Are you measuring pH in food products? How confident are you in the accuracy of your results?
More and more people in the food industry are being required to test pH for both safety and quality reasons. Advances in technology mean that you don't need to be a technician or scientist to deliver precise and accurate results consistently.
- Understanding pH in Food
- Selecting a pH Meter and Electrode Combination
Understanding pH of Foods
In technical terms, pH is the hydrogen ion activity in a solution. It’s measured on a scale of 0 to 14, with 7 being neutral. Effective monitoring of pH in the food industry begins with testing raw materials and continues throughout production to the finished product.
pH is an essential parameter because of how it effects food characteristics such as texture, flavor, aroma, and more. Cheese is a great example of how pH influences the chemical and physical properties of food.
As the pH decreases, these protein crosslinks are altered and the casein loses its ability to interact with water. This results in the harder consistency found in aged cheddars and white mold cheeses.
The casein matrix of cheese is created by protein bonding. In those with an initial pH of higher than 5.0, calcium phosphate crosslinking occurs and the casein interacts strongly with water, creating the elastic, smoother texture found in young Swiss and cheddar cheeses.
Food Safety and Regulation
pH plays a crucial role in inhibiting the growth of microorganisms. It's for this reason that U.S. government agencies, such as the Food and Drug Administration (FDA), regulate pH levels in many commercial food products.
Generally speaking, food falls into two categories according to 21CF114. Those that have a natural pH of less than pH 4.6 are known as acid foods while low acid foods have a pH higher than 4.6.
When an acid is added to lower the finished pH of a low acid food to below 4.6, these foods become known as acidified foods. Examples include canned goods, salsas, and sauces.
In the production of acidified foods, a final pH higher than 4.6 will result in an environment that promotes the growth of harmful bacteria that, when consumed, can cause illness. Not only does ruined food present a health hazard to the public, it can also halt business operations if further evaluation is necessary.
Best Practices in Measuring pH
Determining best practices for measuring the pH of foods can be a challenge due to a variety of different sampling methods, meters, and electrode designs.
Across the globe, there are numerous regulations regarding appropriate sampling methods and levels at which certain parameters must be in order to be considered safe for consumption. Some standard methods are even specific to the type of equipment and the required degree of resolution.
While pH strip indicators and other basic methods are available, the best possible accuracy comes with using a pH meter and electrode. This method provides the highest degree of precision and is less subjective than a chemical based testing method that uses color indicators to determine pH.
Selecting a pH Meter and Electrode Combination
Choosing a pH meter and electrode specific to measuring pH in food is crucial to getting accurate and reliable results.
It is important to consider the sampling points during production at which you will be testing. A durable, portable meter offers the mobility to test pH anywhere, at any time in a production facility, while a high-performance benchtop meter is best suited for stationary lab use only.
Many portable and benchtop pH meters now have storage for data logging and advanced diagnostic capabilities to help make testing as user-friendly as possible.
The minimum requirements for a pH meter recommended by Hanna Instruments for food testing would include:
- 0.01 pH resolution
- 2 point calibration
- Automatic Temperature Compensation (ATC)
While the features in a meter are important it is equally, if not more critical, to choose the appropriate pH electrode for your application.
Hanna Instruments has a full line of Foodcare pH electrodes that are designed for specific applications. The design considerations when making a pH electrode for a specific use include the shape and type of glass, junction material, and body type.
Of the design considerations, the type of reference junction used is one of the most important. The reference junction is an electrical pathway between the sample and the internal reference cell. There are a variety of materials that can be used for the reference junction.
The most common is a porous ceramic that works great in aqueous solutions.
Another material is Polytetrafluoroethylene (PTFE) that repels solids. A sleeve junction made with PTFE is ideal for solutions that have a high solid content such as fruit juices as it helps to prevent clogging.
Hanna offers pH electrodes with an open junction design. With this style of junction, there is a hard gel known as viscolene that separates the sample from the internal reference. The open junction design is extremely resistant to clogging making it the best design for semi-solids, emulsions and direct measurement of food products.
These are just a few of the junction types offered by Hanna Instruments.
Additional electrode features include:
- Built-in temperature sensor for the automatic compensation of temperature variations
- A microprocessor based digital version that stores GLP data and serial number for traceability
- High-temperature glass for use in cooking processes
Having the right pH electrode for your food product will ensure the best possible results and a long life.
When calibrating a pH electrode it is important to choose buffers that “bracket” the expected reading. Bracketing is the process of calibrating a pH meter to points above and below the expected pH value of the sample being tested.
How Calibration Works
When a pH probe is placed in any solution, a voltage is generated and converted to a pH value. A pH buffer is a solution of a known value. It is the voltage response in a pH buffer that is being standardized during the calibration process.
A pH meter will typically accept ±60 mV for a pH 7.01 buffer. This value is known as the offset voltage. Large changes in an offset voltage can indicate that either the buffer is no longer the pH value that is stated on the bottle or that there is a coating on the pH electrode. Hanna Instruments recommends that the offset should be ±30 mV.
The calibration of a pH electrode to pH 4.01 or 10.01 buffer is known as the slope adjustment. This value, relative to the offset, determines the slope of the line used by the meter to correlate between the mV of a sample to its pH reading.
Video: How to Check the Slope and Offset of a pH Meter (3 minutes)
A probe with 100% slope will generate 59.16 mV/pH unit away from pH 7.01 at 25 C. Most pH meters will calibrate to a slope between 85 and 105% (50 mV – 62 mV/pH unit at 25 oC). The slope of a pH electrode should be greater than 90%.
All pH electrodes will have their slope degrade over time. It is typically a gradual process. Any large change in slope from one calibration to the next is an indicator that the pH buffer is contaminated.
Offset and slope sound too technical? We have CAL Check to make it easy for you!
Hanna Instruments offers meters with CAL Check to help you recognize when your probe is deviating from an ideal offset and slope. A meter with CAL Check will display “clean electrode”, “check buffer” and report the overall probe condition or health of the electrode by monitoring the electrode characteristics during calibration. If monitoring offset and/or slope are necessary, then a pH meter with a GLP function can be used to display the information.
It should be noted that once opened, buffers should be changed at least every six months.
Regular maintenance and proper storage are critical to accurate measurements. Fats, oils, and proteins found in foods leave residues on the glass sensing surface of the electrode, clogging the junction and resulting in poor performance.
Application specific cleaning solutions are ideal because they are specially formulated to clean the electrode based on the composition of the sample being tested, such as an enzymatic cleaner for high-protein samples like meat.
For either long or short term storage, it is best to use a storage solution to keep the pH glass bulb hydrated and prevent clogging of the junction from the salts found inside the reference fill solution.
Never store the pH electrode dry or in purified water (distilled, deionized or reverse osmosis) as this is detrimental to it. Storing the pH electrode in purified water will shorten the life of it and contribute to poor performance.