Measuring pH of Yogurt

Measurement of pH presents a critical quality control step in the production of dairy products, especially yogurt. 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 article will discuss electrode selection, calibration techniques, sensor maintenance, and best practices for measuring the pH of yogurt. While the focus is placed on yogurt, the guidelines reviewed can be readily applied to a much broader range of dairy products and quality assurance procedures.

Role of pH Monitoring in Making Yogurt

Monitoring pH is crucial in producing consistent, quality yogurt. Yogurt is made by the fermentation of milk with live bacterial cultures. Following pasteurization and compositional adjustment, milk is homogenized for a consistent texture, heated to the desired thickness, and cooled before inoculation. Most yogurts are inoculated with a starter culture consisting of Lactobacillus bulgaricus and Streptococcus thermophilus. Once the live culture is added, the mixture of milk and bacteria is incubated, allowing for conversion of lactose to lactic acid. As lactic acid is produced, there is a corresponding drop in pH. Due to the more acidic mixture, the casein protein in milk coagulates and precipitates out, thickening the milk into a yogurt-like texture.

Maintaining the ideal fill level for storage solution cap prevents the sensing tip from drying out.

Credit: Hanna

Yogurt producers cease incubation once a specific pH level is reached. Most producers have a set point between pH 4.0 and 4.6 in which fermentation is arrested by rapid cooling. The amount of lactic acid present at this pH level is ideal for yogurt, giving it the characteristic tartness, aiding in thickening, and acting as a preservative against undesirable strains of bacteria. By verifying that fermentation continues to a predetermined pH endpoint, yogurt producers can ensure their products remain consistent in terms of flavor, aroma, and texture. A deviation from the pH set point can lead to a reduced shelf life of the yogurt or a product that is too bitter or tart.

Syneresis is the separation of liquid, in this case whey, from the milk solids; this can occur if fermentation is stopped too early or too late, resulting in yogurt that is respectively too alkaline or too acidic. Consumers expect yogurt to remain texturally consistent, so ensuring fermentation is stopped at the appropriate pH is vital to consumer perception.

Choose the Right Sensor For the Job

Using the correct electrode for your application is one of the first and most important aspects to consider when measuring pH for quality control or analysis. A variety of electrode features can be combined to ensure reliable and repeatable results for specific samples such as yogurt.

Open junction reference. Conventional pH electrodes have a ceramic frit reference junction that allows the internal reference electrolyte to come into contact with the sample. In dairy products, such as yogurt, proteins and other colloidal solids can partially or completely clog this ceramic frit, resulting in slow electrode response or inability to take a reading. For yogurt, it is recommended to use a pH electrode with an open junction rather than the traditional single ceramic junction.

An electrode that has an open junction design utilizes a gel reference electrolyte that comes in direct contact with the sample; because there is no physical junction, potential clogging is no longer an issue. An open junction design offers the additional benefit of a faster response time because of a higher flow rate of electrolyte into the sample. Other types of clog-resistant, high-flow junctions exist, including polytetrafluoroethylene junctions, triple ceramic frit junctions, and ground glass junctions; these designs confer their own advantages, but are typically better suited for other applications.