All experiments were carried out as batch processes. The digesters were glass bottles with a capacity of 3 litres, with two outlet connections, one for gas and another for taking slurry samples. All digesters were kept in a thermostatically controlled water bath. Calibrated gas collecting tubes with a capacity of 5 litres were filled with acidified water (below pH 1). pH values and VFA concentrations of the slurry samples were measured daily. Analyses of the gas were done when sufficient gas (about 2 litres) was present in the gas-collecting tubes. The digesters were manually shaken once a day. Ambient temperatures were also recorded. An average pressure of 730 mm Hg was used to calculate the gas yield at standard temperature and pressure (STP). The specific gas yield from the plant biomass was obtained by subtracting the amount of gas generated from the corresponding amount of inoculum from the total gas produced.