Food Protection Trends - September/October 2023 - 377

it to survive sanitation treatments, demonstrating concern
for the safety of food products (27). Researchers have
been focusing efforts on understanding this pathogenic
microorganism's ability to infect, survive, and persist in
food processing environments and on equipment; however,
the rising numbers of L. monocytogenes outbreaks linked to
fresh produce have hastened the need to understand how to
effectively control this pathogen on produce.
The prevalence, persistence, and diversity through which
L. monocytogenes infects fresh produce is a growing concern,
especially considering the zero-tolerance regulation put in
place by the U.S. Food and Drug Administration for RTE
products (2). Equipment such as picking bags and storage
bins used during the harvesting and handling of produce are
known reservoirs of microbial biofilms (18, 25, 30). Their
regular food-contact surfaces provide a niche environment
for biofilms to persist even after sanitizer application.
The use of chemical sanitizers is a common practice
within the produce industry for controlling Listeria spp.
Most studies have analyzed the effect of commercially
available sanitizers such as peroxyacetic acid (PAA), ozone,
halogen-based compounds, hydrogen peroxide, acid anionic
compounds, and quaternary ammonium compounds on
food processing surfaces (3, 12, 16). The implementation
of technologies using UV-C light has provided a method
for disinfecting surfaces that may eliminate the need for
mechanical scrubbing (1). Today, food safety is moving
toward synergistic processes of simultaneous or sequential
germicidal applications to obtain greater pathogen reduction
(7). Studies analyzing the combined effects of UV light
and chemical treatment have found that such combined
applications have higher antimicrobial effects than singleapplication
treatments. Ding et al. (11) found that when
treating leafy greens, the simultaneous application of UV-A
light and benzoic acid was most effective. The combined
use of chemical and physical means of sanitation has been
presented as a positive strategy to overcome contamination in
food processing environments. Based on a survey conducted
among stakeholders in the tree fruit production industry
(in the Midwest), three favored materials for storing and
harvesting produce were identified: nylon (for picking bags),
wood, and plastic (for bins).
The objective of this study was to investigate the survival of
L. monocytogenes biofilms on wood, nylon, and polycarbonate
produce-harvesting materials after treatment with chemical
sanitizers alone and/or concurrent with UV-C light. The
chemical sanitizers lactic acid, SDC and thymol were
selected to represent nontoxic or generally recognized as safe
chemicals. All of these products are commercially available
and certified organic (GRAS). Their market price places
them in a slightly higher category than bleach; nevertheless,
these products are accessible and commonly used in the
organic farming sector.
MATERIALS AND METHODS
Bacterial strains
Three strains of L. monocytogenes were used in this study to
form biofilms: L2624 (serotype 1/2b) and L2626 (serotype
1/2a), isolated from the 2001 U.S. cantaloupe outbreak,
and J2230 (serotype 4b) from a clinical sample (21). All
strains were kept on CyroCare Bacteria Preserver beads
(Key Scientific, Stamford, TX, USA) and stored at −80°C
until used in experiments. An L. monocytogenes cocktail, as
described by Mendez et al. (21), was created by combining
individual strains in equal ratio to reach an initial population
of ca. 1 × 108
CFU/ml.
Substrate materials
The materials selected for this study included wood,
nylon, and polycarbonate, all of which are representative
of substrates used during the harvesting and handling of
tree fruit production. Coupons made of polycarbonate
(1.27 cm in diameter) were purchased from BioSurface
Technologies (Bozeman, MT, USA). Wood coupons were
made of plywood of unfinished basswood (Tilia americana)
purchased from a local store and cut into coupons of 1.27
cm in diameter. Nylon fabric was obtained from a local
store and was cut into 2- by 2-cm squares to fit within the
rods of the reactor.
Biofilm formation
Based on a protocol developed in our laboratory (21),
with slight modifications, biofilms were grown in a Centers
for Disease Control and Prevention (CDC) biofilm reactor
(BioSurface Technologies) for up to 96 h. To begin the 24-h
batch phase, 1 ml of L. monocytogenes cocktail was used to
inoculate the reactor. This phase was followed by an additional
72 h of continuous nutrient flow at a rate of 8 ml/min by using
tryptic soy broth (Difco, BD, Sparks, MD, USA). The reactor
was maintained at 20 ± 2°C for 96 h of incubation.
Chemical sanitizer and UV-C light
The following chemical sanitizers were used to evaluate
efficacy against L. monocytogenes biofilms: 4% lactic acid
solution (Purac, Corbion, Blair, NE, USA), confirmed using
a lactic acid test kit (ChemWorld, Kennesaw, GA, USA);
5% SDC-based sanitizer, as per label (PURE Bioscience,
Inc., El Cajon, CA, USA); and 0.23% thymol solution, as
per label (Bioesque Solutions, Lighthouse Point, FL, USA).
Because microbial DNA absorbs UV-C light between 200
and 300 nm, with optimal absorption at ca. 260 nm, the
lamp (Lumalier, Memphis, TN, USA) used as the emission
source of the UV-C light was set at 254 nm for germicidal and
disinfection applications. The power output was monitored
using a radiometer (Sper Scientific Ltd., Scottsdale, AZ,
USA) during all of the experiments to ensure constant UV-C
light irradiance.
September/October Food Protection Trends 377
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