Guide to pH Testing in the Food Industry
The 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, pH level maintains a critical factor in the production of so many food products.
Proper pH not only affects the look, taste and quality of products, but maintaining a proper pH level is also a food safety issue. A low pH reading of 4.6 will prevent the growth of deadly bacteria such as botulism in canned or preserved foods. Accurate monitoring of pH during the productions 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 or acidified foods and requirements for this type of equipment are regulated by the FDA and found in the Code of Federal Regulations (CFR) Title 21 Section 114.90.
The requirements set forth in the 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 is the most important factor in measuring pH in the food industry.
Accuracy can range from 0.002 to 0.2 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 an accuracy of 0.2, a reading of 4.5 could actually be 4.3 to 4.7. At 4.7, bacteria can start forming in food products.
Deviations from scheduled processes, CFR 21 subpart E 114.89 show the importance of accuracy and address canned and preserved foods that have pH readings higher than 4.6. In short, the standard calls for one of three actions that must be taken. The first, fully reprocess that portion of the food using a process established by a competent processing authority as adequate, to ensure a safe product; the second, thermally process it as a low-acid food per the guidelines in standard CFR 21 Part 113. Thermally processed low acid foods packaged in hermetically sealed containers are the third and final option; 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. 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 single junction, double junction, Calomel and ISFET type electrodes.
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. For more information on the calibration and care of pH meters, please see Water Samping and Testing.
Testing the pH of Foods
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.
Testing of Non-Homogeneous Foods
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.
Liquid and Solid Mixtures
Mixtures of solid particles in a liquid brine or syrup, such as chunky salsa and pickled vegetables are required to follow specific procedures when testing the pH levels. 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° angle. The weight of the liquid and solid portions need to 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 25C 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.
Marinated Oil Food Products
In this process the oil is separated from the solid product. This is done by allowing the oil to rise to the surface and then 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. 20 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 like 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 20ml 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 which could foul the electrode and lead to drifting or inaccurate readings on future samples testing.
Commonly Asked Questions
What is the proper way clean the electrode?
Apart from rinsing the end of the electrode in water, the best way to make sure the electrode is properly cleaned is to use a cleaning solution specifically designed for the product being tested. Manufacturers have solutions that are designed to clean fats and oils and other solutions for proteins as well as general-purpose cleaners. Always follow the manufacturer's instructions for proper cleaning procedures of the electrode, as well as what solutions to use.
What is the best way to store the electrode?
In order for a pH electrode to give the best response time and accuracy, the measurement bulb of the electrode needs to be kept hydrated. The reason is to keep a constant ionic presence and keep the bulb of the electrode "charged" and ready for use. The ideal solution for this is a storage solution suggested by the manufacturer. This not only keeps the electrode at the optimum ionic level, but also inhibits bacteria and algae growth. If you don't have access to storage solution, you can use tap water on a short-term bases, changing it every couple of days or use a buffer solution. The one solution not used is distilled or deionized water; these contain insufficient ionic activity to maintain the electrode and in effect will stop it from working altogether.
Please Note: The information contained in this publication is intended for general information purposes only. This publication is not a substitute for review of the applicable government regulations and standards, and should not be construed as legal advice or opinion. Readers with specific questions should refer to the cited regulation or consult with an attorney.