Measuring and controlling oxygen, and more specifically, dissolved oxygen (DO), throughout the winemaking process should be part of every winemaker’s quality control protocol, much like free sulfur dioxide (SO2) levels. The chemistry of oxygen reactions notwithstanding, home winemakers have been shying away from measuring oxygen because there have not been any affordable, easy-to-use tools.
But Vinmetrica now offers a DO probe that is both affordable and easy to use. The galvanic-sensor probe connects to an SC-200 or SC-300 analyzer and measures voltage or apparent “pH” value in wine, which is then used to calculate the amount of DO as % saturation, which can be converted into a concentration measurement in mg/L. The calculations are done by comparing measurements in wine with those for calibration, i.e. 0% saturated DO and 100% saturated DO.
I was keen to give this probe a try as I am a strong believer in the need to measure and control oxygen levels in wine. I was curious to compare the performance of this probe to other instruments on the market, which range from a few hundreds of dollars to several thousands. Although I use the more sophisticated instruments for my research-type work, I was really looking for a tool that I could quickly “grab” to measure DO, and which I could recommend to my fellow winemakers. After all, I have been preaching about the need to measure DO in wine.
Those other instruments include the following:
- Extech DO600 Waterproof ExStik II Dissolved Oxygen Meter
- Thermo Scientific Orion Star A223 Portable RDO/DO Meter
- NomaSense O2 P300 Oxygen Analyzer
The following table summarizes key features for each instrument (images are not to scale).
|Electrode type||Temperature compensation||Barometric pressure compensation|
DO probe & SC-300
|Orion Star A223||15772,3||Polarographic
- Probe only; discounted price from Vinmetrica
- From ColeParmer.com
- Complete kit with meter, RDO probe, calibration tools, and carrying case
- From Nomacorc; includes analyzer, optical fiber, sensor spots, starter kit, oxygen dipping probe, ½ -day training and set up, transport by courier and insurance
To measure % saturation or DO mg/L using the Vinmetrica DO probe and SC-200 or SC-300 analyzer, first measure the voltage or apparent “pH” value of a 0% DO calibration solution prepared by dissolving sodium sulfite in water. Sodium sulfite removes all the oxygen from the water. Then measure the voltage or apparent “pH” value of a 100% DO calibration solution, which can be prepared by bubbling air into water using an aeration stone, or more easily, simply relying on air to freely contact the membrane on the electrode and taking a measurement. Then measure the voltage or apparent “pH” value of the wine sample. The difference in voltage or apparent “pH” value is then calculated, relative to the 0% DO calibration solution, for the 100% DO calibration solution and the wine sample.
% saturation is then the percent of the ratio of these differences, and DO (mg/L) is calculated as the product of saturating oxygen level, which is about 8.5 mg/L at 22.5°C/72°F and ordinary air pressure (sea level on a clear day), and % saturation. Since the probe and analyzer do not perform automatic temperature and barometric pressure compensation, it is important that measurements be made under these conditions; otherwise, adjustments are required. Salinity of wine and ethanol content are not important factors. It is also important to stir the probe during the measurement to ensure an adequate flow of oxygen across the membrane on the electrode.
Let’s look at an example.
The measured voltage for the 0% and 100% DO calibration solutions are +8.0 and +302 mV, respectively. The difference then is 294 mV.
The measured voltage for the wine sample is +50 mV. The difference relative to the 0% DO calibration solution then is 42 mV.
Then, DO % saturation is 100 x 42/294, or 14% when measured at 22.5°C/72°F and ordinary air pressure.
DO (mg/L) is calculated as 8.5 mg/L x 14% = 1.2 mg/L.
There are pros and cons, and cautionary notes, performing measurements in samples transferred to beakers vs. in carboys, tanks, or barrels. These are discussed in the Vinmetrica DO Probe User Manual. What is most important is to protect the bulk wine under test or wine sample from air exposure to minimize oxygen uptake, which would otherwise affect measurements.
Although quite difficult to benchmark results for all instruments given the challenges of shielding wine samples against oxygen uptake, the results using the Vinmetrica DO probe and SC-300 were as expected and comparable to all other instruments.
Only the NomaSense P300 can measure DO non-intrusively, i.e. without the need to open the bottle of wine. It can also measure the amount of oxygen (HSO) in the headspace between the cork and wine. By monitoring DO and HSO, one can follow the evolution of the wine specific to that packaging, i.e., bottle plus cork type, or other packaging, such as Bag-in-aBox (BIB).
For the price, the Vinmetrica probe is an excellent addition to the capabilities of the SC-200 and SC-300 analyzers, and which I highly recommend IF you want to make top-notch wines.
To learn more about oxygen in wine, its spoilage effects, its benefits in reds, and how to manage oxygen levels throughout winemaking, please refer to the April-May 2013 and February-March 2015 issues of WineMaker magazine.