Food Protection Trends - July/August 2019 - 298

inoculum tubes were then transferred into 90-ml TSB bottles
in order to have the volume necessary for the experiment.
The three bottles of 100 ml for the same STEC serogroup/
serotype were combined, to obtain 300 ml of inoculum of
each STEC. The 300-ml STEC inoculums were separately
diluted with 2700 ml of BPW, to make 3 l of inoculum of
each STEC type (Table 1). The inoculum preparation was
carried out using the appropriate aseptic techniques to
avoid any cross-contamination between STEC serogroups
and was conducted at the Biosafety Level 2 (BSL2) food
microbiology laboratory of Texas Tech University.
Inoculation
Striploin samples were inoculated by dipping them
for approximately 20 seconds into one of the 3-l STEC
inoculums contained in a plastic tub. After inoculation, the
samples were allowed to sit at 4°C for 30 min to facilitate
bacterial attachment and allow dripping of excess inoculum
from the samples. Bacterial concentration (attachment) at
day zero was sampled via surface swabbing of the inoculated
beef (to be further described). The 3-l inoculums were used
for all the samples in a replication, and the same strains
were used for the three individual trials (replications) of the
study, for uniformity of the experiment. After inoculation,
the samples were vacuum packed in plastic bags (Sealed
Air Corporation, Cryovac® bags 8 in × 22 in, Charlotte,
NC) using a vacuum packaging machine (UltraSource
LLC., Ultravac® 250 Single Chamber Vacuum Packaging
Machine, Kansas City, MO). The samples were labeled
with the treatment type, the storage temperature, and the

sampling day. Inoculation was conducted at the Texas
Tech University Biosafety Level 2 (BSL2) laboratory. The
samples were transported under refrigerated conditions
to the Experimental Sciences Building at Texas Tech
University, where the samples assigned to MW were treated
with microwaves.
Treatment with microwaves
The samples assigned to MW were treated with microwaves
using an industrial, custom-made conveyorized microwave.
The microwave settings were six magnetrons at 95% power
exposing each sample to a wattage of 472.5 kW/s, and the
average surface temperature of samples right after microwave
treatment was 34 ± 4°C. After MW treatment, the samples
were stored either at 4°C (refrigeration) or -18°C (frozen
storage). The microwave treatment settings were chosen
on the basis of preliminary trials (data not shown), which
demonstrated that those settings were non-destructive for
the beef protein, causing no observable changes in color
or texture ("cooked" appearance). Because the objective
of the study did not include the evaluation of meat quality
attributes, surface pH water activity or sensory characteristics
were not monitored or measured.
Quantification of microbial reduction
The objective of this study was accomplished by
quantifying and comparing the concentrations (log
CFU/100 cm2) of STEC on striploin sample surfaces at
each sampling storage interval after an initial post-harvest,
post-packaging microwave treatment. Surface populations

table 1. individual strain identification and concentration (log cfu/ml) of Shiga toxinproducing Escherichia coli serogroups used in the inoculum cocktail and their
respective post-inoculation attachment (cfu/100 cm2) on beef striploins
STEC
Serogroup

Strain 1

Strain 2

Strain 3

Inoculum
Concentration
(log CFU/ml)

SD (±)

Post-inoculation
attachment
(log CFU/100 cm2)

SD (±)

O157:H7

EC 994

EC 922

EC 966

7.64

0.31

6.22

0.28

O145

ECRC 95.0187

ECRC 9.0538

E1-169

7.85

0.39

6.40

0.29

O121

ECRC 3.1064

E1-158

E1-159

7.92

0.11

6.07

0.40

O111

ECRC 4.0005

ECRC 3.1009

BAA 179

7.64

0.36

6.30

0.34

O103

ECRC 97.1377

ECRC 97.1241

ATCC 23982

7.74

0.23

6.00

0.28

O45

ECRC 92.0244

ECRC 2.1064

E1-138

7.63

0.31

5.87

0.34

O26

ECRC 0.1302

ECRC 7.1556

BAA 1653

7.61

0.21

6.12

0.27

Beef striploins (n = 6/serogroup) were inoculated in plastic tubs containing 3 L of inoculum cocktail. The samples were immersed
into the inoculum for 20 s and then allowed to drip for 30 min. Surface swabs (100 cm2) were taken prior vacuum packaging for
verification of STEC attachment.

298

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