Barrel fermentation, Chardonnay, and variation 

Barrel fermentation of Chardonnay offers qualities that are difficult or impossible to achieve otherwise: smooth integration of oak character, enhanced texture, extended lees contact, and micro-oxygenation during fermentation. The downside of barrel fermentation is that each and every barrel is its own miniature barrel lot. With a separate microbiome and possible contamination from the barrel, varying oak characteristics, isolated primary fermentation, and secondary fermentation, there are plenty of opportunities for two barrels of the same Chardonnay to go on separate paths.

Add to this that white wine barrels can often spend a higher portion of their life empty than red barrels, going against the adage that “a full barrel is a healthy barrel”. It is not surprising that Chardonnay has the highest rate of free SO2 outliers of any variety in our database. On average, one in five barrels of Chardonnay is defined as being an outlier for free SO2.

Compared to other major varieties that are common in our database, outlier barrels are twice as frequent in Chardonnay. In fact, one in every three lots of Chardonnay varies by more than 15 ppm between the lowest and highest free SO2 barrel.

Defining Free SO2 Outlier Barrels

What defines an outlier for free SO2? Defining an outlier for free SO2 is tricky to do in an all-encompassing way  – the wine type, time in the wine's life, pH, microbial risk factors, and the winemaker’s stylistic goals all provide important context for free SO2 concentrations. To simplify the analysis, we chose the following definition for an outlier: a barrel that is more than 7.5 ppm away from the average free SO2 concentration of the lot, or has a free SO2 value less than or equal to 13 ppm. For reference, 13 ppm free SO2 is 0.50 molecular SO2 at pH=3.2 and is 0.18 molecular SO2 at pH=3.65. All barrels that are considered in this analysis are post MLF and the winemaker has instructed sulfites to be added for cellaring.

Below are examples of lots with different variations.

The figure above shows three barrel lots from our database. All three lots have an average free SO2 reading around 20 ppm, but very different standard deviation and maximum deviation. The first, Lot A, has no barrels that are considered outliers as all barrels are close to the average value of 21.4 ppm. In Lot B, there are two barrels that would be considered outliers. Both barrels meet both criteria: they are less than or equal to 13 ppm and more than 7.5 ppm less than the 21.2 ppm average. Lot C has three outlier barrels, one of which is at 39 ppm, well above the 18.4 ppm lot average, the other two are at or below 13 ppm.

Chardonnay Barrels Should Be Treated as Individual Lots 

The mini-lot — a barrel of Chardonnay — is treated as an individual during fermentation, why shouldn’t it be treated as an individual once sulfites are added? The individual treatment of barrel fermented Chardonnay is well worth it to optimize the unique characteristics of the variety and maintain the consistency in wine quality for the entire lot.

Barrel fermented Chardonnay doesn’t have to be a dice roll. Careful management of free SO2 in barrel aging reduces the risk of over- or under- oxygenation of the wine and mitigates microbial problems before they cause unwanted sensory impacts. 

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‍Note: These statistics are drawn from the growing database of free SO2 measurements streaming directly from our customer wineries, and might be updated in the future. Check out our BarrelWise Insights project to get similar insights and more information about the project.