Sorghum bicolor [L.] Moench, represents an interesting substrate for methane production. The aim of this study was to evaluate the effect of alkaline pretreatment on chemical composition, physical structure and methane production of five varieties of sorghum (S1, S2, S3, S4, and S5). The pretreatment was conducted in closed bottles, at 55 °C for 12 h. Samples were soaked in a NaOH solution at 4 and 10 gNaOH/100 gTS, with a solid concentration of 35 gTS/L. Sodium hydroxide pretreatment led to a reduction of lignin (50–70%), hemicelluloses (18–35%), cellulose (16–45%) and galacturonic acids (up to 100%), for all varieties of sorghum, as also confirmed by Fourier transform infrared spectroscopy analyses. The reduction of lignin content and thereafter the solubilization of cellulose and hemicelluloses, previously observed, can accelerate the disintegration and the hydrolysis steps during anaerobic digestion. Indeed, an increase in the first order kinetic constant was observed by increasing the alkaline dosage (by 40%, 61%, 64%, 54%, and 40% for sorghum S1, S2, S3, S4, and S5, respectively). Nevertheless, the sodium hydroxide pretreatment had no positive effect in enhancing the methane yields (270 ± 13, 335 ± 11, 294 ± 1, 327 ± 9 and 303 ± 24 mL CH4/gVS for S1, S2, S3, S4 and S5, respectively).
