UV-C doses to reduce pathogen and spoilage bacterial growth in vitro and in baby spinach

The aim of this work was to investigate the use of different doses of UV-C (0, 2.4, 7.2, 12 and 24 kJ m−2) radiation treatments to inhibit microbial growth of Listeria monocytogenes and Salmonella enterica. The spoilage bacteria Pseudomonas marginalis (gram negative) was also tested. These bacteria were studied under in vitro conditions and in baby spinach leaves (in vivo conditions) for 13 and 14 d at 5 °C, respectively. All radiation doses were effective in reducing bacterial growth, although contrary to expectations, high doses did not show the highest microbial inhibition in in vitro experiments. UV-C doses (2.4–24 kJ m−2) were also used on baby spinach (Spinacia oleracea L.) leaf surfaces, stored under humidified air using perforated plastic film. A clear inhibitory UV-C effect was observed on L. monocytogenes for 14 d at 5 °C. Meanwhile, UV-C radiation reduced S. enterica loads until the first 4 d of storage, after which a significant increase was found on radiated leaves compared to the control. P. marginalis counts were slightly reduced in UV-C treated leaves. In addition, significant decreases in psychrotrophic counts and Enterobacteriaceae were found during the first 4 d of storage. Respiration rates of baby spinach leaves were higher in radiated than in non-radiated leaves. Moreover, no obvious damage on the epidermal surface and to cell shape was detected in radiated and non-radiated leaves by scanning electronic microscopy (SEM). In summary, the use of double-sided UV-C radiation, at low doses, was effective in reducing initial microbial counts of the tested bacteria types and psychrotrophic and Enterobacteria counts, and in keeping L. monocytogenes at low levels during the storage period, without affecting the sensory quality of fresh-cut baby spinach leaves.