Researchers use papaya-fed algae to produce biodiesel

US researchers are examining discarded papayas as part of a zero-waste system to produce algal oils for biodiesel.

In Hilo, Hawaii, Agricultural Research Service (ARS) plant pathologist Lisa Keith is leading an effort to produce biodiesel using a type of green algae known as Auxenochlorella protothecoides (formerly Chlorella protothecoides).

She is also using the pulp of discarded papayas, namely, those deemed too blemished, malformed, or damaged to be sold for market.

That’s where the science and technology part comes in—along with support from the state government of Hawaii, which hopes to ease the state’s reliance on imports of petroleum-based oil.

The system devised by Keith and her colleagues calls for growing the algae in giant vats, called “bioreactors.” There, hidden from sunlight, the algae are fed what might be likened to a papaya smoothie.

“While nearly all algae are capable of using energy from light to produce organic molecules from carbon dioxide and water, some algae, including Auxenochlorella protothecoides, can also absorb organic molecules such as sugars from sources such as papaya juice,” said Keith. Keith works with ARS’s Daniel K. Inouye, who is based at the US Pacific Basin Agricultural Research Center in Hilo.

In the process, these industrious algae end up storing 60% of their cellular weight in lipids. These lipids (or oils), in turn, provide material for making biodiesel.

Keith’s team thinks the algae’s remains from the oil-extraction process, called “algal meal,” can offer Hawaiian farmers a low-cost source of feed for fish or livestock, cutting importation costs.  The algae’s fondness for papaya also could offer a way for growers to recoup some of the losses on about one-third of Hawaii’s $11-million (€9.29 million) papaya crop discarded because of defects, according to the Agricultural Research Service.

It’s all part of what Keith and colleagues call a “zero-waste” system.

“The goal is to make agriculture more profitable and to address food- and energy-security issues in Hawaii,” she concluded.

This story was written by Liz Gyekye, editor of Biofuels International.

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