Citation: Fish, W.W., Bruton, B.D., Russo, V.M. 2009. Watermelon juice: A promising feedstock supplement, diluent, and nitrogen supplement for ethanol biofuel production. Biotechnology for Biofuels. 2:18. Interpretive Summary: Twenty percent of each annual watermelon crop is rejected for fresh fruit marketing because of surface blemishes or because they are misshapen. Culls although internally sound, are left in the field. For the 2007 growing season, this would have amounted to around 500 million pounds of watermelon lost to producers as potential revenue. Watermelon is a rich source of lycopene and citrulline that serve a preventative role for a number of diseases. The potential value of these compounds recovered from cull watermelons could provide a source of additional revenue for producers. Processing watermelons to obtain these compounds would produce a waste stream of watermelon juice that contains about 10% fermentation ready simple sugars. A possible use of this juice waste stream would be to ferment it to ethanol. As an initial step toward using watermelon juice as a feedstock in ethanol production, the fermentation properties and biofuel production applications of watermelon juice were investigated. Watermelon juice is readily converted by yeast, a fungus, to ethanol. Watermelon juice could fulfill three key roles in fermentations involving other sugar feedstocks such as molasses or sugar. These roles are: (1) A diluent instead of potable water. (2) A feedstock supplement to reduce primary feedstock usage by up to 10%. (3) A readily available nitrogen supplement that stimulates rapid yeast growth and ethanol production. Results from fermentation of molasses as the primary feedstock, use of watermelon juice as diluent, feedstock supplement, and nitrogen supplement for this fermentation would save about 15% on molasses costs, around 5000 gallons of potable water, and over $500 in nitrogen supplements per 10,000 gallon fermentation run.
Technical Abstract: Processing of watermelons to produce the neutraceuticals lycopene and citrulline yields a waste stream of watermelon juice at the rate of over 500 L/Mt of watermelons. Since watermelon juice contains 7-10% readily fermentable sugars, its potential as feedstock, diluent, and nitrogen supplement was investigated in fermentations to produce bioethanol. 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 approximately 400 mg N/L (approximately 15 umoles/mL amino nitrogen) in watermelon juice was required to achieve maximal fermentation rates when it was employed as nitrogen supplement. The removal of the amino acids from watermelon juice required that another source of nitrogen be added for acceptable rates of 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 (80-90% of theoretical yield of ethanol) or up to 35% (w/v) sugar concentration at pH 5 with a conversion to ethanol of 70-80% of theoretical. Using watermelon juice as a diluent and supplemental feedstock for molasses fermentation, it was estimated that there would be approximately 15% savings on molasses consumption and approximately 22,000 L savings of potable water per 40,000 L fermentation run.