Purpose: This study aimed to assess the effect of several sanitizers and in-package atmospheres on the fate of L. monocytogenes in diced onions stored at 7°C.
Methods: Spanish jumbo yellow onions (Allium cepa) were diced using an Urschel Model H-A dicer, dip-inoculated with a 3-strain avirulent L. monocytogenes cocktail (M3, J22F and J29H) at 5.9 log CFU/g and air-dried for 8 min. The onions were then washed for 2 min in 60 l of 4°C water containing either no sanitizer, 80 ppm free chlorine (XY‑12, Ecolab) adjusted to pH 6.5 with citric acid, 80 ppm peroxyacetic acid (PAA) (Tsunami 100, Ecolab), or 2 ppm ClO2 (CDG Environmental). After centrifugal drying, 100-g samples of diced onion were packaged in 11 x 11 cm polylactic acid (PLA) pouches (EVLON EV-HS1, BI-AX International Inc.) flushed with 99% O2 or air. Periodically during 14 days of storage at 7°C, one package of diced onions per treatment was added to PBS buffer, homogenized by stomaching, appropriately diluted, and then plated on Modified Oxford Agar to enumerate Listeria. Findings from triplicate experiments were analyzed by the Tukey-Kramer HSD test using JMP 10.
Results: Initial Listeria reductions were significantly (P < 0.05) greater for PAA (1.9 log CFU/g) as compared to water, chlorine, and ClO2 (0.9 - 1.0 log CFU/g). After 14 days of storage at 7°C, the total increase in numbers of Listeria was significantly (P < 0.05) lower in 99% O2 (0.6 log CFU/g) as compared to air (1.6 log CFU/g) with no significant (P > 0.05) difference observed between sanitizers.
Significance: Based on these findings, packaging diced onions in PLA bags with a high oxygen atmosphere can reduce the extent of L. monocytogenes growth during extended refrigeration.