Carrying Out pH Testing in the Food Industry

While there are a variety of other variables and protocols to be followed to ensure our food supply remains safe, a basic understanding of pH and how it is measured is crucial to food production.

• By Paul Burnside
• Oct 01, 2013

Monitoring of pH levels during the process of food production is a vital step in the production of high-quality foods. Maintaining a proper pH range is essential in many of the physical and chemical reactions that take place during food processing. Incorrect pH levels during production of yogurt, for example, can lead to discoloration, excessive free whey, and additional or insufficient tartness. In the manufacturing of jelly, pH levels can affect consistency. From wine and cheese to jelly and yogurt, maintaining the proper pH level is a critical factor in the production of many food products.

Proper pH level not only affects the look, taste, and quality of food products, but also maintaining a proper pH level is a food safety issue. A low pH reading of 4.6 will help prevent the growth of deadly bacteria such as botulism in canned or preserved foods. Accurate monitoring of pH during the production of these types of foods may be mandated by state or federal regulations.

The equipment used for monitoring pH during food processing is widely available and relatively inexpensive. The methodology of testing the acidity of foods and the requirements for the type of equipment used in such testing are regulated by the U.S. Food and Drug Administration and found in the Code of Federal Regulations (CFR) Title 21 Section 114.90.

Accuracy is the most important factor in measuring pH in the food industry. The requirements set forth in the above referenced standard include using a pH meter with a resolution of 0.1 pH units. Most small pocket testers, handheld meters, and benchtop meters will offer this resolution.

Accuracy can range from 0.1 to 0.001 pH units on pH meters. Why is accuracy so critical? As mentioned earlier, a pH range below 4.6 will prevent the growth of bacteria when canning or preserving foods. If you have a pH meter with accuracy of 0.1, a reading of 4.6 could actually be 4.5 to 4.7. At 4.7, bacteria can start forming in food products.

The importance of accuracy in meters is shown during quality test procedures. If deviations occur in the testing of pH levels from expected values during a scheduled process or the equilibrium pH of the finished food product is higher than 4.6, the commercial processor of the acidified food, according to CFR 21 subpart E 114.89, must follow one of the following three procedures:

1. Fully reprocess that portion of the food using a process established by a competent processing authority as adequate, to ensure a safe product.

2. Thermally process it as a low-acid food per the guidelines in CFR 21 Part 113.

3. Set aside that portion of the food involved for further evaluation for any potential public health significance. The evaluation shall be made by a competent processing authority and shall be in accordance with procedures recognized by competent processing authorities as being adequate to detect any potential hazard to public health. Unless the evaluation demonstrates the food has undergone a process that has rendered it safe, the food set aside shall either be fully reprocessed to render it safe or be destroyed. A record shall be made of the procedures used in the evaluation and the results.

Choosing the Electrodes
With the expense and time required for the procedures that must be followed when the canning or preserving process has a reading above 4.6, the importance of the accuracy of the meter is critical. It is recommended a meter with a minimum accuracy of 0.1 pH unit or better be used.

Once a pH meter is selected, there is a choice of electrodes to use. The options available include sealed or refillable electrodes that can be single junction or double junction or ISFET (Ion Selective Field Effect Transistors) electrodes.

Sealed or disposable electrodes require the manufacturer's initial conditioning procedure and proper storage and have a service life of 6 to 18 months, depending on materials tested and care. Refillable electrodes have the option of being rejuvenated when the performance starts to wane. This can include readings that drift, longer time to calibrate, or inaccurate readings. Rejuvenating the electrode includes cleaning and replacing electrolyte to the electrode previously consumed during testing. The ability to rejuvenate the electrode ensures accuracy and extends the service life of the electrode.

Either single junction or double junction electrodes can be used in the food testing process. While a single junction electrode is suitable for most laboratory practices, double junction electrodes due to the construction and function of the electrode have an advantage when dealing with oily foods or dairy products. Double junction electrodes have the ability to provide accurate readings in adverse conditions.

