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have also been linked to similar outbreaks (5). The Centers
for Disease Control and Prevention (CDC) has reported
that six O groups of non-O157 STEC (serogroups O26,
O45, O103, O111, O121, and O145) cause 71% of Shiga
toxin-producing E. coli-related illnesses. Currently, non-O157
STEC have little public health surveillance, and many cases
may go undiagnosed or unreported (7).
According to the United States Department of Agriculture
(USDA) Draft Risk Profile for Pathogenic Non-O157 Shiga
Toxin-Producing E. coli, only small numbers of STEC cells
are required on the food product to cause life-threatening
damage to organ systems and to cause illness in children, the
elderly, and other susceptible populations (29).
Because of the high risk associated with STEC, under
the Federal Meat Inspection Act (21 U.S.C. 601(m)(1)),
the Food Safety and Inspection Service (FSIS) considers
all raw non-intact beef and raw intact beef intended for use
in raw non-intact product as contaminated and adulterated
if these pathogens are present on these products (26). One
of the most common non-intact products is mechanically
tenderized beef, a beef product that has been subjected to
a process in which a set of blades or needles penetrate the
meat, piercing and cutting through the muscle fibers and
connective tissue of primal or subprimal cuts, thus improving
tenderness of tough beef cuts (9, 13, 14). If beef primal
and subprimal cuts are intended for use in raw non-intact
products or the intended use is unclear, the FSIS considers
these products subject to the FSIS sampling for Raw Beef
Manufacturing Trimmings (MT60) STEC testing and
sampling program (27).
Mechanical tenderization of beef has the inherent risk
of potentially transferring pathogenic bacteria located on
the beef surface into the previously sterile interior of the
cut (9, 13). For that reason, development of antimicrobial
interventions is necessary, so that such interventions applied
to the surface of the subprimal prior to tenderization,
reduce or prevent the transfer of bacteria to inside the
meat cut, thus decreasing the risk posed by foodborne
pathogens such as STEC. The meat industry has been very
proactive in developing interventions, best practices, and
guidance for pathogen control for operations producing
beef subprimals and whole cuts subjected to mechanically
tenderization/enhancement (10, 13, 14, 16, 31). However,
there is a potential risk to consumers if food retailers, stores,
restaurants and/or institutional settings obtain whole
cuts (not intended for non-intact products) that have not
been tested for STEC and subject these cuts to mechanical
tenderization or enhancement at their establishments (30).
That practice can lead to pathogen translocation during the
mechanical process and survival of the pathogen as the result
of undercooking and consumer preferences (15, 29).
Previous studies in our laboratory have shown that microwave technology has the capability to reduce or eliminate
common bacterial pathogens and quality-affecting organisms

in foods. Published studies by our group include studies of
the elimination of mold from bread (99.9%) (19) and reduction of Salmonella (99%) in shell eggs without cooking the
egg or changing its quality attributes (18). A few studies have
evaluated the antimicrobial effect of microwaves on meat and
meat products (2, 3, 23, 24, 25, 32), but to our knowledge, no
study has evaluated the effectiveness of microwave technology on STEC O157:H7 and non-O157 STEC serogroups on
fresh beef. Because of that, the objective of this study was to
evaluate the effectiveness of a unique microwave treatment,
combined with cold storage conditions (refrigeration or frozen storage), as a post-packaging intervention to reduce Shiga
toxin-producing E. coli on beef intended for use in both intact
and non-intact beef products.
MATERIALS AND METHODS
Product procurement
Beef striploins (Institutional Meat Purchase Specifications
IMPS 180) were procured from a federally inspected meat
processing facility located in Lubbock, TX. Each beef
striploin was portioned into three equal sections. The average
weight of the striploin sections was 1.530 Kg ± 0.240 (SD).
Two of the striploin sections (from the same striploin) were
randomly assigned to the treatments, and the third was used
for measurement of the initial concentration of inoculated
STEC. Six striploins were used per STEC serogroup, with
a total of 42 striploins per replication. The experiment was
conducted in triplicate (three individual, separate trials).
The experiment had two treatments: control (CTR) and
microwave (MW). The treatment CTR was a negative control
in which samples were inoculated with STEC, vacuum
packed, and stored at 4°C or -18°C but not subjected to
any microwaves. For the treatment MW, the samples were
inoculated, vacuum packed, treated with microwaves, and
stored at 4°C or -18°C.
Inoculum preparation
For the inoculum, three strains of each STEC serogroup
(O26, O45, O103, O111, O121, O145, and serotype
O157:H7) were used, and each serogroup was processed
separately. The decision to include a combination of strains
per serogroup was based on the need to "encompass the
variability among organisms" (22) that we could potentially
encounter in a real industrial setting. The STEC strains
used for this study were obtained from the frozen microbial
collection of the TTU Food Safety Laboratory. For the
preparation of each STEC serotype inoculum, a 1µl loop
was transferred from the frozen culture vial to a 10-ml tube
of Tryptic Soy Broth (TSB; EMD Millipore Co., Billerica,
MA), and the tubes were incubated at 37°C for 18 to 24
h. After incubation, 1 ml of each STEC inoculum was
transferred into fresh 9-ml TSB tubes, and the fresh tubes
were incubated for 18 to 24 h. The double enrichment was
used to ensure the recovery of injured STEC cells. The 10-ml

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