is 125 proof.

These lower entry proof limits stem from the minimum proof required for a spirit to be classified as whiskey. In Scotland, a bottle of single malt Scotch must possess a minimum strength of 80 proof. To comply with this requirement, distillers typically barrel their new make spirit at a proof slightly above this minimum, ensuring that even with alcohol evaporation throughout maturation, the whiskey maintains its legal minimum before it is bottled.

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Meanwhile, the global whiskey landscape is witnessing significant transformations. In traditional whiskey regions, craft distillers are challenging age-old practices, while new players in emerging markets are crafting their methods without the weight of tradition. Within these changing dynamics, conventional approaches to barrel entry proof are being critically examined and transformed. Here, SevenFifty Daily delves into the science of this vital phase in the whiskey-making journey, exploring how both seasoned and new distillers approach the intricate decisions surrounding their methods.

Distillers often reference decades-old literature when deciding on their approach to barrel entry proof, even though the information remains relevant today. A seminal study, published in the Journal of the Association of Official Analytical Chemists, details trials conducted in the 1960s and 1970s which focused on congener development—essentially, the flavor compounds present—in bourbon aged at various proofs in new American oak barrels. This research revealed that as the entry proof increases (from 109 to 155 proof), both the color intensity and overall concentration of congeners in the whiskeys decrease. This phenomenon arises because the compounds released from the barrel become more soluble in water.

Although there has been no research that matches the thoroughness of that multi-decade study, several investigations have addressed the topic of cask filling strength and have either supported or expanded upon its conclusions. Some studies emphasize the influence of distillate strength on maturation speed, indicating that while reactions such as the decomposition of polymeric materials require water, the solubility of the resulting compounds improves with increased alcohol concentrations. This implies that the optimal extraction rate happens when the water-to-ethanol ratio is most effectively balanced.

Additional studies highlight that various cask filling strengths significantly impact the color intensity of whiskey, demonstrating that higher ABVs typically result in a lesser degree of color during maturation. Furthermore, a recent doctoral thesis investigated the relationship between cask filling strength and different types of casks. It suggests that casks filled at 125 proof exhibit the highest levels of wood-derived congeners (contrasting with other congeners that prefer lower ABVs), and French oak casks achieve levels akin to those of American oak casks. The research also indicates that a lower entry proof might help minimize the development of off-flavors when utilizing ex-wine casks.

In an effort to further refine their methods and apply academic findings to their unique environments, distilleries frequently supplement scholarly research with their own experiments. For example, Four Roses Bourbon has traditionally filled casks at 120 proof, which is below the maximum permissible limit in the U.S. for bourbon production. In the early 2000s, the distillery undertook a six-year study to experiment with different entry proofs, aiming to determine whether its established method was genuinely the best for achieving the preferred flavor profile.

“Generally, the industry agrees that lower proof yields smoother, more rounded flavors, whereas higher proof can result in brighter, bolder, and woodier characteristics,” explains Brent Elliott, the master distiller at Four Roses. “Our findings mostly supported this theory, demonstrating that a medium-high entry proof of 120 achieved the unique character we aim for at Four Roses.”

As part of his continuous research at Independent Stave Company, Andrew Wiehebrink, director of spirits research and innovation, recently conducted a similar six-year study to explore the development of whiskey at entry proofs of 100, 110, and 125. This study examined around 45 compounds using gas chromatography-mass spectrometry (GC-MS), a method that analyzes small, volatile molecules found in complex chemical mixtures. The results indicated that compounds such as vanillin, trans isoeugenol, phenol, and cis isoeugenol, which contribute to vanilla and spicy notes, were more prevalent at higher entry proofs. In contrast, components like guaiacol, furfural, 5-HMF, and 5-methyl furfural, associated with nutty, caramel, and smoky flavors, were more pronounced in the lower ABV samples.

“Our predictions, based on earlier research, were largely validated,” states Wiehebrink. “However, when we tasted the samples, we observed notable variations in the structure, fattiness, and sweetness of the whiskeys. This indicates that barrel components are not the only elements affected by entry proof during maturation; factors like acids, esters, and aldehydes in the new make are also significant.”

Ari Sussman, the whiskey maker at Outsiders Spirits and Three Chord Bourbon, fills his barrels with spirits at different alcoholic strengths and monitors their progress every quarter. He agrees that a lower entry proof makes whiskeys more accessible at a younger age, stating that this method is particularly well-suited for distillers aiming for a more “modern” whiskey drinker, who prefers spirits highlighted by the raw ingredients rather than the influence of wood and extended aging.

“When working with single grains, it’s essential to use a low entry proof between 105 and 110,” he explains. “This helps manage the wood’s potentially strong influence early in the process while keeping the characteristics of the new make spirit intact. Additionally, the proofing water ages within the barrel, adding much more character to the entire whiskey.”

Nicole Austin, the distiller at George Dickel Tennessee Whisky, mentions that the distillery’s traditional approach of using a lower entry proof of 115 has been advantageous. “Our whiskeys can mature for 15 to 18 years and still retain their vibrancy and liveliness.” However, this method does come with significant drawbacks. The extended maturation period and the requirement for more wooden barrels to age the lower ABV liquid lead to a larger carbon footprint and higher costs for the distillery. This prompted Austin to begin filling some casks at the maximum allowed proof of 125 as well.

On the other hand, Paul Abbott, the founder of Grasmere Distillery in England, contends that lower entry proofs pose a practical challenge. “Our cask entry varies from 94 to 140 proof. At 94 proof, the liquid extracts color and tannin much more quickly,” he explains. “But due to the lower ABV, it lacks long aging potential because of ethanol evaporation. Once the liquid drops below 80 proof, it no longer qualifies as whiskey, so starting at 94 doesn’t leave much time before it must be bottled.”

Despite the challenges, Abbott remains dedicated to using lower entry proofs. “We will probably settle in the low 100s to provide a larger margin for error,” he notes. “[However], I need a few more summers before I can make a complete assessment.”

When discussing entry proof, it’s essential that available data is supported by empirical testing, which considers various factors from warehouse conditions to the characteristics of the new make, in addition to market and financial elements. As Elliott states, “You can’t simply take an experiment and apply it globally. You should adapt it to different conditions. There’s a lot of complexity involved, but it’s precisely that complexity that makes this work so fascinating.”

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Dr. Jacopo Mazzeo is a freelance drinks journalist, consultant, and photographer based in the U.K. He writes for prominent trade and consumer outlets such as Decanter, Wine Enthusiast, Whisky Magazine, and Good Beer Hunting. Jacopo specializes in consumer trends and marketing strategies, has a background as a sommelier, and serves as a judge in international wine, beer, and spirits competitions. Before devoting himself to journalism, he studied musicology at the University of Bologna and earned a PhD at the University of Southampton. Connect with Jacopo on Instagram @jacopomazzeophoto.

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