Whether it be husky coconut shells or slippery mango seeds, between 24 million and 31 million metric tons (MT) of endocarp biomass is created every year, but most is thrown out as waste. Researchers from three U.S. universities - Kentucky,  California and Massachusetts - have found this waste could be an untapped resource for the 1.5 billion people in this world who cannot meet their basic energy needs. At www.freshfruitportal.com we speak with team leader and University of Kentucky associate professor Seth DeBolt about the study and what it could mean for the horticultural industry.

Photo: National Mango Board

Having grown up on a farm in Australia's northern New South Wales, DeBolt says he has always known that plants and how they work was critical to human existence.

However, the recent study he has been involved in shows this on an ever greater level than food, flower and seed production, under the heading "Global bioenergy potential from high-lignin agricultural residue", published in the Proceedings of National Academy of Sciences.

His co-authors were Venugopal Mendu, Tom Shearin, Elliott Campbell Jnr, Jozsef Stork, Junghao Jae, Mark Crocker and George Huber.

"The main thing that spurred the study was that lignin is inedible and it can't be converted into a food product for ruminant livestock as a lot of cellulosic products can; in terms of the nuts industry it’s usually processed at a centralized facility like an almond processor, so there is a reasonably large supply over a pretty constant stream over and that makes an ideal bioelectricity source," says DeBolt.

"We wanted to know where fruits with drupe endocarp [hardened inedible fruit matter such as shells, seeds and stones) were growing and how much was out there, and then apply that to something like a gasifier which is ideally a de-centralized electricity production source.

"You’re not acting with the grid there, you’re making your own electricity, so that can be ideal if you’ve got a big enough cooperative and you’re going to make enough fruit waste to end up putting into that. You’d have to run your numbers pretty carefully though."

Using Food and Agricultural Organization (FAO) data, the group mapped out yields of worldwide endocarp production, finding hotspots in California, Spain, Morrocco, South Asia and South East Asia. DeBolt adds that some of the FAO data may not have been up-to-date or reportedly correctly, particularly in the case of China, so there could be other hotspots that weren't mentioned.

The report found that mangoes and coconuts accounted for 72% of total drupe endocarp, while other horticultural crops with high drupe endocarp material were also included in the study such as almonds, apricots, cherries, olives, peaches, pistachios, plums and walnuts.

Coconuts were found to have the highest maximum level of energy per kilogram at 22.8 megajoules, followed by cherries (22.5Mj/kg), olives (21.8Mj/kg), and apricots and plums (21.1Mj/kg).

Where and how the findings can be applied

While the group had the information about which crops yielded the most energy and accounted for the most global production of endocarp, there was also the issue of practicality.

DeBolt says effective bioenergy generation needs a reliable and large source of endocarp-based crops in a centralized area, where there was a clear need to more affordable energy.

"One of the interesting findings of the study was that when we overlayed where the major commodities were - it was mostly mangoes and coconuts but there were a lot of olives, almonds and stonefruit in general - they were very localized in production," he says.

"But they're often in areas where there’s already fairly centralized energy production; Europe, U.S., Australia, New Zealand, it’s not sort of areas where you have energy crises."

Endocarp Map. Source: Proceedings of National Academy of Sciences

Based on these reasons, the report places a strong focus on the potential for bioenergy in energy poor Asian countries.

"Endocarp biomass used in small-scale decentralized gasification systems (14-50% efficiency) could contribute to the total energy requirement of several countries, the highest being Sri Lanka (8-30%) followed by Philippines (7-25%), Indonesia (4-13%), and India (1-3%)," the report says.

"While representing a modest gain in global energy resources, mitigating energy poverty via decentralized renewable energy sources is proposed for rural communities in developing countries, where the greatest disparity between societal allowances exist.

"The maximum density of coconut endocarp production was observed across developing countries in South Asia, focused in the south of the Indian subcontinent, Sri Lanka, Bangladesh, Philippines, Myanmar, Thailand, Malaysia, Vietnam, Laos and Indonesia."

The report found that minor production potential existed in tropical Africa, Central America and South America, but with more "discreet and spacially constrained production zones".

The other practical issue is how the endocarp is going to be taken from the fruit and processed for mass production. The fact itself means that bioenergy generation may be difficult for many fruits that are sold fresh to consumers.

"When there’s waste the endocarp is rarely used but it is best that the product sold doesn't come with it, because you’re looking at post-consumption recycling, which is more difficult.

"You’re mainly looking at the processing of commodities, so that’s commonly with nuts, coconuts, canned peaches for example, and it has to be centralized.

"In the wartime New York used endocarp material from post-consumption recycling to make gas masks, and that was successful, but I think if you’re going to get something like this to work you need a well-informed population involved."

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