| Xylo-oligosaccharides (XOS), which is a short-chain polymers constituted by 2-6 xylose units with β-1,4-glycoside bond, is a functional polymeric sugar with substantial market prospects, and now widely applied in the field of medicine, health and animal husbandry.Currently, the main methods for the XOS production from xylan are enzymatic hydrolysis, autohydrolysis, and acid hydrolysis. Since autohydrolysis and acid hydrolysis typically require a high temperature environment, it likely to cause degradation of XOS and generate more byproducts. Enzymatic hydrolysis usually requires a stringent substrate. In addition, since the xylan which extracted from lignocellulose obtained a certain amount of cellulose, lignin and other impurities. It is difficult to create an ideal xylan enzymatic hydrolysis substrate environment directly. Therefore, how to control the properties of the substrate and how to effectively combine various hydrolysis types became the focus of this article. The main works of this paper are as follows:1. For basic single factor hydrolysis experiment, this paper studied the effect of enzyme type, enzyme load, enzymatic hydrolysis time and substrate characters on sugar yield for enzymatic hydrolysis; acid type, acid concentration and acid hydrolysis time for acid hydrolysis. First, cellulases and hemicellulases could make xylan more degraded to monosaccharides, and xylanase mainly producted XOS. Acid hydrolysis environment caused more monosaccharide in short time, however alkaline hydrolysis environment would take longer time but caused less monosaccharides. Thus, in order to obtain the target product XOS as much as possible, the enzyme type selection is mainly endo-xylanase. Secondly, because of larger amount of byproducts would be caused, and the reaction condition is violent, this article used acid hydrolysis as one of the assisted method for enzymatic hydrolysis.2. The extracted xylan inevitably contains salt, ethanol, and pigment. In order to investigate the effects of these impurities on XOS production, the alkaline-extracted xylan with different kinds and concentrations of impurities was made and then hydrolyzed using alkaline xylanase (EC 3.2.1.8) to produce XOS. The results showed that a certain concentration of salt (NaCl) promoted the XOS production, while ethanol and pigment inhibited the enzymatic hydrolysis process significantly. The color value mainly ascribed to the phenolic compounds binding to xylan was a key restriction factor in the enzymatic hydrolysis later stage. Using optimal xylan sample (with 10 mg/mL NaCl, color value of 4.6×105, without ethanol) as substrate, the highest XOS yield was obtained of 58.58%. As the substrate of XOS production, prepared xylan should contain colored materials and ethanol as less as possible, however, retains appropriate salt.3. For produce XOS process from raw material corn stover, this paper designed xylan fraction preparation and multi-step hydrolysis method to improve the XOS yield. The "EH-EH", "EH-AH", "EH-AH-EH", and "AUH-EH" hydrolysis processes have been settled. The results are followed:For "enzymatic hydrolysis-acid hydrolysis-enzymatic hydrolysis" process, XOS yield was 62.77%, xylose yield was 15.03%, which significantly improved hydrolytic effect. And for the enzymatic hydrolysis after high-temperature treatment process got 83.81% XOS and 11.23% xylose yield.In summary, this paper investigated that xylan substrate properties and various hydrolysis methods, and got substantial yield of XOS. The results above provided a practical guidance for the efficient production of XOS. |
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