A colorimetric bioassay has been developed enabling separation of wood pulp qualities based on the quantity of methyl-esterified galacturonan exposed on fiber surfaces. Thermomechanical pulp (TMP) and chemi-TMP (CTMP) had, respectively, 3.1 and 0.7 microg galacturonan on the surface per mg wood pulp. The presence of galacturonan on the surface of TMP and CTMP was further confirmed by immunogold localization.