In Pursuit of Fruit

As the food-versus-fuel debate rages, ethanol producers are searching for nonfood and food-based waste products to make renewable fuel. This has led researchers to take a closer look at watermelons that don't ripen in time for harvest or fail to make the grade for grocery store produce aisles.
By Bryan Sims | July 08, 2008
For growers and laborers, harvesting watermelons is no quick and easy task. Workers move through fields, cutting vines and filling trucks by hand, looking for the ripest, most pristine-looking melons. But not all watermelons ripen at the same time and laborers must return later to retrieve them. Even more watermelons might need to be cut later, and after a while, that is costly. Also, watermelons that aren't visually perfect, which are referred to as culls, are rejected and left in the fields. That adds up to a lot of watermelons that never make it to the produce section at the grocery store or the Fourth of July picnic.

"Historically, our industry abandons 20 [percent] to 25 percent of the crop that it produces every year because of labor issues, where you can't afford to go back in the fields for a fourth or fifth time to harvest what's left because it's spread all over creation," according to Bob Morrissey, executive director of the National Watermelon Association. With nine chapters across the country, the NWA is a consortium of watermelon growers, researchers and advocates organized to support the industry.

In 2006, Morrissey met with then U.S. Secretary of Agriculture Mike Johanns and five of his constituents to gather advice about new marketing possibilities for watermelon growers.

For Morrissey, the answer was obvious. Why not create a new revenue stream for the 800 million pounds of watermelons abandoned out of the 400 billion produced each year, and somehow convert them into ethanol or other biofuels? Morrissey's question is laudable, considering that corn-based ethanol production is expected to plateau at 15 billion gallons a

"I told them that we have all this fruit that's loaded with natural sugar and there's really not much opportunity to process it into juice," Morrissey says. "There are a lot of people out there looking at it, especially with everything that's been coming out of Washington. There's an energy component that has now been put into the new Farm Bill, which should help us a little bit."

Looking at the fruit from an ethanol producer's standpoint, watermelons contain all the organic components amenable for ethanol production. Typically, watermelons are comprised of about 10 percent sugar and 90 percent water. The three main natural sugars found in watermelons—fructose, sucrose and glucose—are highly fermentable and have been used for centuries to make a variety of products in the wine and spirit industry, including the infamous "moonshine." In addition, the rind and seeds serve as a feed ingredient.

Producing ethanol out of these simple sugars isn't difficult, but there are a few challenges associated with integrating discarded watermelons into an existing commercial-scale corn-based ethanol plant, particularly in today's market conditions, according to Morrissey.

"It is a viable deal for us," he says. "The concern we've got right now is just the pure economics of it all. Ethanol producers can't afford to give us enough money to cover the farmers' and packers' labor costs." Those costs have increased in tandem with the price of fuel. "We can't afford to ship it and nobody else can afford to ship it anywhere either," Morrissey says. "So, it's become a challenge for us."

After a national convention hosted by the NWA in February, Morrissey met with biochemists and engineers from the University of Georgia to explore strategies that could benefit ethanol producers and watermelon growers.

Researching the Feasibility
Utilizing a watermelon juice waste stream and the watermelon itself as a feedstock for ethanol production would provide simple sugars for direct fermentation to ethanol, and at the same time eliminate a majority of the sewage treatment costs for the waste stream. Like most ethanol feedstocks, these two waste products would have to be broken down into simple sugars, either chemically or enzymatically, before being fermented into ethanol.

Last year, the NWA conducted various research projects in conjunction with the UG to determine the feasibility of using discarded watermelons as an ethanol feedstock. So far, the findings are encouraging.

One option would be to build or acquire a separate facility on site with an ethanol plant that would extract two of the predominant antioxidants found in watermelons—lycopene and citrulline. These two antioxidants could then be shipped for pharmaceutical and/or nutriceutical purposes and the liquid absent the antioxidents would be made into ethanol, according to Elliot Altman, director at the Center for Molecular BioEngineering at UG's Department of Biological and Agricultural Engineering in Athens, Ga. The department also conducts research on other fruits for biofuel production such as peaches and blueberries.

