Food Protection Trends - September/October 2022 - 378

ing consumer exposure to foodborne pathogens. Current regulations
such as the Food Safety Modernization Act Produce
Safety Rule for the United States (19), private standards such
as GLOBALG.A.P. (7), and guidelines from the World Health
Organization (3) require that all agricultural water must be safe
and of adequate sanitary quality for its intended use. The U.S.
Food and Drug Administration (FDA) (19) has clarified that
when agricultural water does not comply with this criterion,
treatment is only one of the options available to comply with
the regulation and avoid safety issues. For safety assurance, water
may be disinfected with traditional approaches, including
chlorine, ozone, UV radiation, and filtration. These methods
are effective for drinking water but are not always suitable for
the treatment of agricultural surface water due to the effects of
factors such as pH, turbidity, dissolved solids, and high microbial
loads on the efficacy of the treatments (9, 10).
UV light inactivates microorganisms by damaging their
nucleic acids and therefore preventing replication (10). UV
light can eliminate human pathogens, including bacteria,
protozoa and most viruses, in drinking water, in water from
nursery settings where recycling is a common method of water
and nutrient conservation, and in certain liquid foods and
beverages including unfiltered fruit juices such as apple cider
(3, 9, 15). Unlike surface water, drinking water is characterized
by low turbidity and low microbial populations (18). UV
light is generally not recommended for disinfection of surface
water with turbidity levels >1.0 nephelometric turbidity units
(NTU) because UV light blocking or absorption may shield
pathogens of concern. However, the commercial UV unit
used in the present study was designed to overcome turbidity
issues even in highly turbid beverages such as unfiltered apple
cider, with a turbidity of >1,000 NTU (9, 10). In a previous
study, Jones et al. (9) evaluated the efficacy of the same commercial
UV processing unit for decontamination of unfiltered
surface water (stream) samples inoculated with bacterial and
oomycete pathogens. The authors reported ≥ 99.9% inactivation
rates for all of the inoculated microorganisms. However,
profiles of agricultural waters, and particularly surface irrigation
waters, are highly variable and may change over time due
to weather events or human activities (9).
The present study was focused on pond surface irrigation
water samples collected over an extended period and included
samples with higher turbidity levels than previously evaluated
and reported. The specific study objective was to evaluate the
effectiveness of UV radiation for reducing levels of E. coli and
Salmonella in longitudinally collected pond water samples. A
commercially available UV juice processing reactor that can
also be used for the industrial treatment of irrigation water was
used to treat the samples. Specific suggestions for measures
that growers can take to reduce microbiological contamination
from agricultural water are still lacking (3). Therefore, this
research was conducted to provide tangible recommendations
regarding the application of UV light as an antimicrobial treatment
for surface agricultural water.
378 Food Protection Trends September/October
MATERIALS AND METHODS
Water samples
To determine efficacy of UV light against E. coli and
pathogenic Salmonella, six water samples (800 mL each;
three for each of the two microorganisms tested) were
collected on each of 16 sampling dates over a 3-year period
(2016 to 2018) at the Texas A&M AgriLife Research and
Extension Center (Weslaco, TX). These samples were tested
on the corresponding testing dates: 16 dates of testing
× 2 microorganisms × 3 replicates = 96 water samples
collected and processed. The irrigation water was collected
from an open pond (used for irrigation of produce crops)
fed by canals from the Rio Grande River and filtered with
sand filters. The average electrical 100% conductivity
was 0.13 S/m. Water was pumped from the pond into
holding tanks, and samples were aseptically collected and
delivered to Cornell University within 24 h. The water was
mixed thoroughly, and the pH (HI 2211 pH/ORP meter,
Hanna, Woonsocket, RI) and turbidity (2100P portable
turbidimeter, Hach, Loveland, CO) were measured.
Sample inoculation
Each of the 96 collected water samples was independently
inoculated immediately before conducting the experiments
with 8 mL of either (i) a three-strain cocktail of Salmonella
enterica (serovars Hartford H0778, Montevideo, and Gaminara)
or (ii) a single strain of E. coli ATCC 25922 (a nonpathogenic
surrogate with UV sensitivity similar to that of E. coli
O157:H7) (13), to reach an initial level of 107
to 108
CFU/
mL. For inoculum preparation, a single isolated colony of each
pathogen strain (E. coli and Salmonella) was grown overnight
in Trypticase soy broth (Difco, BD, Sparks, MD) at 35 ± 2°C
for 20 ± 2 h to stationary phase in an rotary platform shaker
(Innova 2300, New Brunswick Scientific, Edison, NJ) at 250
rpm. The Salmonella cocktail was prepared by mixing equal
amounts of each strain previously grown to stationary phase.
The inoculum was added with the growth medium without a
previous wash but the volume represents 1% of the total volume
so the intrinsic physicochemical properties of the water
samples would not be compromised (2).
UV treatment
A 750 mL volume of inoculated water was immediately treated
at room temperature (25°C) with the UV treatment unit (CiderSure
3500, FPE, Rochester, NY). A thorough description of
the processing device was previously published (15, 16). Based
on information from the UV sensors, the flow rate in the UV
unit was automatically adjusted to overcome differences in water
quality parameters (i.e., solid contents, turbidity, and color) (9).
For water samples with high absorption, the pump flow rate was
automatically reduced so that a constant UV dose of 14.2 mJ/
cm2
at a wavelength of 254 nm was consistently delivered to all
samples while ensuring a turbulent flow regime (Re > 2200).
The maximum flow rate of the UV unit was 378 L/h.
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