HARNESSING PLANT BIOMASS FOR BIOFUELS AND BIOMATERIALSPlant triacylglycerols as feedstocks for the productionof biofuelsTimothy P. Durrett1, Christoph Benning2and John Ohlrogge1,*1Departments of Plant Biology, and2Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48824, USAReceived 5 November 2007; revised 25 January 2008; accepted 30 January 2008.*For correspondence (fax + 1 517 353 1926; e-mail [email protected]).SummaryTriacylglycerols produced by plants are one of the most energy-rich and abundant forms of reduced carbonavailable from nature. Given their chemical similarities, plant oils represent a logical substitute forconventional diesel, a non-renewable energy source. However, as plant oils are too viscous for use in moderndiesel engines, they are converted to fatty acid esters. The resulting fuel is commonly referred to as biodiesel,and offers many advantages over conventional diesel. Chief among these is that biodiesel is derived fromrenewable sources. In addition, the production and subsequent consumption of biodiesel results in lessgreenhouse gas emission compared to conventional diesel. However, the widespread adoption of biodieselfaces a number of challenges. The biggest of these is a limited supply of biodiesel feedstocks. Thus, plant oilproduction needs to be greatly increased for biodiesel to replace a major proportion of the current and futurefuel needs of the world. An increased understanding of how plants synthesize fatty acids and triacylglycerolswill ultimately allow the development of novel energy crops. For example, knowledge of the regulation of oilsynthesis has suggested ways to produce triacylglycerols in abundant non-seed tissues. Additionally,biodiesel has poor cold-temperature performance and low oxidative stability. Improving the fuel character-istics of biodiesel can be achieved by altering the fatty acid composition. In this regard, the generation oftransgenic soybean lines with high oleic acid content represents one way in which plant biotechnology hasalready contributed to the improvement of biodiesel.Keywords: biodiesel, triacylglycerol, oilseeds, fatty acid, bioenergy.IntroductionAn increased necessity for energy independence andheightened concern about the effects of increasing carbondioxide levels have intensified the search for renewablefuels that could reduce our current consumption of fossilfuels. One such fuel is biodiesel, which consists of themethyl esters of fatty acids, usually derived from plant oils,although other sources including animal fat are possible.Plant oils are primarily composed of various triacylglycerols(TAGs), molecules that consist of three fatty acid chains(usually 18 or 16 carbons long) esterified to glycerol(Figure 1a). The fatty acyl chains are chemically similar tothe aliphatic hydrocarbons that make up the bulk of themolecules found in petrol (also called gasoline) and diesel(Figure 1c). The hydrocarbons in petrol contain between 5and 12 carbon atoms per molecule, and this volatile fuel ismixed with air and ignited with a spark in a conventionalengine. In contrast, diesel fuel components typically have10–15 carbon atoms per molecule and are ignited by the veryhigh compression obtained in a diesel engine. Early dem-onstration versions of the diesel engine were designed torun on peanut oil, reflecting the fact that plant-derivedtriacylglycerols and petroleum fuels are chemically similar,with structures consisting largely of chains of reducedcarbons. However, most plant TAGs have a viscosity rangethat is much higher than that of conventional diesel: 17.3–32.9 mm2s)1compared to 1.9–4.1 mm2s)1, respectively(ASTM D975; Knothe and Steidley, 2005). This higherviscosity results in poor fuel atomization in modern dieselª 2008 The Authors 593Journal compilation ª 2008 Blackwell Publishing LtdThe Plant Journal (2008) 54, 593–607 doi: 10.1111/j.1365-313X.2008.03442.xengines, leading to problems derived from incompletecombustion such as carbon deposition and coking (Ryanet al., 1984). To overcome this problem, TAGs are convertedto less viscous fatty acid esters by esterification with a pri-mary alcohol, most commonly methanol (Figure 1b). Theresulting fuel is commonly referred to as biodiesel and has adynamic viscosity range from 1.9 to 6.0 mm2s)1(ASTMD6751). The fatty acid methyl esters (FAMEs) found in bio-diesel have a high energy density as reflected by their highheat of combustion, which is similar, if not greater, than thatof conventional diesel (Figure 2; Knothe, 2005). Similarly, thecetane number (a measure of diesel ignition quality) of theFAMEs found in biodiesel exceeds that of conventionaldiesel (Knothe, 2005).However, some obvious differences exist between themolecules found in conventional diesel and those in biodie-sel, which have an impact on the properties of the fuels. Forexample, biodiesel FAMEs contain two oxygen atoms permolecule and often one or more double bonds (dependingon the triacylglycerol from which they were derived),whereas the hydrocarbons in conventional diesel tend tobe saturated.Advantages of biodieselThe higher oxygenated state compared to conventionaldiesel leads to lower carbon monoxide (CO) production andreduced emission of particulate matter (Graboski andMcCormick, 1998). This latter air pollutant is especiallyproblematic in European cities, motivating temporary cur-fews for diesel-powered vehicles. Biodiesel also containslittle or no sulfur or aromatic compounds; in conventionaldiesel, the former contributes to the formation of sulfuroxide and sulfuric acid, while the aromatic compounds alsoincrease particulate emissions and are considered carcino-gens. In addition to the reduced CO and particulate emis-sions, the use of biodiesel confers additional advantages,including a higher flashpoint, faster biodegradation andgreater lubricity. The higher flashpoint of biodiesel allowssafer handling and storage, whereas the biodegradability ofbiodiesel is particularly advantageous in environmentallysensitive areas where fuel leakage poses large hazards. Thelubricity issue has become increasingly important with thewidespread mandated adoption of low-sulfur diesel fuels.The elimination of sulfur-containing compounds from con-ventional diesel also removes the fuel constituents thatcontribute to the inherent lubricity of the fuel. Biodiesel hasgreater lubricity than conventional diesel, and blendingbiodiesel with low-sulfur fuel restores lubricity (Knotheet