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DISCUSSION
Vacuum-packed subprimals and whole cuts subjected
to mechanical tenderization/enhancement are a potential
source of risk to consumers if the surface of these products
is contaminated with pathogens that are translocated during
the process. Moreover, boxed beef products that processors
or retailers may divide into steaks or other types of cuts
are not necessarily tested for pathogens by the Food Safety
and Inspection Service (FSIS); therefore, interventions
post-packaging are warranted such products to reduce the
presence of harmful microorganisms. The use of microwaves
as an intervention has been tested successfully by our group
with other commodities, such as eggs and bread (18, 19),
and its use on fresh beef intended for non-intact products
was therefore warranted. Studies have evaluated the efficacy
of microwave technology in reducing the concentration of
microorganisms, including E. coli O157:H7; however, those
studies do not present enough evidence to permit drawing
conclusions about the effectiveness of microwave technology
applied at low temperatures. In studies conducted by other
research groups evaluating the effectiveness of microwaves
to decontaminate foods, reduction of inoculated Salmonella
Typhimurium in fresh vegetables (12), E. coli O157:H7 in
chicken (2), Clostridium sporogenes in plastic tray-packed
meals (25), Clostridium difficile in an aqueous suspension
(23), or Salmonella, Yersinia, Campylobacter, Listeria,
Aeromonas, Pseudomonas, and E. coli in beef products was
reported (3). However, the settings were different and the
samples reached temperatures of 63°C or above, which clearly
indicates that the product was cooked after the process,
which in our case was avoided. Moreover, only a few known
studies have been conducted to evaluate the non-thermal
effect of microwaves on animal protein. Shamis et al. (2008)
evaluated the decontaminating effect of low-temperature
microwaves in raw pork samples, obtaining one log reduction
of generic E. coli and Staphylococcus aureus with the product
temperature not exceeding 45°C. Unfortunately, although the
results showed promise, their methodology was not suitable
to be applied in the meat industry (24).
The mechanisms of destruction of microorganisms through
microwave are not well understood, and different hypotheses have been proposed. Some literature indicates that the
microwave's antimicrobial effects are produced entirely by heat,
which causes denaturalization of proteins, nucleic acids, and
vital cell components as well as disruption of membranes (32).
Other authors indicate that there are as many as four different
non-thermal effects of microwaves: the selective heating of

microorganisms, electroporation, cell membrane rupture, and
cell lysis due to electromagnetic energy coupling (11).
The results suggest that the microwave treatment
evaluated in this study as a potential antimicrobial
intervention did not by itself significantly reduce Shiga
toxin-producing E. coli serogroups O157, O145, O121,
O111, O103, O45, and O26. However, a reduction of two
to five log counts (2-5 log10 CFU/100 cm2) of STEC was
achieved in beef striploins vacuum packed and stored at
-18°C for seven or more days. Furthermore, beef samples
stored under refrigeration (4°C) showed about one to two
log counts (1-2 log10 CFU/100 cm2) reduction of STEC,
a reduction that can be attributed to a storage temperature
effect, rather than a microwave effect, as seen on Fig. 1A-1G
and 2A-2G. Furthermore, the STEC serogroups O26, O45,
O103, O111, O121, and O145 survived slightly better in the
beef striploin samples treated with microwaves and stored at
4°C. For frozen storage (-18°), only STEC O45 had higher
survival rates in samples treated with microwaves. This
outcome was in contradiction to our initial hypothesis of a
non-thermal decontaminating effect of microwaves.
Under the parameters used for this study, the use of
microwave radiation was not effective in reducing STEC
inoculated in beef striploins. However, we observed a
significant reduction of STEC in MW and CTR samples
stored under refrigeration and frozen storage (P < 0.05).
This finding suggests that frozen storage could be applied as a
post-packaging intervention by the beef industry to eliminate
potentially 2-5 log CFU/100 cm2 of STEC on beef surfaces
and therefore must be considered, evaluated and validated
by itself in such products with regard to its ability to reduce
STEC (4, 17, 33).
ACKNOWLEDGMENTS
This project was funded by the Foundation for Meat and
Poultry Research and Education (FMPRE). The authors
thank the ICFIE crew for help conducting this project
and Don Stull at MicroZap, Inc. for technical assistance
conducting this project.
CONFLICT OF INTEREST STATEMENT
Authors Echeverry, Sanchez and Cuellar report no conflict
of interest. Authors Brashears reports being a member of
MicroZap Inc.'s board of directors at the time of conducting
the research. Authors Brashears and Brooks report owning
MicroZap Inc.'s stock at the time of conducting the research.

REFERENCES
1. Alnajrani, M., K. Hanlon, A. English,
K. Fermin, M. M. Brashears, and A. Echeverry.
2018. Comparing the recovery of indicator
microorganisms from beef trimmings
using swabbing, rinsing, and grinding
methodologies. Meat Muscle Biol. 2:139-146.

2. Apostolou, I., C. Papadopoulou,
S. Levidiotou, and K. Ioannides. 2005. The
effect of short-time microwave exposures
on Escherichia coli O157:H7 inoculated onto
chicken meat portions and whole chickens.
J. Food Prot. 101:105-110.

3. Aziz, N. H., S. R. Mahrous, and B. M. Youssef.
2002. Effect of gamma-ray and microwave
treatment on the shelf life of beef products
stored at 5°C. Food Control 13:437-444.

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