The main aim of this study was to chemically characterise the polymeric fraction of birch pulp extractives. We showed that 70–96% of the material in extracts of fully bleached birch kraft elemental chlorine-free (ECF) pulp from three Finnish mills consists of compounds that are undetectable by conventional gas chromatography (GC) analysis, i.e. high-molar mass (HM) material. There were small variations in the extractive content and composition between the three mills, but the overall trend was the same. The HM material was present mainly in the acetone extracts, and the molar mass was shown to range between ca. 1000 and 10 000 Da. Alkaline hydrolysis of the hexane and acetone extracts released fatty acids (FAs) especially, but also sterols and triterpenoids. The dominating FAs were palmitic and stearic acids. Pyrolysis and thermally assisted hydrolysis and methylation (THM)-GC-mass spectrometry (MS) analyses confirmed that the HM material consists mainly of FA units, and additionally of sterol, triterpenoid and aromatic units. Nuclear magnetic resonance (NMR) analysis further confirmed the presence of FAs containing aromatic groups, but also glycerol units. The FA chains seemed to be linked to each other by ester linkages mainly, although ether linkages were also present. The FA oligomers and polymers may have a suberin-like structure.
This work was part of the activities at the Johan Gadolin Process Chemistry Center at Åbo Akademi University.
Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
Research funding: None declared.
Employment or leadership: None declared.
Honorarium: None declared.
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