Tufts University biomedical engineers have demonstrated that fruits can stay fresh for more than a week without refrigeration if they are coated in an odorless, biocompatible silk solution so thin as to be virtually invisible. The approach is a promising alternative for preservation of perishable foods—half of which are lost during the food supply chain due mostly to premature deterioration, according to the Food and Agriculture Organization of the United Nations.

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

To investigate silk's use as a preservative, researchers led by Fiorenzo Omenetto, professor in the Department of Biomedical Engineering, dipped freshly picked strawberries in a solution of 1% silk fibroin protein; the coating process was repeated up to four times. The silk fibroin-coated fruits were then treated for varying amounts of time with water vapor under vacuum (water annealed) to create different percentages of crystalline beta sheets in the coating.

Perishable fruit could stay fresher with a coating of silk. Image credit: Pixabay.Perishable fruit could stay fresher with a coating of silk. Image credit: Pixabay.The strawberries were then stored at room temperature. Uncoated berries were compared over time with berries dipped in varying numbers of coats of silk that had been annealed for different periods of time. At 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," says Omenetto. "We saw a statistically significant delay in the decay of the fruit."

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," says fellow researcher Benedetto Marelli, formerly a post-doctoral associate in the Omenetto laboratory and now at MIT.

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