As part of an experimental project, Indian farmers spray mango trees with a hexanal-based solution, a natural compound that helps to preserve fruit.

The mango-saving molecule

In India, the mango is known as "the king of fruits", but it is also very fragile. 
June 29, 2017

Growers lose a third of their crop each year. However, the use of hexanal, a natural compound, could be a complete game changer.

It's the longest stretch of asphalt in India. National Highway 44 crosses the subcontinent from north to south over more than 3,700 km, treating motorists who travel its length to an incredible variety of landscapes. In the southern state of Tamil Nadu, it passes through Krishnagiri, a hotbed of production for what is known in these parts as "the king of fruits". The mango season is just starting in early April, but the stalls that line the highway already display fifteen varieties in multiple colours and shapes

The pale green tottapuri, known as kilimoku because of its curved tip ("parrot beak" in the Tamil language), represents 60% of the region's production and ends up in pulp factories. The same goes for the bengalura, which is small with a skin that often turns red. The banganapalli, golden yellow and rounder, is almost exclusively for export, like its orange-hued cousin alphonso, the most widespread variety in India due to its intense fragrance and very sweet taste. The dark green himanpasand, by far the largest, is produced in such small quantities that it is highly prized.

India is the leading producer of mangoes in the world, with a 40% market share, but unfortunately one third of the country’s mangoes are unfit for sale. Some fruits die on the tree, while others spoil due to inappropriate harvesting methods (trees are shaken and the fruit is damaged when it falls to the ground), storage (carelessly stacked), and transportation (mangoes are crowded or carelessly crammed in nylon bags, causing unnecessary damage). This represents a huge loss of earnings, which experts value at 2 trillion rupees (approximately 40 billion dollars).

This gave rise to the idea of training farmers with more rigorous practices and of harnessing the amazing properties of a compound naturally secreted by injured plants: hexanal. Hexanal has the characteristic odour of a freshly mown lawn or a cucumber that has been cut. It helps to keep the fruit on the tree longer so that it can grow larger, but also so that it can mature more slowly, which prolongs storage. Controlled on a molecular level thanks to the development of nanotechnologies, hexanal could work miracles in the near future.

Night scene at a wholesale market in Krishnagiri, where tonnes of mangoes are sold each day.
Bartay
Night scene at a wholesale market in Krishnagiri, where tonnes of mangoes are sold each day.

Hexanal is raising hope in the Tamil Nadu countryside, which currently produces more than 21 million tonnes of mangoes per year. The effects of this compound have been known for more than a century, but renewed interest by researchers was brought on by the centennial celebrations of the Tamil Nadu Agricultural University (TNAU), located in Coimbatore. "In 2009, the State gave us a billion rupees and we decided to invest this sum in future technologies," says Professor Kizhaeral Sevathapandian Subramanian, director of the nanotechnology department.

One year later a laboratory was opened and, in 2012, a partnership was formed between the TNAU, the Industrial Technology Institute of Sri Lanka, and the University of Guelph, in Ontario. The project is jointly funded by IDRC and Global Affairs Canada, through the Canadian International Food Security Research Fund.

"The use of hexanal is revolutionary and in the end it was logical that its first real application was in the world's largest producer market rather than in Canada, where there are fewer fruits," said biotechnology professor Jayasankar Subramanian from his office in Guelph, where he led the team of Canadian researchers collaborating with the TNAU.

Extended harvests

The sun has only been up for two hours in the Krishnagiri district, but the heat is already oppressive. On the outskirts of the village of Sappanipatti, two growers of around fifty years old are labouring, in shirts and longhis, around their mango trees. Udhayakumar and Varadharajan are brothers and have been working four hectares of land for about a decade. They chose to grow mangoes, "because rice and sugar cane were too water intensive".

On the Deccan plateau, which forms the southern cone of the subcontinent, water is so scarce that Tamil Nadu and its neighbouring state of Karnataka quarrel bitterly over the Kaveri River. Some districts are currently experiencing the worst drought of the last 140 years. Udhayakumar and Varadharajan resigned themselves to planting rows of banganapalli, 800 feet in total, and waiting patiently to start making money. "Mango trees only give their best starting in the seventh year, more than 400kg per year," they stress. The harvest lasts from April to June. "We start the season at 90 rupees per kilo ($1.86), then prices fall to 50, before jumping to 200, when the first of the monsoon rains start falling on us," they say.

This morning, the two brothers are overseeing a rather special operation. Two agricultural workers are copiously spraying the trees in their orchards until drops of whitish liquid fall from each mango. The initial spraying took place two weeks earlier and this second spraying, 15 days before harvest, is the ideal time to treat fruit with a hexanal-based solution known as EFF, short for Enhanced Freshness Formulation. "It will allow us to extend our harvest by three weeks and sell mangoes when the market is at the highest point," confide Udhayakumar and Varadharajan, who on average use a 10L bucket of water per tree, in which they dilute EFF to 2%.

Varadharajan, a grower in the state of Tamil Nadu, dips his mangoes into hexanal-distilled liquid.
Bartay
Varadharajan, a grower in the state of Tamil Nadu, dips his mangoes into hexanal-distilled liquid.

The two growers discovered this process last year when they attended a conference given by the TNAU. "EFF was developed in 2013 and, after organizing training sessions with 3,000 farms in southern India, we have reached the experimental phase," explains Chellappan Sekar, director of the social sciences department at the Tiruchirappalli Research Institute, affiliated with the TNAU.

