Galacto-oligosaccharides (GOS) contain two to five molecules of galactose and one glucose connected through glycosidic bonds. They are classified as non-digestible dietary fibers and widely used in the nutritional market because of their beneficial health effects on stimulating Bifidobacteriain the lower part of the human intestine. GOS can be produced by enzymatic hydrolysis of lactose using β-galactosidase in either free or immobilized form. The goal of this research was to evaluate the feasibility of immobilizing Aspergillus oryzaeand Bacillus circulansβ-galactosidases on commercial nanofiltration (NF) membranes for the production and purification of GOS from lactose. These enzymes were immobilized on NF membranes by adsorption with or without the binding support of polyethyleneimine (PEI).The immobilized enzyme showed the same GOS formation kinetics as that of the free enzyme, indicating that there was no diffusion limitation of the immobilized enzyme reaction with the catalytic membrane.
The nanofiltration membranes with immobilized lactases were then studied in the dead-end filtration system for the simultaneous production and separation of GOS. The results showed that the GOS formation kinetics in the simultaneous reaction/separation at the early stage of lactose conversion was the same as that in the conventional method without separation. However, as lactose was further converted beyond 50% conversion, the GOS content in the simultaneous system increased as a result of the removal of inhibiting glucose from the reaction mixture.The GOS content increased approximately 15% in the simultaneous reaction/separation, with approximately 28-32% of monosaccharides removed.
Nanofiltration of GOS products from the enzymatic process was evaluated for further removing monosaccharides (glucose and galactose) from the GOS product mixture under diafiltration conditions. A high degree of removal of monosaccharides (~94%) from the GOS mixture could be achieved with three to five cycles of diafiltration. The final GOS mixture contained less than 1% of glucose and galactose. The nanofiltration membrane has a great potential to use both as a biocatalytic membrane and as a separating unit to separate monosaccharides from the sugar mixture.