Tufts University researchers in the United States have found a biocompatible silk coating can help fruits can stay fresh for more than a week without refrigeration.

In a release, the university said the coating was odorless and so thin that it was virtually invisible.

It said the approach was a promising alternative for the preservation of delicate foods using a naturally derived material and a water-based manufacturing process.

The work was reported in the May 6 issue of Scientific Reports.

Silk's unique crystalline structure makes it one of nature's toughest materials. Fibroin, an insoluble protein found in silk, has a remarkable ability to stabilize and protect other materials while being fully biocompatible and biodegradable.

For the study, researchers dipped freshly picked strawberries in a solution of 1% silk fibroin protein, with the process repeated four times before the fruit was then treated with water vapor under a vacuum.

The biomedical engineers found the longer the exposure, the higher the percentage of beta-sheets and the more robust the fibroin coating. The coating was 27 to 35 microns thick.

And then the real test - the strawberries were then stored at room temperature, and compared with uncoated strawberries.

After seven days, the berries coated with the higher beta-sheet silk were still juicy and firm while the uncoated berries were dehydrated and discolored.

Tests showed that the silk coating prolonged the freshness of the fruits by slowing fruit respiration, extending fruit firmness and preventing decay.

"The beta-sheet content of the edible silk fibroin coatings made the strawberries less permeable to carbon dioxide and oxygen. We saw a statistically significant delay in the decay of the fruit," said senior and corresponding study author Fiorenzo G. Omenetto, Ph.D.

Similar experiments were performed on bananas, which, unlike strawberries, are able to ripen after they are harvested. The silk coating decreased the bananas' ripening rate compared with uncoated controls and added firmness to the fruit by preventing softening of the peel.

The thin, odorless silk coating did not affect fruit texture.  Taste was not studied.

"Various therapeutic agents could be easily added to the water-based silk solution used for the coatings, so we could potentially both preserve and add therapeutic function to consumable goods without the need for complex chemistries," said the study's first author, Benedetto Marelli, Ph.D.

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