Silky fresh fruit: is silk the secret to fresh fruit?

As the planet’s population continues to grow we need to think how we will feed everyone into the future. That means we need to think about how we will produce our food and how we will then use that food. According to the United Nation’s Food and Agriculture Organisation half of the world’s food supply crops are lost during the food supply chain and that is mostly due to deterioration of perishable food. Of course the best and most desirable option is to eat locally produced fresh food but given that this is not always possible for all people how can we keep food fresh for transportation without refrigeration and the loss of nutrients that the cooling initiates? The answer, according to a new study, may lie in silk.

Silk is a remarkable material, it has a unique crystalline structure that makes it incredibly tough yet flexible at the same time.

Silk is a remarkable material, it has a unique crystalline structure that makes it incredibly tough yet flexible at the same time. Silks is made by silkworms which are caterpillars of (usually) the Bombyx mori moth. During its three to eight day pupating period, the silkworm secretes fibroin, a sticky liquid protein, from its two special salivary glands. Pushed through a spinneret (opening on the mouth), the twin pair of continuous threads harden when they come into contact with the air. Next, the silkworm secretes sericin, a bonding agent, from two other glands to hold the two filaments together. While constructing its cocoon, the silkworm will twist in a figure-8 motion about 300,000 times and produce around one kilometer of filament. It is the fibroin from silk that biomedical engineers have latched onto as a way to preserve fruit.

For the new study researchers dipped freshly picked strawberries in a solution that was one per cent silk fibroin protein. This coating process was repeated up to four times and then the strawberries were treated with water vapour to create varying degrees of crystalline sheets in the coating. In the end the silk protein coating was between 27 to 35 microns thick (a micron is one thousandth of a millimetre).

After the coating was applied the strawberries were stored at room temperature. After seven days the strawberries coated with the edible silk were still juicy and firm while the uncoated strawberries were dehydrated and discoloured.

A similar experiment was conducted with bananas which are different to strawberries in that they can ripen after being picked. The silk coating was found to slow the ripening rate of the bananas and to make them firmer by preventing softening of the peel.

It all comes about because the silk coating makes the fruit less permeable to carbon dioxide and oxygen therefore significantly slowing decay. It might not be the ideal fresh food scenario but if it maintains nutrient levels it could be a step in the right direction.