Fractionation of Extractives and Hemicellulose from Pine Bark via Sequential Aqueous Acetone and Hydrothermal Extraction

Abstract

Abstract Bark is an abundant lignocellulosic side stream from pulp, paper, and wood-processing industries that has recently attracted increasing interest as a renewable resource to produce sustainable chemicals and fuels. However, its heterogeneous composition and relatively high inorganic content pose challenges for its selective valorization in integrated biorefineries. As a result, bark remains largely underutilized for chemical applications, and it is primarily utilized via combustion. In this work, a sequential thermochemical fractionation strategy was applied to Scots pine (Pinus sylvestris L.) bark, comprising a mild solvothermal pretreatment to remove bark extractives, followed by hydrothermal extraction to recover hemicellulosic sugars. Molecular compositions of the resulting liquid fractions were then determined using direct-infusion ultrahigh-resolution mass spectrometry, which enables comprehensive chemical fingerprinting of complex biorefinery streams. Aqueous acetone extraction at 60 °C removed approximately 10 wt% of the bark dry mass, yielding a mixture of lipophilic and semipolar extractives. Subsequent hydrothermal extraction at 170 °C selectively recovered more than 15 wt% of hemicellulosic mono- and oligosaccharides, mainly derived from glucuronoxylan and galactoglucomannan, while largely preserving bark lignin and cellulose in the solid residue. The recovered bark extractives are potential feedstocks to produce bioactive ingredients, antioxidants, functional materials, and renewable chemicals, while the recovered hemicellulosic sugars represent suitable substrates for enzymatic hydrolysis or fermentation, thus highlighting the potential of this sequential fractionation strategy to enhance pine bark valorization within sustainable biorefinery frameworks.