Dr. James Osborne, Associate Professor and Enology Extension Specialist, Dept. Food Science & Technology, OSU
The pH meter is arguable the most important piece of laboratory equipment you have in your winery. It is used for many routine analyses such as pH, titratable acidity, and as part of the analysis of volatile acidity. Accurately knowing your grape and wine pH is also critical in the management of microbial stability. Spoilage microorganisms such as Pediococcus and Brettanomyces are less acid-tolerant than beneficial microorganisms such as Saccharomyces cerevisiae and Oenococcus oeni. Lower pH, lower microbial spoilage risk; higher pH, higher microbial spoilage risk. Furthermore, there is a key relationship between pH and free SO2. At lower pH, a greater proportion of free SO2 is present as molecular SO2, the most effective antimicrobial form of SO2. Because of this, free SO2 concentrations should always be evaluated in conjunction with pH when considering target SO2 levels. A pH meter will provide you with a pH value, but how do you know that the value is accurate? The consequences of using a pH meter that is not functioning properly can be significant. For example, an inaccurate pH meter may lead to over or underestimation of SO2 additions needed to achieve a target molecular SO2 concentration and/or what acid adjustments are required pre- or post-fermentation.
So how can you ensure you are getting accurate and reliable results from your pH meter? Firstly, make sure you are using an appropriate pH electrode probe for grapes and wine. Not all pH electrode probes are suited to the unique physical and chemical composition of grape juice and wine. Many manufacturers have pH probes specific for grape and wine analysis, ensuring you are using the right tools for the job. Secondly, instigate a regularly scheduled calibration schedule in the winery lab. It should be standard practice to perform a calibration the first time the pH meter is used for the day. A logbook kept by the pH meter can be used to keep track of when calibration has been conducted and the result. If you have multiple people using the pH meter, they can quickly see when the last calibration occurred and know whether calibration is needed. Keeping track of the calibration results can help indicate if the probe may need to be cleaned or other reoccurring issues. Typically, you will have a small vial of each buffer for the calibration. These vials of buffer should be regularly changed out for fresh buffer solutions bi-weekly or monthly, depending on how often you use them. Make sure the buffers you are using match the built-in calibration set points for your pH meter. Most commonly, you will be using pH 4.0 and 7.01 set points, but some pH meters use pH 3.0 and 7.01. A buffer of 10.01 can also be used as a third calibration point, although in wine, you will be mainly measuring between pH 3.0 and pH 4.0, so this is where the greatest accuracy is required. The slope should be within ± 5% of the ideal (100%), while ± 10% or greater is considered out of range. Make sure you store the probe in an electrode storage solution (typically a KCL solution). A dried-out probe slows the exchange of ions between the probe and the solution you are measuring and results in false readings. Do not store in water or pH 7.01 buffer as this will result in leaching of the electrolyte solution from the pH probe. Some pH probes allow re-filling of the electrolyte solution, so keep an eye on this level if this is the case and re-fill when necessary (see manufacturers recommendations).
If you are starting to see pH drift and/or calibrations are challenging to conduct or have low accuracy, your pH probe may need cleaning. The build-up of grape and wine deposits on the outside of the pH probe bulb will cause fouling of the membrane and interfere with the interaction of ions in your juice/wine and the electrolyte solution. During heavy use, it may be necessary to clean your pH probe weekly or bi-weekly. This involves soaking the probe in a probe cleaning solution (often provided by probe manufacturers) for 30-60 minutes, followed by rinsing with DI water. This will improve the accuracy of your data as well as extend the life of your pH probe. Keep track of when cleanings occur in your pH meter logbook. By following a regular calibration and cleaning schedule for your pH meter/probe, you can ensure reliable and accurate pH data and improve the lifetime of the probe.