Listeria monocytogenes is a human bacterial pathogen of concern in the fresh produce industry because of its high mortality rate and it is ubiquitous in continually moist processing environments such as produce packing and slicing facilities. The new Preventive Controls for Human Food regulation issued by FDA under the Food Safety Modernization Act requires processors to consider the potential for cross-contamination of equipment during operation and develop sanitation preventive controls proven to minimize risks. Fresh-cut produce operations are especially warned to be aware of the potential for mechanical slicers to harbor and ultimately transfer L. monocytogenes to ready-to-eat produce products.
Current industrial mushroom slicers consist of successive cutting disks aligned and separated by cylindrical spacers (Fig. 1a and Fig. 1b). This design creates niche sites where moisture and soils can enter the spacer-blade interface (Fig. 1c). Frequent deep cleaning and sanitization is impractical for mushroom fresh-cut processors because complete disassembly is time consuming. Current industry practice among some slicing operations is to periodically remove the slicer head from the production line and immerse it in a hot water at temperatures up to 212°F for as long as 1 hour. However, this practice has not been validated for its effectiveness in reducing L. monocytogenes populations at niche sites on the slicer head. A study was conducted to determine parameters for effective thermal sanitization of disk slicers commonly used in the mushroom industry.
In order to safely conduct thermal destruction experiments on a pilot plant scale, the heat tolerance of a 6-strain mixture of L. monocytogenes was compared to a that for a non-pathogenic L. innocua strain. Decimal reduction times (D-values) (time in minutes to achieve a 90% population reduction) were calculated from laboratory data collected using a recirculating water bath set at 50, 60, and 70°C (122, 140, and 158°F). Results showed that the selected L. innocua strain was equally or more heat tolerant than the L. monocytogenes mixture at each of the temperatures (Table 1) and was therefore a suitable surrogate for use in subsequent pilot plant validation experiments.
Thermal properties of slicer head materials and COMSOL® modeling software were used to simulate hot water heating at 65°C (149°F) for up to 2 min. The imaging data showed that the slowest heating surface on the slicer head (cold spot) was the junction between the inner surfaces of spacers that separate the cutting blades and the horizontal axes (Fig. 2). This area is only accessible by completely disassembling the slicer head.
To monitor temperatures at the cold spot, spacers were fitted with thermocouples extending inside so that they were flush with the inner surface and in contact with the axle (Fig. 3a). The inner surface area was spot-inoculated with L. innocua at ~107 CFU /ml (Fig. 3b) and the slicer head was re-assembled. Using D-values for L. innocua obtained in the laboratory study, 7 X D treatment times of 93, 16.4, and 6.51 minutes were calculated for each respective temperature. The slicer head was then immersed in a clean out of place (COP) tank (Fig. 3c) filled with water at 55, 65, 75°C (131, 149, and 167°F). Treatment times began when the thermocouple reached 50, 60, and 70°C. After each treatment, the slicer head was removed from the COP tank and disassembled. The spacers were removed and placed into enrichment broth to determine presence or absence of L. innocua.
No L. innocua was detected after enrichment for each of the temperature/time treatments (Table 2). Therefore, it can be expected that this process will result in at least a 7-log (1/10,000,000) reduction in L. monocytogenes.
These results can be used to make specific recommendations to the mushroom industry on procedures for thermal sanitization of mushroom slicer heads. Current industry practice for boiling slicer heads for an hour or more are likely to be far more severe than necessary to eliminate L. monocytogenes deep within niche sites. Excessive temperatures no doubt contribute to metal corrosion, damaged seals, and leaking bearing lubricants as has been reported by some of the companies that follow these procedures. Each operation can assure adequate destruction of L. monocytogenes by immersing slicer heads in hot water or in moist air and monitoring the temperature at one or more inner spacer sites.
In practice, a set of spacers fitted with thermocouples, as described in this study, could be manufactured for each mushroom slicing facility. Maximum process control could be achieved by monitoring cold spot temperatures each time the slicer head is heat sanitized either by hot water or moist air treatments. For this approach, spacers would have to be fitted with permanent sockets that would allow quick connection and disconnection of thermocouples. Total wash tank treatment time would include the amount of time required for the monitored cold spot to reach the target temperature plus 7 X D minutes at that temperature. Alternatively, a simpler approach would be to conduct initial heat penetration studies using and a dedicated wash tank for a specific slicer head size. The operator would then only be required to monitor and record the wash tank treatment time at the temperature and agitation settings used in the initial study. Recalibration of the system would be required if a different wash tank or slicer head size were used or if changes were made to water temperature or agitation settings.
Different validated temperature and time combinations will provide processors flexibility for incorporating regular and frequent thermal sanitization treatments into their production schedules. For instance, higher temperature and shorter times would allow rapid heat treatments to be conducted between production shifts. Lower temperature and longer time treatments could be done when more time is available with the added advantage of minimizing damage to the slicer head and exposing workers to high water temperatures
This study has demonstrated the feasibility of thermal sanitization treatment to eliminate L. monocytogenes at niche sites within slicer heads without requiring complete disassembly. However, it is recommended that slicers be periodically disassembled for deep cleaning in order to eliminate build-up of soils. Facility-wide sanitation programs should also be evaluated for their effectiveness in eliminating reservoirs of L. monocytogenes on non-food-contact and other food-contact equipment surfaces.
Originally printed in Mushroom News, September 2017.