Silk coating can keep fruit fresh without refrigeration

New York

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 stabilise and protect other materials while being fully biocompatible and biodegradable.

For the study, the researchers dipped freshly picked strawberries in a solution of one percent 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 vapour under vacuum (water annealed) to create varying percentages of crystalline beta-sheets in the coating.

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 discoloured.

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 study author Fiorenzo Omenetto, professor at Tufts University in Massachusetts, US.

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 findings were published in the journal Scientific Reports.

Half of the world's fruit and vegetable crops are lost during the food supply chain, due mostly to premature deterioration of these perishable foods, according to the Food and Agriculture Organisation (FAO) of the United Nations.