Watermelon: Using Everything for Food and Fuel
Over four billion pounds of watermelon were produced in the United States in 2007 for the fresh produce and cut fruit markets [Agricultural Marketing Resource Center, http://www.agmrc.org/agmrc/commodity/vegetables/watermelons/watermelon.htm]. Water-melon is a rich source of at least two compounds that support cardiovascular health, and may serve to prevent a number of diseases. One of these is lycopene, which provides the red color to watermelon, and is a powerful antioxidant carotenoid shown to be important in prostate health. The second compound, citrulline, is a naturally occurring amino acid involved in maintaining the body’s nitrogen balance and indirectly, in cardiovascular health. During extraction of these neutraceuticals a waste stream is developed which produces readily fermentable sugars that can be used in production of bioethanol. Scientists at the Wes Watkins Agricultural Research Laboratory (WWARL) are investigating the fermentation properties and applications for bioethanol derived from watermelon.
Two economic factors relating to watermelon make this fruit worthy of consideration as a possible feedstock for ethanol biofuel production. First, about 20% of each annual watermelon crop is rejected for fresh fruit marketing because of surface blemishes or because they are misshapen; these "culls", although internally sound, must be left in the field. For the 2007 growing season, this would have amounted to around 800 million pounds lost to growers as a potential source of revenue. Second, the neutraceutical value of compounds obtained from watermelon is at a threshold whereby whole and all culls could possibly be employed as starting material from which to prepare these chemicals. The process would produce two waste streams: watermelon juice containing "fermentation ready" simple sugars, and rind tissue composed of a sizeable proportion of sugar polymers such as cellulose that could be hydrolyzed and fermented to ethanol. A watermelon is nominally 60% flesh, and about 90% of the flesh is juice that containing 7-10% sugars; over 50% of a watermelon is readily fermentable liquid. An immediate application is the integration of a watermelon juice processing waste stream to be used in bioethanol production. Since it is mostly water, watermelon juice could serve to dilute concentrated sources of fermentable sugars such as molasses. The ready-to-ferment glucose, fructose, and sucrose, would serve as a supplement to the process and reduce primary feedstock requirements by up to 10%. The rind, about 40% of the watermelon fresh weight is equally rich in citrulline. Once juice has been extracted from the rind and flesh, about 10% of the original weight of the whole watermelon is left as a mixture of complex carbohydrates that potentially can be broken down into fermentable sugars.