Discarded papayas are being examined as part of a zero-waste system to produce algal oils for biodiesel. ARS photo: Peggy Greb

In Hilo, Hawaii, Agricultural Research Service (ARS) Plant Pathologist Lisa Keith is leading an effort to produce biodiesel using the green algae Auxenochlorella protothecoides (formerly Chlorella protothecoides).

She’s also using the pulp of discarded papayas — those deemed too blemished, malformed, or damaged to be sold for market. The system developed by Dr. Keith and her colleagues calls for growing the algae in bioreactors, where they 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,” explains Dr. Keith.

In the process, these industrious algae end up storing 60 percent of their cellular weight in lipids. These lipids, in turn, provide material for making biodiesel. The algal 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, according to the researchers. 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 papaya crop discarded because of defects.

It’s all part of what Dr. Keith and her 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 says.

Posted by Jan Suszkiw, Public Affairs Specialist, Agricultural Research Service in Research and Science