You can extract hemicellulose into either NaOH or KOH (see for example Peng et al Planta 2000 211:406-414. However, NaBH4 is used to reduce the reducing end of the polysaccharide,to prevent peeling.
if you want to remove hemicellulose only,, you can prehyrolysis the raw materials with water only . condition is 150 to 170o C AND 1 hr. and M:L IS 1:5 OR 1:6