We’ve all experienced the prick of remorse when fruits and vegetables spoil before we have a chance to consume them. Recently, investigators from MIT and the Singapore-MIT Alliance for Research and Technology (SMART) have demonstrated that they can prolong the freshness of harvested produce by injecting melatonin using compostable microneedles.

This is significant because the challenge of food waste extends far beyond our salads. Over 30 percent of global food is lost after harvesting — sufficient to nourish more than 1 billion individuals. Refrigeration remains the most prevalent method for food preservation, yet it demands energy and infrastructure that many areas around the world either cannot afford or lack access to.

The researchers believe their technique could serve as an alternative or supplement to refrigeration. At the core of their method are patches of silk microneedles. These microneedles can penetrate the resilient, waxy surface of plants without triggering a stress reaction and can deliver exact quantities of melatonin into the inner tissues of the plants.

“This is the inaugural instance in which we’ve been able to utilize these microneedles to prolong the freshness of a fresh-cut crop,” remarks Benedetto Marelli, the lead author of the study, associate professor of civil and environmental engineering at MIT, and director of the Wild Cards mission of the MIT Climate Project. “We aimed to harness this technology to introduce something that could regulate or influence the plant’s post-harvest physiology. Ultimately, we considered hormones, and melatonin is already employed by plants for similar regulatory functions. The food we discard could sustain around 1.6 billion people. Even within the U.S., this technique could one day enhance access to nutritious foods.”

For the study, which is published today in Nano Letters, Marelli and colleagues from SMART applied small patches of microneedles infused with melatonin to the base of the leafy vegetable pak choy. Following this application, the researchers discovered that melatonin could extend the vegetables’ shelf life by four days at room temperature and 10 days under refrigeration, potentially enabling more crops to reach consumers before they spoil.

“Post-harvest waste is a substantial issue. This challenge is critically important in developing markets throughout Africa and Southeast Asia, where numerous crops are produced but struggle to be preserved during their transit from farms to markets,” explains Sarojam Rajani, co-senior author of the study and a senior principal investigator at the Temasek Life Sciences Laboratory in Singapore.

Plant Stress Relievers

For several years, Marelli’s laboratory has been investigating the use of silk microneedles for purposes such as nutrient delivery to crops and assessing plant health. Microneedles constructed from silk fibroin protein are non-toxic and biodegradable; Marelli’s previous research has detailed methods for large-scale production of these microneedles.

To evaluate the microneedles’ efficacy in prolonging the shelf life of food, the researchers aimed to examine their capability to transport a hormone shown to influence the aging process in plants. Besides aiding human sleep, melatonin is also a natural hormone in various plants that assists them in managing growth and aging.

“The level of melatonin we’re administering is so minimal that it’s fully metabolized by the crops, meaning it won’t substantially increase the amount of melatonin typically found in the food; thus, we would not ingest significantly more melatonin than usual,” says Marelli. “We selected pak choy as it is a crucial crop in Asia and is also highly perishable.”

Pak choy is conventionally harvested by severing the leafy plant from its root system, exposing the shoot base that allows easy access to the vascular bundles responsible for distributing water and nutrients throughout the plant. To initiate their study, the researchers first utilized their microneedles to inject a fluorescent dye into the base to verify that the vascular system could distribute the dye throughout the plant.

The team then compared the shelf life of standard pak choy plants with those that had been treated with melatonin via spraying or immersion, discovering no notable differences.

With the baseline shelf life established, the researchers manually applied small patches of melatonin-filled microneedles to the bottom of pak choy plants. They subsequently stored the treated plants, alongside controls, in plastic containers both at ambient temperature and under refrigeration.

The researchers assessed the plants by tracking their weight, visual quality, and the concentration of chlorophyll, a green pigment that diminishes as plants age.

At room temperature, the untreated control group’s leaves began to yellow within two to three days. By the fourth day, the yellowing accelerated to a point where the plants were likely unsellable. In contrast, the plants treated with melatonin-infused silk microneedles remained green on day five, with the yellowing process significantly postponed. Weight loss and chlorophyll reduction in the treated plants also slowed considerably at room temperature. Overall, the researchers estimated that the microneedle-treated plants retained their marketable quality until day eight.

“We clearly observed that we could extend the shelf life of pak choy without requiring refrigeration,” notes Marelli.

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

“Spectrophotometric analysis revealed that the treated plants exhibited higher antioxidant activity, while gene analysis indicated that the melatonin triggered a protective chain reaction within the plants, safeguarding chlorophyll and modifying hormones to slow down aging,” explains Monika Jangir, co-first author and former postdoctoral researcher at the Temasek Life Sciences Laboratory.

“We examined melatonin’s impacts and found it enhances the plant’s stress response after being cut, effectively reducing the stress the plant endures, thereby extending its freshness,” states Yangyang Han, co-first author and research scientist at the Disruptive and Sustainable Technologies for Agricultural Precision (DiSTAP) interdisciplinary research group at SMART.

Toward Postharvest Preservation

While the microneedles may allow for a reduction in waste compared to other application methods like spraying or dipping crops, the researchers indicate that additional work is essential to implement microneedles on a larger scale. For example, although the researchers applied the microneedle patches manually in this experiment, in the future, these patches could be applied using tractors, autonomous drones, and other agricultural machinery.

This research was backed by the Singapore-MIT Alliance for Research and Technology (SMART) and the National Research Foundation of Singapore.