Survival of Listeria monocytogenes on Blueberries and Raspberries Stored at 4 °C and −18 °C

Raw and minimally processed berries are not subjected to any processing kill steps and are stored under cold conditions to extend shelf-life. This study evaluated the growth and survival of high and low populations of L. monocytogenes on blueberries and raspberries stored under refrigerated and frozen temperatures. Fresh berries (10 g) were inoculated with a five-strain cocktail of L. monocytogenes to target 5.5 or 2 log CFU/g, stored at 4 and −18 ± 2 °C, and enumerated for up to 14 days post inoculation (dpi) at 4 °C and 168 dpi at −18 °C. Significant differences were evaluated (p ≤ 0.05), and die-off was modeled, using Davies test to determine breakpoints. No significant changes (p > 0.05) in L. monocytogenes populations were observed over time on berries stored at 4 °C, regardless of inoculation level. At −18 °C, significant reductions were observed over 168 dpi, with greater declines for high-inoculated blueberries and raspberries (2.63 and 2.13 log CFU/g, respectively), compared to those that were low-inoculated (0.67 and 0.46 log CFU/g, respectively). Die-off modeling indicated linear decay for both low-inoculated berry types and biphasic patterns for both high-inoculated frozen blueberries and high-inoculated raspberries stored at 4 °C. Results suggest that while L. monocytogenes does not grow, blueberries and raspberries support survival during refrigerated and frozen storage, emphasizing the need for stringent pre- and postharvest practices to limit contamination.