Mushrooms: An
Inherently Low
Risk Food
Two Approaches to Risk
Assessment
1. Epidemiological Approach to Risk Assessment
– FDA Fresh Produce Risk Assessment 2009 Used
Epidemiological Approach
– Semi-Quantitative result for pathogen/substrate pairings
2. Agricultural Risk Factor Assessment
– Soil and irrigation water quality
– Surrounding land use
– Domestic and wild animal incursions
Both approaches validate the low risk status of
mushrooms.
FDA Fresh Produce Risk Ranking
(2009)
•
Mushrooms were compared to:
– Leafy Greens (spinach, lettuce, mesclun)
– Berries (blackberries, strawberries,
raspberries)
– Melons (watermelon, honeydew, cantaloupe)
– Green onions
– Peppers (capsicums)
– Herbs (cilantro, basil, parsley)
– Tomatoes (Roma, globe)
– Cruciferous vegetables (broccoli, cabbage)
– Non-citrus fruits (pineapple, mango)
FDA Fresh Produce Risk Ranking
Mushrooms were ranked among the lowest risk of all
produce items considered by the FDA study.
•
•
•
•
Lowest pathogen prevalence
Weakest epidemiological link
Fewest illness cases
Fewest outbreaks (no documented outbreaks ever)
U.S. Food & Drug Administration, CFSAN, College Park, MD. 2009. Fresh Produce Risk
Ranking Tool Summary: Identification of Priority Pathogen-Commodity Combinations for
Quantitative Microbial Risk Assessment.
Agricultural Risk Factors
Risk Factor
Field Crops
Mushrooms
1. Irrigation Water Quality
Water source & quality
are highly variable.
Irrigation water meets EPA
drinking water standard.
2. Wildlife Incursion
Frequent contact with deer,
birds, pigs, rodents (and
droppings) possible.
Mushroom houses protect
the product from contact with
wildlife.
3. Flooding of Fields
Flooding contaminates
plants and soil.
Mushroom houses wellprotected from floods.
4. Domestic Animal Contact
Concentrated animal feedlots (or runoff) may be in
close proximity to field.
Mushroom house prevents
contact with domestic
animals.
5. Use of Soil Amendments
Highly variable use of
manure in farming,
composting often not wellcontrolled.
Mushroom compost is steampasteurized before it is
inoculated with mushroom
spawn (process highly lethal
to vegetative pathogens).
Irrigation Water Quality: Field Crops
•
•
•
•
•
Surface water is commonly
used – overhead irrigation
Tremendous variability in
microbiological and chemical
quality
Open ponds and ditches are
exposed
Overhead irrigation most
common method
Mushroom irrigation water
meets EPA drinking water
standards for coliforms and E.
coli.
Wild, Feral & Domestic Animal
Incursion
•
List of animals that can impact
field crops is almost endless: e.g.
deer, pigs, birds, raccoons, dogs,
lizards, insects
•
Mushroom houses are block wall
construction, and are not
susceptible to animal incursion.
•
Pest management programs are
required for MGAP certification.
•
No threat of runoff or seepage
from adjacent feedlots.
Flooding of Fields
• Floodwater can carry
raw sewage
• Toxic chemicals
• Debris of all types
Domestic Animals
• Used intentionally to
graze in orchards
• Frequent unintentional
presence of dogs,
cows, goats, pigs
Mushroom Houses
In figure above, note concrete floors with drainage,
impervious walls and floors.
In figure at right, door to growing room is surrounded
by filter material. Note wires leading to data-logger – thermocouples are used to monitor air
and substrate temperature during and after Phase II pasteurization.
Mushroom Bed Composition
Casing soil (peat moss)
has distinct antimicrobial properties.
Mushrooms form within
this layer.
Substrate is fermented
to an avg. temp of 170oF
during Phase1 of
compost preparation,
and is steam treated and
held many hours at ≥
150oF during Phase 2
Casing Soil
Substrate
In Phase 3, the steam-pasteurized substrate is inoculated with
aseptically produced mushroom spores that are carried on sterile
grain. The mycelium develops within the substrate and then sends
reproductive bodies (mushrooms) up through the casing soil.
Mushroom Growth Cycle
Phase I – Substrate Preparation
Phase I – Substrate Preparation
Exothermic fermentation during
Phase I raises compost
temperature to 170oF.
Compost filled into mushroom
beds or transferred to tunnels
for Phase II treatment.
PhaseIIII––Substrate
SubstratePasteurization
Pasteurization
Phase
Compost Temp. Equalization: O2,
Compost & Air Temp Monitored
Compost Temp gradually
increased to 140oF +with live
steam treatment.
Compost Temp held at 140 –
142oF for at least 2 hrs. for
pasteurizing effect, then cooled.
Air temp, substrate temp, %
R.H., air circulation are controlled
in growing rooms.
Mushroom spores germinate
and mycelium runs through
substrate.
Mushroom Buttons appear 16
days after casing. Hand
harvesting begins and continues
over 21 days.
Growing Room emptied,
cleaned, sanitized and beds
treated with steam to prepare for
new crop.
Phase III – Spawn Run
Phase III – Spawn Run
Mushroom spawn on sterile grain
introduced onto the pasteurized
substrate.
Casing and Cropping
Top Dressing of casing soil (peat
moss) applied to beds to support
growth of primordial fruiting
bodies
Raw Materials
•
Raw materials: Hay, Straw, Soybean and Cottonseed
Meal, Gypsum, Water (Some Farms Still Use Stable
Bedding)
Phase 1: 17 Days
• Raw materials
are blended and
watered.
• Formed into
ricks to create
optimal aerobic
conditions
• Exothermic
fermentation
process is
mediated by
fermentative
organisms.
• Core temperature of ricks can reach 170°-175°F during
this process
Phase 2: 5 Days
• Fill compost into tunnels
trays, or beds
• Heated to 120° - 165°F
for about 4-9 hours (on
average) to pasteurize
• Different techniques
used to convert
ammonia to useable
nitrogen
• Slowly reduce
temperatures over next
4 days to promote
growth of thermophiles
Phase II Pasteurization of
Substrate
• At a treatment temperature of
65oC (150oF), it takes about 3.2
minutes of exposure to produce
a 1 log reduction against
Salmonella spp. and Listeria
spp.
• In a typical pasteurization cycle
– 12 hours at a compost temp.
of 150oF, more than 200 log
reductions would be expected.
• Even with variations grower to grower, the safety factor is extremely high.
• Growers are obsessive over temperature monitoring since it is directly
related to productivity and yield.
Casing - 18 Days
• Trays are covered
with a 13/4” inch
layer of peat moss
• This provides
support structure
for mushrooms to
grow
• Peat helps
maintain moisture
in the compost
• Humic acid and
other compounds
in peat have a
biostatic effect
(documented in recent
Penn State study)
Harvesting Mushrooms - 25 Days
• Mushroom grow in flushes,
or breaks
• We harvest 1st, 2nd and 3rd
breaks
• Near harvest, mushrooms
double in size every 24
hours
• Cypress wood is traditional
– used for its resistance to
decay
• Many growing rooms are
now using metal trays
instead of wooden beds.