Immobilization of whole cells of Lactococcus lactis containing high levels of a hyperthermostable β-galactosidase enzyme in chitosan beads for efficient galacto-oligosaccharide production

Galacto-oligosaccharides (GOS) are prebiotic food ingredients that are proposed to stimulate the growth of beneficial gut microorganisms, particularly bifidobacteria. Previously, we developed a method for efficient GOS production using whole cells of Lactococcus lactis containing high levels of a hyper-thermostable β-galactosidase enzyme from Sulfolobus solfataricus. In this study, a recombinant DNA removal and whole-cell enzyme immobilization process was developed to produce GOS from lactose before removal of the immobilized whole-cell enzyme, which could be reused for subsequent applications. Chitosan was found to be a superior immobilization material compared with alginate, as it retained its bead structure during the high temperature (90°C) used here for GOS production. Prior to immobilization, the recombinant DNA was degraded in the whole cells using UV treatment, resulting in an immobilized whole-cell enzyme that was free of recombinant DNA and with minimum effect on the efficiency of the enzyme. The optimum pH and temperature for GOS synthesis using the chitosan beads was pH = 5.5 and 90°C. The highest GOS production using the chitosan beads occurred with 40% initial lactose resulting in 150 g/L of GOS (tri-oligosaccharides and tetra-oligosaccharides) in addition to di-oligosaccharide GOS products that were not quantified. Notably, the highest lactose conversion rate was found using lower starting lactose concentrations, with more than 60% conversion into tri-oligosaccharides and tetra-oligosaccharides. The immobilized enzyme retained ∼50% activity after 2 cycles of GOS production. In conclusion, the chitosan-immobilized whole-cell enzyme can be used for efficient GOS production that is free of the whole-cell enzyme as well as detectable recombinant DNA.