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In our previous post, we discussed the importance of measuring and adding Free SO₂ in wine to control the level of oxidation that can occur throughout the maturation and aging process. Although several methods and tools are available for making these measurements, the aeration-oxidation method is generally considered the most accurate and reliable by the winemaking industry – a gold standard of sorts. For this reason, BarrelWise used aeration-oxidation as the calibration benchmark for development of our proprietary Free SO₂ sensor.
Unfortunately, there are two downsides to using aeration-oxidation for measuring free sulfites in wine. Firstly, it takes a long time – between 12 and 15 minutes per sample. Secondly, it requires a skilled operator who not only knows the process and how to use the equipment but can also ensure that the environment and reagents are adequate for generating accurate results.
So how much of an issue will arise if one of your solutions or protocol is off? Through a set of experiments conducted in the BarrelWise laboratory by our R&D Enologist Brittany Goldhawke, we have identified several issues that can cause significant deviations of Free SO₂ measurements from your true values.
Common issues with aeration-oxidation evaluated in this experiment include:
· Measuring Free SO₂ in wine at common winery/cellar temperatures
· Improper concentration of Phosphoric acid
· Improper concentration of Sodium Hydroxide (NaOH)
· Improper concentration of Hydrogen peroxide (H2O2)
The effect of temperature in measuring Free SO₂ in wine with aeration-oxidation
Not only does temperature play a role in determining the amount of wine evaporation and ullage a wine barrel may experience, it is also a key parameter in influencing the SO₂ equilibrium in wine. Temperature is one of the factors that determines the percentage of SO₂ that will be present in the molecular phase. At lower temperatures less molecular SO₂ will be present, and at higher temperatures, more molecular SO₂ will be present. At very high temperatures, like those used for total SO₂ analysis, hydrolysis of bound SO₂ to free SO₂ can occur, resulting in significantly false high readings. Standard aeration-oxidation protocol by the OIV calls for an adjustment of wine temperature to room temperature (20-21°C).
It would certainly reduce the time required for the analyses if you could simply measure Free SO₂ in wine without waiting for it to come to temperature. So what would happen if you disregarded the protocol?
Holding all other parameters constant, we measured the Free SO₂ in a red wine sample at 5 different temperatures that would commonly be found in a winery ranging from 10°C to 25°C. We found that as temperature decreased so did the Free SO₂ values (~1-2mg/L for every 5°C decrease). If you are experiencing significant deviations from what your true Free SO₂ values are, temperature (at these values) may be the culprit. The major takeaway here is to keep your analyses consistent and run your wine samples at the same temperature each time.
Inaccuracy in wine Free SO₂ measurement due to improper concentration of phosphoric acid
Another key element of aeration-oxidation is the acidification of the wine sample by the addition of a 25% phosphoric acid solution. Why do we need to acidify the wine sample first? Aeration-oxidation runs on the premise of pushing molecular SO₂ gas out of the wine sample by aspiration, where it will travel through the column and into the H2O2 trap. At this point the SO₂ is oxidized by H2O2 to sulfuric acid, where it can be titrated with NaOH. To get all the Free SO₂ in the wine into the gas phase, we must first convert it to molecular SO₂. By adding 25% phosphoric acid, we can reduce the pH so that all available Free SO₂ shifts to the molecular form of SO₂. But what happens if you make a mistake and make your phosphoric acid outside of the recommended concentration of 25%?
Since the pH doesn’t change much at concentrations higher than 25%, we evaluated the use of 5 different concentrations of phosphoric acid below 25% We used the same red wine with a known value of 27mg/L Free SO₂ and held all other parameters constant. We found that our Free SO₂ remained consistent until the acid concentration dipped below 10%. If you are experiencing out of range Free SO₂ values when using aeration-oxidation, it is likely that your acid concentration is not the culprit, unless you think it may be less than 10%. We advise to test and troubleshoot other parameters first to find your issue.
Inaccuracy in wine Free SO₂ measurement due to improper concentration of NaOH
After all the Free SO₂ is out of the wine solution and oxidized to sulfuric acid, the next step in the analysis is to titrate with 0.01N Sodium Hydroxide. This step can create several opportunities for errors. The first problem is that accuracy and precision rely on the technical skill of the technician performing the titration. Secondly, NaOH in solid form is very hygroscopic, meaning it absorbs moisture. If you are making your 0.01N NaOH solution from weighed out solid NaOH, you must ensure that its container is kept airtight. NaOH as a standard solution can also absorb CO2 from the air, which will form sodium carbonate, inevitably causing errors in your acid-base titrations. To ensure that your standard 0.01N NaOH is always up to par, perform an acid base titration daily of your 0.01N NaOH to a set volume of 0.01N HCl. If your solution is good, you should titrate equal volumes of base to your acid. But what happens if you take a short-cut, skip the NaOH check, and just assume that your solution is perfectly good?
We evaluated a standard 40mg/L SO₂ solution (KBMS in distilled water) for Free SO₂ with aeration-oxidation. We ran the test on the standard in 5 separate reactions and using 5 different concentrations of NaOH for the final titration step. We found that a difference of 0.02N from the standard 0.01N NaOH concentration resulted in a difference of 8-9mg/L of Free SO₂ from the true value. When considering what is causing the source of error in your analysis, an incorrectly made or expired solution of NaOH is a big culprit. Consider checking your NaOH concentration first when troubleshooting problems with your aeration-oxidation setup.
Inaccuracy in wine Free SO₂ measurement due to improper concentration of Hydrogen Peroxide (H2O2)
The H2O2 trap is another crucial step in the aeration-oxidation analysis. The concentration of H2O2 must be enough to trap and ensure full oxidation of SO₂ to sulfuric acid. Common procedures are to use a saturated solution of 3% H2O2, although some protocols suggest that even 0.3% is enough. But what happens if you accidentally miscalculate your volumes and make a solution that is lower than 0.3%? Another problem might arise if your H2O2 solution is exposed to sunlight. In this case the H2O2 decomposes to water and oxygen, and the solution strength is reduced. What would be the consequences in your Free SO₂ values?
To assess what would happen if this problematic scenario occurred, we performed an experiment with a range of concentrations from 3% to 0.1% H2O2. We found that at all concentrations, there was sufficient H2O2 to trap all SO₂ and oxidize it to sulfuric acid. Even though lower concentrations of H2O2 worked for us, we still advise that a 3% solution is a good protocol to stick to, to allow room for making potential errors in making/expiration of your H2O2 solution.
Optimizing your aeration-oxidation analysis is the key to quality SO₂ management
If your wine laboratory has an aeration-oxidation setup, it is incredibly important to get the most out of your analyses by optimizing every step of the protocol. Ensuring that your wine temperature is at room temperature prior to running the analysis helps to provide accuracy and consistency in results. To ensure problems do not arise and to save yourself the time in troubleshooting, always check your solutions daily. Start your day by checking your 0.01N NaOH solution by titrating against 0.01N HCl. Prepare a standard SO₂ solution to calibrate your setup daily and assess the accuracy of your phosphoric acid and H2O2 solutions. Ensure your H2O2 is stored in the dark, in the fridge, and is always at the perfect starting color prior to analyses.
Follow the above steps, and you can better optimize the SO2 management and quality of your wine.
This article was written by Brittany Goldhawke
PS: It is with great sadness that we say goodbye to Brittany as she leaves BarrelWise to pursue further education, and explore new opportunities. Brittany has contributed a great deal to the BarrelWise work on understanding the complexities of barrel ageing wine and her blogs remain some of the most visited and appreciated pages on our website. We wish Brittany all the best in what is sure to be a stellar future career!
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