Food Protection Trends - May/June 2024 - 183

properties of the foods; however, this limits the reduction
of microbial load during the drying process (2). Dried
mushroom products are sold widely in retail grocery stores
in plastic pouch packages. In some Asian traditional markets,
it is common to see bulk quantities of dried mushrooms
displayed in open air to be weighed by request and sold to
customers (1).
Generally, mushrooms, such as the wood ear variety, may
be considered ready-to-eat in the United States, as they
are frequently raw, according to the Mushroom Council
(19). Dried mushrooms are commonly used as ingredients
or toppings in soups and cold desserts (e.g., snow fungus
dessert soup), especially in Asian cuisines. In this instance,
the product is rehydrated in water and applied directly
onto food, sometimes without any additional cooking
step (17). In addition, there is also increasing popularity
in using dried mushroom powders as coffee alternatives;
the powders are steeped in warm water for an indefinite
amount of time before consumption (24). However,
pathogens might potentially contaminate fresh mushrooms
and therefore the final dried product. Mushrooms can be
potentially contaminated during seeding or harvesting,
as the substrates are made from raw agricultural materials
(31). This contamination risk is further exacerbated, as
mushrooms are rarely subjected to postharvest washing to
preserve quality and shelf life (5, 29). Improperly sanitized
processing equipment (i.e., slicer, dryer), as well as improper
handling in storage, retail, and household settings (i.e., crosscontamination
with raw meat and vegetables via a cutting
board or utensils) can also introduce pathogens to the food
product (26). Once contaminated, pathogens can survive in
these desiccated, low-water-activity storage conditions for
over 6 months (7, 10).
Contaminated dried and semidried mushrooms have
been linked to numerous outbreaks and product recalls (27,
28). In 2020, a multistate outbreak of Salmonella enterica
Stanley in dried black fungus (i.e., wood ear mushroom)
occurred, indicating that this food product can be a suitable
vehicle for foodborne pathogens (3, 27). However, there are
currently limited guidelines to ensure safe preparation of
dried mushroom products in household settings. Only one
federal recommendation from a German institution exists,
which outlines that dried mushrooms be heated to 80°C
for at least 10 min (9). Besides this, preparation methods
suggested by online recipes or instructions found on product
packaging remain inconsistent, invalidated, and vague
for the reduction of foodborne pathogens (Table 1). This
raised a safety issue, as the common practice of rehydrating
mushrooms prior to cooking can allow Salmonella enterica to
grow to a considerable degree, potentially causing diseases
(1, 13). This study will address this issue by evaluating three
different consumer preparation methods of dried (sliced
and powdered) mushrooms on Salmonella Stanley survival.
Time-temperature combinations were selected on the basis
of available recipes and instructions listed on the product
packaging. Results from this study will be useful to provide
recommendations for best practices to consumers of these
products and control for potential Salmonella contamination.
MATERIALS AND METHODS
Salmonella serotype
Salmonella Stanley H1256 (Department of Food Science
and Technology, University of Georgia, Athens) was used in
this study. The strain was maintained at −80°C on protectant
beads in tryptic soy broth with glycerol (Microbank, ProLab
Diagnostics, Richmond Hill, Ontario, Canada), and a
working culture was maintained on tryptic soy agar (TSA;
Difco, BD, Sparks, MD) plates at 4°C with monthly transfers
to maintain viability. A single bead for the bacterial strain
was removed and added to 10 ml of tryptic soy broth (Difco)
and incubated at 37°C for 24 h. The broth culture was used
to streak bacterial lawns of each strain on multiple plates
of TSA. After incubation for 24 h at 37°C, the lawns of
confluent growth were harvested by using a sterile L-spreader
(VWR, Radnor, PA) and 1 ml of 0.1% peptone water (Hardy
Diagnostic, Santa Maria, CA) to obtain highly concentrated
inoculum (ca. 10 log CFU/ml).
Sample preparation and inoculation
Dried sliced mushrooms (dried black fungus slice;
water activity [aw] 0.56) were obtained from an online
store (Amazon; product of People's Republic of China,
distributed by Wei-Chuan USA, City of Industry, CA) and
stored at room temperature until use. Prior to inoculation
with Salmonella Stanley, the sliced dried mushrooms
were autoclaved at 121°C for 15 min to inactivate relevant
background microbial populations, which did not
significantly alter the aw
(0.49). The background microbial
populations of autoclaved wood ear mushroom were
checked prior to inoculation, and no colonies were found on
modified TSA with yeast extract plates that were incubated
at 35°C for 24 h. Sliced dried mushrooms were aseptically
weighed (180 g) and directly inoculated with Salmonella
Stanley. An inoculum of 18 ml was sprayed onto sterile
sliced mushrooms and homogenized manually in a sterile
bag (Whirl-Pak, Nasco, Fort Atkinson, WI) for 2 min, after
which the inoculated mushrooms were transferred into an
aluminum foil tray and left to dry in the biosafety cabinet
for 1 h. Next, 21 g of sliced mushrooms was aseptically
separated and grinded until powdered form (mesh no. 25,
0.7 mm) using a spice grinder for 30 s and repeated three
times at the same grinder setting (SG10 Electric Spice and
Nut Grinder, Cuisinart, Stamford, CT). Both sliced and
powdered mushrooms were stored in custom-designed
relative humidity (RH) chambers equipped with water and
silica beads columns, as well as a digital humidity sensor
(SHT85, Sensirion AG, Stäfa, Switzerland) (14). The RH
chambers were set to the predetermined RH (30%), and
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