First of all, there is a body of research on pathogen
regrowth in composts, because there is indeed a real problem with this, and
especially in large scale composting. It has been researched with respect to
biosolids because of the nature of that material, and taxpayer dollars
have been spent looking at the problem.
See for example the extensive discussions in The Science and
Engineering of Composting, Hoitink and Keener eds, 1992, Renaissance Press. See
especially the Farrell piece and his discussion of Yanko's work.
Large compost piles often reach high temperatures, high enough
to crash the populations of mesophilic organisms and 'quasi-sterilize' the
piles. Especially in big windrows, this means that the 'sterilized' inner
portions of the piles, where the competitive microflora that would inhibit
pathogen regrowth have been destroyed by overheating, are re-inoculated with
pathogens that may have survived on the cooler outer surfaces when the piles are
turned, and thus conditions for pathogen regrowth are created.
For open air composting operations seagulls and other birds
can often be the source of recontamination, as can equipment that has not been
cleaned between use for windrows well along in the process, and windrows in the
early stages with high populations of pathogens.
Smaller windrows, in vessel systems, static pile systems, and
flow through vermicomposting systems, properly managed, should offer better
alternatives for the production of high quality composts with a full foodweb
present to inhibit and resist pathogen regrowth. (Full foodweb, aka
'competitive microflora')
A too hot compost pile that has lost a large measure of its
foodweb (competitive microflora) is indeed a perfect host for pathogen regrowth.
A compost pile that has been monitored to prevent overheating, and has a full
foodweb present, is not a perfect host for pathogen regrowth.
Vermicomposting destroys pathogens and unless it is thermally
sterilized, will not support pathogen regrowth upon reinoculation because it
relies on the competitive microflora entirely throughout the process.
See
"No pathogen regrowth has been detected in
any resampled material over 30 months of operation at our established sites.
Regrowth is highly unlikely for two
reasons. Firstly a high degree of stabilisation occurs. Secondly the end product
is microbially active containing over 70 million beneficial soil microbes/gram
on harvest and over 10 million at point of use after 12
months"
My own viewpoint is that, considering the small
amounts of compost needed to produce large amounts of tea, that compost tea
users growing fresh fruit and produce should use tested, clean composts of high
quality; or should use composts made under cover from materials of known and
contaminant free origin.
A compost made to process standards aimed at
crashing fecal coliforms to below 1000 MPN per gram, may not be adequate to
achieve the much lower OMRI standard of less than 3 CFU per gram. So a process
based set of compost standards that were designed to acheive the
less than 1000 MPN/g EPA biosolids standard may not guarantee acheivement
of OMRI performance standards.
The study Elaine references below is a Chinese
bench sized in vessel study which operated at below the required 55 C
temperature requirement and still found destruction of E. coli 0157:H7 after two
weeks.
Other researchers have found strains of E.coli
that could adapt in thermophilic conditions, but whose pathogenicity is not
known. When Dennis Avery made the claim, some years ago, that O157 was surviving
hot composting, Dr Patricia Millner of the USDA-ARS and other researchers
pointed out that this was not known to occur. (I will refrain from saying that
the survival of pathogenic E. coli in thermophilic composting was 'hotly
denied'.....;-)
See:
"Question 4:
Heat tolerant strains of E. Coli are reported to develop in thermic composting operations. Please discuss data sets that address coliform bacterial trends in thermic and vermicomposts. Answer 4:
Several researchers have demonstrated that E. coli and other human pathogens can genetically adapt to the high temperatures in compost piles and survive. Some have survived temperatures as high as 65 degrees C. What is not well known is the pathogenicity of these genetic variants relative to the normal heat-susceptible ones. Recolonization of compost by pathogens can also occur from sources outside the pile and from the cool edges of the pile. Researchers have identified three factors which influence recolonization - moisture content, carbon availability, and microbial diversity. Optimizing these conditions can help reduce the potential recolonization by pathogens. As far as vermicompost, Dr. Clive Edwards of Ohio State University has conducted a number of studies on the fate of human pathogens during vermicomposting. He has extensive data to show that 30-50 day vermicomposting eliminates human pathogens".....
References:
Brinton, W.F., and M.W. Droffner. 1994. Microbial approaches to characterization of composting processes. Compost Science and Utilization 12:12-17. Soares, H., B. Cardenas, D. Weir,
and M. Switzenbaum. 1995. Evaluating pathogen regrowth in biosolids
compost. BioCycle. June 1995, p. 70-75.
So there is a literature out there about E. coli that survive
hot composting, and of E. coli regrowth in composting.
This literature would seem to support a 'precautionary
principle' approach to compost tea, namely, to use composts that either test
negative for E. coli or that are made from materials that do not contain E. coli
in protected environments and under appropriate parameters, eg, aerobic, not too
hot but hot enough, or vermicomposted properly.
For many growers buying in suitable, tested composts may
be a relatively cheap insurance policy and necessary for
certification.
The use of tested recipes and equipment, and/or the subsequent
testing of teas for at least a few production runs prior to use, when using
untested recipes and equipment, would seem to be minimally prudent for anyone
using CT on fresh fruit and produce for raw consumption.
Water quality is also an issue, and pond and well water
might need either filtration, UV treatment, or ozonation, to eliminate pathogens
in some cases, before using it in tea production.
Rumor has it that New York vegetable growers using well or
pond water for irrigation of raw consumption produce may be required to use a UV
system to ensure pathogen destruction.
Given that kind of thing in the regulatory pipeline, it
behooves the tea making community to be proactive and supportive of reasonable
compost standards, and to provide clear evidence in support of the science of
pathogen prevention along the aerobic pathway both in the composting and the
teamaking processes.
My two cents,
Frank Teuton
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