A small volume bioreactor comprising a cultivation vessel that - has a working volume of from 20 ml to 350 ml, - comprises a stirrer shaft with at least one impeller affixed thereto, - comprises a feed pipe comprising i) a sparging tube connected to a sparger at its end, and ii) at least one feed line with an opening at its end, - comprises two or more baffles (114; 126) extending from the wall of the cultivation vessel perpendicular in the direction to the center of the cultivation vessel, and - a reactor head plate, wherein the reactor head plate comprises - a fitting for connecting the drive axis of a motor to the stirrer shaft, - a sparger gas inlet connected to the sparging tube in the feed pipe, optionally a gas inlet connected to the headspace, - a gas outlet connected to the headspace of the cultivation vessel, - at least one inlet for liquids of a feed line being part of the feed pipe, - one in-situ sensor port with a pH electrode mounted thereto, and - a supply port area comprising the sparger gas inlet and the inlet of the at least one feed line, wherein the cultivation vessel and the reactor head plate are both substantially made of non-metal material, whereby the small volume bioreactor comprises an in-situ glucose sensor. a cultivation vessel that - has a working volume of from 20 ml to 350 ml, - comprises a stirrer shaft with at least one impeller affixed thereto, - comprises a feed pipe comprising i) a sparging tube connected to a sparger at its end, and ii) at least one feed line with an opening at its end, - comprises two or more baffles (114; 126) extending from the wall of the cultivation vessel perpendicular in the direction to the center of the cultivation vessel, and - a reactor head plate, - has a working volume of from 20 ml to 350 ml, - comprises a stirrer shaft with at least one impeller affixed thereto, - comprises a feed pipe comprising i) a sparging tube connected to a sparger at its end, and ii) at least one feed line with an opening at its end, - comprises two or more baffles (114; 126) extending from the wall of the cultivation vessel perpendicular in the direction to the center of the cultivation vessel, and - a reactor head plate, wherein the reactor head plate comprises - a fitting for connecting the drive axis of a motor to the stirrer shaft, - a sparger gas inlet connected to the sparging tube in the feed pipe, optionally a gas inlet connected to the headspace, - a gas outlet connected to the headspace of the cultivation vessel, - at least one inlet for liquids of a feed line being part of the feed pipe, - one in-situ sensor port with a pH electrode mounted thereto, and - a supply port area comprising the sparger gas inlet and the inlet of the at least one feed line, - a fitting for connecting the drive axis of a motor to the stirrer shaft, - a sparger gas inlet connected to the sparging tube in the feed pipe, optionally a gas inlet connected to the headspace, - a gas outlet connected to the headspace of the cultivation vessel, - at least one inlet for liquids of a feed line being part of the feed pipe, - one in-situ sensor port with a pH electrode mounted thereto, and - a supply port area comprising the sparger gas inlet and the inlet of the at least one feed line, wherein the cultivation vessel and the reactor head plate are both substantially made of non-metal material, whereby the small volume bioreactor comprises an in-situ glucose sensor.