Nisin application delays growth of Listeria monocytogenes on fresh-cut iceberg lettuce, while the bacterial community structure changes within one week of storage.. Lettuce bacteria

The bacteriocin Nisin, produced naturally by various Lactococcus lactis strains, is commonly used in the cheese industry as it displays broad-spectrum activity against many Gram-positive bacteria including Listeria monocytogenes. In the ready-to-eat food sector L. monocytogenes poses a risk to minimally processed foods due to its ability to grow under refrigeration conditions. Many natural antilisterial products render Iceberg lettuce unsuitable for consumption within 2 days of storage and thus this study investigated the efficacy of Nisin as antilisterial agent and its sensory impact on lettuce. In-vitro assays confirmed the efficacy of Nisin to inhibit growth of a three strain mix of L. monocytogenes. Subsequently, this L. monocytogenes strain mix was added to lettuce that was treated either with Nisin, L. lactis DSM20729 or was kept treatment-free. Incubation took place at 4 and 8°C under various atmospheres. On days 0, 2, 5 and 7, Listeria was extracted from the food surface and enumerated on selective agar and a sensory panel graded the lettuce on product appearance. At 4 and 8°C a 10-100 fold reduction of Listeria growth was achieved with 5 ppm Nisin over a seven day period, while lettuce kept an acceptable sensory appearance over the first 5 days. Direct application of Nisin producing L. lactis had no detectable effect on Listeria growth in vitro and in situ. The bacterial community structure changed substantially over the 7 days of incubation with the genus Pseudomonas being most abundant and increasing to around 90% of relative abundance by day 7.