A study by researchers at the US Department of Agriculture (USDA) found that although watermelon juice would have to be concentrated 2.5- to 3-fold to serve as the sole feedstock for ethanol biofuel production, it could easily integrate with other more concentrated feedstocks—either as whole juice or as a waste stream from neutraceutical production—to serve as diluent, supplemental feedstock, and nitrogen supplement for the yeast.
Watermelon is worthy of consideration for use in ethanol production for two basic economic reasons, the team wrote in an open-access paper published in the journal Biotechnology for Biofuels. First, about 20% of each annual US watermelon crop is left in the field because of surface blemishes or because they are misshapen. That represents about 360,000 tonnes lost as a source of revenue to growers.
Second, the neutraceutical value of lycopene and L-citrulline obtained from watermelon is at a threshold whereby watermelon could serve as starting material to extract and manufacture these products. The waste stream of juice resulting from the processing could be used in ethanol production.
A watermelon is nominally 60% flesh, and about 90% of the flesh is juice that contains 7 to 10% (w/v) sugars; i.e., more than 50% of a watermelon is readily fermentable liquid.
There are three potential process benefits to integrating watermelon juice into ethanol production, the authors note:
- 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% (w/v) sugars before fermentation.
- The 7 to 10% (w/v) ready-to-ferment sugars in watermelon (glucose, fructose, and sucrose) would supplement the primary feedstock and reduce primary feedstock requirements proportional to the volume of watermelon juice used to dilute the concentrated feedstock.
- The free amino acids at 15 to 35 µmol/mL in watermelon juice could serve as a nitrogen supplement for yeast in those feedstocks such as cane sugar and molasses that possess inadequate available nitrogen levels to maintain maximal yeast growth and ethanol production.
The researchers concluded that the 8.4 t/ha of unmarketable watermelons left in the field at harvest would produce about 220 L/ha of ethanol for on-farm use or as an additional revenue stream for the grower.
Wayne W Fish, Benny D Bruton and Vincent M Russo (2009) Watermelon juice: a promising feedstock supplement, diluent, and nitrogen supplement for ethanol biofuel production. Biotechnology for Biofuels doi: 10.1186/1754-6834-2-18