Gary Breitenbeck, Charles Graham and Javed Iqbal

The expansion of the U.S. biodiesel industry in the past few years has been astounding. Biodiesel production increased from 0.5 million gallons in 1999 to 450 million gallons in 2007. The combined capacity of existing U.S. biodiesel facilities exceeds 2.2 billion gallons with an additional 1.2 billion-gallon capacity scheduled to be completed by the end of 2010. The U.S. biodiesel industry is now suffering from acute growing pains as production capacity has outpaced the supply of suitable vegetable oil to produce biodiesel.

Recent data from the National Biodiesel Board suggest U.S. biodiesel facilities have operated at about 20 percent capacity because of a lack of affordable feedstocks. Feedstock accounts for 70-80 percent of the cost of biodiesel production, and its cost is the principal factor determining the competitiveness of biodiesel with petroleum-derived diesel. Soy oil has been the feedstock of choice, but high soybean oil prices have caused many U.S. biodiesel refineries to halt operations or to shift to alternative feedstocks such as animal fats and waste cooking oil. It is far more difficult to process these alternatives into biodiesel that meets rigorous American Society for Testing and Materials (ASTM) standards. There is a critical need to identify additional affordable feedstocks to preserve the fledgling U.S. biodiesel industry.

One answer may be pecans. Pecan nuts contain a high amount of oil consisting primarily of oleic acid, a fatty acid well-suited for biodiesel production. But planting pecans for biodiesel is impractical for several reasons. It takes a decade or more for a young pecan tree to begin producing nuts. And given their high value as a foodstuff, the nuts are too valuable to be crushed for oil. Even so, significant acreage of both wild and improved pecans is not harvested and marketed as food. In addition, a substantial portion of the harvested crop (10-20 percent) is culled and accumulates as waste at processing facilities. Many acres are not harvested each year when disease and insect damage reduces the quality of the crop below the rigorous standards of the food industry.

Production of food-quality pecans requires a high level of management, and a significant portion of current pecan acreage consists of neglected or abandoned trees. Abandoned fields typically contain a large number of young trees crossed with nearby native species or closely related hickories that produce nuts of low quality. In addition, many acres of native pecan trees are found growing wild in wet, bottomland forests along rivers.

As a result, LSU AgCenter researchers began to assess the feasibility of using nonmarketed pecans as a feedstock to supplement the U.S. biodiesel industry.

Commercial production of pecans is primarily in the southeastern United States, with Georgia accounting for as much as 40 percent of U.S. production. Between 2006 and 2008, total U.S. pecan production ranged from 119,000 to 208,000 tons (unshelled equivalent) and averaged 155,000 tons. These yields are based on utilized nuts, not the total harvested. Unfortunately, records of total harvest or total acreage planted to pecan trees are not available; therefore, it is not possible to estimate the amount of pecans available to extract oil for biodiesel.

The oil content of pecan and some closely related hickory nuts ranged from 17 percent in water hickory to 40 percent in Little Page, a commercial native pecan. On a tonnage basis, oil yields ranged from 53.3 to 128 gallons per ton of fresh nuts. Though pecan yields vary among years and regions, 1,000 to 1,500 pounds per acre is a reasonable estimate of harvested yield in the Southeast, indicating the oil yield from improved and native pecan trees ranges from 54 to 96 gallons per acre. Nut and oil yields of pecan orchards under minimal management are likely to be much lower. Also, nut and oil yields of related species and native pecans crossed with hickory are substantially less.

The fatty acid composition of pecan oil consists primarily of oleic and linoleic acids with less than 1 percent unsaturated fatty acids. This composition is similar to that of soybean oil except pecan oils sampled in this study also contain 17-34 percent of an omega-9 monounsaturated fatty acid. This mixture of fatty acids suggests pecan oils can be used to produce biodiesel capable of meeting ASTM standards, though additional tests are needed to fully characterize biodiesel derived from pecan and hickory oils.

The dollar value of pecan nuts based solely on their oil content, assuming a target price of $2 per gallon for oil, would range from $107 to $256 per ton. These estimates do not consider the costs of cracking nuts and extracting oil, nor do they consider the value of the defatted meal. Though high in protein, defatted meal has no current market as an animal feed supplement or other use.

Harvesting costs typically average $170 per acre and are likely to be higher for harvesting native pecans and water hickory in naturalized stands. These values suggest that pecans have little likelihood of serving as a viable biodiesel feedstock with the exception of processing culls, where harvesting costs are not a consideration. Assuming an average annual U.S. yield of 155,000 tons of processed pecans and culling rates of 10-20 percent, 15,500 to 31,000 tons of culls containing 1.2 to 2.5 million gallons of oil are available annually. While not a vast amount, these culls represent a disposal problem that could be used beneficially to help address the critical need for biodiesel feedstocks.

Dry shells represent about 42-60 percent of the weight of fresh nuts, and in a processing facility, shells represent a resource far larger than culls. Solvent extraction of ground shells yields about five percent oil. Processing these shells for oil may be worth consideration at a facility equipped to extract oil from culls.

While the composition of pecan oil appears to be suitable for biodiesel production, low yields and the high costs of harvesting suggest that collecting and processing pecans as a biodiesel feedstock would require a per-gallon oil cost far in excess of soy oil, even at today’s elevated prices. The exception is processing culls, which are seasonally available in centralized locations. The supply of these culls is limited and unlikely to have a national impact on biodiesel production, but processing them into oil may offer an attractive and profitable alternative for disposal of these nuts at large pecan processing facilities.

Gary Breitenbeck, Professor, School of Plant, Environmental & Soil Sciences, LSU AgCenter, Baton Rouge, La.; Charles Graham, Associate Professor, Pecan Research & Extension Center, Shreveport, La.; Javed Iqbal, Laboratory Manager, W.A. Callegari Environmental Center, LSU AgCenter, Baton Rouge, La.

(This article was published in the fall 2009 issue of Louisiana Agriculture.)