Silky sheen keeps fruit firm and fresh

Fruit dipped in edible silk protein stayed juicy and firm for more than a week without refrigeration, new research shows.

The odourless, virtually invisible silk layer, which staves off decay, can be applied to delicate strawberries as well as ripening bananas.

A team of scientists in the US led by Benedetto Marelli at Tufts University (now at Massachusetts Institute of Technology) published their two-step technique in Scientific Reports.

Rotting fruit is a huge problem worldwide. The United Nations Food and Agriculture Organisation estimates around half the world’s fruit and vegetable crops are lost in the supply chain, with most due to rot.

Applying edible coatings to fruit and vegetables, then, is an attractive preservation technique – as long as it doesn’t spoil the food’s texture or taste, that is. So Marelli and his colleagues turned to fibroin – an insoluble protein found in silk that not only protects other materials but is also completely biodegradable.

They chopped up silk from silkworm cocoons and made a solution of 1% silk fibroin protein. They then dipped fresh strawberries in the mixture up to four times.

Some of the coated fruits were “water-annealed” – that is, placed in a vacuum with water vapour which crystallises the fibroin into its tough structure.

The longer the annealing, the more robust the coating. No annealing produced a layer comprising 23% fibroin sheets; 12 hours of annealing produced 58% fibroin sheets.

The strawberries were stored at room temperature. After seven days, the best-looking fruit by far were those dipped and annealed. They were still plump, red and juicy while those uncoated, or coated but not annealed, shrivelled and browned.

The experiment was repeated on bananas.

When bananas become overripe, they turn brown and mushy. But the silk fibroin coating slowed the bananas’ ripening rate, kept the skin relatively yellow and the flesh firm and cream-coloured, even after nine days.

So what magical anti-rot power does silk fibroin bestow?

By creating a barrier that blocks gases such as oxygen, water vapour and carbon dioxide, cells in the fruit drop their respiration rate. More respiration means higher metabolism which, in turn, means more decay.

But by blocking gases that contribute to respiration, the fibroin coating dropped the rate of rot.