Food Protection Trends - September/October 2021 - 482

TABLE 5. Average temperature of lasagna prepared with S. enterica and pathogenic
E. coli-inoculated no-boil lasagna noodles after baking treatment
Pathogenic organism
S. enterica
Top layera
Middle layera
Bottom layera
Pathogenic E. coli
Top layera
Middle layera
Bottom layera
a n = 2 for each layer.
At the end of all three cooking treatments evaluated in
this study, the water activity of the inoculated pasta products
was >0.97 (Table 4). At this water activity, S. enterica and
pathogenic E. coli are expected to be much less resistant to
heat than in a low-water-activity matrix. Although the heat
resistance of S. enterica and pathogenic E. coli is a function
of the organism strain and the characteristics of the matrix,
in general the relative heat resistance of these organisms in
high-water-activity matrices is expected to be as follows:
S. enterica Senftenberg 775W > E. coli > Salmonella species
(not including S. enterica Senftenberg 775W) (13, 15, 21, 26).
The relatively high heat resistance of S. enterica Senftenberg
775W in high-water-activity matrices is an interesting
phenomenon and was the primary reason this particular
strain was included in our study.
A review of studies on the heat resistance of Salmonella
spp. conducted by Doyle and Mazzotta (13) found that of
the high-water-activity matrices assessed (which included
eggs, milk and dairy, poultry, and other meats), the heat
resistance of Salmonella spp. was greatest in liquid eggs and
liquid egg yolks. A study by Humphrey et al. (17) found
that S. enterica Senftenberg 775W was comparatively more
resistant in homogenized whole egg than five other S. enterica
strains (Enteritidis and Typhimurium) that had been isolated
from egg sources. Thus, the heat resistance of S. enterica
Senftenberg 775W in a liquid egg matrix may provide
context when assessing the lethality at the end of the cooking
treatments evaluated in the present study. The heat resistance
of S. enterica Senftenberg 775W in liquid whole eggs has been
quantified by Humphrey et al. (17) and Mañas et al. (19);
the D-values determined in these studies at 63 to 64°C were
2.8 min (calculated z-value of 8.2°C) and 1.2 min (z-value of
5.2°C), respectively. Considering the Humphrey et al. (17)
482 Food Protection Trends September/October
D-value as a worst-case scenario, a 5-log reduction of S. enterica
Senftenberg 775W is predicted in <1 s at 88.3°C. This suggests
that a final temperature of 88.3°C or higher may be sufficient to
deliver adequate reduction of S. enterica and pathogenic E. coli
instantaneously in a high-water-activity matrix.
In our study, the average temperature after the application
of the microwave treatment was >88.3°C in each trial
executed (data not shown). In addition, although the
temperature after the application of the stovetop treatment
was not measured, it may be surmised that the flat noodles
had reached a temperature >88.3°C in the boiling water by
the end of the cooking treatment, when the water activity
of the noodles was high. Both of these cooking treatments
resulted in >6-log reductions of S. enterica and pathogenic
E. coli; thus, these results are in agreement with the predicted
lethality of the microwave and stovetop cooking treatments.
The average temperature of the inoculated lasagnas after
the application of the baking treatment differed between
layers (Table 5). In all trials executed, the temperature of
the top layer of the inoculated lasagna was the lowest of the
three, followed by the middle layer, and finally the bottom
layer. The lowest average temperature of the top, middle,
and bottom layer recorded was 73.3, 84.7, and 92.2°C
respectively (n = 2). Although a 5-log reduction of S. enterica
Senftenberg 775W is predicted by Humphrey et al. (17) to
be instantaneous at temperatures >88.3°C, a 5-log reduction
at 73.3°C is expected to occur within 62 s (i.e., the top
layer). The observance of >6-log reductions of S. enterica
and pathogenic E. coli when the three layers of lasagna were
evaluated as one sample indicates that the baking treatment
was capable of delivering adequate reductions of these two
pathogens. It is reasonable to assume that, considering the
relatively long length of the baking treatment (i.e., 50 min),
Average temperature (°C) for each lasagna layer after baking treatment
Trial 1
Trial 2
73.3
85.3
92.2
78.6
84.7
96.1
77.1
85.5
94.1
83.9
95.0
96.4
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