According to new research, applying antimicrobial coatings to whole cantaloupes during storage significantly reduces contamination and has the potential to improve their microbiological safety and extend their shelf life.
According to new research, applying antimicrobial coatings to whole cantaloupes during storage significantly reduces contamination by pathogenic bacteria, and has the potential to improve their microbiological safety and extend their shelf life. Qiumin Ma, PhD, from the University of Tennessee in Knoxville, and colleagues published the results of their study in the International Journal of Food Microbiology.
Chitosan-based coatings “significantly inhibited the growth of E[scherichia] coli O157:H7, L[isteria] monocytogenes and S[almonella] enterica cocktails on whole cantaloupes during 14-day storage at ambient temperature (21°C),” the authors write. “Coatings also significantly reduced total mold and yeast counts on whole cantaloupes.”
Cantaloupes are particularly susceptible to microbial contamination because they grow on the ground and can therefore come into contact with foodborne-pathogens associated with polluted irrigation water, uncomposted manure, or animal droppings. They can also become contaminated during harvesting, handling, and preparation. The rough cantaloupe skin also allows bacteria to easily attach to the surface of the fruit.
“These pre- and post-harvest safety factors have directly or indirectly contributed to more than 25 outbreaks of foodborne illnesses associated with the consumption of cantaloupes between 1973 and 2003 in the United States and Canada,” the authors state.
Food preservation, safety, and quality maintenance therefore represent mounting concerns for the food industry. Antimicrobial food coatings represent just one form of technology that has been investigated as a tool to help improve food safety in various food types, including fresh produce and meats.
Chitosan, derived from the polysaccharide chitin, is known to have film-forming properties as well as antimicrobial activity. Chitosan-based coatings have also been shown to improve food safety—in fresh produce, for example.
With this in mind, Dr. Ma and colleagues conducted a study to investigate the efficacy of such antimicrobial coatings in reducing bacterial populations from the surface of cantaloupes. In addition to chitosan, they investigated two generally-recognized-as-safe antimicrobials—lauric arginate (LAE; which inhibits a broad spectrum of foodborne pathogens) and cinnamon oil (CO; an essential oil that has shown activity against Listeria species, gram negative bacteria, E. coli 0157:H7, and Salmonella species). They also investigated ethylenediaminetetraacetic acid (EDTA; an agent that chelates divalent calcium ions that are important to bacterial structures; it also enhances the activity of some antimicrobials).
The researchers coated the surface of whole cantaloupes with mixtures containing chitosan, LAE, CO, and EDTA, and then inoculated the fruits with foodborne pathogens: L. monocytogenes, S. enterica and E. coli O157:H7. After storing the cantaloupes at room temperature (21 °C), the researchers investigated changes in the number of bacteria on the surface of the fruit. They also assessed characteristics of the cantaloupes that reflect fruit quality.
During 14-day storage, “[c]hitosan coating with 0.1% LAE, 0.1% EDTA, and 1% CO was the most effective for inactivating foodborne pathogens inoculated on cantaloupes,” the authors write. S. enterica was particularly sensitive: “This coating caused a >3 log CFU/cm2 reduction of Escherichia coli O157:H7 and Listeria monocytogenes immediately after coating, and reduced Salmonella enterica to below the detection limit during a 14-day storage.”
In addition, they state that the antimicrobial coating reduced the levels of total molds and yeasts on the surface of cantaloupes to the detection limit, and delayed the appearance of visible molds. But visible mold growth was seen on the coated fruits after 4 days of storage. One possible reason for this is that “the coating may not totally block access to oxygen which is required for mold growth,” they note. Other strategies are therefore needed to effectively inhibit molds and yeasts on cantaloupes during their normal shelf life.
Dr. Ma and colleagues also found that the antimicrobial treatments—especially 0.1% LAE, 0.1% EDTA, and 1% CO—significantly slowed ripening of the cantaloupes during storage. After day 6, compared with uncoated fruits, coated cantaloupes were significantly more reddened, less yellowed, and more firm. Although the reason for delayed ripening was unclear, the authors suggest that the treatments may decrease respiration rate or enzyme activity, both of which are responsible for the ripening process. However, the treatments did not affect other quality-associated characteristics such as cantaloupe weight or total solids content.
“These observations suggest that these novel coating formulations have potential to improve the safety and quality of whole cantaloupes,” the authors conclude. “However, the specific role of the antimicrobials must still be elucidated.”
Dr. Parry graduated from the University of Liverpool, England in 1997 and is a board-certified veterinary pathologist. After 13 years working in academia, she founded Midwest Veterinary Pathology, LLC where she now works as a private consultant. She is passionate about veterinary education and serves on the Indiana Veterinary Medical Association’s Continuing Education Committee. She regularly writes continuing education articles for veterinary organizations and journals, and has also served on the American College of Veterinary Pathologists’ Examination Committee and Education Committee.