Purification of highly concentrated galacto-oligosaccharide preparations by selective fermentation with yeasts

Galacto-oligosaccharides (GOS) were synthesised from 50% (w/w) lactose with Aspergillus oryzae β-galactosidase. Reacted medium without nutrient supplementation (raw GOS) was fermented with Kluyveromyces marxianus cells obtaining GOS of 95% purity containing mostly tri- and tetrasaccharides with total recovery of GOS after 24h. The effect of the initial concentration of carbohydrate was assessed in the range of 10-50% (w/w), such purity being obtained even at 50% (w/w) (undiluted raw GOS) though being strongly influenced by the dry cell to total carbohydrate mass ratio (R_CC). The process of purification by selective fermentation of raw GOS at 20% (w/w) was optimised in terms of the specific productivity of unwanted carbohydrate removal, considering temperature, pH and R_CC as operational variables. According to the experimental design, maximum specific productivity predicted was 0.16gg^-1h^-1, obtained at the optimal operating conditions of 40°C, pH 3.5 and R_CC 1.55gg^-1.