The instant invention relates to the use of a L-arabinose isomerase for converting D-galactose into D-tagatose in a dairy product which contains D- galactose, for instance during lactic acid fermentation of milk or by lactose hydrolysis, to a method of converting D-galactose into D-tagatose during lactic acid fermentation of milk, to a method of producing a sweetened fermented dairy product, and to a sweetening dairy product additive which comprises a L- arabinose isomerase and/or a L-arabinose isomerase-expressing microorganism. The invention also relates to a microorganism expressing an exogenous L- arabinose isomerase, and to a mutated L-arabinose isomerase US100 from Bacillus stearothermophilus, in particular to a mutant with enhanced acidotolerance and stability at acid pH or to a mutant with lower optimal temperature.
