Colorimetric behavior and seasonal characteristic of xylem sap obtained by mechanical compression from silver birch (Betula pendula)

This study investigated the color development in fresh birch xylem sap (Betula pendula) squeezed by mechanical compression, which was not seen in birch exudate and squeezed spruce sap. Altering the pH of the colored xylem sap demonstrated distinctive patterns of light absorption bands which suggested the formation of quinonoid intermediates by the enzymatic activity of polyphenol oxidases (PPO). Comparison with other PPOs by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) suggested the presence of the enzyme in the sap. The major phenol which acts as a substrate for PPO was identified as (−)-epicatechin. Inhibition of the color development was also attempted using Na₂SO₃, ethylenediaminetetraacetic acid (EDTA), and N₂ gas. The results showed that Na₂SO₃ was the most effective inhibitor. Knowledge on the differences between squeezed and exuded sap is important because squeezing can be envisaged as a pretreatment step for any process—such as a biorefinery—that utilizes biomass and has the advantage of utilizing trees that have recently been felled.