Enzymatic hydrolysis with commercial enzymes of a xylan extracted from hardwood pulp

Abstract

In the forest products industry the opportunity exists to extract currently under-utilised compounds from the process or waste streams and thereby derive more value from the wood entering the process. A big portion of the hemicellulose content of wood does not form part of the final product. Extracting the hemicelluloses from the waste streams or other locations in the process would allow them to be used more effectively. The predominant hardwood hemicellulose, xylan, is polymeric xylose. Xylose is an important platform sugar in bioconversion strategies and can be converted to fuels and other valuable chemicals. The xylan polymer can be hydrolysed to its xylose monomers by a number of conversion strategies; the most widely known being chemical and enzymatic digestion. Chemical conversion is usually done using acid at elevated temperatures, but high yields are often offset by degradation of the product. On the other hand, enzymatic hydrolysis can be better regulated to prevent unwanted degradation of the monomeric sugar products. Enzymatic hydrolysis has been pronounced the environmentally friendly choice of technology, although it is hampered by low conversions and high cost of enzymes. To date commercial enzymes for biomass conversion are not readily available most of which are still in development. In understanding how to best utilise a xylan, recovered from the pulping process, the potential to convert hardwood xylan to xylose with enzymes currently available on the market was studied. A hardwood xylan extracted from fully bleached Eucalyptus pulp with a chelating agent, Nitren, was used as substrate to evaluate the ability of some commercial enzymes to degrade the extracted xylan to xylose monomers. The enzymes used in this study were not dedicated biomass conversion enzymes, but rather chosen for their xylan degrading potential, i.e. xylanase content. By means of hydrolysis profiles on commercial Birchwood and Oat Spelts xylan as substrates and enzyme characterisation, Multifect xylanase was identified as most promising enzyme for xylan conversion. Multifect contained high levels of xylanase and xylosidase activity in the enzyme preparation. Commercial Birchwood xylan and the extracted Eucalyptus xylan were found to be chemically similar, both composed predominantly of xylose. The hydrolysis profiles obtained on Birchwood xylan could therefore serve as a benchmark against which the hy-drolysis of Eucaluptus xylan could be compared. Full conversion of the Eucalyptus xylan with Multifect could not be achieved, although Multifect completely degraded the Birchwood xylan. The maximum xylose yield that could be obtained on Eucalyptus xylan was 80 % and it was concluded that the remaining 20% was unhydrolysable by the enzyme, most likely due to the limitations in the employed extraction method. It was however noted that up to the point of 80 % conversion higher hydrolysis rates were observed on Eucalyptus xylan than Birchwood xylan with equal charges of Multifect. The differences in hydrolysis rates may have indicated that the Eucalyptus xylan is more accessible to enzyme attack than the Birchwood xylan, likely as a result of the extraction methods used to prepare the xylans. A simple economic evaluation illustrated the weight of various costs in process profitability. The most economic operation of a continuous steady state reactor is at a low enzyme charge, 17 IU/ℓ, and a long retention period, five days, due to the high cost of the enzyme compared to other factors. For a reduced retention time, an investigation into enzyme immobilisation and the use of a packed-bed type reactor is recommended.