Effect of the structure of galactooligosaccharides on their adsorption by activated carbon

Abstract

Gibson and Roberfroid introduced in 1995 the concept of prebiotics as non-digestible oligosaccharides that reach the colon without being hydrolyzed and are selectively metabolized by health-positive bacteria such as bifidobacteria and lactobacilli. Since then, intensive research on the role of intestinal microbiota in many aspects of health highlighted the potential importance of prebiotics and a great diversity of compounds are already used as prebiotics. However, despite the increasing demand and the broad diversity of structures of all proposed prebiotics, limited knowledge is available regarding structure function relationships in prebiotic carbohydrates. This is mainly due to the fact that most prebiotics are synthesized by enzymatic processes giving rise to a complex mixtures of chemical compounds difficult to separate as in the cases of lactose-derived galactooligosaccharides (GOS) (chains of β-D-galactose residues with a glucose end unit) and lactulose-derived GOS (chains of β-D-galactose residues with a fructose end unit). The aim of this study was to evaluate the effect of GOS structure on its adsorption by activated charcoal. Lactulose-derived GOS were synthesized using Duphalac (800 mL) and β-galactosidase from Aspergillus oryzae (16 U mL-1) following the method described by Anadón et al. Lactose-derived GOS were commercial. Fractionation of lactose-derived GOS and lactulose-derived GOS was performed on activated charcoal (3g) and different ethanol/water solutions (0, 1, 5, 8, 10 or 15%, v/v). The resulting mixture was stirred for 30 min at 25ºC and filtered under vacuum. The activated charcoal was washed with 25 mL of corresponding ethanolic solution. For both GOS samples, total monosaccharides were desorbed with water, as well as about 20% of unreacted disaccharides. Desorption of di- and oligosaccharides increased with ethanol content and decreased as the molecular weight of carbohydrates increased. In all assays lactose-derived GOS were more strongly adsorbed on activated charcoal than lactulose-derived GOS. The significant influence of the chemical structure of carbohydrates on the adsorption onto activated charcoal makes possible its use for separating carbohydrates with the same molecular weight but different structural formula.