Purpose: This study was conducted to examine the effect of temperature and humidity of storage on the survival of Salmonella on three different varieties of tree nuts.
Methods: Three different tree nut varieties - pecans, hazel nuts and pine nuts - were chosen for examination based on differences in their typical fat content. Five different serotypes of Salmonella were grown to stationary phase on trypticase soy agar supplemented with 0.6% yeast extract and harvested for use as inoculum for each tree nut variety. Inoculated nuts were stored at 4, 10, and 25°C at two different humidity levels (57% RH and 34% RH). Each nut variety at each storage condition was monitored for one year by periodically removing 10-g quantities for testing for surviving Salmonella populations (plating on selective and non-selective media) and water activity.
Results: Salmonella populations were reasonably stable at both humidity levels when stored at 4 and 10°C with a less than 1.5 log CFU/g population loss over the entire year tested. Population changes were detected at 25°C, with larger changes occurring at the higher humidity used. There was a significant difference (P < 0.0001) based on the variety of nut, with pine nuts showing approximately a 2 log CFU/g decline in Salmonella populations at 25°C and 57% RH versus 3.5 to 4-log decline under the same conditions for hazel nuts and pecans.
Significance: Differences in Salmonella survival on the different tree nut varieties may result in differences in the risk of illness associated with the consumption of these different nut varieties.