Background: Two economic factors make watermelon worthy of consideration as a feedstock for ethanol biofuel production. First, about 20% of each annual watermelon crop is left in the field because of surface blemishes or because they are misshapen; currently these are lost to growers as a source of revenue. 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. Processing of watermelons to produce lycopene and L-citrulline, yields a waste stream of watermelon juice at the rate of over 500 L/t of watermelons. Since watermelon juice contains 7 to 10% (w/v) directly fermentable sugars and 15 to 35 micromol/ml of free amino acids, its potential as feedstock, diluent, and nitrogen supplement was investigated in fermentations to produce bioethanol.
Results: Complete watermelon juice and that which did not contain the chromoplasts (lycopene), but did contain free amino acids, were readily fermentable as the sole feedstock or as diluent, feedstock supplement, and nitrogen supplement to granulated sugar or molasses. A minimum level of ~400 mg N/L (~15 micromol/ml amino nitrogen) in watermelon juice was required to achieve maximal fermentation rates when it was employed as the sole nitrogen source for the fermentation. Fermentation at pH 5 produced the highest rate of fermentation for the yeast system that was employed. Utilizing watermelon juice as diluent, supplemental feedstock, and nitrogen source for fermentation of processed sugar or molasses allowed complete fermentation of up to 25% (w/v) sugar concentration at pH 3 (0.41 to 0.46 g ethanol per g sugar) or up to 35% (w/v) sugar concentration at pH 5 with a conversion to 0.36 to 0.41 g ethanol per g sugar.
Conclusion: 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.