Food Protection Trends - May/June 2024 - 185

TABLE 2. Sample descriptions and treatment parameters
Mushroom product
Sliced
Powdered
Type of treatment
Soak sliced mushrooms in 90 ml of sterile distilled water at 80°C
Soak sliced mushroom in 90 ml of sterile distilled water (30 min) +
submerge in water bath at 100°C
Soak powdered mushrooms in 230 ml of sterile distilled water at 80°C
0, 2, 4, 6, 8, 10
Time interval (min)
2, 10 g of sliced mushrooms was soaked in ambient water
(90 ml; 20 ± 2°C) for 30 min in a filter bag. The filter
bag was submerged in a water bath held at 100°C for 10
min. Sampling was done directly after the ambient water
soaking (before treating at 100°C) to evaluate the effect of
presoaking and after treating at 100°C for a predetermined
time interval. In treatment 3, powdered mushrooms (3 g)
were weighed and combined with 230 ml of sterile distilled
water at 80°C in a filter bag. The sample filter bag sat at
room temperature, and sampling was conducted similar to
treatment 1. In all treatments, multiple bags were prepared
to assess different time intervals, and samples were analyzed
at intervals of 2, 4, 6, 8, and 10 min. Temperatures of
samples were recorded by using the Thermapen, and at
the end of each time point, the filter bag was immediately
cooled in ice water to halt pathogen inactivation. Each
sample was serially diluted with sterile 0.1% peptone
water and was homogenized for 30 s by using a stomacher
(Seward, Weber Scientific, Hamilton, NJ). Table 2 presents
a summary of the treatments used in this study.
Enumeration of surviving bacteria
The samples were serially diluted with 0.1% peptone
water and spread plated onto modified TSA. Garcia et al.
tested several media recipes for the recovery of potentially
injured Salmonella and found that TSA with yeast extract and
sodium thiosulfate, modified for the selective and differential
recovery of Salmonella (9, 12, 15, 30), resulted in recovery
that was better or similar than other recipes tested. All plates
for the treatments were incubated at 35°C for 24 h.
Statistical analysis
Three replicates of each treatment were conducted.
Bacterial counts were converted to log CFU per gram before
statistical analysis. The data were analyzed using JMP Pro
16.2.0 software (JMP Statistical Discovery, Cary, NC). The
analysis of variance and Tukey's pairwise comparisons were
conducted to determine statistical significance.
RESULTS AND DISCUSSION
This study focused on the inactivation kinetics of
Salmonella Stanley on sliced and powdered mushrooms
under various rehydration thermal treatments. The rationale
behind selecting the time-temperature combinations in the
study was based on several considerations. At the time of the
study, recommendations or best practices of dried mushroom
preparations were not widely available or documented.
Hence, this study based one of the rehydration methods on a
recommendation made by the German Federal Institute for
Risk Assessment that suggested heating dried mushrooms
to 80°C for at least 10 min (9). This method was tested
against presoaking mushrooms with cold water, treating
them in boiling (100°C) water, and allowing them to steep
for 10 min, as suggested on the product packaging. The
same instructions could also be found on a different brand's
packaging of the same product (i.e., Asian Taste). Note that
only a handful of dried wood ear mushroom products sold
in the market are accompanied with cooking instructions.
Some that do, often list varying instructions than the one
used in this study (e.g., soak for 6 to 8 h in cold water or soak
in hot water for 15 min; Table 1). Before experimentation,
the aw and initial population of Salmonella in both sliced and
powdered mushrooms were measured after exposure to RH
of 50% for 2 days and are tabulated in Table 3.
Table 4 shows the Salmonella Stanley reductions for the
three treatments across time intervals. At every sampling
time point, sliced mushrooms that were soaked at ambient
water for 30 min and cooked at 100°C exhibited significantly
higher Salmonella reduction than other preparation methods,
except at 10 min when the reduction was only significantly
higher compared with that of powdered mushrooms treated
at 80°C (P < 0.05). This method yielded >5 log CFU/g
reductions of Salmonella Stanley populations after 2 min
of treating at 100°C. Specifically, Salmonella populations
were reduced by 7.3 ± 0.6 log CFU/g (initial population
8.6 ± 0.1 log CFU/g) after 10 min. Sliced mushrooms
treated at 80°C water exhibited an eventual 5-log reduction
after 10 min (5.2 ± 0.3 log CFU/g reduction). In contrast,
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