Purpose: To quantify the transfer and survival of Salmonella enterica from contaminated roots to the leaves of butterhead lettuce packaged as “living lettuce” in a clamshell with intact roots stored at 4°C and 12°C throughout the 18-day shelf life.
Methods: Butterhead lettuce cultivar Buttercrunch was grown hydroponically. Mature lettuce (n = 48) was harvested by removing the entire plant including roots. Roots were soaked in Salmonella enterica Typhimurium LT2 solution. The roots were wrapped in a knot and the lettuce placed in plastic clamshells that were stored at 4°C and 12°C to simulate proper holding and temperature abuse conditions, respectively. Periodically three packages were removed and lettuce destructively processed. Roots were removed from the head, homogenized in peptone water, serially diluted and plated onto XLT-4 agar. Plate counts were also performed using 25 g of randomly selected leaves.
Results: On average 5.06 ± 0.02 log CFU/g of Salmonella was transferred to the roots from solution and 2.99 ± 0.11 log CFU/g transferred to the edible tissue. Decrease of 0.5 log CFU/g Salmonella on roots occurred on day 6 at 4°C, but no further decrease occurred throughout shelf life (P = 0.0001). Storage at 12°C was associated with 2 log CFU/g increases in Salmonella on roots after 18 days storage (P = 0.0002). At 4°C and 12°C, Salmonella persisted on the leaves after 18 days.
Significance: Salmonella was transferred to leaves from inoculated roots and persisted on living lettuce stored at 4°C for 18 days. Growth occurred only under temperature abuse conditions. This reinforces the need for maintaining temperature control and highlights the importance of identifying contamination risks associated with hydroponic production and distribution.