ISFET electrodes are solid state electrodes that rely on a silicon chip. ISFET electrodes are durable and easy to maintain. The probes can be made of stainless steel and, because there is no glass, they are ideal for the food industry. Disadvantages associated with the ISFET electrode include cost; they are two to three times more expensive, they do not offer the stability and accuracy of traditional glass electrodes, and they work only with meters adapted to electrode ISFET.

Meter Calibration
To ensure the meter is reading properly, calibration of the meter should be conducted daily or in accordance with the manufacturer's instructions.

Calibration can be done at one, two, or three points on the pH scale, depending on the meter. A buffer solution of 7.0 is used for one-point calibration. Meters with two- or three-point calibration allow them to be calibrated at 4.0, 7.0, and 10.0 to ensure the meter is reading accurately in more than one range.

When testing the pH levels of acidified foods, a two-point calibration of 7.0 and 4.0 will ensure the meter is reading accurately in that range.

If the meter used to test a sample does not have an Automatic Temperature Compensation (ATC) feature, the food sample being tested should be at room temperature (25° C) at the time of testing.

For foods that have a uniform consistency or homogeneous foods, a sample is considered to be representative of the whole. These foods are usually wholly liquid or contain only very small particles. According to CFR 21 114.90 methodology, two readings should be taken. If it is truly homogeneous, the readings should agree with each other and the values should be reported to the nearest 0.05 pH unit.

Some food products may consist of a mixture of liquid and solid components that differ in acidity. Other food products may be semisolid in character. Others may be coated or mixed with oils. Special preparation procedures of the samples are required for pH testing of these samples. The following are examples of the preparation procedures for pH testing of each of the categories.

Specific procedures are required for testing the pH levels of mixtures of solid particles in a liquid brine or syrup, such as chunky salsa and pickled vegetables. Requirements include separating the liquids from the solids by draining off the contents of the container for two minutes on a No. 8 stainless steel sieve. This needs to be inclined at a 17-20 degree angle. The weight of the liquid and solid portions must be recorded and each portion retained separately.

If the liquid contains oil that could lead to the fouling of the electrode, the layers need to be separated with a separatory funnel and the aqueous layer retained. At this time, the oil layer can be discarded. The aqueous layer needs to be adjusted to 25° C and the pH level tested.

To test the pH levels of the drained solids, they are removed from the sieve and blended to a uniform paste. The temperature of the paste needs to be adjusted to 25° C and the pH level tested.

Next, the solid and the liquid fractions are blended in the same ratio as found in the original container and then blended to a uniform consistency. The temperature should be adjusted to 25° C and the equilibrated pH is determined. Alternatively, the entire contents of the container are blended to a uniform paste, the temperature of the paste is adjusted to 25° C, and the equilibrated pH determined.

For testing of marinated oil food products, the oil is separated from the solid product. This is done by allowing the oil to rise to the surface, and then it is removed by skimming or pouring it off. If the oil is not easily separated from the food, cooling the sample in a refrigerator may solidify the oil so that it can be removed.

Once the oil is separated from the solid food, the food portion is placed into a blender and blended to the consistency of a paste. It may be necessary to add a small amount of distilled water to some samples to facilitate blending; a small amount of distilled water will not alter the pH value of most foods. Twenty milliliters (ml) of distilled water is then added to each 100 grams of food product. The temperature of the paste is adjusted to 25° C and the probe is inserted into the paste to determine the pH level.

Food products classified as a semisolid have the consistency of products such as puddings, thick sauces, and potato salads. For this test, the sample is blended to a uniform paste consistency. As with marinated oil food products, it may be necessary to add distilled water to obtain the fluidity required for testing. Adding from 10 to 20 ml of distilled water to 100 grams of product is acceptable. Again, the temperature of the paste is adjusted to 25° C to determine the pH.

Upon completion of testing, proper cleaning of the electrode is vital to remove any residual oils or food product that could foul the electrode and lead to drifting or inaccurate readings on future samples testing.

While there are a variety of other variables and protocols to be followed to ensure our food supply remains safe, a basic understanding of pH and how it is measured is crucial to food production.

This article originally appeared in the October 2013 issue of Occupational Health & Safety.