An instrument with electrochemical sensors for measuring oxygen, carbon dioxide and glucose is described. The instrument has been used to monitor the variation of the three analytes in the fermentation of beer both in the laboratory and in a 1700 dm3 vessel in a pilot plant. Typical results are presented. The control of oxygen levels at the start of the process is important. The concentration of glucose passes through a maximum. The time to this maximum and the initial rate of CO2 production provide early indicators of yeast vitality.