Determining pH During Cheesemaking

The production of cheese begins with the metabolism of milk by microbial cultures. These cultures ferment the milk under controlled conditions, producing lactic acid from the sugars that are naturally present. Fermentation causes a decrease in pH and allows for flavors to develop. Next, the milk is separated into solid and liquid components through the use of rennet, an enzyme complex that is responsible for the curdling of proteins in milk. The resulting solid component is known as curd while the liquid component is called whey. Once the milk is coagulated, the curd is fermented until it reaches a pH of 6.4. It is subsequently separated from the whey and left to form into a mat. At this point, the mat is cut into sections and layered to expel more liquid. Fermentation continues in this layered form until the pH reaches pH 5.1 to 5.5 and is then salted or brined. Additional treatment may be performed based on the desired style of the cheese before the product is stored, aged, and packaged.

It is best practice to measure pH frequently throughout the cheesemaking process from raw milk to finished product, as pH influences the microbial community that contributes to flavor and texture development. If pH during cheese processing is too low, the cheese may be prone to a brittle or pasty texture, and can potentially harbor the growth of mold after packaging. If pH is too high, the cheese may become too firm, and can be dangerous for consumption due to risk of pathogen formation.

When pH is measured in dairy products, electrode fouling is a common challenge. Electrode fouling occurs when fats and proteins obstruct the reference junction or attach themselves to the sensing glass of the electrode.

Electrode fouling can be minimized with proper maintenance, storage, and cleaning. Buildup on the sensing glass causes inaccurate and sluggish measurements, as it directly affects the glass’ impedance. An offset outside of the acceptable range of ±30 mV usually indicates the pH glass bulb is dirty or coated. Cleaning solutions are effective at both disinfecting and removing oil and protein deposits. It is also recommended to store electrodes in storage solution when not in use. This ensures that the sensing glass stays hydrated and ready for measurement.

The Design

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 milk and cheese, 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 dairy, it is recommended to purchase a pH electrode with an open junction rather than the traditional single ceramic junction. The open junction design utilizes a gel reference electrolyte that comes in direct contact with the sample; because there is no physical junction, clogging is no longer a potential issue. The open design also offers an added benefit of a faster response time due to a higher flow rate of electrolyte into the sample. Other types of electrode junctions exist, including PTFE junctions, triple ceramic frit junctions, and ground glass junctions; these designs confer their own advantages, but are more suited for other applications.

Conventional pH electrodes have a spherical sensing bulb that provides an increased surface area for the sample to interact with the sensing glass; this bulb shape is ideal for measurement in aqueous solutions. However, other tip designs exist on the market, and each shape offers an advantage in certain applications. For example, conical tipped pH electrodes are pointed so that they may easily penetrate semisolid objects, such as cheeses.