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Watermelon Juice As Feedstock Supplement, Diluent, and Nitrogen Supplement for Ethanol Production

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.

Resources

  • 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

Comments

Aureon Kwolek

Algae, Duckweed, and Watermelons:

Harvesting and shipping the scrap watermelons is a big factor in cost effectiveness. You would be shipping mostly water. ‘How about processing them Onsite or nearby?

Another possibility is growing heterotrophic algae on the sugar water, and then using the algae for the primary feedstock. That amount of sugar fed to algae would triple the biomass in less than 48 hours, under ideal conditions. You need a minimum sugar solution of only 1-2%. Heterotrophic algae is grown in dark tanks on a relatively small footprint, and it’s up to 1,000 times more concentrated than algae grown in sunlight.

The water component would be a good growth medium for the algae. Just add an NPK growth supplement or integrate chickens, to provide the fertilizer. Algae loves chicken poop. You can also feed the complete high protein algae component back to the chickens, maybe even the biomass residues leftover after you juice the melons. Or, you’ll have the cellulose residues to dispose of or convert. That could go into onsite methane digesters or gasifiers. Wet algae can now be efficiently gasified, without drying (GeniFuel). That gives you “synth gas”, a fuel feedstock, a natural gas replacement, or even an alternative to coal burning.

Onsite Duckweed is another fast growing feedstock. That’s also good for making ethanol, because it’s about 40% starch. Duckweed can also be grown heterotrophically on the watermelon juice. Again, the complete high protein component could be fed back to the poultry providing the manure to fertilize the duckweed. Researchers in North Carolina tested duckweed on hog manure and got 6,000 gallons of ethanol per acre per year, grown on sunlight. With ultra-concentrated heterotrophic duckweed living on watermelon juice and fertilized with poultry manure, you may get a lot higher production than that.

The idea of algae and duckweed grown on watermelon juice is worth testing. Chlorella is a good candidate, because it’s been know to double in less than 6 hours on less than 3% sugar water.

This concept is not limited to watermelons. Heterotrophic algae and duckweed can be grown on many forms of sugar – sugar beets, sweet sorghum, sugar cane, sugar palm juice, sugary waste, starches converted to sugar – what have you. Even cellulose converted to sugars and fed to heterotrophic algae and duckweed, would multiply the feedstock overnight – Instead of using the sugar itself as a direct biofuel feedstock.

For example, converting wood waste into sugars, and then feeding them to algae, would not only multiply the feedstock – It would also give you a much more versatile feedstock: oils for biodiesel, bio-plastics, omega 3 nutriceuticals and exotic medicines; digestible complete proteins for humans and animals; and carbs for ethanol or whatever. We may one day see our entire corn ethanol industry exploiting its corn sugar to triple or quadruple algae or duckweed, onsite – overnight, instead of fermenting corn sugar directly into ethanol.

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