Douglas-fir (Pseudotsuga menziesii) forest residues were physically fractionated through sieving. The bark and wood were separated for large-sized fractions (>12.7 mm) and their contents were determined. The chemical compositions of the large fractions were calculated based on the contents and chemical compositions of the bark and wood. The chemical compositions of the fine fractions were analyzed. The bark and wood content in the fine fractions were calculated based on the measured glucan and lignin contents in each fraction. It was found that fractionation by particle/chip size can effectively fractionate bark and wood and therefore lignin from carbohydrates. The large sized-fractions (>12.7 mm) represent approximately 60% of the collected forest residues but only contain approximately 37% of the total bark and 35% of the total ash, or a selectivity over bark and ash of 1.6 and 1.7, respectively. Pretreatment of forest residues by Sulfite Pretreatment to Overcome Recalcitrance of Lignocelluloses (SPORL) and subsequent enzymatic hydrolysis revealed the presence of 14.3% bark can reduce substrate enzymatic digestibilities (SED) 16% compared with that from a bark free sample. The SED of a bark is 41% compared with 73% for wood when pretreated under the same conditions. Separating pretreatment of bark from wood is beneficial for producing a more enzymatically digestible substrate. The results from the present study could have significant implications for harvesting forest residues.
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