Oxidative stability of biodiesel has been the subject of considerable research [41-62]. This issue affects biodiesel primarily during extended storage. The influence of parameters such as presence of air, heat, traces of metal, antioxidants, and peroxides as well as nature of the storage container was investigated in the aforementioned studies. Generally, fac­tors such as the presence of air, elevated temperatures, or the presence of metals facilitate oxidation. Studies performed with the automated oil stability index (OSI) method have confirmed the catalyzing effect of metals on oxidation; however, the influence of the compound structure of the fatty esters, especially unsaturation as discussed below, was even greater [52]. Numerous other methods, including not only wet-chemical ones such as the acid value and peroxide value, but also pressurized dif­ferential scanning calorimetry, nuclear magnetic resonance (NMR), and so forth, have been applied in oxidation studies of biodiesel.

Two simple methods for assessing the quality of stored biodiesel are the acid value and viscosity since both increase continuously with increasing fuel degradation, i. e., deteriorating fuel quality. The peroxide value is less suitable because it reaches a maximum and then can decrease again due to the formation of secondary oxidation products [48].

Autoxidation occurs due to the presence of double bonds in the chains of many fatty compounds. Autoxidation of unsaturated fatty compounds proceeds with different rates, depending on the number and position of double bonds [63]. Especially the positions allylic to double bonds are susceptible to oxidation. The bis-allylic positions in common polyun­saturated fatty acids, such as linoleic acid (double bonds at. C-9 and. C-12, giving one bis-allylic position at C-11) and linolenic acid (double bonds at. C-9, .C-12, and C-15, giving two bis-allylic positions at C-11 and C-14), are even more prone to autoxidation than the allylic positions. The relative rates of oxidation given in the literature [63] are 1 for oleates (methyl, ethyl esters), 41 for linoleates, and 98 for linolenates. This is essential because most biodiesel fuels contain significant amounts of esters of oleic, linoleic, or linolenic acids, which influence the oxida­tive stability of the fuels. The species formed during the oxidation process cause the fuel to eventually deteriorate.

A European standard (EN 14112; Rancimat method) for oxidative stability has been included in the American and European biodiesel standards (ASTM D6751 and EN 14214). Both biodiesel standards call for determining oxidative stability at 110°C; however, EN 14214 pre­scribes a minimum induction time of 6 h by the Rancimat method while ASTM D6751 prescribes 3 h. The Rancimat method is nearly identical to the OSI method, which is an AOCS (American Oil Chemists’ Society) method.

Besides preventing exposure of the fatty material to air, adding antiox­idants is a common method to address the issue of oxidative stability. Common antioxidants are synthetic materials such as ieri-butylhydro — quinone (TBHQ), butylated hydroxytoluene (BHT), butylated hydrox — yanisole (BHA), and propyl gallate (PG) as well as natural materials such as tocopherols. Antioxidants delay oxidation but do not prevent it, as oxidation will commence once the antioxidant in a material has been consumed.