A coating of silk keeps fruit fresh

Despite its delicate appearance, silk is one of the natural world’s toughest materials. Now, that strength is being harnessed to protect delicate produce such as berries and bananas to lengthen their shelf life.

Engineers from Tufts University in Massachusetts, USA, have demonstrated that fruits can stay fresh for more than a week without refrigeration if they are coated in an odourless, biocompatible silk solution so thin as to be virtually invisible.

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 Organization (FAO) of the United Nations. The use of silk coating offers a naturally derived and water-based alternative for the preservation of delicate foods.

Silk’s toughness stems from its unique crystalline structure. 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, published in Scientific Reports, researchers dipped freshly picked strawberries in a solution of 1% silk fibroin protein, repeating the coating process 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 longer the exposure, the higher the percentage of beta sheets and the more robust the fibroin coating. The coating was 27 to 35 µm thick.

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 and corresponding study author Fiorenzo G Omenetto, PhD.

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, odourless 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, PhD.

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