Whiskey professionals tend to agree that the primary factor influencing a whiskey’s character is the time it spends in wooden barrels. Throughout maturation, the whiskey engages with the wood, absorbing various compounds and undergoing complex chemical reactions, all interactions that gradually shape and refine its flavor, aroma, texture, and appearance.
Among all factors that influence this process, however, the effect of cask filling strength is one of the least understood. Most distilleries base their processes on conventions that owe their current state to economic and historical factors. For example, in Scotland, new make spirit destined to become single malt is typically barreled at 127 proof. In other whiskey-making regions, conventions are often dictated by law, such as in the U.S., where the maximum strength for freshly distilled spirits entering barrels is 125 proof.
Lower entry proof thresholds are based on the minimum strength required for a spirit to be legally called whiskey. For example, in Scotland, a bottle of single malt Scotch must have a minimum strength of 80 proof. To meet this requirement, producers barrel their new make spirit at a proof somewhat above this threshold, ensuring that, even with alcohol evaporation during maturation, the whiskey does not fall below the legal minimum before bottling.
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Yet, the global whiskey industry is undergoing transformative changes. In traditional whiskey-making regions, craft distillers are challenging established conventions, while new producers in emerging markets are developing their processes from the ground up, free from the constraints of tradition. Among these evolving practices, established approaches to barrel entry proof are being questioned, debated, and reimagined. Here SevenFifty Daily explores the science behind this crucial step in the whiskey-making process and looks at how distillers—seasoned and novel—navigate the complex decisions involved in choosing their approach.
The Academic Consensus on Barrel Entry Proof
Distillers deciding on their approach to barrel entry proof often draw on decades-old—albeit still relevant—literature. A seminal study, published in the Journal of the Association of Official Analytical Chemists, reports on trials conducted between the 1960s and 1970s that examined congener development—essentially, the amount of flavor compounds—in bourbon matured at various proofs in new American oak barrels. The study found that as the entry proof increases (from 109 to 155 proof), both color intensity and overall concentration of congeners in the whiskeys decreases. This occurs because the compounds released from the barrel are more soluble in water.
While no subsequent research has matched the scope of that multi-decade study, several have touched on the issue of cask filling strength, often corroborating—and occasionally shedding further light on—its findings. Some, for example, emphasize the impact that distillate strength has on maturation speed, highlighting that while reactions such as the breakdown of polymeric materials require water, the solubility of the resulting compounds increases with higher alcohol concentrations. This suggests that the optimal rate of extraction occurs at the point where the water-to-ethanol ratio is most effectively balanced.
Further studies stress that different cask filling strengths significantly affect the color intensity of whiskey, showing that higher ABVs tend to reduce the level of color developed during maturation. Meanwhile, a recent doctoral thesis examined cask filling strength in relation to different types of casks. The research indicates that casks filled at 125 proof have the highest levels of wood-derived congeners (as opposed to other congeners which favor lower ABVs), with French oak casks reaching levels similar to those of American oak casks. It also reveals that a lower entry proof may reduce the risk of developing off-flavors when using ex-wine casks.
Discovering the Nuance Between Filling Strengths
To refine their processes and adapt insights to their specific conditions, distilleries often complement academic research with their own trials. Four Roses Bourbon, for instance, has long been filling casks at 120, which is below the maximum allowed in the U.S. for bourbon production. In the early 2000s, the distillery conducted a six-year experiment to test different entry proofs, aiming to determine if its longstanding approach was truly optimal to achieve the desired flavor profile.
“Industry consensus generally suggests that lower proof yields softer, more rounded flavors, while higher proof may result in brighter, more robust, and woody characteristics,” says Brent Elliott, Four Roses’ master distiller. “Our experiment largely confirmed this theory, finding that a medium-high entry proof of 120 produced the distinctive Four Roses character we seek.”
As part of his ongoing research at Independent Stave Company, director of spirits research and innovation Andrew Wiehebrink recently conducted a similar, six-year-long experiment to evaluate the development of whiskey at 100, 110, and 125 entry proof. It involved investigating approximately 45 compounds through gas chromatography-mass spectrometry (GC-MS), a technique that allows the analysis of small, volatile molecules in complex chemical mixtures. The experiment revealed that components like vanillin, trans isoeugenol, phenol, and cis isoeugenol, which are associated with vanilla and spicy aromas, tended to favor higher entry proofs. Conversely, elements such as guaiacol, furfural, 5-HMF, and 5-methyl furfural, which are associated with nutty, caramelly, and smoky flavors, were more pronounced in the lower ABV samples.
“Our expectations based on previous studies were largely confirmed,” says Wiehebrink. “But, when we then tasted the samples, we noticed significant differences in the whiskeys’ structure, fattiness, and sweetness. This suggests that barrel components are not the sole factors influenced by entry proof during maturation. Elements like acids, esters, and aldehydes [in the new make] also play a role.”
The Benefits and Challenges of a Low Entry Proof
Ari Sussman, the whiskey maker at Outsiders Spirits and Three Chord Bourbon, fills his casks with liquid at varying alcoholic strengths and evaluates their progress quarterly. He confirms that a lower entry proof makes whiskeys more palatable at a younger age, arguing that this approach is especially suited for distillers targeting a so-called more “modern” whiskey drinker, keen on spirits that focus on the raw ingredient, rather than wood character and length of aging.
“When working with single grains, it’s crucial to use a [low] entry proof between 105 and 110,” he explains. “It helps control the wood’s potentially aggressive influence early on while preserving the character of the new make spirit. Plus, the proofing water is also aged in the barrel, which imparts much more character to the entire whiskey.”
Nicole Austin, the distiller at George Dickel Tennessee Whisky, notes that the distillery’s historical practice of using a lower 115 entry proof has proven beneficial. “Our whiskeys can age for 15 to 18 years and still remain vibrant and lively.” There are, however, significant downsides to this approach. The longer maturation time and the need of more wooden casks to mature lower ABV liquid result in both a higher carbon footprint and increased costs for the distillery. This led Austin to start filling some casks at the maximum allowed proof of 125, too.
Meanwhile, Paul Abbott, the founder of England’s Grasmere Distillery, argues that lower entry proofs present an additional, more practical challenge. “Our cask entry ranges from 94 up to 140 proof. At 94 proof, the liquid is extracting color and tannin much faster,” he says. “But because the ABV is low, it does not have a long aging potential due to ethanol evaporation. Once the liquid falls below 80 proof it is no longer legally whiskey so if you start at 94, you don’t have many years before it has to be bottled.”
Despite the downsides, Abbot remains committed to using lower entry proofs. “We will likely settle in the low 100s to allow more margin for error,” he says. “[But] I need a few summers before I can make a full assessment.”
Indeed, when it comes to entry proof, available data must be backed by empirical testing that takes into account specific factors from warehouse conditions to the new make’s character, alongside market and financial considerations. As Elliott puts it: “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.”
Dispatch
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Dr. Jacopo Mazzeo is a U.K.-based freelance drinks journalist, consultant, and photographer. He contributes to leading trade and consumer publications including Decanter, Wine Enthusiast, Whisky Magazine, and Good Beer Hunting. Jacopo consults on consumer trends and marketing strategies, is a former sommelier, and judges international wine, beer, and spirits competitions. Before he embraced full-time journalism, he studied musicology at the University of Bologna and took a PhD at the University of Southampton. Follow Jacopo on Instagram @jacopomazzeophoto
