NOVELTY - Process for quenching a hydrothermal, dilute acid hydrolysis reaction of a biomass feedstock, involves: slowing down or stopping degradation of an aqueous monomer and/or oligomer sugar mixture by flash cooling of the sugar mixture. The flash cooling ensures that a fraction of dissolved and volatile degradation byproducts are removed by forming vapor stream, and where a lignin component, if present, is solidified into a structure with good de-watering characteristics, allowing for subsequent removal of the lignin component by separation.    USE - For quenching a hydrothermal, dilute acid hydrolysis reaction of a biomass feedstock (claimed), particularly for quenching a liquefaction reaction of a lignocellulosic biomass starting material.    ADVANTAGE - The process avoids continued detrimental decomposition of monomer and/or oligomer sugar mixture solution, and provides high yields of monomers and/or oligomers in the final solution. It is an optimal method for quenching a biomass material which is undergoing liquefaction in a sub- or super-critical condition. The entire method is fast, and it prevents, minimizes or eliminates clogging and/or fouling of sticky biomass components in process equipment, such as by an alkaline liquid being washed through the process equipment, either as a sole solution between regular process operations of a biomass process flow in a liquid solution, or as added directly into the liquid solution for dissolving biomass components which are or otherwise become sticky. Such an alkaline liquid can be processed separately from the biomass process flow solution after the washing or the addition. Moreover, the alkaline liquid can be recovered after the washing or addition, for further washing or addition.    DETAILED DESCRIPTION - Process for quenching a hydrothermal, dilute acid hydrolysis reaction of a biomass feedstock, involves: slowing down or stopping degradation of an aqueous monomer and/or oligomer sugar mixture by flash cooling of the sugar mixture, where the flash cooling ensures that a fraction of dissolved and volatile degradation byproducts are removed by forming vapor stream, and where a lignin component, if present, is solidified into a structure with good de-watering characteristics, allowing for subsequent removal of the lignin component by separation. The process results in a hydrolyzed solution of sugar monomers and/or oligomers.