Pullulan is an edible, tasteless, naturally available polysaccharide used in breath fresheners and mints.
To produce the food-safe fibers, a scientist group creates a polymer known as pullulan.
The thought of translating the study from wound dressing to food packing was accepted in partnership with Philip Demokritou, the former co-Director of the Center for Nanotechnology and Nanotoxicology (NanoCenter) at Harvard’s Chan School. The NanoCenter is a collective initiative between Harvard and the NTU of Singapore. (the Nanyang Technological University)
“As it depended out, wound dressings have the same objective, in some methods, as food packaging — sustaining tissues, shielding them against bacteria and fungi, and maintaining moisture,” said Huibin Chang, a postdoctoral fellow at SEAS and first author of the paper.
Surprisingly, the new food packing method has its roots in battleground medicine. For more than a decade, Parker and his Disease Biophysics Group have been producing antimicrobial fibers for wound dressings. Their fiber manufacturing platform, known as Rotary Jet-Spinning (RJS), was explicitly invented.
RJS works like 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.
“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 processing to increase both the longevity and freshness of the food items and doing so in a sustainable model.”
The researchers dissolved the pullulan polymer in water and mixed it with various naturally derived antimicrobial agents, including thyme oil, nisin, and citric acid. The solution is 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 a 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 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 seven days on a lab bench, 90 percent of unwrapped avocados were rotten, while only 50 percent 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.