Galactooligosaccharides formation during enzymatic hydrolysis of lactose: towards a prebiotic-enriched milk.

The formation of galactooligosaccharides (GOS) in skim milk during treatment with several commercial β-galactosidases (Bacillus circulans, Kluyveromyces lactis and Aspergillus oryzae) was analysed in detail, at 4 and 40°C. The maximum GOS concentration was obtained at a lactose conversion of approximately 40-50% with B. circulans and A. oryzae β-galactosidases, and at 95% lactose depletion for K. lactis β-galactosidase. Using an enzyme dosage of 0.1% (v/v), the maximum GOS concentration with K. lactis β-galactosidase was achieved in 1 and 5h at 40 and 4 °C, respectively. With this enzyme, it was possible to obtain a treated milk with 7.0 g/L GOS - the human milk oligosaccharides (HMOs) concentration is between 5 and 15 g/L--and with a low content of residual lactose (2.1g/L, compared with 44-46 g/L in the initial milk sample). The major GOS synthesised by this enzyme were 6-galactobiose [Gal-β(1 → 6)-Gal], allolactose [Gal-β(1 → 6)-Glc] and 6'-O-β-galactosyl-lactose [Gal-β(1 → 6)-Gal-β(1 → 4)-Glc].