Sphagnum peat-based casing soils do not permit the survival of Listeria monocytogenes & Salmonella sp

Mushroom News, Sept, 2006 by Naveen Chikthimmah, Robert Beelman, Luke LaBorde

ABSTRACT

In the mushroom growing process, sphagnum peat amended with calcium carbonate added on top of mushroom mycelia-colonized substrate (compost) is called casing soil. The objective of the present study was to determine the survival of the foodborne pathogens Listeria monocytogenes and Salmonella sp. inoculated into sphagnum casing soils. Batches of casing soils were either untreated or autoclaved at 121[degrees]C for 90 min to destroy populations of native casing microflora. The casing soils were inoculated with L. monocytogenes and/or Salmonella sp., maintained under simulated mushroom-growing conditions (80 percent moisture, 23[degrees]C), and periodically sampled for enumerating populations of the foodborne pathogens. Inoculated population levels of L. monocytogenes and Salmonella sp. remained largely unchanged in autoclaved (sterile) casing soil over the sampling period (35 days). However, populations of the foodborne pathogens rapidly declined in untreated casing soil. A 5.7-log population of L. monocytogenes was reduced to undetectable levels within 14 days of introduction into the untreated (unsterile) casing soil. Results demonstrate that commercial sphagnum-peat casing soils (that are not subjected to pasteurization or heat treatments) are effective in destroying introduced foodborne pathogens. The results also suggest that the native sphagnum-peat casing microflora are beneficial for food safety. Cultural practices such as thermal pasteurization may negatively affect the native casing microflora and hence compromise food safety. Therefore thermal pasteurization of sphagnum-casing soils is not recommended.

INTRODUCTION

In the mushroom growing process, Agaricus bisporus (button mushroom) starter culture (spawn) is mixed into pasteurized mushroom substrate and allowed to grow throughout the substrate for about 14 days. After mycelial colonization of the substrate is complete, an approximate 2-inch layer called casing soil is applied on top of the substrate (Figure 1). The function of the casing soil is to provide an environment for fruit body formation and to maintain a source of water for the mushrooms. A successful mushroom crop with good yield, optimal size and good mushroom quality is dependent upon the amount of moisture in the substrate and casing soil (Kalberer 1985 and Schroeder and Schisler 1981). Hence, high-water holding capacity is a necessary attribute for casing soils. In the commercial mushroom community, sphagnum-peat is a preferred material for use in casing soil mixes. Casing soil mixes are made of sphagnum-peat amended with calcium carbonate (to neutralize pH) and water (80 percent moisture).

Commercial preparation of substrate for mushroom growing involves a Phase II substrate protocol. This protocol includes a substrate pasteurization step designed to eliminate mushroom pathogens (Weil 2004), weeds and insect pests. A successful pasteurization requires that the air and substrate temperature reach 60[degrees]C (140[degrees]F) for at least 2 hours (Wuest and Bengston 1982). Studies at Penn State demonstrated that complete inactivation of human bacterial pathogens in mushroom substrate (compost) can be achieved during a Phase II protocol where pasteurization temperatures reach 60[degrees]C for at least 2 hours (Weil 2004). Hence, properly prepared mushroom substrate (compost) is not a food safety concern. However, the microbial food safety of fresh mushrooms as influenced by the casing soil is largely unknown.

[FIGURE 1 OMITTED]

Most commercial mushroom growers do not currently employ a process to disinfect casing soils primarily due to anecdotal evidence that it increases incidence of crop disease and reduces crop yields. Increase in crop disease incidence has been reported when growers employ casing soil steam-treatments of 63-70[degrees]C, presumably because populations of native microflora that inhibit mushroom pathogens are reduced or eliminated or they indirectly influence primordial formation. Mushrooms form in close proximity to the casing soil. Particles of casing soil often adhere to the mushroom surface and are evident at retail display. Hence, there are new demands from buyers for growers to implement food safety control measures for casing soils. However, a recommendation to implement a food safety control measure for casing soils has been reserved, since the microbial ecology of the casing soil and its influence on the survival of foodborne pathogens is not known.

The goal of the present project is to study the microbial ecology of the casing soil and determine the survival of L. monocytogenes and Salmonella sp. inoculated in casing soils maintained under mushroom growing conditions.

MATERIALS & METHODS

Casing Soil

Casing soil was prepared at the Mushroom Test Demonstration Facility (MTDF) on the Penn State University Park campus. Sphagnum peat soil (11 bales, 0.17 m[.sup.3] each, Acadian Peat Moss Ltd., New Brunswick, Canada) was amended with calcium carbonate (205 Kgs, Graymont pulverized lime stone, Bellefonte, PA) and normal tap water (2090 liters) to adjust for pH and moisture content respectively. The pH of the casing soil was 7.2. The (oven-dried) moisture content of the casing soil was 80 percent. The casing soil was determined to have a Carbon : Nitrogen ratio of 88:1.