Understanding the Effects of Water on Thermal Inactivation of Salmonella in Low Moisture Food Systems


Learn from two esteemed food science and engineering professors as they discuss the intricacies of defending against salmonella in low-moisture pasteurization processes.

This webinar operates under the assumption that all participants have a working knowledge of water activity and moisture absorption. Therefore, the webinar content will establish basic principles of the effects of water activity on salmonella water activity. To learn the basics of water activity and how it’s used, please refer to Water Activity 101: Mastering the Basics.


Salmonella within a product

While salmonella cannot grow or proliferate in an environment held below a certain water activity level, it can survive and remain present. One particularly tricky aspect of food production is that many products are subsequently used as ingredients in other food products. If one ingredient or inclusion contains bacteria, the final product will too.

Figure 1: Time is essential to reducing a bacterial population within a substance.

Thermal resistance of bacteria

The higher the thermal resistance of a bacteria, the harder it is to inactivate that bacteria within a substance. Therefore, we performed a study of thermal resistance using two different types of substances: all-purpose flour and peanut butter. We created isotherms to map the relationship between the water activity and moisture content in these two samples.

In this experiment, we examined the impact of temperature on the capacitance of polymer film, relative humidity, and water activity. The thermal resistance of the flour and the peanut butter differed, even though they were subject to the same temperature increase. This reinforces that in food production, it is important to remember that the thermal resistance of bacteria will differ depending on the substance. 

Inactivation of pathogens is achieved through a complicated process: inoculation, equilibration (e.g., control humidity), fabrication (e.g., grind, mill, etc.), re-equilibration (five to twelve days), and treatment through bench scale (iso- and non-iso-) and pilot-scale.


Low water activity and low moisture content products

When working with any substance, inoculation is a crucial step and affects the resistance of any bacterial organisms. Several product treatments are available to inoculate and further test bacterial thermal resistance. These treatments include bench scale, isothermal, and pilot scale. All of these involve monitoring the temperature and humidity in an environment. A product’s starting water activity level will directly impact the thermal resistance of the present bacteria.

In an example using almonds, roasting the nuts resulted in a low moisture system, low water activity, a high temperature, and a low bacterial population. In further experiments, it was determined that higher humidity resulted in double the kill rate of the bacteria.

Figure 3: The results of a study that observes the effects of roasting on various nuts.

The relationship between water activity, moisture content, and humidity is complex and requires precise measurements and full attention.


Theoretical alternatives

The webinar presents five theoretical alternatives to validate a process: (1) pathogen challenge in actual plant; (2) surrogate challenge in actual plant; (3) pathogen challenge in pilot-scale; (4) surrogate challenge in pilot-scale; and (5) time-temperature measurement and inactivation model. 

Figure 4: Aggregate RMSE across all thermal treatments, comparing to actual Salmonella lethality.

The first option is a non-starter because we never want to introduce an actual pathogen into our facilities, so the presenter focuses on the latter three options. The three key takeaways from this webinar are (1) Water affects salmonella thermal resistance through water activity, moisture content, and process humidity; (2) the relationships between water activity, moisture content, and process humidity are complex; and (3) other factors (e.g., composition, structure) also affect resistance.

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