Measurement of pH presents a critical QC step in the production of dairy products. The pH offers an indication of contamination from bacteria or chemicals while also providing a convenient method to estimate the acid development of a dairy product. As there are a myriad of different sampling methods, electrode care guidelines, and electrode designs, determining best practices for pH measurement can be a challenge. This guide will discuss electrode design, maintenance, and practices for measuring the pH of dairy products. While the focus is on cheese, the guidelines can be applied to a much broader range of dairy products.
Cheese is a versatile food that is valued worldwide for its high nutritional value and long shelf life. There are hundreds of cheese varieties, all prepared with differing compositions and techniques. Due to this variance and complexity in production, cheese is the most diverse form of dairy product. The cheesemaking process involves multiple pH quality control steps to ensure the desired flavor is achieved and batches remain consistent.
Prior to measurement, pH meters must be calibrated. Calibration adjusts how pH values are assigned to incoming millivolt (mV) readings from the electrode. The pH electrodes generate a mV potential based on hydrogen ion activity; this activity is determined by pH glass, which is specially formulated to measure the hydrogen ion. Hydrogen ions contribute to how acidic a sample is, while hydroxide ions contribute to how basic a sample is. The pH scale ranges from 0 to 14, with pH values less than 7 being acidic, pH values greater than 7 being basic, and pH 7 being neutral. As pH glass breaks down and changes over time due to normal wear and tear, calibration of the meter corrects for these changes in the glass; the quality and frequency of calibration procedures will ultimately determine the accuracy of data. For best results, calibrate the pH meter at least once per day with standards that bracket the expected pH range of the samples. Because cheese typically has a pH value between pH 5.0 and 6.7, ideal calibration standards are pH 4.01 and 7.01; a third buffer may be incorporated for higher precision.
The theoretical relationship between pH and mV is defined by the Nernst equation. Based on this equation, a theoretical electrode will read 0 mV in pH 7.0 buffer (the value of which is known as the offset), and will have a slope of -59.16 mV per change in pH unit. Calibration corrects for deviations of electrode behavior from this theoretical relationship, but can only correct for so much before the accuracy of the measurement is affected. Many meters will have indications of electrode condition or slope condition, but it is recommended to use the mV mode on a pH meter to periodically check electrode offset and slope. To perform an electrode offset and slope check, first measure and record the mV value in pH 7.0 buffer; this is the electrode offset. Next, measure the mV value of a second buffer, such as pH 4.0. To determine the electrode slope, calculate the difference in mV between the two buffers and then divide this by the number of pH units between buffers. To convert this to electrode slope percentage, divide the electrode slope by the theoretical slope of 59.16, and multiply by 100. Acceptable ranges for offset is ±30 mV and slope percentage is 85 percent to 105 percent; anything outside of these ranges may result in inaccurate measurement.
Before cheesemaking begins, milk must be tested for quality. Milk is slightly acidic, typically in the range of pH 6.5 to 6.7, due to the presence of phosphate, citrate, and other buffering salts present. Values above this range may suggest illness in the cow; values below this range may indicate that lactic acid fermentation has begun. Although lactic acid fermentation is commonly performed as part of the cheesemaking process, it is undesirable in the raw milk because potentially pathogenic bacteria, rather than the starter cultures, are performing the fermentation.