Microbial Challenge Testing

For consumers, the concept of shelf life of food is quite simple: Follow the storage instructions indicated on the label, and you can be reasonably sure that the product will still be good to eat at least until the expiration date. The work that goes on behind the scenes to make that happen, however, is more complicated.

There are two important measures that food manufacturers must apply to ensure microbiological safety: killing unwanted microorganisms during processing and preventing their growth during shelf life. Under FSMA, these measures fall into the category of process preventive controls, which are the only ones requiring written validation of their effectiveness.

How well a kill step or a food formulation will work depends on several variables. For example, heat resistance of Salmonella will vary depending on the strain and the combination of macronutrients, water, salt, and pH. These intrinsic factors, together with the use of preservatives, will also have a direct effect on the growth of microorganisms during shelf life.

The interaction of these variables can make validation challenging: “Every organism has an absolute limit: it won’t grow below a certain level of pH, salt content or water activity. But when these are combined, it can be difficult to know what will happen,” says Peter Wareing, a food safety consultant based in the U.K. “If the pH is high enough to allow the organism to multiply, but salt content is also high and water activity is low, then those two factors might prevent the organism from growing.”

Quite often, food manufacturers can use published literature to validate process preventive controls. For example, the efficacy of milk pasteurization is well-established and will require no further evidence; however, with many recent products there is not sufficient data to know whether the combination of kill steps, formulation, and storage conditions will ensure shelf stability. The only way to find out is to adopt an empirical approach and conduct a microbiological challenge test.

In a challenge test, a food sample is inoculated (challenged) with a known microorganism or a surrogate, to observe how an antimicrobial treatment or the composition of the food itself will affect lethality and growth. The two main types are inactivation and growth challenge tests.

Will It Die?

In an inactivation challenge test, a food sample is first inoculated and then put through the intended kill step, such as thermal treatment or high-pressure processing (HPP), and finally analyzed to verify how many cells survived.

When this test is conducted in a lab, says Alvin Lee, PhD, associate professor of food science and nutrition at Illinois Institute of Technology in Chicago, it’s important to closely mimic the conditions the product will go through. “If you’re validating the cooking process in the production of soup, for example, you’ll have to apply not only the same time and temperature but also the ratios between produce and water,” he adds.

Inactivation challenge tests can also be done directly at the manufacturer’s plant. “On-site challenge tests allow you to use the actual processing equipment and take into account all of the unknowable variables,” says Rob Limburn, group manager of industrial process microbiology at Campden BRI, a food and beverage research organization based in the U.K. “In this case, we wouldn’t use an actual pathogen, but a non-pathogenic surrogate with similar inactivation characteristics.” Typical examples of surrogates, says Limburn, include Enterococcus faecium or Pediococcus spp for Salmonella, Listeria, or E. coli in dry products. In some cases, non-pathogenic strains, such as Listeria innocua, may be appropriate.

To make on-site test results more reliable and efficient, says Limburn, it’s important to identify the points in a static system, or the path through a continuous processing line, where the product receives the least severe process: “For example, if you have a conveyor belt with several lines of products going into an oven, there’s likely going to be a gradient of temperature, depending on where the fans are positioned and other factors. In that case, rather than placing more samples across the belt, the best point will be the coldest channel: if it’s effective there, it will be effective anywhere else on the belt.”