Eggs that would otherwise be wasted can be used as the base of an inexpensive coating to protect fruits and vegetables, according to Rice University researchers in Houston, Texas, US. Researchers found when the micron-thick coating was applied to produce by spraying or dipping, it showed a remarkable ability to resist rotting for an extended period, comparable to standard coatings like wax. The innovation that tackles food waste for both eggs and produce is a promising step to creating more bioprotective films on fruits and vegetables that reduce superfluous plastic or otherwise inedible packaging.

“Reducing food shortages in ways that don’t involve genetic modification, inedible coatings or chemical additives is important for sustainable living,” says Pulickel Ajayan, Materials Scientist at the Rice University’s Brown School of Engineering Lab. “The work is a remarkable combination of interdisciplinary efforts involving materials engineers, chemists and biotechnologists from multiple universities across the US.”

The coating relies on eggs that never reach the market. As the US produces more than 7 billion eggs a year and manufacturers reject 3 percent of them, the researchers estimate more than 200 million eggs end up in landfills.

Along with being edible, the multifunctional coating retards dehydration, provides antimicrobial protection and is largely impermeable both to water vapor to retard dehydration and to gas to prevent premature ripening. The coating is all-natural and washes off with water.

Egg whites, otherwise known as albumen, and yolks account for nearly 70 percent of the coating. Most of the rest consists of nanoscale cellulose extracted from wood, which serves as a barrier to water and keeps produce from shriveling. A small amount of curcumin is added for its antimicrobial powers, as well as some glycerol for additional elasticity.

Invisible packaging slows ripening process
Lab tests on dip-coated strawberries, avocados, bananas and other fruit showed they maintained their freshness far longer than uncoated produce. Compression tests showed coated fruit were significantly stiffer and more firm than uncoated and demonstrated the coating’s ability to keep water in the produce, slowing the ripening process.

An analysis of freestanding films of the coating showed it to be extremely flexible and able to resist cracking, which better protects the produce. Tests of the film’s tensile properties showed it to be just as tough as other products, including synthetic films used in packaging. Further tests proved the coating to be nontoxic and solubility tests showed the thicker-than-usual film is washable.

“If anyone is sensitive to the coating or has an egg allergy, they can easily eliminate it,” says undergraduate student Seohui Jung, a researcher on the study.

“Rinsing in water for a couple of minutes can completely disintegrate it,” affirms Ajayan. The researchers continue to refine the coating’s composition and are considering other source materials. 

“We chose egg proteins because there are lots of eggs wasted, but it doesn’t mean we can’t use others,” says co-corresponding author Muhammad Rahman, a research scientist in Ajayan’s Rice lab, who mentored and led the team.

Even before the impact of the new coronavirus, the world wasted a third of the food produced around the globe, the researchers note.

Jung noted the team is testing proteins that could be extracted from plants rather than animal produce to make coatings. The work by Rice undergraduate students Jung and Yufei Cui is detailed in the peer-reviewed journal Advanced Materials.

Earlier this year, Israel-based Sufresca announced the development of edible coatings that “eliminate fresh produce plastic packaging entirely and reduce food loss and waste.” The tailor-made edible formulations are recognized by the European Food Safety Authority (EFSA) as food additives (E Number) and by the US Food and Drug Administration (FDA) as Generally Recognized As Safe (GRAS). They can also be stored at ambient temperatures, do not require dilution and extend shelf life up to six months.