Plant-Scale Validation of Physical Heat Treatment of Poultry Litter Composts Using Surrogate and Indicator Microorganisms for Salmonella

Abstract

This study selected and used indicator and surrogate microorganisms for Salmonella to validate the processes for physically heat-treated poultry litter compost in litter processing plants. Initially laboratory validation studies indicated that 1.2- to 2.7-log or more reductions of desiccation-adapted Enterococcus faecium NRRL B-2354 were equivalent to > 5-log reductions of desiccation-adapted Salmonella Senftenberg 775/W in poultry litter compost, depending on treatment conditions and compost types. Plant validation studies were performed in one turkey litter compost processor and one laying hen litter compost processor. E. faecium was inoculated at ca.7 log CFU g^-1 into the turkey litter compost and at ca. 5 log CFU g^-1 into laying hen litter compost with respectively targeted moisture contents. The thermal processes in the two plants yielded 2.8 - > 6.4 log CFU g^-1 (> 99.86%) reductions E. faecium of the inoculated. Similarly, for the processing control samples, reductions of presumptive indigenous enterococci were in the order of 1.8-3.7 log CFU g^-1 (98.22% to 99.98%) of the total naturally present. In contrast, there were less reductions of indigenous mesophiles (1.7-2.9 log CFU) and thermophiles (0.4-3.2 log CFU g^-1). More indigenous enterococci were inactivated in the presence of higher moisture in the poultry litter compost. Based on the data collected under the laboratory conditions, the processing conditions in both plants were adequate to reduce any potential Salmonella contamination of processed poultry litter compost by at least 5 logs, even though the processing conditions varied among trials and plants.IMPORTANCE Poultry litter compost, commonly used as a biological soil amendment, is subjected to a physical heat-treatment in industry setting to reduce pathogenic bacteria such as Salmonella and produce a dry product. According to the FDA Food Safety Modernization Act (FSMA) Produce Safety Rule, the thermal process for poultry litter compost should be scientifically validated to satisfy the microbial standard requirement. To the best of our knowledge, this is the first validation study in commercial poultry litter compost processing plants, and our results indicated that Salmonella levels, if present, could be reduced by at least 5 logs based on the reductions of surrogate and indicator microorganisms, even though the processing conditions in these commercial plants varied greatly. Furthermore, both indicator and surrogate microorganisms along with the custom-designed sampler can serve as practical tools for poultry litter compost processors to routinely monitor or validate their thermal processes without introducing pathogens into the industrial environments.

FIG 1

Survival of desiccation-adapted S. Senftenberg 775/W and E. faecium NRRL B-2354 in turkey litter compost with 20% (A), 30% (B), and 40% (C) moisture contents at 75°C. Inactivation curves during come-up times (to the left of the vertical dotted line) and during holding times (to the right of the vertical dotted line) are shown. The horizontal dotted line indicates that Salmonella was detectable only by enrichment (detection limit by direct plating, 1.3 log CFU g⁻¹). Data are averages of values from two trials.

FIG 2

Survival of desiccation-adapted S. Senftenberg 775/W and E. faecium NRRL B-2354 in laying hen litter compost with 15% moisture content at 75°C. Inactivation curves during come-up times (to the left of the vertical dotted line) and during holding times (to the right of the vertical dotted line) are shown. The horizontal dotted line indicates that Salmonella was detectable only by enrichment (detection limit by direct plating, 1.3 log CFU g⁻¹). Data were expressed from the average of two trials.

FIG 3

Custom-designed sampler (top) and the inoculation procedure for plant B (bottom).

FIG 4

Average log reduction of mesophiles and thermophiles in the processing control samples during heat treatment in plants A and B.