Renewable energy from watermelons

Photo: Steve EvansWatermelon juice can be a valuable source of biofuel. Researchers writing in BioMed Central's open access journal Biotechnology for Biofuels have shown that the juice of reject watermelons can be efficiently fermented into ethanol.

Wayne Fish worked with a team of researchers at the USDA-Agricultural Research Service's South Central Agricultural Research Laboratory in Lane, Oklahoma, US, to evaluate the biofuel potential of juice from 'cull' watermelons, those not sold due to cosmetic imperfections, and currently ploughed back into the field.

He said, "About 20 per cent of each annual watermelon crop is left in the field because of surface blemishes or because they are misshapen. We've shown that the juice of these melons is a source of readily fermentable sugars, representing a heretofore untapped feedstock for ethanol biofuel production".

As well as using the juice for ethanol production, either directly or as a diluent for other biofuel crops, Fish suggests that it can be a source of lycopene and L-citrulline, two 'nutraeuticals' for which enough demand currently exists to make extraction economically worthwhile. After these compounds have been removed from the 'cull' juice, it can still be fermented into ethanol.

The researchers conclude, "At a production ratio of ~0.4 g ethanol/g sugar, as measured in this study, approximately 220 L/ha of ethanol would be produced from cull watermelons".

Although watermelon juice would have to be concentrated 2.5- to 3-fold to serve as the sole feedstock for ethanol biofuel production, the results of this investigation indicate that watermelon juice, either as whole juice fermented on-site or as a waste stream from neutraceutical production, could easily integrate with other more concentrated feedstocks where it could serve as diluent, supplemental feedstock, and nitrogen supplement.

A watermelon is nominally 60 per cent flesh, and about 90 per cent of the flesh is juice that contains 7 to 10 per cent (w/v) sugars. Thus, over 50 per cent of a watermelon is readily fermentable liquid.

A logical use for a watermelon juice processing waste stream would be integration into bioethanol production. Since it is water-based, watermelon juice could serve as a diluent for concentrated sources of fermentable sugars such as molasses that require an approximate dilution to ~25 per cent (w/v) sugars before fermentation.