Iowa Wine Lab Comparison Project: Summer 2022
Results compiled by the Midwest Grape and Wine Industry Institute
Participating Iowa wineries were provided bottles of the same commercially available wine with identifying information removed. The same wine was also sent to an external lab for analysis. Participants were given a list of chemical parameters to measure, but instructed to skip any that were not within their lab’s capacity. Results were reported anonymously using Qualtrics.
Results were collected between June 22, 2022 and July 29, 2022.
Complete results for each parameter are included below, along with brief discussion for each.
Some results of note include:
- All of the participating wineries reported results for free sulfur dioxide, pH, and
- titratable acidity.
- None of the participating wineries reported results for acetic acid / volatile acidity (VA).
- Alcohol results show good agreement among labs.
- pH results showed a surprisingly large discrepancy between labs.
- Free sulfur dioxide (FSO2) results show a wide range of values, mostly due to two main
- categories of measurement methods.
- • These results provide further context for the findings of the Survey of Iowa Wine
- Quality, reported in January 2021, which indicated substantial numbers of commercially
- available Iowa wines with low FSO2 levels and sensory issues related to oxidation and
- Extra attention is warranted for proper calibration, the choice of appropriate equipment
- for the application, and reporting with the correct units.
- The MGWII is grateful for the participants who volunteered their time for this project.
- We welcome any feedback to improve this program, or any of our other services and programs.
If there is continued interest, a similar program may be offered again in the future and any
suggestions for improvement would be appreciated.
MGWII staff are always available by phone or by emailing email@example.com with questions and concerns related to lab analysis or general winemaking.
If you did not participate in 2022 but would like to in the future, please let us know!
Looking forward, we plan to continue to focus on areas related to the fundamentals of wine
quality. Based on these results, we will likely continue to emphasize the importance of accurately monitoring sulfur dioxide and VA in wines as two important wine quality parameters.
Free sulfur dioxide
Free sulfur dioxide discussion
All participating wineries reported free sulfur dioxide.
Labs used one of two basic categories of methods to measure free sulfur dioxide: aeration/oxidation (A/O) or a titration-based method. In the reported results, there is a clear separation between values obtained with one method vs. the other.
For comparison, the result reported by the external lab is grouped at the lower end of the range, along with the values reported by labs using A/O.
It is likely that labs using a titration-based method may overestimate the free sulfur dioxide level in their wines, which can have consequences such a higher risk of oxidation and microbial instability.
Total sulfur dioxide
Total sulfur dioxide discussion
Fewer labs reported results for total sulfur dioxide than for free sulfur dioxide. Results ranged from 55 ppm to 113 ppm, with the result from the external lab (102 ppm) falling toward the higher end of that range.
Because total sulfur dioxide is less directly involved in protecting wine from oxidation and microbial spoilage, measuring it may be a lower priority for winery labs.
However, if winery labs underestimate the level of total sulfur dioxide in
their wines, they may be at risk of exceeding the legal limit of 350 ppm set
by the TTB (27 CFR § 4.22).
Alcohol/ Ethanol Discussion
The reported results for alcohol showed good agreement, with a total range of 11.38% to 12.30%, a difference of 0.92 percentage points.
The majority of wineries reported using ebulliometry to measure alcohol, and the results generated with that method do not show a clear separation from results obtained using other methods.
Acetic acid / Volatile acidity
None of the participating wineries reported results for acetic acid or volatile acidity (VA)
VA is a major quality parameter for wine, and the TTB has set legal limits in wine (1.4 g/L for red wine and 1.2g/L for white wine).
VA is also a “red flag” compound for other quality issues in wine, including microbial spoilage and oxidation.
For more information, please see the Wine Faults Series: Volatile Acidity fact sheet at
Brix measurements ranged from 2 to 3 degrees brix.
Brix is most useful for monitoring grape ripening and tracking the progress of fermentation, so exact values may not be crucial. However, accurate brix
measurements are particularly useful for detecting dryness at the end of fermentation.
Not all wineries reported brix results, possibly due to volume limitations of the 750 mL wine bottle.
Glucose + Fructose / Residual sugar
Glucose + Fructose / Residual sugar discussion
Note that the first two results appear to be outliers, for two different reasons. The first line, with a value of 0.4% residual sugar (RS), is likely a reporting or calculation error representing a shift by a factor of 10. The second line, with a value of 1.176% RS, was reported as 11.76 g/L and converted to % for comparison with the other values. The listed method includes refractometry, which cannot be used to measure sugar once ethanol is present in the sample due to the refractive index of ethanol.
Therefore, averages and box plots are presented both with and without the first two values.
Excluding the first two values, the RS values range from 3% to 5%, with an average of 4.44% showing good agreement with the external lab’s reported value of 4.5%.
All participating wineries submitted results for pH.
The range of pH measurements was surprisingly wide, with the lowest reported as 2.99 and the highest reported as 3.44.
Accurate pH measurement has consequences for wine’s microbial stability and the amount of free sulfur dioxide required to reach the protective level of 0.8 ppm molecular SO2.
pH probes must be properly cleaned and stored, and probes and meters require calibration before each day of use.
Titratable acidity discussion
All participating wineries submitted results for titratable acidity (TA).
The value shown in the first line, 0.61 g/L, is an outlier that may be due to a
reporting error by a factor of 10. Averages are calculated both with and without this value.
When the first value is excluded, reported values still range from 5.60 g/L to 9.87 g/L. When 9.87 g/L is also excluded as an outlier at the high end, the range becomes 5.60 g/L to 7.14 g/L, representing a difference of 1.54 g/L.
Malic Acid and Lactic acid
Only one winery reported results for malic acid and lactic acid.
Using paper chromatography, they reported the presence of both malic and lactic acids.
The external lab also detected both acids, with a value of 1.17 g/L for L-malic acid and a value of 1.58 g/L L-lactic acid.
No wineries reported results for these parameters
- Sorbic acid
- Dissolved carbon dioxide
- Other (blank options)