NOVELTY - Carrying out liquefaction of a biomass slurry in a reactor using enzymes, where the reactor includes alternating plug flow and continuously-stirred regions, comprises measuring pH of the biomass slurry in multiple plug flow regions of the reactor; after each operation of measuring pH of the biomass slurry, adjusting the pH of the biomass slurry in the reactor as needed to a value within a desired range; and adding enzymes to the biomass slurry in the reactor.    USE - The methods are useful for carrying out liquefaction of biomass slurry in reactor using enzymes; producing bio fuel from lignocellulosic material; and for carrying out liquefaction of pre-treated biomass slurry in liquefaction reactor having multiple alternating plug flow and continuously-stirred regions (all claimed).    ADVANTAGE - The pH of the biomass slurry can be maintained within a desired range, for example, between about 4 and about 6.5, throughout the liquefaction process to help promote efficient liquefaction (and cellulolysis) of the biomass slurry and help avoid denaturing of the cellulase enzymes.    DETAILED DESCRIPTION - Carrying out liquefaction of a biomass slurry in a reactor using enzymes, where the reactor includes alternating plug flow and continuously-stirred regions, comprises measuring pH of the biomass slurry in multiple plug flow regions of the reactor; after each operation of measuring pH of the biomass slurry, adjusting the pH of the biomass slurry in the reactor as needed to a value within a desired range; and adding enzymes to the biomass slurry in the reactor; or carrying out liquefaction of a biomass slurry in a reactor using enzymes, where the reactor includes alternating plug flow and continuously-stirred regions, comprises measuring pH of the biomass slurry in multiple regions of the reactor; after each operation of measuring pH of the biomass slurry, adjusting the pH of the biomass slurry in the reactor as needed to a value within a desired range; and adding enzymes to the biomass slurry in the reactor. INDEPENDENT CLAIMS are:    (1) a method of producing bio fuel from lignocellulosic material, comprising pre-treating the lignocellulosic material using physical and/or chemical processes to liberate cellulose from the lignocellulosic material and form a pre-treated biomass slurry; processing the pre-treated biomass slurry in a liquefaction reactor using cellulase enzymes to form a liquefaction slurry; and saccharifying the liquefaction slurry to produce the biofuel;    (2) a method for carrying out liquefaction of a pre-treated biomass slurry in a liquefaction reactor having multiple alternating plug flow and continuously-stirred regions comprising measuring an initial pH of the biomass slurry before the biomass slurry enters the liquefaction reactor, and adjusting the initial pH as needed to a value within a desired range; adding enzymes to the biomass slurry after adjusting the initial pH of the slurry to the value within the desired range and before the biomass slurry enters the liquefaction reactor; in a plug flow region of the liquefaction reactor, measuring pH of the biomass slurry; and in the liquefaction reactor, adjusting the pH of the biomass slurry measured in the plug flow region as needed to a value within the desired range; and    (3) a tower reactor for use in carrying out liquefaction of a biomass slurry, comprising alternating plug flow regions and continuously-stirred regions; agitators for moving the biomass slurry in the continuously-stirred regions; probes positioned in multiple plug flow regions and configured to measure pH of the biomass slurry in the plug flow regions; first fluid lines positioned in communication with multiple ones of the continuously-stirred regions and configured to deliver an acid and/or a base to the biomass slurry in the multiple ones of the continuously-stirred regions; and second fluid lines positioned in communication with multiple ones of the continuously-stirred regions and configured to deliver enzymes to the biomass slurry in the multiple ones of the continuously-stirred regions.