Manuela Callari
Scientists have harnessed the versatile properties of silk proteins to create edible coatings that shield against spoilage and reduce food waste and energy consumption.
Every few months, researchers at Tufts University’s SilkLab in Boston take off their labcoats to don a chef’s hat and step into a culinary battleground. Professor Fiorenzo Omenetto, principal investigator and director of the SilkLab – and a food enthusiast – orchestrates the intriguing event where silk takes centre stage as the star ingredient.
We are not talking about organza and chiffon, but the protein silk is made from.
In one such contest, Associate Professor Benedetto Marelli, a former postdoctoral associate at SilkLab, attempted to recreate chocolate-dipped strawberries, replacing chocolate with silk. The audacious goal was to engineer intricate diffraction gratings on the strawberry surface using silk’s self-assembly properties. The desired outcome was a visual effect akin to Pink Floyd’s “The Dark Side of the Moon” album cover, where a strawberry would diffract light into several beams, much like a prism.
The result fell short of Marelli’s expectations. The thin, transparent silk coating did not produce the anticipated visual effect.
Disheartened, Marelli left his strawberries on his bench and forgot about them; little did he know, his culinary experiment would take an unexpected turn. A week later, he noticed that the silk-coated strawberries resisted decay and maintained freshness. Meanwhile, the uncoated berries were mouldy and spoiled. What began as an unconventional culinary experiment has evolved into an innovation of significant impact – an edible silk coating that can extend the shelf life of perishable foods, saving food waste and energy consumption.
Silk is an abundant, natural fibre produced by the mulberry silkworm (Bombyx mori) a domesticated moth that feeds on mulberry leaves. When silkworms spin their cocoons, they make a protein called fibroin from specialized glands. As the fibroin is extruded, it solidifies and forms the basis of the cocoon’s structure, which protects the silkworm as it undergoes its metamorphic process. This cream-coloured, black-striped caterpillar can produce up to 1km of silk in its lifetime.
The textile industry has used mulberry silkworms for millennia to craft luxurious fabrics. Once the silkworms have spun their cocoons, they are harvested, boiled, and unwound. The silk fibres are then reeled into a continuous thread, ready to weave glamorous textiles.
About two decades ago, scientists found a way to reverse-engineer the cocoon-making process to obtain a solution of water and proteins.
“The same [building blocks] the caterpillar has in its guts,” says Omenetto.
Fibroin has remarkable strength and durability, but what makes it especially interesting is its unique ability to be spun into a variety of different fabrics and materials. It can fold into various forms, like a crumpled string or a flat sheet. It can produce materials that dissolve in water and materials that can be stable in water for years. It is biocompatible, edible, and possesses optical properties. These features depend on how the fibroin molecules arrange in space and interact with one another.
“We have worked with every biopolymer under the sun. Silk still turns out to be very much one of the most versatile biopolymers,” Omenetto says.
Now an associate professor at MIT, Marelli is studying how to recover silk proteins from textile discards and re-assemble them into clear coatings around complex objects.
“Silk is a commodity. There is already a large production of silk, and we can easily tap into the waste of the silk industry to upcycle it and make a technical material,” he says.
Fibroin molecules dissolve in water, and through techniques like spray drying and dip coating, commonly used in the agro-food industry, they effectively adhere to surfaces of various shapes and can be applied to a wide range of food products, including fruits, vegetables, meats, and dairy products.
There is already a large production of silk, and we can easily tap into the waste of the silk industry to upcycle it and make a technical material.
Benedetto Marelli
The silk fibroin layer is imperceptible to sight and taste, yet it acts as a barrier that prevents oxygen and moisture from reaching the food, keeping it fresher for longer. It also shields against harmful microorganisms that typically cause spoilage. This means that food products can last much longer without the need for refrigeration or other preservation methods.
Silk coating has several advantages over traditional food preservation methods, such as preservatives, wax coatings, or plastic wraps. It is an entirely natural and safe method that does not rely on chemical additives or preservatives. Unlike packaging, it doesn’t produce waste. The production process only requires water, sodium carbonate, and a little energy.
“The processing to obtain fibroin is very benign, with no toxic agent whatsoever used at any point,” explains Omenetto. “[The coating] is a very inert, thin layer that you don’t even know it’s there.”
Food waste is a significant problem today, with millions of tons of food discarded daily. This waste harms the environment, affects global food security, and contributes to economic losses. According to a 2019 report from the Food and Agriculture Organisation of the United Nations (FAO), environmental factors and microbial agents like bacteria, mould, and yeast contribute to substantial food losses, with around 14% of the world’s food, valued at $400 billion, lost annually before it even reaches the market.
Consumers and retailers waste an additional 17%. This wastage deprives millions of people of sustenance and contributes to global greenhouse gas emissions. FAO estimates that the lost and wasted food could feed 1.26 billion hungry people yearly, accounting for up to 10% of global greenhouse gas emissions. Meanwhile, the number of people affected by hunger is rising, with an estimated 3.1 billion people who do not have access to a healthy diet.
The edible silk coating technology holds the “generally recognised as safe” designation in the United States and has acquired “non-novel” food status from Health Canada. It also aligns with the historical consumption of Bombyx mori in many Asian countries.
“We have been able to design edible coatings to extend the shelf life of foods that are now a product in the United States. While the use of silk as an edible coating has been approved in 12 countries and covers more than 1 billion people in terms of approval,” Marelli says.
In 2018, Mori, a US-based company built upon the intellectual property developed through Marelli’s research, spun off the SilkLab. Mori’s silk-coated baby spinach and kale are sold in US supermarkets today.
We need pragmatic solutions to help tangibly manage extreme events and reduce our carbon footprint.
Fiorenzo Omenetto
Beyond extending shelf life and reducing food waste, the technology boasts energy-saving benefits. With decreased reliance on refrigeration, it could also play a pivotal role in disaster response scenarios and areas with limited access to power, such as in a climate event or a conflict zone.
“Just having a couple of drums of solution can give you an extra week of food resilience,” says Omenetto. “The world is in a bit of trouble, so we need pragmatic solutions to help tangibly manage extreme events and reduce our carbon footprint.”
Marelli and his team are now looking at developing silk seed coatings that can encapsulate, preserve and deliver nitrogen and phosphate-based fertilisers.
“There are not going to be enough fertilisers to feed the plants,” says Marelli. These coatings can also encapsulate biofertilizers, such as bacteria that act as probiotics for plants: “…these microbes can promote plant health when there are abiotic stresses like heat, drought, and salinity, which are a big problem in a warming climate.”
