Prebiotic Oligosaccharides from Xylan: Heat Pretreatment, Xylanase Hydrolysis, Utilization by Potential Probiotics and Evaluation in a Mouse Study

University dissertation from Chemistry Division of Biotechnology

Abstract: Modulation of the gut microbiota with prebiotic oligosaccharides from partially hydrolyzed xylans can be part of the solution for reducing the occurrence of many health problems such as diabetes, cardiovascular disease and cancer. Prebiotics are carbohydrates that selectively stimulate health beneficial bacteria in the colon, restoring homeostasis to the gut environment. Xylans are hemicelluloses from the cell wall of plants, which can be enzymatically degraded into short oligosaccharides with prebiotic properties. Production of prebiotics from xylans requires an understanding how the raw material, pretreatments and choice of xylanase impact the final product composition. Therefore we set out to make xylan-derived prebiotics from different raw materials and evaluated their prebiotic potential. In the first study we compared two types of xylans as substrates for a thermostable xylanase RmXyn10A to make carbon sources for bifidobacteria and lactobacilli. Results showed that arabinoxylooligosaccharides derived from cereal arabinoxylans are more selective than xylooligosaccharides from hardwoods in stimulating bifidobacteria over lactobacilli (Paper I). In the second study potential probiotic strains belonging to the genus Weissella were able to utilize xylooligosaccharides. This experiment also confirmed that short oligosaccharides are better than longer ones for fermentation by lactic acid bacteria (Paper II). In the third study we used an environmentally friendly pretreatment method (using only water) on wheat bran to increase the content of water-soluble arabinoxylan to make prebiotics that can selectively stimulate healthpromoting bacteria. Depending on the temperature and treatment time, different yields and arabinoxylan fragments could be obtained (Paper III). In the fourth study we used heat pretreatment together with xylanase hydrolysis to improve the prebiotic potential of cereal byproducts. Mice fed a high fat diet supplemented with prebiotics from rye bran showed increased growth of bifidobacteria, which can be linked to improved metabolic functions (Paper IV). In the fifth study we aimed at improving the product quality of arabinoxylan from heat pretreated rye bran and comparing the product profiles when using two different xylanases. Removal of starch and proteins from the product before or after the heat pretreatment significantly improved the purity. Xylanases from family 10 were found to be better than for family 11 in producing short oligosaccharides from arabinoxylan (Paper V). In the last study a novel β-xylosidase belonging to a Weissella strain was shown to preferentially hydrolyze short xylooligosaccharides xylobiose and xylotriose (Paper VI).