Measuring pH in food: theory and pitfalls

The pH value is one of the units of measurement used to characterize a food product. Usually, this value is considered simple to measure and is added to the list of analyses to be performed. However, don't be misled: this can be costly!

pH is a measure of the acidity of an aqueous solution. Dry substances, powders, pure fats, and oils do not contain water; therefore, their pH cannot be measured. Measuring pH provides insight into one or more properties of the (final) product, such as structure, foaming ability, shelf life, colour, and/or taste. pH can range from 0 to 14, with 1 being strongly acidic and 13 highly alkaline. A neutral solution has a pH of 7.

pH is expressed as the negative logarithm of the concentration of H3O+ ions in the solution, defined in formula form as:

pH = -log [H₃O⁺]

It's often stated that pH measures H+ concentration. The correct reference ion is the hydronium ion (H3O+), which forms as

H⁺+ H₂O ↔ H₃O⁺

Measurement methods

pH can be measured using strips, a pH indicator in the solution, or a measuring instrument.

Strips

Measurement with a strip is based on the property of the chemical compound litmus, which turns red in an acidic and blue in a basic solution. pH can also be determined by dipping a piece of litmus paper into the liquid. The colour-changed strip is then compared with the provided colour scale.

Indicators

In (chemical) processes, it is often necessary to know the pH in the liquid or to adjust it to a different value. This can be done by adding a pH indicator liquid that changes colour within a narrow pH range. However, these pH indicators are not food grade.

Measuring instruments

Instruments for measuring pH all operate on the same principle: a sensor (electrode) sensitive to the hydrogen ion determines the concentration of hydronium ions and compares this with a reference signal. The measuring electrode gives a potential based on the present hydrogen ions, while the reference electrode gives a constant potential. The potential difference between these two is a measure of the pH of the solution.

It's important to know that the temperature factor affects the potential difference between the electrodes. At lower temperatures, the voltage decreases, and it increases at higher temperatures. For accurate measurements you must compensate this. Modern pH sensors usually have Automatic Temperature Compensation (ATC).

There are various types of pH sensors, including sensors with a pointed tip for measuring in 'solid substances' and sensors with a knife in front of the tip for making incisions in, for example, meat.

Correct measurement

Contamination on the sensor can significantly disturb the measurement. pH meters still display a value despite contamination, but it's crucial to understand that this value is often incorrect. Cleaning the electrode should only be done with a special cleaning solution to avoid damaging the sensitive membrane.

Calibrating the pH meter is done with buffer solutions. Often a two-point calibration is sufficient, using a buffer solution in the acidic range (pH 4) and in the neutral range (pH 7). Sometimes a three-point calibration is performed, adding a measurement in an alkaline buffer (pH 10). Calibration should be performed at least every day (prior to measurement).

Key points in pH measurement

• Measuring the pH of juices is a relatively easy task. However, be mindful of the juice temperature if using an electrode without ATC.
• In liquid dairy products, there's a high chance of fats and proteins contaminating the sensor of the pH meter. With a contaminated sensor, the measured value slowly moves to a final value, which may not be the correct pH.
• When measuring pH in semi-solid products like meat, it's important to consider the measurement location. Cutting into the meat releases moisture from the cells, which mixes with intercellular fluid. If the pH inside and outside the cell differs, it affects the measured pH. If the sensor ends up in a fatty part of the meat, the measurement will be incorrect, as fat is not an aqueous solution.
• Immediately after slaughter, various biochemical processes occur in meat. The rate of these processes depends on the environmental temperature and the carcass temperature. To understand these processes, measuring in the same place on a carcass is necessary. Meat contains, besides fat and protein, a significant amount of moisture, including hydronium ions. Some of the moisture is bound, some of it is freely available. A pH measurement will measure the concentration of hydronium ions in this freely available moisture.
• The structure at the core of fruits and vegetables can differ from the outer structure. One option is to measure deliberately at a specific location, or another is to mash the product and determine an average pH. When reporting pH, the measurement method used should be specified.
• In products like cheese, where there's almost no moisture, there's a risk that the pH meter's value does not reflect the 'real' pH as the measurement isn't in an aqueous solution. Moreover, the sensor quickly becomes contaminated by the present fats and proteins. Frequent cleaning of the sensor tip with a cleaning solution is necessary to continue measuring accurate values.

Conclusion

pH measurement must be performed correctly. The used methodology and approach should be clearly reported. Only this way can the data be accurately interpreted and the measurement replicated by others.

REFERENCES:
HTTPS://NL.WIKIPEDIA.ORG/WIKI/LAKMOES
HTTPS://NL.WIKIPEDIA.ORG/WIKI/PH-INDICATOR
PH THEORY GUIDE METTLER TOLEDO
HANNA INSTRUMENTS, WHAT IS PH