Santhakumar, 67, was one of the first to agree to test hexanal four years ago. His 15 hectare farm is in Santhur, some ten kilometres from highway 44.

"After I sprayed the fruit on my trees, I noticed that the leaves were greener, meaning that they produce more chlorophyll and therefore more nutrients for the fruit," he marvels. The yield of each mango tree increased by 5 kg, 10% more than before. "EFF costs me 40 rupees per tree but, for each rupee invested, I earn four additional rupees," he explains.

His neighbour Madhavan, 69, is also a convert to hexanal: "Not only are my mangoes shinier, more colourful and sweet, but in addition, I can store them in my warehouse at room temperature for seven to 10 days longer without damaging them." In terms of earnings, Madhavan claims his are even higher. "For one rupee spent on EFF, my income increases by six rupees," he affirms.

A molecule that hates water

From laboratories to Indian plantations, the use of hexanal has taken a long road with several detours. "Awareness of the value of this molecule dates back to the 1970s, when we learned that hexanal could block the enzyme responsible for aging the skin of fruit, while creating a physical protection against pathogens," says professor Jayasankar Subramanian. Although French researchers managed to synthesize this aldehyde for the first time in 1907, it would take until 2007 to obtain the necessary patents for its use in Canada, the United States, and India. Even a few years more before interest was shown in its practical application. Hexanal being extremely volatile, it was still necessary to find a way of having it act upon fruit as quickly as possible.

Except that scientists came up against a major obstacle: hexanal doesn't like water. The only way to use it was to work at a nanometric level to trap the hydrophobic molecule inside a hydrophilic membrane to allow it to dilute in an aqueous solution. The formula finally developed by the Indians consisted of adding one part hexanal to10 parts ethanol and 10 parts Tween 20, a dispersant made of oleic acid and sorbitol. "Thus, we multiplied the effect of the hexanal and made it 24 times more potent than in its natural state," remarks Kizhaeral Sevathapandian Subramanian, walking the halls of the Coimbatore laboratory where other uses for hexanal are being explored, including dipping mangoes in a bath of EFF right after picking, for example.

Carefully gathered in a net, the fruit is washed in salt water, immersed in the solution for five minutes, then left to dry on burlap for about half an hour. They are placed head down to prevent the sap flowing from the tail from blemishing the fruit. Kizhaeral Sevathapandian Subramanian also shows a large, transparent box inside which researchers expose the mangoes to EFF vapour for an hour or two.

Another approach is "nano packaging", which would overcome the high volatility of hexanal in its liquid state. The Coimbatore laboratory has developed a fibre obtained from a polymer solution of hexanal sprayed in a chamber subjected to a magnetic field. "With 1g of fibre, you could connect two distant points over 2,000km," explains the professor to illustrate the enormous size of the area of contact with ambient air. The idea is to enclose a few grams of the fibre into little bags that are stuck inside the packing cases of mangoes destined for shipping. The hexanal then evaporates inside the box and slowly impregnates the fruit during shipping. The TNAU is also contemplating manufacturing hexanal tablets that would be used in mini-bags placed in boxes.

Kizhaeral Sevathapandian Subramanian is clear: all of these processes are safe. Although farm workers spraying EFF on the mango trees of Tamil Nadu wear face masks, it's "to protect them from the strong odour of the product," he affirms. Hexanal was declared harmless after successful testing on human cells, and on those of bees, green lacewings, and earthworms — among the animals most sensitive to nano elements. Several authorities — the Organization for Economic Co-operation and Development, the U.S. Food and Drug Administration, the Food Safety and Standards Authority of India, and India’s Central Insecticide Board — have approved its use. Furthermore, the product evaporates very quickly, so consumers have little chance of coming into contact with it. In Canada, for the time being, only manufacturers of chewing gum and candied fruit are authorized to use it. "To my knowledge, India and the countries that have joined its program — Sri Lanka, Kenya, Tanzania, and Trinidad and Tobago — are the first to experiment with it in agriculture," says Jayasankar Subramanian.

Women learn to use mangoes that are unfit for sale by cooking them. Juice, jam, fruit paste, chips, and chutneys: all of these will be sold at market, bringing in additional income for the families.
Bartay
Women learn to use mangoes that are unfit for sale by cooking them. Juice, jam, fruit paste, chips, and chutneys: all of these will be sold at market, bringing in additional income for the families.

The support of Tamil farmers could not, however, end there. With help from Myrada, a non-governmental organization, the TNAU also supports rural women's cooperatives. It teaches them how to make use of mangoes that are unfit for sale and shares recipes for juice, jam, fruit paste, chips, and other chutneys. All around Krishnagiri, in the villages of Solari, Moramadagu, and Alapatty, the wives of mango growers are learning these skills to increase their household income. "It's another way of reducing waste and it’s an intelligent complement to the TNAU's global initiative for producers," remarks social scientist Chellappan Sekar. Before we part he reveals two or three cooking tips, while Kizhaeral Sevathapandian Subramanian savours a mango before returning to his laboratory — as if to prove that hexanal pairs well with a love of good food.

The original French version of this article was published in the July-August 2017 issue of Québec Science.

 

Photo: Bartay