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 4 to 16 carbon atoms, but is most active with C8 to C12 compounds . The enzyme from rat does not accept C16 at all and is most active with C6-C8 compounds . cf. EC 220.127.116.11, short-chain acyl-CoA dehydrogenase, EC 18.104.22.168, long-chain acyl-CoA dehydrogenase, and EC 22.214.171.124, very-long-chain acyl-CoA dehydrogenase.