Heat from composting kills pathogens in mushroom industry
Strict requirements on the use of animal manures in fresh produce production from the US federal food safety law threatened to adversely impact the mushroom industry.
Commercial production of white button mushrooms requires a specialized growth substrate, or organic medium, from composted agricultural by-products, which widely use horse and poultry manure.
Weil et al said the results show that a phase II composting process can be an effective control measure for eliminating risks with the use of composted animal manures during production.
This means there will be no restrictions on the mushroom industry composting process.
Two step composting process
Most mushroom growers in the US have adopted a two-step substrate preparation process known as phase I and phase II composting.
Phase I sees agricultural raw materials mixed, wetted, and formed into long rows on outdoor concrete wharfs where they are turned periodically, watered, and reformed.
Rapid thermophile growth causes core pile temperatures to reach 70 to 80oC within 24 to 72 hours, with composting times ranging from six to 25 days.
Phase II is a controlled, indoor composting process designed to continue thermophilic breakdown of organic matter, eliminate any surviving pests and fungal pathogens, and disperse toxic ammonia gas that decrease crop yields and postharvest quality.
Pathogen cocktail
In the study, partially composted substrate was inoculated with a pathogen cocktail (log 106 to 108 CFU/g) containing Listeria monocytogenes, E.coli O157:H7, and Salmonella.
Pathogen and indicator-organism reductions were followed at temperatures that occur during a six-day phase II pasteurization and conditioning procedure.
Controlled-temperature water bath studies at 48.8, 54.4, and 60oc demonstrated complete destruction of the three pathogens after 36, 8 and 0.5 hours, respectively.
Elimination of L. monocytogenes and E. coli O157:H7 at 54.4oc occurred more slowly than E. coli, total coliforms, Enterobacteriaceae and Salmonella.
“The high levels of E. coli, total coliforms, and Enterobacteriaceae in substrate used for phase II inoculation studies confirms our assumption that phase I composting is not an adequate microbial hurdle for eliminating pathogens associated with horse and poultry manure,” said the researchers.
They also warned that pathogens present in phase I ingredients are more resistant to phase II composting temperatures because they have been exposed to high, but not temperatures so may have increased thermotolerance and cultures used in the study were grown in the laboratory.
Source: Journal of Food Protection, Vol. 76, No. 8, 2013, Pages 1393–1400
Online ahead of print DOI: 10.4315/0362-028X.JFP-12-508
“Inactivation of Human Pathogens during Phase II Composting of Manure-Based Mushroom Growth Substrate”
Authors: Jennifer D Weil, Catherine N. Cutter, Robert B. Beelman and Luke F. LaBorde
