In order to study the detailed water extract of wood components in birch under hydrothermal treatment, xylem sap obtained by mechanical compression—squeezing—was analyzed. Comprehensive physical and chemical characteristics of birch log under soaking were investigated in a previous study where limited permeation of soaking water into the log was observed, and glucose and fructose (dominant free monosaccharides in the squeezed sap) were increased during the experiments. In this paper, focus was placed on the effect of treatment time and the water extract of chemical compounds which were not determined in the previous study under controlled laboratory condition using small wood blocks. Laboratory-scale experiments were carried out with vacuum-packed birch blocks heated in water at 70 °C, which is a soaking temperature sometimes used in veneer manufacturing. The squeezed sap was analyzed for acetate, formate, sulfate, and sulfite using ion chromatography, and epicatechin, which is the most abundant phenolic compound in the sap, was determined by gas chromatography analyses with a flame ionization detector following solvent extraction with ethyl acetate. The results indicated that the concentration of free monosaccharides and other organic substances increased proportionally with soaking time, as well as the concentration of carboxylates and sulfur compounds. Epicatechin, on the other hand, was reduced during soaking. It was suspected that nutrients existing in parenchyma cells and the wood cell wall became more squeezable under the treatment due to the structural integrity of the wood.