Evaluation of Selective and Nonselective Plating Media for Recovery of Salmonella Enteritidis PT 30, Salmonella Seftenberg 775W, and Salmonella Typhimurium DT 104 Colonies from Heat-treated Almonds

Introduction: Validation studies aimed at determining the effect of a thermal process on survivability of a pathogen requires a reliable enumeration method to successfully determine the survivability during food processing. Direct plating method have been utilised in many research for determining the initial and final level of microorganisms in order to calculate the microbial log reduction achieved during a process validation study. Also research have shown that Salmonella species could become heat injured during thermal processing which makes it difficult to recover their cells.

Purpose: This study was designed to assess the efficiency of a non-selective agar; Tryptone Soya Agar (TSA) and five selective media; Bismuth Sulfite Agar (BSA), Chromogenic Agar (CHR), Hektoen Enteric Agar (HEA), MacConkey Agar (MAC) and Xylose Lysine Deoxycholate Agar (XLD) for the detection, enumeration and recovery of Salmonella before and after heat treatment.

Methods: Artificially inoculated almonds were roasted in rapeseed oil heated to approximately 120°C for 95 seconds. The population of Salmonella before and after heat treatment was then enumerated by direct plating on TSA, BSA, CHR, HEA, MAC and XLD.

Results: The mean level of Salmonella (n=4) before heat treatment ranged from 8.69 to 9.68 log cfu/g of almonds for the three serovars. After heat treatment (n=4) log cfu/g level of Salmonella Enteritidis PT 30 (SEPT 30), Salmonella Seftenberg 775W (775W) and Salmonella Typhimurium DT 104 (DT 104) recovered ranged from 5.01±0.01(HEA) to 5.55±0.26(TSA), 4.01±0.20(XLD) to 5.16±0.30(TSA) and 4.82±0.15(CHR) to 5.84±0.05(TSA), respectively. The media that were optimal for the recovery of SEPT 30 and 775W included TSA, BSA and MAC, which recovered more cells (P≤ 0.01) than CHR, HEA and XLD. For DT 104, TSA, HEA and MAC recovered more cells.

Significance: The result obtained in this study suggests the use of TSA in combination with BSA and/or MAC agar for maximum recovery of Salmonella cells after oil roasting process.