Laboratory unbleached Kraft pulp fibers based on spruce were obtained through a conventional Kraft pulping process (18% EA, 40% sulfidity, 1190 H-factor,160 °C temperature, and 167 min time), using a pilot-scale forced-flow circulation digester (500g of wood chips per run). Fibers were separated in a water jet defibrator from NAF (Nordiska Armaturfabriken).Unbeaten Kraft fibers, with original fiber structures maintained as intact as possible, were used as reference for the holocellulose fibers.