Assessing dry inoculation carriers and Salmonella transfer in low moisture foods: a peanut-based model investigation

Salmonella has been responsible for several foodborne outbreaks associated to low moisture foods (LMFs) worldwide, including peanut based products. In this study the performance of calcium carbonate (a_w 0.331), non-fat milk powder (a_w 0.226), soil (a_w 0.388), crushed peanut skin (a_w 0.357) and crushed peanut shell (a_w 0.341) as dry carriers for Salmonella was evaluated. In addition, Salmonella dry transfer from soil and crushed peanut skin to peanut kernels was assessed. Immediately after the dry inoculation, the highest Salmonella count was obtained in calcium carbonate, ca. 7 log CFU/g, followed by soil and peanut shells, both with 6.4 log CFU/g, powdered milk, with 6.2 log CFU/g and peanut skin, with 6.0 log CFU/g. However, there was no significant difference (p > 0.05) among the carriers. The stability of Salmonella on the carriers was also evaluated for 7 days at 37 ºC. At the end of the storage time, only peanut skin showed a significant decline in the inoculum load (p < 0.05), with reduction of 2.7 log CFU/g. For the other carriers the Salmonella counts varied by up to 1.2 log CFU/g. Moreover, the Salmonella transfer rate from soil and crushed peanut skin to peanut kernels was 0.14% and 0.10%, corresponding to ca. 4 log CFU/g. After 30 days at 25 ºC, reductions of 2 log CFU/g in the peanut samples were observed. Neither the carriers nor the culture media used to recover the inoculum from peanuts had significant effect on the results (p > 0.05). In conclusion, four out of the five carriers displayed good performance. The indirect inoculation method optimized in this study reduced the inoculum preparation time. In addition, soil and crushed peanut skin showed potential for dry transfer of Salmonella to peanuts, illustrating a representative scenario of cross-contamination of peanuts.