Project Initiated for Validating Thermal Sanitization of Mushroom Slicing Equipment

Listeria bacteria are common in soil, vegetation, water, sewage, silage, and in animal feces. They are also readily found in continuously damp, cool indoor environments such as produce packing houses.

Only one species, Listeria monocytogenes, is capable of causing serious human disease. Although L. monocytogenes infections are rare, up to 20 percent are fatal. Although there have been no reported outbreaks of listeriosis associated with consumption of fresh mushrooms, contamination can occur as evidenced by several retail product surveys and industry recalls.

A 13 month Penn State study to survey non-food contact surfaces in a cooperating commercial mushroom packing and slicing facility revealed an average detection rate of over 20.1%. As expected, most Listeria positive sites were found on continuously wet floors and drains.

Now that we know that Listeria monocytogenes can become established on non-food contact surfaces in mushroom packing and slicing facilities, we must take all possible measures to prevent bacterial cross contamination to food contact equipment, such as mushroom slicers. If sporadic contamination does occur, effective slicer disinfection procedures must be developed to prevent transmission of Listeria to mushrooms.

Sanitary Equipment Design

In the food industry, sanitary design standards can be defined as methods and specifications for fabrication, construction, and maintenance of food processing equipment to minimize the occurrence of chemical and microbiological cross contamination during processing operations (Schmidt, 2013).

Briefly stated, all food contact surfaces should be smooth, durable, free of cracks and crevices, non-porous, non-reactive, and readily accessible for easy cleaning and sanitizing. In the U.S., 3-A Sanitary Standards, Inc., and the National Sanitation Foundation (NSF) have recently collaborated to develop standards for meat and poultry equipment for review by the International Organization for Standardization (ISO). Already, 3-A standards are referenced in the Grade A Pasteurized Milk Ordinance (PMO). More broadly, the U.S. Food and Drug Administration (FDA) has established minimal regulatory sanitary design standards within "Current Good Manufacturing Practice in Manufacturing, Packing, or Holding Human Food (21CFR Sec. 110.40 - Equipment and Utensils).

In Europe, the European Hygienic Engineering and Design Group (EHEDG) is the main organization for sanitary design standards for food equipment, many of which have been incorporated into international trade and equipment manufacturing association standards (EHEDG, 2004).

In response to increasing incidences of food contamination linked to improper design and use of processing equipment, interest among all food processor for assuring consistent and effective cleaning and sanitizing procedures has increased. Validation studies, defined as "obtaining documented evidence that cleaning and/or disinfection processes are consistently effective at reaching a pre-defined level of hygiene" (Timmerman, 2013), are therefore needed to assure that equipment does not become a source of food contamination.

Under authority of the recently enacted Food Safety Modernization Act of 2011 (FSMA) [P.L. No. 111-353], FDA has proposed "Current Good Manufacturing Practice and Hazard Analysis and Risk-Based Preventive Controls for Human Food". This proposes specific new requirements for documenting validated preventative control measures that go far beyond the general requirements established in 21CFR Part 110. In particular, the rule mandates HACCP based validation of sanitation preventative controls, including cleaning and sanitizing of food contact equipment.

Disk Slicers Used By The Mushroom Industry

The mushroom industry uses specialized slicing equipment that properly aligns the mushrooms before high speed disk slicing. The primary worldwide supplier of mushroom conveying, orienting, and slicing units is DutchTechSource headquartered in the Netherlands.

The key component is a removable slicer head which consists of a series of circular stainless steel cutting blades placed along a metal axis and which are aligned by slotted high density polyethylene (HDPE) "fingers". It is evident upon cursory inspection that disc slicers are not designed for easy cleaning and sanitizing. In practice, the slicer blades and fingers may not be completely dissembled, cleaned, sanitized, and reassembled as frequently as might be needed because the time taken to complete the process can conflict with production schedules. Instead, the slicer heads are removed several times during daily production, taken to a designated location, and various high pressure water and chemical cleaning and sanitizing methods are employed.

It has become apparent to the industry that there are multiple locations that are inaccessible to liquid sanitizers and it is speculated that adequate disinfection may not be consistently achieved. Therefore, some operations have adopted a hot water immersion technique for disinfection. However there is no evidence that this technique is effective and thus there are no consistent industry-wide standards for this practice.

Under FSMA, facilities that slice fresh mushrooms will be mandated to provide evidence that the equipment they use to slice mushrooms can be effectively cleaned and sanitized such that it will not become a source of contamination. For these reasons, the American Mushroom Institute's Food Safety Task Force has identified validation of disinfection treatments for mushroom slicers as a priority research area. FDA has established a "zero tolerance" standard for ready-to-eat (RTE) foods that support the growth of L. monocytogenes (FDA 2008). As government and commercial buyers increase their scrutiny of fresh produce for the presence of Listeria, the mushroom industry must have validated methods in place for sanitization of food contact mushroom slicers.

Thus, with contributions from the Giorgi Mushroom Company Fund for Mushroom Research, the Mushroom Council, the Penn State Department of Food Science, Monterey Mushrooms, and the loan of a mushroom slicing head by DutchTechSource, this fall, Drs. L.F. LaBorde and R.C. Anantheswaran we will begin a research project to validate thermal disinfection of mushroom disk slicers. Our objectives are to:

  • Determine temperature/time treatments required to achieve at least 5-log reductions in Listeria monocytogenes on the surface of materials used to construct the slicer (e.g. stainless steel, HDPE plastic),
  • Conduct a nondestructive "tear down analysis" and dye penetration study of the slicing head to make a preliminary assessment of entry points for contamination and potential harborage sites for bacteria
  • Conduct a heating profile study of slicer heat components during hot water or steam treatments in a Clean-Out-of-Place (COP) tank using thermocouples placed at previously identified harborage sites.
  • Conduct an inoculation study to validate a 5-log destruction of L. monocytogenes at all points within the slicer head.

Based on our results we will make recommendations to the industry for slicer thermal sanitization procedures. We will also make recommendations on how the design and fabrication of mushroom slicing heads can be improved so that, in the future, cleaning and sanitization procedures can be made more efficient and effective. The results will be presented to the AMI Mushroom Food Safety Task Force, at the annual Mushroom Short Course, and the annual Mushroom Food Safety Workshop as well as in industry and scientific journals.

Readers can update their knowledge of potential Listeria harborage sites in mushroom packing and slicing facilities by visiting the Penn State Mushroom Food Safety website and clicking on the link to Sanitary Practices and Design Considerations for Mushroom Packing and Slicing Operations, a 1.5 hour recording of a webinar from the Penn State Department of Food Science originally presented on May 4, 2011.

Selected References

  • EHEDG. 2004. Hygienic equipment design criteria. Document No. 8. 2nd edition. European Hygienic Engineering and Design Group. April 2004.
  • FDA. 2008. Guidance for Industry - Control of Listeria monocytogenes in refrigerated or frozen ready-to-eat foods. U.S. Food and Drug Administration. Center for Food Safety and Applied Nutrition. February 2008.
  • Schmidt, R. 2013. Food equipment hygienic design: An important element of a food safety program. Sanitation. December 2012/January 2013.
  • Timmerman, H. 2013. Cleaning validation, practical considerations. Journal of Hygienic Engineering and Design, 2, 3-5. Chicago, USA.

Mushroom News 60(9):6-8. September 2014

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Project Initiated for Validating Thermal Sanitization of Mushroom Slicing Equipment

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Contact Information

Luke LaBorde
  • Professor of Food Science - Plant Based Products.
Phone: 814-863-2298