Researchers inject vegetables with melatonin using microneedles to extend shelf life

RESEARCHERS from the US and Singapore have developed a method to extend the shelf life of vegetables by injecting them with biodegradable microneedles containing the hormone melatonin.

The researchers, based at Massachusetts Institute of Technology (MIT) and the Singapore-MIT Alliance for Research and Technology, found their method was able to roughly double the unrefrigerated shelf life of the vegetable pak choi.

Melatonin is a natural hormone that delays growth and ageing in plants. In the study, the researchers boosted the pak choi’s melatonin concentration by inserting the chemical into patches of silk microneedles which could penetrate the plant’s skin without causing a stress response. They placed the patches at the bottom of the stem to allow the melatonin to spread up through the vegetable’s inner tissues.

Their results, published in Nano Letters last month, showed that the shelf life of pak choi at room temperature was roughly doubled to around eight days when fortified with melatonin microneedles. When refrigerated, the shelf life was extended by ten days, remaining “relatively green” after 25 days. They measured shelf life by monitoring the pak choi’s weight, visual appearance and concentration of chlorophyll – the pigment that gives plants their green colour, which decreases as plants age and wilt.

Benedetto Marelli, associate professor of civil and environmental engineering at MIT and the study’s research director, said: “We clearly saw we could enhance the shelf life of pak choi without the cold chain.”

Around 30% of the world’s food is lost after harvest – a figure the researchers hope their technology will help reduce. Marelli added: “The food we waste could feed about 1.6bn people. Even in the US, this approach could one day expand access to healthy foods.”

As well as naturally occurring in plants, melatonin is also a commonly prescribed drug to help people with insomnia and jetlag, although doctors warn against taking too much. However, Marelli said the amount they added to the pak choi did not significantly raise its concentration above the normal level.  

Originally developed for drug delivery in humans, microneedles have since been explored by Marelli’s group for enhancing crops – a method they believe is more efficient and environmentally friendly than conventional agrochemical techniques like pesticide spraying, which can pose risks to humans and wildlife. In a study in April, researchers injected iron into crops using microneedles to treat chlorosis, as well as using the technology to boost vitamin B12 in tomato plants. 

The postdoctoral researchers who led that study, Yunteng Cao at Yale University and Doyoon Kim at MIT, believe their microneedle design is easily scalable owing to its “simple” fabrication process. The microneedles are made by combining silk fibroin protein with a salt solution inside tiny, cone-shaped moulds. As water evaporates from the solution, the silk solidifies while the salt forms crystalline structures inside the moulds, which can then be removed to form a hollow needle into which the active chemical can be placed.

Marelli hopes that applying microneedles to the stalks of plants will one day become standard agriculture procedure. He said: “We’re going to continue to analyse how we can increase the impact this can have on the value and quality of crops. We’re also going to continue looking into scaling up the technology so this can be used in the field.”