Abstract: A liquefaction reactor includes multiple alternating plug flow and continuously-stirred regions. And, liquefaction of a biomass slurry in the reactor includes measuring pH of the biomass slurry in the reactor and, after each operation of measuring the pH, adjusting the pH of the biomass slurry in the reactor as needed to a value within a desired range. Liquefaction of the biomass slurry also includes adding enzymes to the biomass slurry in the reactor after measuring the pH of the biomass slurry in the reactor and adjusting the pH of the biomass slurry as needed. In addition, the pH of the biomass slurry may also be measured before the biomass slurry enters the reactor and, as needed, adjusted to a value within the desired range. Enzymes may then also be added to the biomass slurry before the biomass slurry enters the reactor.