Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Harvard T.H. Chan School of Public Health have developed a biodegradable, antimicrobial food packaging system that can extend the shelf life of fresh foods and eliminate microbial contamination.
“One of the biggest challenges in the food supply is the distribution and viability of the food items themselves,” said Kit Parker, the Tarr Family Professor of Bioengineering and Applied Physics at SEAS and senior author of the paper. “We are harnessing advances in materials science and materials processing to increase both the longevity and freshness of the food items and doing so in a sustainable model.”
The new food packing system has its roots in battlefield medicine. For more than a decade, Parker and his Disease Biophysics Group have been developing antimicrobial fibers for wound dressings. Their fiber manufacturing platform, known as Rotary Jet-Spinning (RJS), was designed specifically for that purpose.
RJS works likes a cotton candy machine — a liquid polymer solution is loaded into a reservoir and pushed out through a tiny opening by centrifugal force as the device spins. As the solution leaves the reservoir, the solvent evaporates, and the polymers solidify to form fibers, with controlled diameters ranging from microscale to nanoscale.
The idea to translate the research from wound dressing to food packing was born of a collaboration with Philip Demokritou, the former co-Director of the Center for Nanotechnology and Nanotoxicology (NanoCenter) at Harvard’s Chan School. The NanoCenter is a joint initiative between Harvard and Nanyang Technological University of Singapore.
“As it turned out, wound dressings have the same purpose, in some ways, as food packaging — sustaining tissues, protecting them against bacteria and fungi, and controlling moisture,” said Huibin Chang, a postdoctoral fellow at SEAS and first author of the paper.
To make the fibers food-safe, the team turned to a polymer known as pullulan. Pullulan is an edible, tasteless and naturally occurring polysaccharide commonly used in breath fresheners and mints.
The researchers dissolved the pullulan polymer in water and mixed it with range of naturally derived antimicrobial agents, including thyme oil, nisin and citric acid. The solution is then spun in an RJS system and the fibers are deposited directly on a food item. The researchers demonstrated the technique by wrapping an avocado with pullulan fibers. The result resembles a fruit wrapped in spiderweb.
The research team compared their RJS wrapping to standard aluminum foil and found a substantial reduction of contamination by microorganisms, including E.coli, L. innocua (which causes listeria), and A. fumigatus (which can cause disease in people who are immunocompromised).
“The high surface-to-volume ratio of the coating makes it much easier to kill dangerous bacteria because more bacteria are coming into contact with the antimicrobial agents than in traditional packaging,” said John Zimmerman, a postdoctoral fellow at SEAS and co-author of the paper.
The team also demonstrated that their fiber wrapping increased the shelf life of avocado, a notoriously finicky fruit that can turn from ripe to rotten in a matter of hours. After 7 days on a lab bench, 90% of unwrapped avocados were rotten while only 50% of avocados wrapped in antimicrobial pullulan fibers rotted.
The wrapping is also water soluble and biodegradable, rinsing off without any residue on the avocado surface.