title: Development of a Dry Inoculation Method for Thermal Challenge Studies in Low-Moisture Foods by Using Talc as a Carrier for Salmonella and a Surrogate (Enterococcus faecium).
abstract: The objective of this study was to obtain dry inocula of Salmonella Tennessee and Enterococcus faecium, a surrogate for thermal inactivation of Salmonella in low-moisture foods, and to compare their thermal resistance and stability over time in terms of survival. Two methods of cell growth were compared: cells harvested from a lawn on tryptic soy agar (TSA-cells) and from tryptic soy broth (TSB-cells). Concentrated cultures of each organism were inoculated onto talc powder, incubated at 35 °C for 24 h, and dried for additional 24 h at room temperature (23 ± 2 °C) to achieve a final water activity of ≤ 0.55 before sieving. Cell reductions of Salmonella and E. faecium during the drying process were between 0.14 and 0.96 log CFU/g, depending on growth method used. There was no difference between microbial counts at days 1 and 30. Heat resistance of the dry inoculum on talc inoculated into a model peanut paste (50 % fat and 0.6 water activity) was determined after 1 and 30 days of preparation, using thermal death time tests conducted at 85 °C. For Salmonella, there was no significant difference between the thermal resistance (D(85 °C)) for the TSB-cells and TSA-cells (e.g. day 1 cells D(85 °C) = 1.05 and 1.07 min, respectively), and there was no significant difference in D(85 °C) between dry inocula on talc used either 1 or 30 days after preparation (P > 0.05). However, the use the dry inocula of E. faecium yielded different results: the TSB-grown cells had a significantly (P < 0.05) greater heat resistance than TSA-grown cells (e.g. D(85 °C) for TSB-cells = 3.42 min versus 2.60 min for TSA-cells). E. faecium had significantly (P < 0.05) greater heat resistance than Salmonella Tennessee regardless what cell type was used for dry inoculum preparation; therefore, it proved to be a conservative but appropriate surrogate for thermal inactivation of Salmonella in low-moisture food matrices under the tested conditions.
authors.affiliation: Grocery Manufacturers Association, 1350 I Street N.W., Suite 300, Washington, D.C. 20005, USA. eenache@gmaonline.org.
authors.forename: Elena
authors.initials: E
authors.lastname: Enache
authors.affiliation: Grocery Manufacturers Association, 1350 I Street N.W., Suite 300, Washington, D.C. 20005, USA.
authors.forename: Ai
authors.initials: A
authors.lastname: Kataoka
authors.affiliation: Grocery Manufacturers Association, 1350 I Street N.W., Suite 300, Washington, D.C. 20005, USA.
authors.forename: D Glenn
authors.initials: DG
authors.lastname: Black
authors.affiliation: Grocery Manufacturers Association, 1350 I Street N.W., Suite 300, Washington, D.C. 20005, USA.
authors.forename: Carla D
authors.initials: CD
authors.lastname: Napier
authors.affiliation: Grocery Manufacturers Association, 1350 I Street N.W., Suite 300, Washington, D.C. 20005, USA.
authors.forename: Richard
authors.initials: R
authors.lastname: Podolak
authors.affiliation: Grocery Manufacturers Association, 1350 I Street N.W., Suite 300, Washington, D.C. 20005, USA.
authors.forename: Melinda M
authors.initials: MM
authors.lastname: Hayman
mesh_terms: D010367:Arachis / Q000382:microbiology; D015169:Colony Count, Microbial; D003890:Desiccation; D016984:Enterococcus faecium / Q000254:growth & development*; D005516:Food Microbiology; D006358:Hot Temperature; D050296:Microbial Viability; D012475:Salmonella / Q000254:growth & development*; D019779:Salmonella enterica / Q000378:metabolism; D013627:Talc
chemical_list: D013627:Talc