Brief Description:
Prior to the invention of the pH meter, pH values of solutions could be found using litmus paper (which changes color depending on the pH of the solution), or other complicated potentiometric techniques (which were often very complicated and time consuming). However, these methods often had limitations that made them unreliable for determining pH (litmus can be bleached by some solutions, i.e. sulfur dioxide). These problems were resolved with the invention of the “acidometer” by Arnold Beckman in the 1930’s. He created what would become the basis for today's modern pH meters; a device consisting of electrodes and amplifiers that could measure a change in electronic potential equivalent to the pH of the solution being measured.
Today the pH meter is a common and essential device found in wineries around the world. It generally consists of a single glass probe that can be inserted into the solution to be measured. The probe consists of a reference electrode, often comprised of Ag/AgCl, that maintains a constant potential regardless of the surrounding solution. An indicator electrode , often a glass membrane comprised of sodium silicate molecules, which acts as a cation exchange surface with an HCl solution held inside the glass membrane. The amount of exchange that takes place is determined by the pH of the solution being measured. This change in potential can then be measured against the reference electrode to find the pH of the solution.
For accurate pH measurement, it is often necessary to calibrate pH meters against solution standards. This is done by measuring pH standards, often of pH 2, 7, and 10, to obtain a pH curve used to calibrate the device. The calibration process correlates the voltage produced by the probe (approximately 0.06 volts per pH unit) with the pH scale. Proper rinsing of the probe between measurements is also essential to obtaining proper readings. This is preferably done using deionized or distilled water that is rinsed over the probe. Excess water is then removed by patting dry, not rubbed, so as not to produce any static charge that could skew results. The probe is also stored in a salt solution to prevent drying of the sensitive probe head that needs to remain wet to work correctly.
Application in Wine Microbiology:
Proper measurement techniques for the pH of grape juice and wine are essential to quality wine production as well as microbial control. The pH of a juice generally is desired to be below 3.6. This is due to the fact that bitartrate precipitation above 3.8 will raise the pH of the solution. This will be detrimental to the wine in several ways. Firstly, pH is the main antimicrobial condition of the wine; many microbes cannot survive at pH levels below 3.6. However as pH rises to 4.0, many problematic microbes can survive and possibly ruin a wine. Knowing the pH is also essential to sulfite additions. Sulfite exists as three forms at the pH levels found in wine: the molecular SO2 form, the bisulfate form and the sulfite form. The most important form to the winemaker is the molecular form: the anti-microbial form as well as the form that reacts with hydrogen peroxide (which is important in preventing acetaldehyde formation). As pH increases from 3.0 to 4.0, the molecular form decreases 10 fold. Thus as pH increase, all molecular SO2 is lost along with its effective functions.
Overall, the pH of a juice of wine is one of the most important factors needed to be known by the winemaker; mostly because of the antimicrobial effect a low pH will produce as well as allowing for more molecular SO2. A winemaker will often add tartaric acid to lower pH to a level (generally 3.6 or less) to allow for proper microbial action and a desired concentration of molecular SO2. pH will also have a profound effect on protein stability (which is important in haze formation), malolactic fermentation (low pH can inhibit the bacteria responsible for ML fermentation), and potassium tartrate precipitation, amongst other things.
References:
- Boulton, R., V. Singleton, L. Bisson, and R. Kunkee. 1999. Principles and Practices of Winemaking. Chapman & Hall, New York.
- Ebeler, S. 2009. Potentiometric Techniques. VEN 126 Course Notes. UC Davis. Davis, CA.
- Guerzoni, M. E., M. Sinigaglia, F. Gardini, M. Ferruzzi, S. Torriani. 1995. Effects of pH, Temperature, Ethanol, and Malate Concentration on Lactobacillus plantarum and Leuconostoc oenos: Modelling of the Malolactic Activity. Am. J. Enol. Vitic. 46: 368-374.
- Unknown Author. An Instrumental pHenomenon: Beckman Invents the Acidimeter. 2000. Chemical and Engineering News. 78:15 19.