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BACKGROUND: Advances in industrial biotechnology offer potential opportunities for economic utilization of abundantly available agricultural crop waste such as wheat straw, which is of great interest for advancement in xylo-oligosaccharides (XOS) and fermentable monosaccharides. In the present study, crude XOS and fermentable monosaccharides were sequentially produced by a combined chemical and enzymatic process from wheat straw.RESULTS: Maximum yield of XOS was 38.2% and the corresponding yield of glucose was 82.7% despite the wax barrier on the surface of wheat straw, while the acetic acid concentration was decreased from 20% to 5%. Pilot scale run produces 91.6 g XOS, 92.5 g xylose and 316.0 g glucose from 1000 g of wheat straw that means the utilization of 78.9% of xylan and 89.7% glucan constitutes.CONCLUSION: By removal of 65% xylan from wheat straw, enzymatic hydrolysis of cellulose was also improved efficiently although its polymer structure was almost kept inert during acetic acid pretreatment. ABBREVIATIONSAA acetic acid DC degraded chemicals HMF hydroxymethyl furfural SSTs stainless steel tubes XOS xylo-oligosaccharides Analytical method The sugars and degraded chemicals were detected by high-performance liquid chromatography (HPLC, Agilent 1260, J Chem Technol Biotechnol 2020; 95: 694-701Figure 3. (a) Enzymatic hydrolysis at various cellulase loading; (b) main sugar contents variations in 15 FPIU/g enzymatic hydrolysis; (c) changes of polysaccharide components in wheat straw before and after enzymatic hydrolysis and pretreatment. J Chem Technol Biotechnol 2020; 95: 694-701

Background Methods to produce XOS have been intensively investigated, including enzymatic hydrolysis, steam explosion, and acid hydrolysis. Acid hydrolysis is currently the most widely used method to produce XOS due to its advantages of fewer processing steps, stronger raw material adaptability, higher yield, and better reproducibility. Especially, organic acids such as acetic acid, formic acid and xylonic acid work better as compared with mineral acids. However, the catalytic mechanism of different organic acids has been little studied. In this paper, four different organic acids, including formic acid, glycolic acid, lactic acid, and acetic acid were selected to compare their hydrolytic effects. Results Using pKa values as the benchmark, the yield of xylo-oligosaccharide (XOS) increased with the increasing value of pKa. The yield of XOS was 37% when hydrolyzed by 5% acetic acid (pKa = 4.75) at 170 ℃ for 20 min. Combined severity (CS), a parameter associated with temperature and reaction time was proposed, was proposed to evaluate the hydrolysis effect. The results of CS were consistent with that of pKa values on both the yield of XOS and the inhibitor. Conclusion The results based on pKa values and combined severity, a parameter associated with temperature and reaction time, concluded that acetic acid is a preferred catalyst. Combining the techno-economic analysis and environmental benefits, acetic acid hydrolysis process has lower factory production costs, and it is also an important metabolite and a carbon source for wastewater anaerobic biological treatment. In conclusion, production of xylo-oligosaccharides by acetic acid is an inexpensive, environment-friendly, and sustainable processing technique.

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