Coming across SO2 Stratification

When developing our new free SO2 measurement technology, the FS1, we ran thousands of comparative measurements across dozens of wineries. We started to notice replicate measurements, sampled from the same barrel and analyzed in the same wine lab, coming back with different free SO2 results. This only seemed to happen when the two samples drawn from the barrel separately. If one larger sample was drawn from the barrel and split into two for duplicate analysis, the results came back the same. We started taking samples at different depths in the barrel and noticed that some barrels had a 25 ppm difference based on depth, and some had no difference.

Experiment: Setup & Results

To investigate this in more detail, we examined the distribution of free SO2 in oak barrels after a sulfite addition in a winery. The aim was to understand how sulfite concentrations develop throughout the barrel over time.

Experiment details:

  • Oak barrels (228 L American Oak) were used and instrumented with taps at seven different locations inside the barrel.
  • Samples were drawn from these locations to measure free SO2 concentration using the aeration-oxidation method.
  • Two common methods of sulfite additions were considered: addition by aqueous KMS solution and addition with pre-dosed effervescent KMS tablets.
  • A 40 mg/L free SO2 addition was made to each barrel, using an artificial wine matrix with pH = 3.4 and constant temperature of 18°C.
  • The barrels were not stirred, topped, or moved during the experiment.


  • Samples were taken at different time intervals to track the development of free SO2 concentration distribution.
  • Two figures below show the spatial free SO2 concentration distributions at one hour and six days after the respective sulfite additions.
  • Stratification of sulfite addition remained significant after six days for both methods.
  • In the aqueous KMS solution addition, most of the sulfites remained in the lower quarter of the barrel after one hour.
  • The effervescent tablet method distributed about half of the sulfites vertically within one hour, but the other half remained concentrated at the bottom surface.
  • After six days, 21 mg/L of the aqueous KMS addition and 24 mg/L of the tablet addition reached the centroid of the barrel.
  • The majority of the remaining sulfite addition was concentrated at the bottom of the barrel in both cases.
The spatial free SO2 concentration distribution over time — addition by aqueous KMS solution
The spatial free SO₂ concentration distribution over time — addition by pre-dosed effervescent KMS tablets

Implications for the winemaking process

  • The depth at which a sample is drawn from a barrel can affect the measurement of free SO2 concentration.
  • There is no definitive answer to the "correct" depth for sampling, but consistency between barrels is crucial for accurate comparisons.
  • Sampling near the center or biased towards the top captures a more conservative measurement.
  • If a barrel shows an unexpected free SO2 concentration, resampling at a different depth can help determine if it is caused by stratification or if the barrel is an outlier.
  • Stirring the barrel after a sulfite addition can homogenize the distribution but also increases free SO2 consumption and introduces other factors.
  • The decision to stir should be based on winemaking goals, considering the stratification of free SO2 as one factor among many.