Contaminated feed represents one of the first introduction pathways for Salmonella and other Enterobacteriaceae into the food value chain. One potential measure to reduce this risk is targeted heat treatment of the feedstuff. But for the implementation of heat treatment as a validated “kill-step” systematic studies are still lacking.

Dr. Edyta MARGAS Food and Feed Safety Leader Bühler AG

In the previous articles the importance of Salmonella control in feed as one of the first entry pathways into the food value chain was discussed. To control Salmonella in feed, heat inactivation is one of the key steps. During conditioning, superheated steam is added to increase the temperature and the moisture of the product. These conditions are maintained in the retentioner for a certain time and cause Salmonella inactivation. Studies providing process parameters that ensure a certain inactivation effect exist but are mainly not sufficient.

The first practical step in defining the best approach for validation is a laboratory scale study as described in part 2 of the article series. In the work, conducted by Buhler in collaboration with Campden BRI, the most heat resistant Salmonella strain associated with feed was defined as S. Agona. The time that is needed to achieve a reduction of 90% (1 log) of the initial cell count of S. Agona, also named “D-value”, was tested at different temperature and moisture levels under controlled conditions. For further large scale studies, Enterococcus faecium was defined as a non-pathogen surrogate which has a comparable heat resistance like S. Agona that can be used outside of a laboratory environment.

In a next step, the data assessed in the laboratory served as a basis for a pilot scale study which was conducted in the feed pilot plant of Bühler AG, Uzwil. A conditioning-retentioning system (Bühler AG, Uzwil), equipped with heating mats to prevent heat loss during processing (active thermal insulation) and additional temperature sensors for improved temperature control was used. Product moisture was adjusted with a NIR system in the batch mixer before conditioning (Bühler AG, Uzwil). Material inoculated with Enterococcus faecium in high cell counts (~108 cfu/g) was added into the process directly before the conditioning system. It passed through the conditioning and retentioning system, was collected after the retentioner and analyzed by an external lab. The samples were processed at three different temperatures (75°C, 80°C, 85°C) and different retention times. Retention times at different temperatures were calculated with the D-values gained in the laboratory study. Capacities were adapted to achieve a constant filling level of the retentioner and varied between 1-5 t/h depending on the retention time. The product moisture was kept stable at 12% moisture during all trials representing the minimum common moisture during feed processing and therefore a “worst case scenario”.

In comparison to the results from laboratory scale trials, Salmonella reduction was significantly lower in laboratory scale. Not all settings defined on the basis of the laboratory studies achieved the expected 5 log reduction of the surrogate through heat treatment. This demonstrates the importance of validation studies in a larger scale, combined with correct mapping of processing conditions. Laboratory trials can just give indications but do not include the influences of natural process fluctuations and product inhomogeneities.

From the other hand, validation trials in industrial scale are challenging due to the high throughputs, the missing accessibility of the process to supply inoculated material and sample taking, the need for aseptic sampling, a strict control of process conditions and the broad variety of recipes influencing bacterial inactivation. Therefore, the use of representative pilot scale equipment which is specifically tailored to perform such studies, is the best method to assess process parameters for a trusted “kill-step”.

The pilot scale study can be assessed as representative for industrial scale due to same process conditions and influences. In industrial scale it is important to ensure stable process conditions and to continuously monitor critical parameters at previously defined key locations to ensure a reliable Salmonella inactivation. Therefore, adequate sensor technique positioned at the right spot has to be implemented in the equipment. By combining the knowledge about inactivation kinetics with process control and monitoring, a specific microbial reduction can be ensured and therefore a trusted “kill-step”.