Contains FAD as prosthetic group. One of several enzymes that catalyse the first step in fatty acids beta-oxidation. The enzyme from pig liver can accept substrates with acyl chain lengths of 6 to at least 16 carbon atoms. The highest activity was found with C12, and the rates with C8 and C16 were 80 and 70%, respectively . The enzyme from rat can accept substrates with C8-C22. It is most active with C14 and C16, and has no activity with C4, C6 or C24 . cf. EC 220.127.116.11, short-chain acyl-CoA dehydrogenase, EC 18.104.22.168, medium-chain acyl-CoA dehydrogenase, and EC 22.214.171.124, very-long-chain acyl-CoA dehydrogenase.