Lab Report: Turning Up the Heat on Plant-Based Cheese

Editor's Note: This article is the final installment in a three-part series on university-based studies. Read the first installment here, the second installment here, and more about food safety's university-industry connection here.

With a steady stream of plant-based food products entering the market, how do processors know if the thermal inactivation of Salmonella and Listeria is the same as in comparable, traditional foods?

Kristin Schill and Kathleen Glass were principal investigators for an internally funded project evaluating the thermal lethality of foodborne pathogens in plant-based process cheese. The team worked with University of Wisconsin-Madison food science major and undergraduate researcher Calvin Slaughter, Harneel Kaur and other researchers affiliated with the Food Research Institute’s Applied Food Safety Laboratory.

“We work with a lot of different food products, setting up food challenge studies where we inoculate foods with various pathogens,” Schill said, noting that this project required a series of testing formulations with different water activity and pH combinations at various temperatures.

Because plant-based foods are relatively new, Schill explained, there’s little research or evidence-based direction related to pasteurization other than referring to standards for typical processed cheese products.

The question was, should the industry be issuing different thermal processing guidance for plant-based cheese?

PROJECT AND PURPOSE. The project is formally titled “Thermal Inactivation of Listeria monocytogenes and Salmonella species in a plant-based dairy analog product as a function of pH and water activity.” Its purpose is scientifically complex. Determine D- and Z-values for both pathogens in plant-based process cheese.

D-value is basically the time it takes at a given temperature or conditions — such as water activity — to kill 90% of microorganisms. In this case: Listeria and Salmonella. The Z-value is the number of degrees required to change the D-value by a factor of 10. Really, the Z-value takeaway is this: How susceptible is a microbial population to changes in temperature?

Now, introducing the variables.

Four different plant-based dairy formulations were prepared and inoculated with either a cocktail of Listeria monocytogenes or Salmonella. They were flattened into a film, sealed in moisture-impermeable pouches and then vacuum-packaged.

First, the submersion process — in pre-heated water baths to achieve one of five temperatures ranging from 57.2° C to 68.3° C. Each of these four packages had an almost-twin of a different pH that was also submerged but pulled out during varying time intervals and then chilled. pH samples were 4.8 and 5.8 for each sample of plant-based dairy product for every temperature.

“We conducted three trials at each temperature,” Schill said, adding, “The pH and water activity are the main drivers that determine difference in resistance to [bacteria].”

FACTS AND FINDINGS. The study confirmed there is variability in thermal inactivation rates in plant-based dairy process cheese analogs with respect to pH and water activity. So, this means additional studies are necessary across plant-based products so preventive controls can be developed to ensure the health and safety of plant-based alternatives during shelf life.

“We are seeing that the thermal resistance of Listeria in plant-based process cheese analogs is greater than that of Salmonella at least for the formulations with the water activity and pH combinations we tested,” Schill said, noting the dual purpose of the study.

“The thermal resistance of both of these pathogens increases with decreasing water activity,” she added. “And we observe higher, increased resistance of both pathogens in formulations with a higher pH.” 

WHAT’S NEXT? “Having some thermal inactivation data for plant-based cheese helps us evaluate the safety of plant-based process cheese formulations so companies can file their products,” Schill said.