In the art of whisky making, seemingly minor decisions at the production stage can have profound effects on the final spirit. One such decision is the alcohol strength at which whisky is filled into barrels for maturation. While the industry standard in Tasmania is typically around 63.5% ABV, at Greenbanks we have adopted a different approach: filling at 57% ABV. This practice, informed by our unique local climate and a deep understanding of oak extraction chemistry, yields a more refined and balanced spirit over time.
The Climate Factor: Low Humidity and Its Consequences
Southern Tasmania is known for its cool, maritime climate, but a less commonly discussed characteristic is its relatively low ambient humidity. In whisky maturation, this has significant implications.
In drier conditions, ethanol evaporates more slowly than water. This leads to an unusual phenomenon where, instead of the ABV decreasing over time – as it commonly does in Scotland and other places with high relative humidity – it increases during maturation in Tasmania. If whisky is filled into barrels at the conventional 63.5% ABV, it can climb to even higher concentrations, pushing past 68% or more over time. Such high strengths can suppress the extraction of certain flavour compounds and exacerbate harshness in the final product.
By filling at 57% ABV, we allow the whisky to mature within a more favourable ethanol concentration range, maintaining balance and encouraging a broader extraction of desired oak-derived compounds.
Oak Chemistry: Water-Soluble vs Alcohol-Soluble Compounds
Barrel maturation is not just a matter of aging – it’s a dynamic exchange between spirit and wood. Oak contributes hundreds of chemical compounds to whisky, and the type and balance of these compounds depend heavily on the alcohol concentration in the barrel.
- Alcohol-soluble compounds, such as vanillin, lactones, and certain tannins, are readily drawn out at higher ABVs. These lend sweetness, spice, and structural grip to the whisky.
- Water-soluble compounds, including sugars, phenols, and organic acids, are more extractable at lower ABVs. These contribute to roundness, fruitiness, and complexity.
At 63.5% ABV and rising, extraction favours the alcohol-soluble fraction, often leading to an oak-forward profile with dominant tannins and less nuance. In contrast, filling at 57% ABV promotes a more balanced extraction, capturing both water- and alcohol-soluble elements. This results in a whisky with richer mouthfeel, layered flavours, and better integration of oak characteristics.
Benefits of Lower Fill Strength
- Greater maturation harmony: Lower starting ABV helps moderate the upward drift in strength over time, particularly in dry environments like southern Tasmania.
- More balanced flavour extraction: Encourages contribution from both alcohol- and water-soluble compounds in oak.
- Faster development of complexity: With improved solubility and interaction at lower ABV, some flavour development milestones may be reached sooner.
- Lower dilution required at bottling: Preserving more of the spirit’s natural character and reducing the risk of over-dilution.
Our decision to fill whisky barrels at 57% ABV is not driven by tradition, but by a deliberate understanding of local climate and oak maturation chemistry. In the low-humidity environment of southern Tasmania, this approach prevents excessive ABV rise and enhances the extraction of a broader spectrum of oak compounds. The result is a whisky that matures with greater balance, complexity, and elegance—a reflection not just of where it’s made, but how thoughtfully it’s crafted.
