Food contamination generally depends on two major variables: how easy it is for a particular product to become contaminated and how difficult it is to discover the contamination through the testing methods in use. With milk and dairy products, the combination of these two factors make the probability of food safety incidents much higher than with other foods.
The risk starts at the very source. “Milking happens in a non-sterile environment that can harbor different pathogens,” says Dino Demirovic Holmquist, vice president of business development at Eurofins. “For bacteria to grow, you need humidity, the right temperature, and food. Milk has the perfect combination: It’s liquid, nutritious, and is drawn from the cow at a temperature between 32º and 34ºC.”
The other variable is not favorable either: Pathogens in dairy products can be difficult to detect because of their complex matrices and the interaction among different microorganisms. One of the effects of this interaction is a phenomenon called metabiosis, which happens when a microorganism creates the right conditions for the growth of another one. A typical example, says Holmquist, is a pathogen that lowers the pH in milk, creating a perfect environment for another pathogen that was already there, but in very small quantities. As this second pathogen grows, it produces a substance or other favorable conditions in which a third one can flourish and make a product unsuitable for consumption, he adds.
In fermented products, these interactions may have the opposite effect of keeping bacteria undetected when using standard plating techniques. “Fermentation often uses lactic acid bacteria,” says Luke Thevenet, a pathogen technical sales specialist at 3M. “These can produce antimicrobial compounds that compete for resources with the pathogen that you’re trying to detect, preventing it from growing.”
The same phenomenon occurs in dairy powders: “Powdered dairy is probably one of the most difficult matrices to recover pathogens from and prevent interference if using an unvalidated detection method. Their low-water-activity environment is not conducive for low numbers of pathogens to survive and grow rapidly, which affects the detection and recovery rate of molecular platforms,” says Celina To, regional technical sales manager at Hygiena.
Whether it’s metabiosis or competition between microorganisms, the result is that the pathogen is there, but invisible to standard plating methods. “You can have the best technology, but if the pathogen hasn’t grown to levels above the limit of detection, it is not going to provide valuable information,” says Thevenet.
To complicate this situation even further, dairy is one of the most dynamic segments in the food industry, with new products and formulations launched every week: “If you’re introducing new ingredients all the time, you might not have data on their pathogenic risk, their interaction with the rest of the formulation, or whether the tests you’ve been running are still valid for that new matrix,” says Thevenet.
Using an aggressive heat treatment such as ultra-high temperature (UHT) to sterilize milk in all products would not be a viable solution, says Holmquist: “Ultra-high temperatures oxidize lipids and caramelize sugars [and] will change the taste, which is the main reason we buy milk and dairy products these days. What’s more, the dairy industry has always claimed to interfere very little with milk and keep it very close to its natural state. With the clean label trend, this has become even more important.”
The Need for Speed
To be sure, plated methods are not any less valid because of these challenges. With the right strategy, the right enrichment process can always be found. For example, says Thevenet, “You might have to adjust the pH or select antibiotics to target the competing microorganisms, while promoting a positive growth environment for the pathogen.”