Practical tip of the month
Nicole Martin
Gram-negatives provide more information about post-pasteurization contamination than coliforms
Do you want to improve your fluid milk quality? Start testing your finished product for total Gram-negative bacteria in addition to coliforms.
It’s not surprising that after a century of monitoring processed dairy products for coliforms, much of the industry has become adept at eliminating these contaminants. In our long-term monitoring program, the Voluntary Shelf-Life (VSL) program, we’ve seen coliform prevalence in pasteurized fluid milk steadily drop over the last 20 years. But that doesn’t mean that rates of post-pasteurization contamination (PPC) in fluid milk have declined, on the contrary, this is still a major issue for many fluid milk processors. So, you may be asking yourself if coliforms are not frequently detected in the finished product, how do we know there is PPC? The answer starts with the bacterial cell wall. There are two main types of bacterial cell walls, we call these Gram-positive and Gram-negative – named for Danish bacteriologist Hans Christian Gram who developed a method for classifying bacteria by their cell wall properties. The important difference between Gram-positive and Gram-negative bacteria as it pertains to PPC, is that Gram-negative bacteria are very heat sensitive, meaning they are easily killed by pasteurization. When Gram-negative bacteria are detected in pasteurized dairy products it’s very likely that they were introduced after the heat treatment – they are therefore indicators of PPC.
In current fluid milk supplies, the primary types of bacteria that are contaminating pasteurized fluid milk after pasteurization are non-coliform Gram-negative bacteria, especially Pseudomonas. In fact, a recent study from the Milk Quality Improvement Program determined that of 132 samples with PPC, Pseudomonas was the primary cause in 101 of those samples. Fluid milk processors who aren’t testing for Gram-negative bacteria are not likely to catch PPC when it happens – this leads to marketplace failures and unhappy customers.
If you’re convinced that monitoring your pasteurized fluid milk for Gram-negative bacteria is the right move, there are a couple of methods that you can use. The standard method for detection of Gram-negative bacteria uses Crystal Violet Tetrazolium Agar (CVTA), but this method may be challenging for processors without the capability of making traditional agar in-house. Fortunately, our team recently developed a method to detect Gram-negative bacteria in fluid milk by modifying the Coliform Petrifilm method. In short, simply incubating Coliform Petrifilm for an additional 24 hours (a total of 48 hours) then counting any growth, with or without gas production, provides a simple way to quickly determine if the product is contaminated with Gram-negative bacteria like Pseudomonas.
We’ve seen great results from fluid milk processors who implement Gram-negative testing in their overall quality program. If you have any questions on how to get started with this method, feel free to reach out for more information.
