Protective cultures are an emerging and growing area in the food industry, including for use in dairy products. Among other benefits, they can help reduce food waste and spoilage. In yogurt and cheese applications, we use starter cultures to produce lactic acid. However, protective cultures aren’t added to produce acid and help with the fermentation; they instead can provide some degree of protection against pathogens and spoilage organisms. 

There are a couple of ways that protective cultures work (i.e., bio-preservation). One of these mechanisms is that protective cultures can produce short chain organic acids, like propionic or acetic acids, that are inhibitory to other unwanted organisms. This can be a more label friendly approach favored by some consumers, instead of adding organic acids directly as separate ingredients during food processing. Metabolites from bacteria that have antimicrobial activity also include hydrogen peroxide, acetoin and diacetyl. 

Another mechanism that some types of protective cultures use is the production of bacteriocins, which are inhibitory proteins/peptides that can kill or inhibit other bacteria. They can also be considered a natural food preservative. Presumably, bacteria developed this ability to produce bacteriocins because it allows them to better compete for spaces and nutrients with their local bacterial competitors. This is an example of the competitive exclusion approach.
 
These bacteriocins often act by disrupting the bacterial cell membrane, or the integrity of the cell wall, and are generally considered safe ingredients, if they are derived from common lactic acid bacteria. Nisin is the most widely used (isolated) bacteriocin ingredient and nisin is effective when added at low concentrations against a range of gram-positive pathogens like Listeria monocytogenes. Other types of bacteriocins (e.g., reuterin) have also been investigated as antimicrobial agents in dairy products. 

The effectiveness of these bioprotective approaches depends on the level of microbial contamination as they have the potential to slow or delay further contamination or growth (e.g., during handling or storage) but are much less effective if the initial level of contamination is very high. Some commercial protective cultures target yeasts and molds to help extend the shelf life and reduce off-flavor development in fresh dairy products. These are sometimes used as potential alternatives to antifungal ingredients like sorbates. 

Another bio-protective approach is the use of fermentates, where the bacteria are usually removed or inactivated after an initial fermentation process and the supernatant, or fermentation mixture (if the inactive bacteria are included), is used as an antimicrobial in a food application. These fermentates may contain organic acids (e.g., lactic, propionic), bacteriocins, and other bacterial metabolites. Suppliers have developed various types of fermentates, which would need to be tested for their effectiveness in different types of applications. High usage levels can be an issue if the fermentates contain shorter chain organic acids due to their impact on the sensory properties of the food. 

Researchers here at the University of Wisconsin-Madison, specifically Dr. Kathy Glass, have researched fermentates and how they can be used in different dairy products to inhibit unwanted organisms. Another research project taking place at the university is being led by Dr. Tu-Ahn Huynh of the Department of Food Science. Her lab is investigating wood boards used for cheese aging. Interestingly, Dr. Huynh’s lab has found that when some cheese varieties are aged on wood boards, certain unusual types of bacteria can be isolated that are inhibitory against Listeria. The discovery of additional types of protective cultures is an ongoing research focus. 

Fresh dairy products like yogurt, cottage cheese and cheeses that are considered riskier for contamination or spoilage (e.g., those with high pH and moisture content) are the most common choices for the use of protective cultures or other bio-preservative approaches. 

One thing to keep in mind if using these bio-preservative cultures is that they need to be tested to ensure that they do not inhibit starters, desirable gas formers in eyed cheeses, and/or flavor producing adjuncts. 

Finally, it’s important to emphasize that these bio-preservative approaches are meant to be used as an additional hurdle step in the production of safe and high-quality products. They can provide additional protection to dairy products. They are not a substitute if the dairy plant uses poor quality ingredients, lacks good sanitation practices, or uses improper storage temperatures. Protective cultures and bio-preservatives have become more popular as they are viewed as a clean-label ingredient, which can be attractive to consumers. They appeal to consumers as a more “natural” ingredient that can help the dairy industry produce high-quality, safe products.