Three-dimensional epicuticular wax on plant surface reduces attachment and survival rate of salmonella during storage

Authors: CHIU, YU-CHUN - West Virginia University; SHEN, CANGLIANG - West Virginia University; Farnham, Mark; KU, KANG-MO - Chonnam National University

Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/30/2020
Publication Date: 6/21/2020
Citation: Chiu, Y., Shen, C., Farnham, M.W., Ku, K. 2020. Three-dimensional epicuticular wax on plant surface reduces attachment and survival rate of salmonella during storage. Postharvest Biology and Technology. 166:111197. https://doi.org/10.1016/j.postharvbio.2020.111197.
DOI: https://doi.org/10.1016/j.postharvbio.2020.111197

Interpretive Summary: The occurrence of Salmonella contamination on vegetables has become a concern in recent years, and particular contamination events have caused significant economic losses to vegetable producers and processors in the United States. Work is needed to better understand host plant factors that may impact contamination of produce going to market. Thus, investigators at the USDA-ARS-U.S. Vegetable Laboratory in Charleston, South Carolina, cooperated with scientists at West Virginia University in Morgantown, West Virgina, in studies to examine important host cole crop vegetable factors that are likely to impact potential contamination by Salmonella. These scientists used unique broccoli and collard varieties that differ in the amount of wax that occurs on the leaf surfaces to examine the effect these waxes have on Salmonella adhering to leaf surfaces and persisting over time. They found that varieties with a waxy surface were much less likely to exhibit contamination by the bacteria when it was introduced on the leaf surfaces than were varieties that lack a waxy surface. A lack of wax on leaf surfaces also resulted in longer adherence of the bacteria in cold storage following harvest. This information is very important in understanding how the host plant can influence potential contamination of leafy vegetables and could influence the choice of variety that vegetable producers may consider when contamination might present a problem.

Technical Abstract: Salmonella is the second most common foodborne pathogen for leafy vegetables, therefore understanding how to reduce Salmonella attachment onto produce surface is crucial to combat salmonellosis. Epicuticular wax is the outermost layer on the leaf surface that directly interacts with food pathogen attachment. The hydrophobic nature of epicuticular wax was found to increase pathogen resistance, however, there is limited study on whether three-dimensional epicuticular wax on the leaf can reduce Salmonella attachment. This study aimed to test whether the presence of three-dimensional epicuticular wax crystals decreases the attachment of Salmonella on leafy green surfaces. Using gum arabic paste, three-dimensional epicuticular wax was removed from three waxy plants (USVL188-NG, USVL115-NG, and ‘Top Bunch’ collard). Leafy surfaces in disks were dip-inoculated with a mixture of Salmonella Typhimurium and Tennessee at day 0, followed by aerobic storage at 4.0 ± 0.2 °C for 14 d. After 30-minute inoculation, significantly lower (P<0.05) Salmonella were attached to plants with three-dimensional epicuticular wax, resulting in 3.27, 2.76, and 4.51 log10 CFU cm-2, respectively when compared to three glossy plants (USVL188-GL, USVL115-GL, and ‘Green Glaze’ collard greens) on XLT-4 agar. Attached Salmonella on gum arabic-treated plants were significantly lower than the untreated plants, suggesting that three-dimensional epicuticular wax reduced the attachment efficiency of Salmonella. The survival rate of Salmonella populations on three waxy plants were usually significantly lower (P<0.05) than glossy plants during storage. From day-9 to day-14, the Salmonella population on ‘Top Bunch’ collard greens decreased faster and resulted in lower (P<0.05) numbers than ‘Green Glaze’ collard greens (2.88 to 3.47 vs 4.41 to 4.82 log10 CFU cm-2). The results implied that plant cultivars with three-dimensional epicuticular wax may be a safer choice for producers in terms of minimizing foodborne outbreak risks.