Barrel-by-barrel sampling of free SO2 can potentially help growers avoid unnecessary and costly barrel downgrades.
–David Sommer, CTO, BarrelWise Technologies
Free sulfur dioxide (SO2) is unique among common chemical measurements in wine as it is both a symptom and a treatment. SO free2 protects wine by scavenging oxygen and the interruption of microbiological activity is itself consumed in the process (ignoring a lot of complex chemistry). This allows accurate measurement and proper management of free SO2 particularly important levels during aging in barrels.
So free SO2 is too low, the wine has a high risk of developing microbial or oxidative defects. Excessive addition can also cause problems, as high SO2 levels begin to collide with sensory and, eventually, regulatory thresholds.
In a recent survey, conducted at BarrelWise, 97 percent of winegrowers (n = 70) agreed that maintaining the correct free SO2 levels in aging wine barrels is important to maximize wine quality. Despite almost unanimous agreement on its importance, the most common methods in the industry for measuring and managing free SO2 in large groups of barrels are based on an underlying assumption: all barrels are the same.
All barrels are not the same. There is significant variability between them, which is, after all, why barrel groups are made up of a variety of oak types, cooperages and ages, and why assembly from barrels is arguably one of the most important roles of a winemaker. The barrel-to-barrel variance of sensory characteristics is sought after, as it leads to a wide range of aromatic blending opportunities, ultimately leading to more complex and high-quality wines.
The apparent contradiction between free SO2 the management strategy and the sensory variety observed barrel by barrel led us to wonder if this barrel-by-barrel variation extended to free SO2 levels and, if so, how does this correlate with barrel performance and sensory results. The problem being that the barrel-by-barrel variation of the free SO2 within a group may result in some drums presenting a high risk of microbial or oxidative defects and / or excessive addition of sulphites in other drums.
To investigate, we sampled each individual barrel in a 56 barrel group of Merlot, made up of a variety of coopers, with barrels ranging from first to third fill. A typical winery process of collecting a composite sample from a few ‘random’ barrels (which often consists of the barrels easiest for the winery team to reach) would most often indicate a free SO2 level of about 35 ppm, a reasonable set point of 0.45 ppm molecular SO2 for this wine.
Analysis of each barrel in the group revealed that the barrels ranged from 14 to 41 ppm free SO2. Four of the barrels were rated at a critical level by the winemaker (less than 0.22 ppm molecular SO2) and required individual additions to correct them.
This limited data set suggests that a rate of about 7 percent barrels is sufficiently far from the composite group average. If we assume that only half of these premium casks develop an unwanted oxidative or microbial impact during their aging period that results in a downgrade to a lower price tag, the financial impacts become quite striking. If four barrels from a hundred barrel group go from a flagship $ 60 SKU to a midrange $ 18 SKU, the result is a lost revenue opportunity of $ 50,000.
What causes some barrels to bind to SO2 faster than the others? The reasons are complicated, but can include a barrel’s usage and cleaning history, different microbiomes between barrels, human error (i.e., missed additions, open bungs, contamination) and the construction of the barrel. BarrelWise conducts active research with our wine partners to better understand the cause and effect relationship between barrel factors, wine chemistry, sensory results, and wine quality.
Barrel-by-barrel sampling of free SO2 can help identify outliers and potentially give winemakers the tools to avoid unnecessary and costly barrel downgrades. Barrels are where many of the more expensive wines spend most of their life in the cellar, but so far the management of barrels has generally been done in the dark, with a very dim flashlight. .
David Sommer is the CTO of BarrelWise Technologies. He holds a BA and MA in Mechanical Engineering from the University of Waterloo and is currently pursuing his PhD at the University of British Columbia, where his research focuses on optical diagnostics and energy systems. He worked as a commercial engineer for Toshiba, a space systems engineer for the Canadian Space Agency and a consultant for RDH Building Engineering. Sommer’s goal at BarrelWise is to lead product development and manufacturing, and to ensure the technology portfolio is aligned with the needs of current and future customers.
Sommer will speak at WINExpo 2021. His session, Variation of fSO2 between barrels and impact on the quality of precision dosing delve into winery-specific case studies regarding deviations in barrel specifics regarding their effects on free SO2 and the quality of the resulting wine. Learn more about WINExpo and Register.