Influence of modified atmosphere packaging on survival and recovery of sublethally heat- and acid-injured Listeria monocytogenes

This research was conducted as two separate studies. In the first study, the suitability of PALCAM and modified Oxford (MOX) agars for recovering sublethally heat- and lactic acid-injured Listeria monocytogenes was investigated. L. monocytogenes LMIOIM, LM103M (meat isolates), and Scott A were suspended in tryptose phosphate broth (TPB), heated for up to 40 min at 54°C, and surface plated onto tryptose phosphate agar(TPA), TPA + 4% NaCl (TPAS), PALCAM, and MOX. TPA and TPAS were used to determine total viable and sublethally injured populations, respectively. Heat-injured LM103M was recovered in the highest numbers on all media, followed by Scott A and LMlOlM (P<0.01). TPA allowed best recovery of all test strains, followed by PALCAM and MOX, which were not different, and TP AS (P<0.01). For acid-injury studies, uninjured and heat-injured (54°C for 20 min) test strains were suspended in phosphate-buffered TPB + 0.85% lactic acid (bTPBLA) at 25°C for up to 24 h and plated as described above. Uninjured and heat-injured L monocytogenes were recovered better from bTPBLA on MOX than on PALCAM (P<0.05). Heat injured L. monocytogenes LM103M was recovered better than LMIOIM but similar to Scott A on MOX and PALCAM (P<0.05), whereas Scott A was recovered similarly to LMIOIM and LM103M on MOX and PALCAM (P>0.05). Acid-injury of L. monocytogenes LM103M was enhanced by prior heat stress. In the second study, the effect of package atmosphere on survival of uninjured and sublethally heat-injured L. monocytogenes, inoculated onto tryptose phosphate agar containing 0.85% lactic acid and 2% NaCl (TPALAS) was investigated. Inoculated TPALAS plates were packaged in air, 100% N₂ (N₂), 30% CO₂/70% N₂ (CO₂/N₂), and vacuum and stored at 4 and 20°C for up to 31 days. Recovery of monocytogenes from TPALAS was influenced by the injury status (i.e., injured and uninjured) of the inoculum, storage atmosphere (air, N₂, CO₂/N₂, and vacuum), storage temperature (4 and 20°C), and recovery media (tryptose phosphate agar [TPA] and modified Oxford agar [MOX]) (P<0.05). Storage in CO₂/N₂ atmosphere was more inhibitory to uninjured L. monocytogenes stored at 20°C than air or vacuum, but less than N₂ (P<0.05). When sublethally heat-injured L. monocytogenes was stored at 20°C, CO₂/N₂ atmosphere allowed better recovery than N₂ (P<0.05) but was not different than air (P>0.05). At 4°C, uninjured and sublethally heat-injured L. monocytogenes were recovered in highest numbers from samples packaged in N₂ followed by CO₂/N₂ (P>0.05). Overall, non-selective TPA allowed greater recovery of L. monocytogenes than the selective MOX (P<0.05). Uninjured cells stored at 4°C were recovered better than sublethally heat-injured cells on TPA (P<0.05). On TPA, L. monocytogenes stored under air was recovered better than L. monocytogenes stored under N₂ or CO₂/N₂ (P<0.05). However, recovery on MOX was best when L. monocytogenes was stored under N₂ (P<0.05)