Biodiesel is biodegradable, creating long-term storage and fuel stability issues.  When exposed to high temperatures, oxygen or sunlight, or placed in contact with non-ferrous metals, FAME will deteriorate.  The oxidative stability of a particular biodiesel sample is dependent on the type of fatty acid chains found in the feedstock oil/fat.  Feedstocks containing a significant amount of fatty acids with double bonds are especially susceptible to oxidation.  Oxidation in biodiesel can result in the formation of various acids or polymers, leading to fuel thickening, filter clogging, injector plugging and fuel system malfunctions.  Untreated biodiesel will have relatively poor oxidation stability, restricting storage time to around 4 months.  Once oxidation has occurred, the biodiesel cannot be salvaged.  In order to improve the oxidation stability of biodiesel, additives that have anti-oxidant properties (such as BHT and BHA) can be added, the fatty ester composition can be modified through changing of the alcohol or changing of the fatty acid profile through physical procedures, genetic modification or the use of alternative feedstocks.

Significance

Test method EN 14112 measures the oxidation stability of a biodiesel sample under accelerated conditions in a FAME oxidation stability bath.  A stream of purified air is passed through the biodiesel sample at a specific temperature.  The vapours that are released during the oxidation process, along with the air, are passed into a flask containing distilled water.  There, an electrode measures the conductivity in the water.  When the conductivity begins to increase rapidly, this is an indication of the dissociation of volatile carboxylic acids produced during the oxidation process.  ASTM D6751 specifies that the minimum acceptable time allowed for a biodiesel sample before oxidation occurs under these conditions (known as the Rancimat method) is 3 hours.