Injecting veggies with sleep hormone can extend shelf life by up to 10 days

By Stephen Beech

Injecting vegetables with the sleep hormone melatonin can extend their shelf life by up to 10 days, according to new research.

American scientists used biodegradable microneedles to add the drug to freshly-cut crops and delay spoilage.

Researchers from the Massachusetts Institute of Technology (MIT) and the Singapore-MIT Alliance for Research and Technology (SMART) say the system can extend the shelf life of harvested plants.

It is estimated that, currently, more than 30% of the world’s food is lost after it’s harvested - enough to feed more than one billion people.

Refrigeration is the most common way to preserve food, but it requires energy and infrastructure that many regions of the world can’t afford or lack access to.

Melatonin is produced naturally in the brain to signal to the body that it is time to sleep.

In many countries, the hormone is classified as a drug and available by prescription only, but in the United States, it is available over the counter in child-friendly gummies.

The MIT team believes their system could offer an alternative or complement to refrigeration.

Central to their approach are patches of silk microneedles that can penetrate the tough, waxy skin of plants without causing a stress response, and deliver precise amounts of melatonin into the plants’ inner tissues.

Study senior author Professor Benedetto Marelli said: “This is the first time that we’ve been able to apply these microneedles to extend the shelf life of a fresh-cut crop.

“We thought we could use this technology to deliver something that could regulate or control the plant’s post-harvest physiology.

"Eventually, we looked at hormones, and melatonin is already used by plants to regulate such functions.

"The food we waste could feed about 1.6 billion people.

"Even in the US, this approach could one day expand access to healthy foods.”

For the study, published in the journal Nano Letters, Marelli and his team applied small patches of the microneedles containing melatonin to the base of the leafy vegetable pak choy.

Following application, the researchers found that melatonin was able to extend the vegetables’ shelf life by four days at room temperature and 10 days when refrigerated, which could allow more crops to reach the shops before they’re wasted.

Study co-senior author Dr. Sarojam Rajani, of the Temasek Life Sciences Laboratory, Singapore, said: “Post-harvest waste is a huge issue.

"This problem is extremely important in emerging markets around Africa and South East Asia, where many crops are produced but can't be maintained in the journey from farms to markets.”

Marelli’s lab has been exploring the use of silk microneedles for applications such as delivering nutrients to crops and monitoring plant health for several years.

The microneedles, made from silk fibroin protein, are non-toxic and biodegradable.

To test their ability to extend the shelf life of food, the researchers wanted to study their ability to deliver a hormone known to affect the ageing process.

As well as helping humans sleep, melatonin is also a natural hormone in many plants that helps them regulate growth and aging.

Marelli said: “The dose of melatonin we’re delivering is so low that it’s fully metabolised by the crops, so it would not significantly increase the amount of melatonin normally present in the food; we would not ingest more melatonin than usual.

“We chose pak choy because it's a very important crop in Asia, and also because pak choy is very perishable.”

Pak choy is usually harvested by cutting the leafy plant from the root system, exposing the shoot base that provides easy access to vascular bundles, which distribute water and nutrients to the rest of the plant.

The researchers first used their microneedles to inject a fluorescent dye into the base to confirm that the vasculature could spread the dye throughout the plant.

The team then compared the shelf life of regular pak choy plants and plants that had been sprayed with or dipped into melatonin, finding no difference.

Researchers applied small patches of the melatonin-filled microneedles to the bottom of pak choy plants by hand.

They then stored the treated plants, along with controls, in plastic boxes both at room temperature and under refrigeration.

The team evaluated the plants by monitoring their weight, visual appearance, and concentration of chlorophyll, a green pigment that decreases as plants age.

At room temperature, the leaves of the untreated control group began yellowing within two or three days.

By the fourth day, the yellowing accelerated to the point that the plants likely could not be sold.

But plants treated with the melatonin-loaded silk microneedles remained green on day five, and the yellowing process was "significantly" delayed.

The weight loss and chlorophyll reduction of treated plants also slowed "significantly" at room temperature.

Overall, the researchers estimated the microneedle-treated plants retained their saleable value until the eighth day.

Marelli said, “We clearly saw we could enhance the shelf life of pak choy without the cold chain."

In refrigerated conditions of about 40 degrees Fahrenheit, plant yellowing was delayed by about five days on average, with treated plants remaining relatively green until day 25.

Analysis indicated the treated plants had higher antioxidant activity, while gene analysis showed the melatonin set off a protective chain reaction inside the plants, preserving chlorophyll and adjusting hormones to slow the ageing process.

Co-first author Dr. Yangyang Han, of SMART, said: “We studied melatonin’s effects and saw it improves the stress response of the plant after it’s been cut, so it’s basically decreasing the stress that the plant experiences, and that extends its shelf life."

The researchers say more work is needed before microneedles are deployed at scale, such as by autonomous drones or other farming equipment.

Marelli said, “For this to be widely adopted, we’d need to reach a performance versus cost threshold to justify its use."

He added: “This method would need to become cheap enough to be used by farmers regularly.”

The research team plans to study the effects of other hormones on different crops using its microneedle delivery technology.