Samples of maize silage, olive pomace, and pig slurry were collected at an anaerobic digestion plant in Central Italy, operating under mesophilic conditions, in which they are daily used for feeding the digester. Samples of the substrate of the primary digester (i.e., the acidogenic reactor) were also taken. Becauseoftheirrapidacidificationthesampleswereimmediatelyanalyzedandmaintainedina refrigerated environment until they were used. The chemical–physical properties of the substrate and of the inoculum (Table 1) were measured by means of TGA 701 LECO (LECO Corporation, St. Joseph, MI, USA) (see Table 1) [24,25]. Moisture, ash, total solids and volatile solids content were measured according to References [26–28].