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Clay-based antimicrobial packaging keeps food fresh

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Credit: Hayriye Ünal

WASHINGTON, Aug. 21, 2017 — Sometimes it seems as if fresh fruits, vegetables and meats go bad in the blink of an eye. Consumers are left feeling frustrated, often turning to less expensive processed foods that last longer but are less nutritious. Now scientists report that they have developed a packaging film coated with clay nanotubes packed with an antibacterial essential oil. The film provides a one-two punch, preventing over-ripening and microbial growth, which could help improve the shelf life of perishables.

The researchers are presenting their results at the 254th National Meeting & Exposition of the American Chemical Society (ACS). ACS, the world's largest scientific society, is holding the meeting here through Thursday. It features nearly 9,400 presentations on a wide range of science topics.

"Food packaging that is capable of interacting with food can contribute to safety and prevent economic losses from spoilage," Hayriye Ünal, Ph.D., says. "Specialized films that can preserve a wide array of foods are highly sought after."

People around the world have been trying to preserve fruits, vegetables and meats since ancient times. From the traditional methods of salting or fermenting to the more modern methods of canning, freezing or wrapping in plastic films, food preservation has always been important. According to the U.S. Department of Agriculture, about 30 to 40 percent of the food that farmers produce in the country goes to waste. Sometimes, busy people just don't have enough time to eat all of the fresh food they buy, but other times food spoils at the grocery store because consumers avoid purchasing imperfect fruits, vegetables and meats.

Over the past two decades, scientists have been developing alternative ways to preserve food, with an extensive focus on packaging. Numerous issues, primarily the versatility of the materials, have hindered progress. "While companies can already make many films that prevent fruit and vegetables from drying out, incorporating additional properties in the same film is a challenge," says Ünal, who is at Sabanci University in Turkey.

Bacterial contamination and permeability to both oxygen and water vapor are major issues that scientists are addressing. Another challenge is to prevent too much ethylene from building up around foods. Ethylene is a compound naturally released by fruits and vegetables that aids in the ripening process. But an excess of ethylene trapped underneath the packaging film can cause food to over ripen and rot.

To meet the demand for multi-functional packaging, Ünal's team started with a polyethylene film. To scavenge for ethylene and provide a gas barrier the group incorporated clay "halloysite nanotubes," which are small, hollow cylinders. The nanotubes prevent oxygen from entering the film, and prevent water vapor and other gases from escaping. In addition they keep ethylene from building up by absorbing it. The researchers loaded these nanotubes with a natural antibacterial essential oil found in thyme and oregano called carvacrol and coated the inner surface of the packaging film with the loaded nanotubes to kill microbes.

The team wrapped tomatoes, bananas and chicken in the film to test its effectiveness over varying amounts of time compared to foods wrapped in plain polyethylene. After 10 days, tomatoes wrapped with the new film were better preserved than the control vegetables. In addition, the new film helped bananas stay more firm and keep their vibrant yellow color after six days compared to the control fruit. And chicken enveloped with the experimental film and refrigerated for 24 hours showed significantly less bacterial growth than chicken in plain polyethylene.

But moving this technology to industry will require some additional work, Ünal says. As a next step toward that, her team will test the new film to make sure it is safe and nontoxic.

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A press conference on this topic will be held Tuesday, Aug. 22, at 1 p.m. Eastern time in the Walter E. Washington Convention Center. Reporters may check-in at the press center, Room 154A, or watch live on YouTube http://bit.ly/ACSLive_DC. To ask questions online, sign in with a Google account.

Ünal's work was funded by the Scientific and Technological Research Council of Turkey.

The American Chemical Society, the world's largest scientific society, is a not-for-profit organization chartered by the U.S. Congress. ACS is a global leader in providing access to chemistry-related information and research through its multiple databases, peer-reviewed journals and scientific conferences. ACS does not conduct research, but publishes and publicizes peer-reviewed scientific studies. Its main offices are in Washington, D.C., and Columbus, Ohio.

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Title
Halloysite nanotube/polyethylene nanocomposites as multifunctional active food packaging materials

Abstract
Active food packaging materials that are designed to interact with food can greatly contribute to food safety and prevent economic losses caused by spoilage of food products. Halloysite nanotubes/polyethylene (HNT/PE) nanocomposites will be presented as active food packaging materials that can release antibacterial agents to prevent bacterial spoilage and absorb ethylene gas secreted by fruits and vegetables to prevent spoilage caused by overripening. Halloysite nanotubes that are natural clay nanoparticles with hollow tubular structures were utilized as the active component enabling the antibacterial and ethylene scavenging behaviors. Antibacterial essential oil loaded HNTs were coated onto the surface polyethylene films resulting in sustained release of antibacterial agents and reduce the viability of A. hydrophila up to 90%. HNTs were furthermore shown to absorb ethylene gas with a capacity of up to 0.85wt% as determined by gravimetric analysis, which results in corresponding HNT/PE nanocomposite films with significantly higher ethylene scavenging capacity than neat PE films. Antibacterial and ethylene scavenging properties of HNT/PE nanocomposite films were also demonstrated on food samples. Growth of pathogenic bacteria on surfaces of chicken samples packaged with HNT/PE films were reduced compared to samples packaged with neat PE films. HNT/PE nanocomposite films were also shown to slow down the ripening of bananas and retain the hardness of tomatoes and strawberries for longer time than PE films.

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