The fresh-cut produce industry has been the fastest-growing portion of the food retail market during the past 10 years, providing consumers with convenient and nutritious food. However, fresh-cut fruits and vegetables raise food safety concerns, because exposed tissue may be colonized more easily by pathogenic bacteria than intact produce. This is due to the higher availability of nutrients on cut surfaces and the greater potential for contamination because of the increased amount of handling. We found that applied Listeria monocytogenes populations survived and increased only slightly on fresh-cut Red Delicious apples stored at 10°C but increased significantly on fresh-cut honeydew melons stored at 10°C over 7 days. In addition, we examined the effect of lytic, L. monocytogenes-specific phages via two phage application methods, spraying and pipetting, on L. monocytogenes populations in artificially contaminated fresh-cut melons and apples. The phage mixture reduced L. monocytogenes populations by 2.0 to 4.6 log units over the control on honeydew melons. On apples, the reduction was below 0.4 log units. In combination with nisin (a bacteriocin), the phage mixture reduced L. monocytogenes populations by up to 5.7 log units on honeydew melon slices and by up to 2.3 log units on apple slices compared to the control. Nisin alone reduced L. monocytogenes populations by up to 3.2 log units on honeydew melon slices and by up to 2.0 log units on apple slices compared to the control. The phage titer was stable on melon slices, but declined rapidly on apple slices. The spray application of the phage and phage plus nisin reduced the bacterial numbers at least as much as the pipette application. The effectiveness of the phage treatment also depended on the initial concentration of L. monocytogenes.
Penicillium expansum, P. digitatum, and P. italicum acidify the ambient environments of apple and citrus fruit during decay development. They use two mechanisms for this: the production of organic acids, mainly citric and gluconic, and NH(4)(+) utilization associated with H(+) efflux. Exposure of P. expansum and P. digitatum hyphae to pH 5.0 increased their citric acid production, compared with the production of organic acids at acidic ambient pH. In decayed fruit, both pathogens produced significant amounts of citric and gluconic acids in the decayed tissue and reduced the host pH by 0.5 to 1.0 units. Ammonium depletion from the growth medium or from the fruit tissue was directly related to ambient pH reduction. Analysis of transcripts encoding the endopolygalacturonase gene, pepg1, from P. expansum accumulated under acidic culture conditions from pH 3.5 to 5.0, suggesting that the acidification process is a pathogenicity enhancing factor of Penicillium spp. This hypothesis was supported by the finding that cultivars with lower pH and citric acid treatments to reduce tissue pH increased P. expansum development, presumably by increasing local pH. However, organic acid treatment could not enhance decay development in naturally acidic apples. Conversely, local alkalinization with NaHCO(3) reduced decay development. The present results further suggest that ambient pH is a regulatory cue for processes linked to pathogenicity of postharvest pathogens, and that specific genes are expressed as a result of the modified host pH created by the pathogens.
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