Cheese is a “living” food due to the presence of different types of bacteria and sometimes yeast and mold, depending on the cheese variety. Like any other living thing, the bacteria used in cheese can get sick.
Bacterial viruses, known as bacteriophage or phage, can infect the starter bacteria used in cheese. These infections lead to bacterial lysis and, consequently, the loss of the necessary lactic acid production that is required for the cheesemaking process.
Small, easily spread and heat-resistant enough to survive routine pasteurization, bacteriophage is difficult to eliminate. However, the incidence of phage can be greatly reduced when a cheesemaker takes preventative measures.
In cheese plants, bacteriophage primarily spreads through whey movement — either in aerosols of whey in air or spillage on floors — or simply by having whey processing located in the same room as cheesemaking. Therefore, it is important to have equipment such as fine savers, clarifiers and whey cream separators in rooms separate (or segregated) from cheesemaking because these pieces of equipment can generate whey aerosols.
Since phage can spread via whey aerosols, it is also important to have a positive airflow pattern in place with the correct airflow direction (airflows from more sensitive rooms to less sensitive rooms).
In addition to proper airflow, cleaning and sanitation of the plant throughout the day is also essential. This includes cleaning each vat and all affected equipment, including using a sanitizer effective against phage. Floors should also be thoroughly sanitized, especially in areas where whey can accumulate.
Keeping the floors as dry as possible is also effective because phage need to infect living bacterial cells, and bacteria cannot multiply without a source of water such as in whey. Additionally, employees can carry the virus from room to room on their clothes/shoes, so utilizing a proper traffic flow pattern and shoe sanitizer stations between rooms is essential.
Finally, employing culture strain rotation will help to avoid an outbreak. Culture rotation is using the same starter culture blend for a limited number of consecutive vats before switching to an entirely new blend of strains. This method works because, like all viruses, each strain of bacteriophage has a unique “fingerprint” that recognizes a specific type of bacterial cell. When multiple cultures are rotated, the bacteriophage cannot attack all of the strains.
Keeping track of phage numbers in the whey is the best way to know when to rotate cultures and to lessen the likelihood of developing new phage strains. It is imperative that all cheese manufacturers regularly monitor and test the phage levels of the whey in their plants, either through testing in an in-house laboratory or by regularly sending whey samples to their culture supplier for testing.
Culture suppliers have also been steadily improving the phage resistance of their strains, but cheesemakers should still take the preventative measures described here.