"If you really wanted a process to do this right you would probably need to extract the lycopene and citrulline first and then produce the ethanol," he says. "What we have found is that watermelons don't have any inhibitors that would affect the [ethanol production] process so that's feasible."

The only problem is what to do with the rind. Altman says he has a way to circumvent this potential hurdle. "We have actually found chemical hydrolysis techniques that are pretty benign, where you can actually extract more sugar from the rind," he says. "So, we would suggest including that as part of the process."

Another positive property of watermelon juice is that if a producer doesn't extract the citrulline and other amino acids prior to production, the juice itself is rich in nitrogen, which is a critical component to the propagation of yeast in the fermentation process, according to Wayne Fish, lead research chemist with the USDA Agricultural Research Service in Lane, Okla. Since August 2007, Fish and his research team have been exploring the use of watermelon juice, rind and pulp waste streams as feedstocks for ethanol production. "We have found by fermenting watermelon juice, or by taking watermelon juice and adding molasses or sugar to it, those amino acids in the juice have a high affinity to the yeast," Fish says.

As for improvements in production efficiency of the rind, Fish says that part of the research is ongoing. He says that if the rind were to be coprocessed with the watermelon juice, it could add to the amount of ethanol produced per pound of watermelon by 50 percent to 75 percent. "We have a process to remove the lycopene that's industrially scalable, and we are finishing up the last aspects to remove the amino acids, including citrulline, which will also be industrially scalable," he says. "We almost have things in place so that watermelon juice could be implemented with another ethanol plant as a source of some sugars and water, which would be a nice fit. This looks like it does have definite potential."
Ethanol producers are cautiously optimistic about the economics of using fruit-based waste feedstocks for ethanol production on a commercial scale. "If there are ethanol plants in heavy watermelon growing regions then it does become feasible," he says. "We're looking at a process or a system whereby the watermelons could be processed in the field. So, instead of hauling individual watermelons, you're hauling concentrated products, including the juice." Another possibility is to have the watermelon processing plant next to the ethanol plant so the juice can be processed on site and pumped into the ethanol plant.

One reason ethanol producers shy away from taking in waste fruit-based feedstocks, such as watermelons, is because it's seasonal and would interrupt their operations, according to Altman. "The problem with watermelons is you're probably going to have a three- to four-month window where that's available," he says. "It's not going to be a 12-month process and you definitely don't want to let your watermelons lie around before you use them. They're just not interested in something that's going to interrupt their operation."

Boca Raton, Fla.-based Citrus Energy LLC is locating its ethanol operations adjacent to a fruit-based waste feedstock. In conjunction with FPL Energy, Citrus Energy is currently breaking ground on a 4 MMgy commercial-scale citrus-peel-to-ethanol plant in Florida where it will utilize its proprietary processes. FPL specializes in wind energy and operates the two largest solar fields in the world. According to David Stewart, president and vice president of engineering, he formed his technology company two years ago to focus on converting citrus waste to ethanol and received a $250,000 grant from Florida's Farm to Fuel Initiative last year.

"Clearly, to turn anything that's not coming from the food supply chain like waste watermelon into ethanol is a good idea," he says. "But, the economics are tough because when people look at the amount of fruit waste available you're talking about pretty small-scale plants. That's the predominant challenge right now."

As for Morrissey, the prospect of using discarded watermelons for ethanol production is at a standstill as growers' top priority today is food quality and safety. "I think just looking at watermelon alone it can have a small impact," Morrissey says. "If we could take the lost production from watermelon, citrus peels and from any other fruit or vegetable that can be converted into some type of biofuel, it's going to help all of us in the long run. Is it going to be the total answer? No. But every little bit helps."

Bryan Sims is an Ethanol Producer Magazine staff writer. Reach him at or (701) 738-4962.