Purpose: In response to this outbreak, the impact of packaging atmosphere and package size on growth of Listeria monocytogenes on fresh-cut celery was assessed.
Methods: Locally purchased celery was cut into 10-cm sticks and inoculated by immersion for 30 min in a 3-strain cocktail of avirulent L. monocytogenes (J22F, M3, J29H) containing 6.96 log CFU/ml to obtain 2.74 log CFU/g. After centrifugal drying and 18 – 22 h of storage at 4°C, the celery was immersed in 60 l of tap water containing 50 ppm available chlorine (XY-12, Ecolab, St. Paul, MN) for 1 min, air-dried for 1 h, and packaged in two different sized ultra-high barrier film pouches (53 x 25.5 or 25.5 x 20 cm) containing air, 99% O2, 99% N2, or 15% CO2/5% O2/80% N2. Following 0, 1, 3, 5, 7, 10, and 14 days of storage at 7°C, single celery sticks from these packages were added to 100 ml of neutralizing buffer, homogenized by stomaching, appropriately diluted, and plated with or without prior membrane filtration on Modified Oxford Agar for quantification of L. monocytogenes after 48 h of incubation at 37°C.
Results: After the large pouches were stored for 14 days at 7°C, Listeria populations were significantly lower (P < 0.05) using 99% O2 (0.92 log CFU/g) as compared to 99% N2 (4.87 log CFU/g) with no differences observed between air (1.61 log CFU/g) and 15% CO2/5% O2/80% N2 (3.65 log CFU/g). However, no significant differences (P > 0.05) in numbers of Listeria were seen for the small pouches during storage. Only two instances occurred where package size impacted Listeria growth, with populations 1.51 and 1.50 log CFU/g higher in small pouches containing 15%CO2/5% O2/85% N2 at day 5, and 99% O2 at day 14.
Significance: Based on these findings, packaging fresh-cut celery sticks in oxygen may reduce the level of growth achieved by L. monocytogenes during refrigerated storage, while package size should not impact pathogen growth.