L. casei subsp. rhamnosus (DSM 20021) was maintained on MRS-agar (Merck) medium and subcultured fortnightly.
Synthetic medium used for lactic acid production contained (per l) 10 g yeast extract, 0.5 g
and 0.005 g ascorbic acid; and 100 g sugar comprising de®nite amounts of both glucose and lactose. Sterilization of the
medium components was performed by autoclaving at 120°C for 20 min. Sugar solution was autoclaved separately prior to mixing with the rest of the medium.