Concentrations of acetic (a) and propionic acid (b) in the slurry samples of preliminary experiments using P. stratiotes homogenisate as substrate in batch cultures. Substrate: inoculum ratio: 1 = 1 : 15, 2 = 1:7'5, 3 = 1:5, 4 = 1:4, 5 = control (inoculum only). Experimental conditions and determination of acids as described in text.