Editorial Feature

pH Sensors to Monitor pH Levels During Fermentation of Alcohol

Brewing alcohol is a complex process where yeast converts carbohydrates into alcohol and carbon dioxide. It requires a series of sensors to monitor a range of parameters during the fermentation process including carbon dioxide, oxygen, pH, temperature, and pressure transducers to measure the density of the fermenting alcohol. 

For the scope of this article, the focus will be on pH sensors and their application in the fermentation process of alcohol. pH measurements can be an early sign of complications in the fermentation process, indicating the presence of undesirable acid producing bacteria or autolysis, the process of yeast cells dying and fragmenting

pH Sensor – Functional Principle

The main purpose of a pH meter is to ensure the pH level is maintained during fermentation, which allows the optimal catalytic conversion of sugar to alcohol. A classic pH meter measures the acidity or alkalinity of a liquid. The pH meter involves the application of glass electrodes. Any pH meter will need to be calibrated before being used to test for the acid/alkalinity of a test solution and requires regular calibration so that the glass electrode generates an accurate pH reading.   

A standard calibration process should be carried out with two buffer solutions that cover a spectrum of pH values, usually between a pH value of 4 and 10. It is important to set the pH meter to two control pH values that correspond to the two pH buffer values. The pH meter is also adjusted to a control value for temperature. By calibrating the pH meter for all three control values, the meter can achieve linear accuracy.

A standard pH meter involves glass electrodes and reference electrodes (such as Ag/AgCl [silver/silver chloride]) that are important for measuring the pH of a test liquid. Electrical circuits containing a glass electrode, a reference electrode, and an instrument to measure the electrical potential between opposing electrical fields are the basic units to a pH meter. 

The main purpose of the glass electrode is vital to measuring the pH of a solution and carries an electrical current via a wire submerged in the test liquid. The wire and liquid are enclosed by a thin glass tube (figure 1). 

During a standard testing procedure, this electrode is exposed to an acidic solution that forces the positive ions from the acid solution to bind to the glass electrode. Whilst submerged, the electrodes need to maintain a neutral solution and so the electrons from the inner surface of the electrode move to the outer solution, but this will change the electrical potential of the testing solution. The purpose of the reference electrode is to carry the electrical current to a meter that can measure the difference in electrical potential. 

Working principle to a pH Meter.

Figure 1. Working principle to a pH Meter. Image Credit: Nielsen, S.S. (2010). Food Analysis. 4th Edition. New York: Springer Science and Business Media, LLC.

When brewing wine, it is important to measure total acidity. A graduated syringe is normally used to inject dilute sodium hydroxide (a neutralizer) to the wine solution together with phenolphthalein. The indicator used changes the wine to a pink color to ensure that the solution is no longer acidic. The one problem with red wine is the dark color, which can obscure the color change and make it difficult to determine the complete neutralization of the wine solution. This is where the pH meter becomes useful – this device can work to provide a neutral endpoint on a digital monitor.

There are currently a number of pH meters on the market. Ocean Optics provides two types of pH sensors (In Situ Transmissive pH Sensors and Non-Intrusive Reflective pH Sensors) that deliver an accurate response time, are low maintenance, and have a precise reading of pH content.

In Situ Transmissive pH Sensors

This type of pH sensor uses a sol-gel solution combined with a colorimetric pH indicator dye that can reflect light through a read fiber, which will then provide an estimation of color change to a sample solution at a certain wavelength. pH sensors are typically susceptible to changes in salinity, though the In Situ Transmissive pH Sensor is built to avoid this problem. This type of pH sensor works well with organic solvents including acetone, alcohols, and aromatic and so would be of benefit to the beverage brewing industry.

Non-Intrusive Reflective pH Sensors

These sensors are novel and have evolved pH sensor technology. The sensor uses an electroformed mesh material which adds a coating of metal on a non-metallic surface of the pH sensor; this creates a reflective ion permeable membrane allowing for pH measurement through a clear wall to a cavity containing reflective probes. Again, this type of sensor is applicable to the food and beverage processing industry.

Sources and Further Reading

  • Ocean Optics Sensors
  • Warrick, S.F. (2010). The Way to Make Wine: How to Craft Superb Table Wines at Home. England, London: University of California Press, Ltd.
  • Gopalan, R., Sugumar, R.W. (2008). A Laboratory Manual for Environmental Chemistry. New Deli: I.K. International Publishing House Pvt. Ltd.
  • Nerantzis E T, et al. Winemaking process engineering: Ïnline fermentation monitoring - sensors and equipment. E-Journal of Science and Technology. 2007: page 29–36.
  • The Wine and Spirit Trade Association
  • Nielsen, S.S. (2010). Food Analysis. 4th Edition. New York: Springer Science and Business Media, LLC.

This article was updated on 14th February, 2020.

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