Galactooligosaccharide production by culture of bullera singularis and beta-galactosidase

Abstract

The production of galactooligosaccharide (GOS) by microorganisms and enzymes with transgalactosylation activity was studied. A number of microorganisms with the ability to produce GOS was screened and a fungal strain of Bullera singularis ATCC 24193 was selected as the best GOS-producing strain considering GOS yield and enzyme activity. The cultural conditions of B. singularis were optimized for the production of GOS. Optimum temperature was 25℃, the pH was 6.0, and the inoculum size was over 5% (v/v). GOS production by the culture of B. singularis was shown to be proportional to cell growth. The production rate increased most sharply at an exponential growth phase. Through optimization of pH and initial lactose concentration, a maximum GOS% (defined as the ratio of GOS to total saccharides in the medium) of 92% was reached with 10% lactose at an initial pH 4.0. This value corresponds to 70 g-GOS/l medium. The cultural products were mainly of trisaccharides and tetrasaccharides as confirmed from the HPLC and TLC analysis. A new mechanism of GOS production was proposed based on the metabolism of the carbon source. Enzymatic production of GOS was performed by partially purified -galactosidase. Ammonium sulfate precipitation and ultrafiltration methods were used to prepare the enzyme. A maximum yield of 40% GOS, corresponding to 120 g GOS/l, was reached from the 300 g/l lactose solution at 45 C and pH 3.7. The yield of GOS did not increase with initial lactose concentration and pH change. Reaction products were found to be comprised of a disaccharide and trisaccharides. The structure of the major product, a trisaccharide, was proposed to be ο-β-D-galactopyranosyl-(1-4)-o-β-D-galactopyranosyl-(1-4)-β-D-glucose (4``-galactosyl lactose). GOS were continuously produced using lactose and immobilized -galactosidase in a packed bed reactor. Partially purified -galactosidase was immobilized onto a Chitopearl BCW 3510 bead (970 GU/g-resin) by simple adsorption method. Amou...