The invention relates to controlled-release metal microparticles comprising a nucleus consisting of at least one metal salt selected from nickel salt, cobalt salt, copper salt, manganese salt, magnesium salt and mixtures of same and a polymer and a coating of a polymer derived from polyethylene glycol. The invention also relates to the use of same for producing biogas from biodegradable material and to a method for producing biogas, comprising the use of said microparticles.