Abstract: Preparation of oligosaccharides from straw fiber were studied by ultrasonic hydrolysis and condensation classification.The results show that significant impacts on cellulose crystal structure, glycosidic bond breaking and molecular polymerization degree of different ultrasonic irradiation intensity were observed.When the ultrasonic power 180W, reaction time 60 min, settling time 120min, the hydrolysates were mainly distributed at cellotriose and cellotetraose, total yield of oligosaccharides were up to 42.9%, which were 3.46 times compared with the absence of ultrasonic hydrolysis, and reaction and condensate time were reduced by 1/3 and 1/2. In addition, the classification technology can be used to classify oligosaccharides from different polymerization degree.

Key words: straw fiber, ultrasonic hydrolysis, oligosaccharides, condensation classification