Efficacy of gossypol as an antioxidant additive in biodiesel Bryan R. Moser * Bio-Oils Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, United States Department of Agriculture,1815 N. University St., Peoria, IL 61604, USA article info Article history: Received 28 January 2011 Accepted 5 September 2011 Available online 5 October 2011 Keywords: Antioxidant Biodiesel Butylated hydroxytoluene Fatty acid methyl esters Gossypol g-tocopherol abstract The efficacy of gossypol as an antioxidant additive in fatty acid methyl esters (FAMEs) prepared from soybean oil (SME), waste cooking oil (WCME) and technical grade methyl oleate (MO) was investigated. Gossypol is a naturally occurring polyphenolic aldehyde with antioxidant properties isolated from cottonseed that is toxic to humans and animals. At treatment levels of 250 and 500 ppm, gossypol exhibited statistically significant improvements in the inductionperiods (IPs; EN 14112) of SME, WCME and MO. Efficacy was most pronounced in SME, which was due to its higher concentration of endogenous tocopherols (757 ppm) versus WCME (60 ppm) and MO (0 ppm). A comparison of antioxidant efficacy was made with butylated hydroxytoluene (BHT) and g-tocopherol. For FAMEs with low concentrations of endogenous tocopherols (WCME and MO), g-tocopherol exhibited the greatest efficacy, although treat- ments employing BHT and gossypol also yielded statistically significant improvements to oxidative stability. In summary, gossypol was effective as an exogenous antioxidant for FAMEs investigated herein. In particular, FAMEs containing a comparatively high percentage of endogenous tocopherols were especially suited to gossypol as an antioxidant additive. Published by Elsevier Ltd. 1. Introduction A principle disadvantage of biodiesel versus petroleum diesel fuel is greater susceptibility to oxidative degradation during storage [1,2]. Oxidative stability is specified in the biodiesel standards ASTM D6751 [3] and EN 14214 [4] (Table 1) and is determined following EN 14112 [5]. The Rancimat method (EN 14112) leverages increased exposure to heat (110  C) and air (10 L/h) to accelerate oxidative degradation. Oxidation is measured indirectly as the induction period (IP, h) and fuels with longer IPs are more stable to oxidation. Minimum limits for IP of 3 and 6 h are specified in ASTM D6751 and EN 14214, respectively. The common ground state of oxygen is the diradical triplet state 3 O 2 and is implicated in autoxidation of lipids. The excited singlet state, 1 O 2 , is 94.2 kJ/mol more reactive and participates in photo- xidation [6e8]. Autoxidation initiates at methylene carbons allylic to olefinic moieties along the hydrocarbon tails of biodiesel and consists of initiation, propagation and termination [6e9]. Autoxidation commences with production of resonance-stabilized alkyl radicals by allylic hydrogen abstraction caused by an initiator (Fig. 1). Direct oxidation is spin-forbidden as a result of the opposite spins of 3 O 2 and lipids in their ground states, thus requiring initiators such as light, heat, peroxides, hydroperoxides and transition metals. Propagation then occurs as a two-step sequence as alkyl radicals react exother- mically with 3 O 2 to produce peroxyl radicals, which in turn react with biodiesel to yield additional alkyl radicals and hydroperoxides (Fig.1) [6e9]. The conversion of alkyl radicals to peroxyl radicals is compar- atively fast whereas transformation to hydroperoxides is rate- limiting. The rate constant for the rate-limiting step of propagation depends primarily on the ease with which a hydrogen atom can be abstracted from the methylene group [6e8]. The bond dissociation energy of an allylic CeH bond (such as in methyl oleate) is approxi- mately 41.9 kJ/mol greater than that of a bis-allylic (such as in methyl linoleate or linolenate) CeH bond [6e9]. Propagation continues so long as there is sufficient supply of reactants. Termination occurs when two radicals react to form stable products, such as aldehydes, shorter-chain fatty acids (FAs), ketones, epoxides, alcohols, dimers, oligomers and polymers, among others [6e9]. The volatile low molecular weight decomposition products are what EN 14112 indi- rectly measures to determine IP. Polyunsaturated FA methyl esters (FAMEs) containing one or more bis-allylic methylene positions oxidize quicker than mono- unsaturated or saturated FAMEs, as seen by the relative rates of Abbreviations: AV, acid value; BHT, butylated hydoxytoluene; FA, fatty acid; FAME, fatty acid methyl esters; IP, induction period; IV, iodine value; MO, methyl oleate; PV, peroxide value; RBD, Refined bleached and deodorized; SME, soybean oil methyl esters; WCME, Waste cooking oil methyl esters. * Tel.: þ1 309 681 6511; fax: þ1 309 681 6524. E-mail address: Bryan.Moser@ars.usda.gov. Contents lists available at SciVerse ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene 0960-1481/$ e see front matter Published by Elsevier Ltd. doi:10.1016/j.renene.2011.09.022 Renewable Energy 40 (2012) 65e70