Rapeseed oil is used as diesel fuel, either as biodiesel, straight in heated fuel systems, or blended with petroleum distillates for powering motor vehicles. Rapeseed oil was produced in the 19th century as a source of a lubricant for steam engines. Natural rapeseed oil contains 50% erucic acid. Wild seeds also contain high levels of glucosinolates (mustard oil glycosides), chemical compounds that significantly lowered the nutritional value of rapeseed press cakes for animal feed.