Palm kernel was dried at 50 °C and then crushed to powder. A fraction (100 g) with a particle size smaller than 60 mesh was dispersed in 1000 mL of water, and the system was heated to reflux for 3 h, after which time concd sulfuric acid (5 mL) was added dropwise at 100 °C. After 30 min of reaction, the system was cooled to room temperature and then neutralized with Ba(OH)2. The insoluble solid was filtered out and washed twice with 100 mL of deionized water. The washings were combined with the previous filtrate, and the mixture was then evaporated to 100 mL. The concentration of d-mannose in the solution was determined by HPLC.
The hydrolysate was then added with 1 g of mannan-degrading enzymes (0.01 g/mL) and fermented at 60 °C and pH 7 for 60 min. The concentration of d-mannose in the new solution was detected under the same conditions and compared with the previous results.