Similar to fruit crops on the mainland, a sizable portion of the fruit grown in Hawaii never gets to the supermarket. A staggering 35% of the state’s multi-million-pound papaya harvest is culled from packinghouses due to reasons like disease and pest pressure or post-harvest damage.
Lisa Keith, a research plant pathologist with the USDA Agricultural Research Service in Hilo, Hawaii, sees potential in all that wasted papaya pulp. Keith is feeding the rejected fruit to a type of green algae called auxenochlorella protothecoides in the hopes that it will help move Hawaii closer to energy independence.
Instead of getting energy from sunlight, the algae are kept in the dark so they will consume sugars in the papaya instead. The algae grow and store 60% of their weight in lipids (or oils), which can be harvested to produce biodiesel.
Making biofuel from papaya-fed algae has a few advantages to other forms of alternative fuels. First, the feedstock — wasted food — is inexpensive and sustainable. Second, it doesn’t require much land. “The facilities to grow the algae could also be built up instead of over large expanses of land,” said Keith. “The current vision for papaya is to have the algae tanks and equipment at the packinghouses where the papaya culls are available.”
In Hawaii, an isolated state with high imported fuel prices, finding ways to be more energy independent is a necessity. Hawaii has the most aggressive renewable energy goals in the country, aiming for 100% of electricity coming from renewable sources by 2045, and there’s a push to extend that goal to ground transportation fuel by the same year.
Getting more Hawaii-made biofuel to market will help make that goal possible. The process Keith and her team is piloting needs fine-tuning in order to make papaya-fueled biodiesel production feasible, but they are working on making that happen. “Using papaya and algae as our model system, we are currently in the process of scaling up in terms of tank size and upstream and downstream processing capabilities,” said Keith. She hopes to have data that proves the concept is feasible within a couple of years.
Keith’s algae project has potential to work with other crops in Hawaii, like Okinawan sweet potato, cacao pulp and banana. “We are also looking into using additional oil-producing microorganisms,” said Keith.
Extracting the oil from the algae produces byproducts, but even those have uses, making the process truly zero-waste. The “algal meal” that’s leftover can be used for fish or livestock feed in Hawaii, which would provide a homegrown feed source and cut down on the costs of shipping feed in. A scaled-up papaya-to-fuel project could also provide papaya farmers with another source of revenue for fruit that now goes unsold.