Food Protection Trends - May/June 2023 - 218

TABLE 1. Average log CFU per gram or square centimeter (±SD) of E. coli recovered
from inoculated chicken, mask A and B contaminated from chicken, and lettuce
contaminated from mask B
Log CFU/g or log CFU/cm2
Inoculation level Slicing time (min)a
Chicken
High
Low
a
1
5
10
1
5
10
5.9 ± 0.2
3.8 ± 0.4
Mask Ab
3.2 ± 0.8
4.8 ± 0.2
4.6 ± 0.5
1.4 ± 0.3
2.6 ± 0.2
2.8 ± 0.4
Mask Bc
2.3 ± 1.1
4.1 ± 0.5
3.4 ± 0.7
0.9 ± 0.2
1.5 ± 0.4
2.1 ± 0.6
Lettuce
2.0 ± 0.8
3.2 ± 0.2
2.8 ± 0.5
1.0 ± 0.0
1.1 ± 0.2
1.4 ± 0.5
Slicing time refers to the number of minutes the chicken was sliced while touching masks once per minute. All lettuce was chopped
for 5 min while touching the contaminated mask once per minute.
bMask A was sampled immediately after chicken was sliced.
cMask B was sampled following chopping lettuce, in which it was used as the contamination source.
± SD
excluded because no effect was detected. The response (Yijk)
was investigated in two parts: 1) transfer from chicken to
mask and 2) transfer from mask to lettuce. Rank statistics
were used to evaluate the ratio of the transferred bacteria
from chicken to mask because the levels were on vastly
different scales. Use of ranks allowed better understanding
of data values with different scales, was free of assumptions
(such as the assumption of normality in a statistical model),
and is invariant to the log transformations. Analysis of
variance was used for subsequent analyses. All data analyses
were conducted in R Studio (R version 3.6.3) (18).
The interaction term (inoculation × mask-touching)ij
RESULTS
In studies that characterize cross-contamination, data may
be log-transformed and compared, or the percent transfer
may be calculated (11, 12). Although this transformation can
produce normalized data, it is not always a transformation
that yields normalized data that can be meaningfully
compared. The present study chose to log-transform CFU
per gram or square centimeter, calculate a transfer ratio,
and conduct rank statistics to evaluate significance after
normalization. The rank statistics allowed meaningful
comparison regardless of the difference in inoculation level
so that conclusions would not be drawn simply based on the
magnitude of difference between high and low inoculation.
Table 1 shows the log CFU per gram or square centimeter of
E. coli recovered from each chicken, mask, and lettuce sample,
and Fig. 2 shows the general progression of transfer (recovery
of E. coli) for each contamination and slicing scenario in the
218 Food Protection Trends May/June
was
form of a scatterplot. Transfer of E. coli was observed from
inoculated chicken to masks and from contaminated masks to
lettuce. The difference in inoculation level did not affect the
ranking of bacterial transfer. However, the number of touches
between inoculated chicken and mask did result in greater
bacterial transfer. The ranked statistical approach allowed
normalization of data and highlighted that contaminating
a mask 10 vs. 1 time contributed to significant transfer of
E. coli but that the difference between touching a mask 5
vs. 10 times was not significant. Furthermore, the number
of times the mask was touched during chicken slicing did
not proportionally affect bacterial transfer from the mask to
the lettuce. Mean and standard deviation (SD) of bacterial
transfers are reported in Table 1.
Transfer of E. coli from inoculated chicken to masks
Overall, the slicing time and subsequent number of
touches to the mask while slicing the inoculated chicken
significantly affected E. coli transfer. Evident in the
normalized ranking statistics, the more times a mask was
contaminated, the greater the bacterial transfer (P < 0.05).
In the case of slicing high-inoculated chicken breasts, the
transfer to the mask when touched every minute for 1, 5, and
10 min was 3.2, 4.8, and 4.6 log CFU/cm2
, respectively. In
the case of low-inoculated chicken breasts, touching masks
every minute for 1, 5, and 10 min resulted in recovery of 1.4,
2.6, and 2.8 log CFU/cm2
, respectively. Some touching time
intervals (1, 5, and 10 min) had significant effects on transfer
of E. coli. Significant differences were found between slicing
times of 1-5 min (P < 0.05) and 1-10 min (P < 0.05), with

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