A method of crystallizing α-lactose monohydrate from an aqueous solution comprising dissolved α-lactose and β-lactose, said method comprising: circulating a first volume of said aqueous solution in a mutarotation loop in a mutarotation loop system; said mutarotation loop system comprising: - a crystallization tank operating at a crystallization temperature, T cryst, from 0°C to 30°C for crystallizing α-lactose monohydrate from said aqueous solution, said crystallization tank comprising a second volume of said aqueous solution, said second volume larger than said first volume, preferably much larger than said first volume; - a mutarotation tank operating at a mutarotation temperature, T muta, from 30°C to 110°C for promoting mutarotation of β-lactose to α-lactose; - a heater for heating said first volume from said crystallization temperature, T cryst, to said mutarotation temperature, T muta; - a cooler for cooling said first volume from said mutarotation temperature, T muta, to said crystallization temperature, T cryst; and - at least one circulating means for circulating said first volume in said mutarotation loop system from said crystallization tank to said heater to said mutarotation tank to said cooler and back to said crystallization tank; wherein - said mutarotation tank is operated under iso-volumetric conditions; and - said first volume is retained in said mutarotation tank for a mutarotation time, τ muta, of less than 1 hour. circulating a first volume of said aqueous solution in a mutarotation loop in a mutarotation loop system; said mutarotation loop system comprising: - a crystallization tank operating at a crystallization temperature, T cryst, from 0°C to 30°C for crystallizing α-lactose monohydrate from said aqueous solution, said crystallization tank comprising a second volume of said aqueous solution, said second volume larger than said first volume, preferably much larger than said first volume; - a mutarotation tank operating at a mutarotation temperature, T muta, from 30°C to 110°C for promoting mutarotation of β-lactose to α-lactose; - a heater for heating said first volume from said crystallization temperature, T cryst, to said mutarotation temperature, T muta; - a cooler for cooling said first volume from said mutarotation temperature, T muta, to said crystallization temperature, T cryst; and - at least one circulating means for circulating said first volume in said mutarotation loop system from said crystallization tank to said heater to said mutarotation tank to said cooler and back to said crystallization tank; - a crystallization tank operating at a crystallization temperature, T cryst, from 0°C to 30°C for crystallizing α-lactose monohydrate from said aqueous solution, said crystallization tank comprising a second volume of said aqueous solution, said second volume larger than said first volume, preferably much larger than said first volume; - a mutarotation tank operating at a mutarotation temperature, T muta, from 30°C to 110°C for promoting mutarotation of β-lactose to α-lactose; - a heater for heating said first volume from said crystallization temperature, T cryst, to said mutarotation temperature, T muta; - a cooler for cooling said first volume from said mutarotation temperature, T muta, to said crystallization temperature, T cryst; and - at least one circulating means for circulating said first volume in said mutarotation loop system from said crystallization tank to said heater to said mutarotation tank to said cooler and back to said crystallization tank; wherein - said mutarotation tank is operated under iso-volumetric conditions; and - said first volume is retained in said mutarotation tank for a mutarotation time, τ muta, of less than 1 hour. - said mutarotation tank is operated under iso-volumetric conditions; and - said first volume is retained in said mutarotation tank for a mutarotation time, τ muta, of less than 1 hour.