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

•Milk with a significant content of galactooligosaccharides and a low amount of lactose was obtained.•β-Galactosidase from Kluyveromyces lactis is the most appropriate enzyme.•GOS concentration compares well with that of human milk oligosaccharides in human milk.•The process can be performed at 4°C in 5h with 0.1% enzyme dosage. 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 7.0g/L GOS − the human milk oligosaccharides (HMOs) concentration is between 5 and 15g/L – and with a low content of residual lactose (2.1g/L, compared with 44–46g/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].