A method for separating off compounds in gaseous aggregate state from a polymer melt which contains these compounds in liquid or solid aggregate state and which contains polylactide and/or a poly(co)lactide, with a thin-film treatment apparatus, comprising - a housing with a heatable and/or coolable housing casing which encloses a rotationally symmetrical treatment chamber extending in the axial direction, - at least one inlet port arranged in an inlet region of the housing, for introducing the material to be treated into the treatment chamber, - at least one outlet port arranged in an outlet region of the housing, for discharging the material from the treatment chamber, and - a drivable rotor shaft arranged in the treatment chamber and extending coaxially therewith for producing a film of material on the inner surface of the housing casing and for conveying the material in the direction from the inlet region towards the outlet region, wherein the rotor shaft comprises a central rotor shaft body and rotor blades arranged on the periphery of said rotor shaft body, the radially outermost ends of which blades are spaced apart from the inner surface of the housing casing, and also - at least one temperature sensor arranged in the treatment chamber for measuring the temperature of the film of material, in which the polymer melt in a viscous aggregate state is introduced into the treatment chamber by way of the at least one inlet port, a film of the polymer melt is produced on the inner surface of the housing casing, wherein at least a portion of the compounds contained in the polymer melt in liquid or solid aggregate state is converted into the gaseous aggregate state and is discharged from the thin-film treatment apparatus in the gaseous aggregate state, and also the treated polymer melt is discharged from the thin-film treatment apparatus by way of the at least one outlet port, characterised in that a permanent or temporary determination of a local temperature value of the film of the product is carried out at at least one point by means of the at least one temperature sensor. - a housing with a heatable and/or coolable housing casing which encloses a rotationally symmetrical treatment chamber extending in the axial direction, - at least one inlet port arranged in an inlet region of the housing, for introducing the material to be treated into the treatment chamber, - at least one outlet port arranged in an outlet region of the housing, for discharging the material from the treatment chamber, and - a drivable rotor shaft arranged in the treatment chamber and extending coaxially therewith for producing a film of material on the inner surface of the housing casing and for conveying the material in the direction from the inlet region towards the outlet region, wherein the rotor shaft comprises a central rotor shaft body and rotor blades arranged on the periphery of said rotor shaft body, the radially outermost ends of which blades are spaced apart from the inner surface of the housing casing, and also - at least one temperature sensor arranged in the treatment chamber for measuring the temperature of the film of material, in which the polymer melt in a viscous aggregate state is introduced into the treatment chamber by way of the at least one inlet port, a film of the polymer melt is produced on the inner surface of the housing casing, wherein at least a portion of the compounds contained in the polymer melt in liquid or solid aggregate state is converted into the gaseous aggregate state and is discharged from the thin-film treatment apparatus in the gaseous aggregate state, and also the treated polymer melt is discharged from the thin-film treatment apparatus by way of the at least one outlet port, characterised in that a permanent or temporary determination of a local temperature value of the film of the product is carried out at at least one point by means of the at least one temperature sensor.