Preparation of high-purity lactulose through efficient recycling of catalyst sodium aluminate and nanofiltration: a pilot-scale purification.

BACKGROUND: Lactulose, a valuable lactose-originated 'bifidus factor' product, is exclusively produced by chemical-based isomerization commercially. A complexing agent of sodium aluminate exhibiting high conversion efficiency and strong recyclable stability is more practical for industrial applications. In this study, efficient purification of high-purity lactulose through recycling of sodium aluminate and further desalination by nanofiltration (NF) was implemented on a pilot scale. RESULTS: Over 99.5% of the catalyst was prior recycled in the form of Al(OH)3 precipitate by pH-induced precipitation and centrifugation; residual aluminum was further absorbed by ion exchange resin to an acceptable level (≤10 mg kg-1 ). Subsequently, impurities (monosaccharides and NaCl) were ideally separated from lactulose syrup by NF based on their significant retention differences (lactulose 94.8-97.2% > lactose 86.2-93.5% > monosaccharides 36.3-48.7% > NaCl 9.5-31.1%). High-purity (>95%) lactulose was obtained with >90% yield in both constant and variable volume diafiltration (CVD and VVD) modes when the volume dilution ratio (Vc /Vf ) was 4.0 and 2.5 respectively. Both experimental and predicted results showed that the VVD mode was more water-saving in practice. CONCLUSION: This is the first trial purification of lactulose syrup from chemical isomerization of lactose catalyzed by sodium aluminate, and the applied methodology is a promising industrial-scale purification strategy. © 2018 Society of Chemical Industry.