Thermal process validation for low water activity food – what is an in-process validation study?
Low water activity foods (LWAF) are foods with a water activity at or below 0.85. Examples include:
- Dried fruits and vegetables
- Breakfast cereals
- Nut butter
- Teas and herbs
- Spices and condiments
While most pathogenic organisms cannot actively grow in LWAFs, they can survive for extended periods of time in or on the product. Since these organisms, like Salmonella and E. coli, have low-infectious doses, it is crucial to remove them completely from the products prior to distribution. To achieve this goal, thermal processing, which uses heat to disrupt pathogen survival, is added to the LWAF production procedures. Thermal processing can be:
- Drying
- Dry roasting
- Superheated steam
- Baking
- Pasteurization and blanching
- Radiofrequency
Each thermal process approach has varying levels of effectiveness depending on the product matrix, specific equipment, and temperature and time settings. Thermal process validation studies investigate the process’s effectiveness for pathogen inhibition. On top of the initial process assessment, processes may need re-validation in the case of:
- New pathogen risks in similar products
- Routine monitoring shows failure
- Changes in product intrinsic parameters or formulation
- Regulatory agency alerts
- Consumer complaints
Since thermal validation studies are a key component of product safety and reliability, it is important to understand how researchers investigate a process and draw conclusions about its effectiveness.
There are three main stages in a thermal validation study. We have outlined them below, with a focus on Phase 3 (in-plant validation).
Phase 1: Product/process assessment – building the foundation
The first step of a thermal validation study is a critical evaluation of the regulatory requirements for a specific product
and process. A streamlined validation workflow 
is developed based on the specific product’s regulations. In phase 1, the process parameters, including equipment, manner of heat application, temperature, and time are considered. Additionally, the intrinsic (like pH and moisture) and physical (like size and shape) properties of the product are evaluated to better understand how the thermal process acts on the product. 
A key part of the foundational research for a validation study is the selection of target organisms. Based on previous outbreaks in similar products, the current food safety plan, and regulatory requirements, target pathogens that need to be inhibited during the thermal process are selected for investigation. In LWAF, Salmonella is the most common target organism.
Finally, the worst-case scenarios are investigated. These are the product and process conditions that are least likely to prevent pathogen growth and lead to process failure. The worst-case scenario conditions are used in subsequent work to ensure that even when things are not perfect, the process is still acceptable.
Phase 2: Surrogate evaluation
The goal of a thermal process is to inhibit target organisms, selected in phase 1, to ensure product safety and reduce the risk of foodborne illness. However, target organisms, like Salmonella and E. coli, cannot be used for in-plant validation studies to measure the effectiveness of a thermal process. Instead, a surrogate organism represents the target. Surrogates must have similar or greater resistance to the thermal process compared to the target organism in the context of the product matrix. Additionally, they must be non-pathogenic, stable, easy to grow, and easily differentiated in the lab. A common surrogate for Salmonella in LWAF is E. faecium, which is often more resistant in thermal processing conditions.
Phase 3: In-process validation – executing the validation study
The overall goal of an in-process validation study is to determine if a certain thermal process effectively inhibits the growth of pathogenic (target) organisms and is acceptable for use in the production process. To achieve this goal, scientists test the worst-case conditions of the product (the form of the product that is most likely to foster microbial growth) and the process (the process conditions, like temperature or position on the conveyer belt, that are least likely to inhibit growth).
First, temperature mapping identifies any cold spots in the equipment, like a dryer or steamer, which is used to apply heat. These spots are either fixed for even heat distribution or used as the worst-case position for analysis.
Next, the worst-case product is inoculated with the surrogate organism. Ideally, the entire product would be inoculated, but this is not always feasible due to size, shape, or other parameters. Instead, the inoculated product is marked with dye or carefully labeled to differentiate the samples that have the surrogate organism from those that do not. In the image on the left, the inoculated product is dyed red, and the control product is undyed. A high level of inoculum is used in a matrix that is stable in the product (no antimicrobials used).
The in-plant validation study is conducted by subjecting the worst-case product to the processing procedure according to the facility’s SOP. The time, temperature, and amount of product in/on the unit used for normal processing are used for the validation study, ensuring any cold spots are included in inoculated product placement. A positive and negative control is included in the study, and all process parameters are recorded for accuracy when drawing conclusions (temperature, time, etc.).
After the thermal process is complete, the inoculated samples and controls are transported to a lab to count the number of surrogate organisms.
Process lethality calculation:
Once the surviving bacteria are quantified, the final step is calculating the log reduction of the surrogate organism using the following equation:
Log reduction = log(number of bacteria in positive controls) –
log(number of bacteria in inoculated samples)
Using the log reduction of the surrogate organism, the log reduction of the target pathogenic organism can be determined. The most common performance criteria for the validation of LWAF is a five-log reduction of Salmonella.
The final report:
The final report generated at Eurofins provides background and objectives for the study. Additionally, every step of the protocol is described, including methods for inoculation and enumeration, process description and parameters, product description, and controls. The results include reports of the pH and water activity of the samples and controls, initial and final levels of the surrogate organism in each sample, and the log reduction calculations. The final conclusion describes the efficacy of the process on the log reduction of surrogate organisms and selected target pathogens.
Do you have an LWAF with a thermal process? We are here to help you validate your process to ensure safe and reliable products. Check out this webinar to learn more about thermal process validation studies, or contact us to get started!
