Barrios et al. Biotechnology for Biofuels and Bioproducts
(2025) 18:48
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Fig. 4 Hardwood xylan a before and b after kraft pulping
Furthermore, the combination of enzyme and cati- onic starch treatment (Fig. 3d) results in increased fiber entanglement and more pronounced cell wall disloca- tions and cracks. These morphological changes contrib- ute to improved fiber bonding and entanglement, which are critical for enhancing the mechanical properties of the paper. The visible disruptions on the fiber surface may also indicate areas where the enzyme has penetrated the fiber structure, further contributing to the improved inter-fiber bonding. Enzymatic cleavage of HexA groups Removing hexenuronic acid (HexA) groups is critical to secure efficient and cost-effective bleaching operations during papermaking [80]. These are harmful to kraft bleaching systems in the papermaking process as they reduce bleaching efficiency by consuming a dispropor- tionate amount of bleaching chemicals. Hardwood pulps are characterized by a higher xylan content [81]. Degra- dation of HexA dominates over its formation only in the last part of the hardwood pulping and not in the begin- ning, as is the case for softwoods. Removal of HexA can be carried out by bleaching at acidic conditions or by acidic treatment at high temperatures without bleaching chemicals. The reactive double bond in the hexenuronic acid reacts with several bleaching chemicals such as chlo- rine, chlorine dioxide, ozone, and peracid but not with alkaline oxygen and hydrogen. Enzymes—xylanase and laccase-mediator systems—also effectively remove hex- enuronic acid from kraft pulp [82]. Although the removal
of HexA is typically conducted before or during the bleaching process to reduce the consumption of bleach- ing chemicals and improve the brightness and stability of the pulp, it is not 100% efficient. HexA groups are hydro- philic due to their carboxylic acid functionality, which can interact with water molecules, as shown in Fig. 4. Before bleaching, hardwood kraft pulps typically con- tain HexA in the range of 40–100 μmol/g [83, 84]. After undergoing oxygen delignification, a common pre- bleaching step, this content is usually reduced to 20–60 μmol/g [85]. However, it is after the complete bleach- ing process—whether through elemental chlorine-free (ECF) or total chlorine-free (TCF) sequences—that the HexA content is significantly lowered, typically ranging from 5 to 20 μmol/g [86]. The results for the untreated fibers indicate extensive or very aggressive bleaching sequences, as shown in Fig. 5. The slight reduction in HexA content following mechanical refining aligns with the understanding that refining can impact HexA lev- els by removing hemicellulose [87]. HexA levels below 5 μmol/g require specific treatments targeting HexA, such as using xylanase enzymes or additional oxidative bleaching stages [88]. In that sense, the enzyme treat- ment led to a significant decrease in the HexA content of the fibers. This reduction is likely due to the enzymatic cleavage of the HexA groups, particularly by xylanase enzymes, which target the xylan chains in the fiber and break down the hemicellulose portion that contains the HexA groups [89, 90]. The mechanism involves the selec- tive hydrolysis of xylan and associated components,
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