Food Protection Trends - March/April 2023 - 130

such as freeze-thaw cycles and low nutrient availability also
affect pathogen survival (83, 120, 170).
Bardsley et al. (10) reported that Salmonella survived longer
in clay-loam soils than in sand-loam soils. When manure was
applied to produce farms in New York State within 1 year
before sampling, L. monocytogenes was detected significantly
more often in terrestrial (soil and drag) samples than in
fields where manure was never applied (177). In a 2-year
survey of manure-amended soil from 19 organic produce
farms in California, Maine, Maryland, and Minnesota, 1
(0.04%) of the 2,460 samples was positive for E. coli O157,
7.3% were positive for non-O157 STEC, 1.1% were positive
for Salmonella, and 5.0% were positive for L. monocytogenes
(130). Twenty-five percent of soil samples taken from Iowa
fields were positive for Salmonella 1 year after application of
contaminated poultry manure (71). Soil samples collected
from West Coast fields in which sheep grazed on cover crops
contained E. coli strains closely related to strains isolated
from sheep fecal samples and were detected through 139 days
postgrazing (126).
The presence of nearby sources water or water from rainfall
or irrigation also can influence L. monocytogenes isolation in
produce fields (68, 129, 176-178). Pires et al. (129) reported
that increased soil moisture and precipitation were associated
with higher levels of E. coli in manure-amended soils on
organic produce farms. The odds of detecting L. monocytogenes
in soil samples collected from spinach fields on
New York State farms were 25 times greater at 24 h after a
rain or an irrigation event than at 6 to 8 days after such events
(178). Terrestrial samples taken from New York produce
farms within 3 days of irrigation were 27 times more likely
to be positive for L. monocytogenes than were samples taken
10 days after irrigation (177). In a study of L. monocytogenes
distribution in two fields on a New York State produce
farm, the likelihood of isolating the pathogen increased for
soil samples as proximity to streams increased and for drag
samples when samples were collected within 5 days of rainfall
(68). These findings suggest that wet soils have a higher
probability of pathogen presence following precipitation or
irrigation. Thus, the potential for contamination increases for
such equipment as heavy mechanized harvesters (i.e., used
for tender leaf commodities) that may sink in the furrows and
contact the soil on the planted bed. Enhanced cleaning and
sanitation procedures should be considered after events that
introduce moisture because these conditions are associated
with increased soil deposits on equipment.
Equipment design, construction, and surface
cleanability
Three features of equipment that have an impact on a
pathogen's ability to persist are the design, construction
materials, and surface cleanability. These three features are
interdependent both in how they relate to each other and in
how they affect the microbial safety of equipment used in
130 Food Protection Trends March/April
produce harvesting operations. How equipment and tools are
designed and constructed affect the likelihood of attachment
or entrapment of pathogens and subsequent persistence
or growth on equipment surfaces (29, 108). Food safety
guidelines have typically provided guidance addressing how
design and construction of equipment used in a packing or
processing facility affect the microbial safety of produce (80,
166, 167, 179). The majority of published academic studies
have been focused on equipment design and construction
and the steps needed when food processing and packing
equipment used in facilities becomes contaminated and leads
to cross-contamination of product (17, 26, 29, 98, 137, 144).
Although food safety is most certainly taken into account by
equipment manufacturers, any food safety research related to
harvesting equipment design, construction, and cleanability
typically has been conducted in-house by food companies
and/or the equipment manufacturers and has not appeared
in peer-reviewed publications. However, although produce
harvesting equipment is used differently and in different
settings compared with food processing equipment, much
relevant information can be obtained from studies on how
equipment design and construction affects equipment
cleanability and, ultimately, the microbial safety of the food
(108).
Equipment design
Of all the equipment and tools used in produce harvesting,
hand-held tools used to cut and trim produce crops have
been the most frequently studied. In particular, the device
used to harvest and field core head lettuce has been the
subject of several studies (45, 161, 188). During field coring
of lettuce, the core is cut out of the lettuce head with a knife
that has a flat cutting blade at one end and a coring ring at the
other end. This practice was first introduced in the 1990s to
reduce costs by removing waste product in the field before
transport and processing. To evaluate pathogen transfer
from contaminated soil to lettuce via the field coring knives,
Taormina et al. (161) inoculated the knives by exposing
them to soil containing two levels of E. coli O157:H7 (2.72
and 1.67 log CFU/g). After inoculation, the contaminated
knives were used repeatedly to cut and core 10 lettuce heads,
resulting in E. coli O157:H7 transfer to 10 and 5 consecutively
cored heads for the two bacterial levels, respectively.
In two other studies, Dev Kumar et al. (45) and Zhou et
al. (188) further explored the contamination of field coring
knives that were being used by industry at that time. In both
studies, rough welding joints on commercially available
knives provided a favorable site for bacterial pathogen
attachment. Once attached, bacterial cells were resistant to
removal with a chlorine solution, a sanitation step commonly
used by industry. Both groups of researchers designed and
constructed modified coring knives, creating prototypes
without a welding joint or with a smoothed, polished
welding joint between the cutting blade and the coring ring.
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