The present invention relates to milk products comprising a high amount of in situ produced galactooligosaccharides (GOS) and methods of producing same. Milk substrate having a lactose content of at least 20wt% lactose is treated with an enzyme having transgalactosylating activity. The transgalactosylating activity of said enzyme may have been increased via glycation of lysine and/or arginine residues by incubation of said enzyme with high concentrations of glucose at elevated temperatures.