Growth Potential of Listeria monocytogenes on Refrigerated Spinach and Rocket Leaves in Modified Atmosphere Packaging

Abstract

Minimally processed ready-to-eat (RTE) vegetables are increasingly consumed for their health benefits. However, they also pose a risk of being ingested with food-borne pathogens. The present study investigated the ability of RTE spinach and rocket to support the growth of Listeria monocytogenes as previous studies provided contradicting evidence. Findings were compared to growth on iceberg lettuce that has repeatedly been shown to support growth. Products were inoculated with a three-strain mix of L. monocytogenes at 10 and 100 cfu g−1 and stored in modified atmosphere (4 kPa O2, 8 kPa CO2) at 8 °C over 7–9 days. Spinach demonstrated the highest growth potential rate of 2 to 3 log10 cfu g−1 over a 9-day period with only marginal deterioration in its visual appearance. Growth potential on rocket was around 2 log10 cfu g−1 over 9 days with considerable deterioration in visual appearance. Growth potential of iceberg lettuce was similar to that of rocket over a 7-day period. Growth curves fitted closely to a linear growth model, indicating none to limited restrictions of growth over the duration of storage. The high growth potentials of L. monocytogenes on spinach alongside the limited visual deterioration highlight the potential risks of consuming this raw RTE food product when contaminated.

Microbiological safety of spinach throughout commercial supply chains in Gauteng Province, South Africa and characterization of isolated multidrug-resistant Escherichia coli

“…Furthermore, no spinach samples from the current study harboured L. monocytogenes, which corresponds to a previous study of retailed fresh produce sold formally and informally (Richter et al, 2021). However, previous studies have confirmed that spinach supports the growth of L. monocytogenes, with the retailed product not showing any obvious deterioration (Culliney et al, 2020). This poses a serious health risk to consumers, making surveillance of L. monocytogenes together with potential pathogenic Enterobacteriaceae in food supply crucial, as leafy greens have previously been implicated in listeriosis outbreaks, including a multistate outbreak in the U.S. (Self et al, 2019).…”

Cultivation Conditions of Spinach and Rocket Influence Epiphytic Growth of Listeria monocytogenes

“…Dilutions with phosphate-buffered saline (PBS, pH 7.3, BR0014, Oxoid) were carried out to mix the three strains at equal cell densities to aim for inoculation at cell densities of 100 cfu g −1 . This was confirmed by enumeration on Listeria selective agar (LSA), conforming to ALOA [17,21,26].…”

“…Likewise, baby spinach displayed considerably higher growth at 9 • C compared to 4 • C [20]. An additional lower initial inoculation density of 1 log L. monocytogenes was associated with greater growth potentials on lettuce, spinach, and rocket, compared to 2 logs [21]. Moreover, the same study revealed that there are factors other than incubation conditions that influence the development of L. monocytogenes on RTE leafy vegetables.…”

“…However, cultivation conditions were not revealed on the packaging of the vegetables purchased from the supermarket. Moreover, contradicting data of L. monocytogenes on spinach and rocket still exists with some supporting growth [21], while others did not [22,23]. Additional studies identified conflicting trends of L. monocytogenes growth with rocket supporting more growth than spinach [18], whereas a recent study found more growth on spinach compared to rocket [21].…”

“…Moreover, contradicting data of L. monocytogenes on spinach and rocket still exists with some supporting growth [21], while others did not [22,23]. Additional studies identified conflicting trends of L. monocytogenes growth with rocket supporting more growth than spinach [18], whereas a recent study found more growth on spinach compared to rocket [21]. Kale had no significant changes over 6 days in L. monocytogenes populations when inoculated with 4 logs at 4 and 13 • C [24].…”