IMPACT OF CARBON-DIOXIDE EVOLUTION ON THE CALORIMETRIC MONITORING OF FERMENTATIONS

During fermentation, carbon dioxide formation usually takes place. The absorption of carbon dioxide in water is an exothermic reaction; its release from the medium is endothermic. These effects were first analy sed in a fermenter-calorimeter using model media and then monitored du ring anaerobic batch and continuous cultivation of Zymomonas mobilis c ells. The heat effects proved to be functions of the carbon dioxide pa rtial pressure, the pH value of the medium, the gas flow rate through the liquid, and the pump rate of the medium through the fermenter (con tinuous culture). The heat of biological growth is usually diminished by carbon dioxide desorption. The maximum error introduced in the on-l ine measurement was calculated to be 48% for anaerobic conditions; the average measured was 35%. The worst case calculated for aerobic condi tions was 4%. In the case of a pH-controlled fermentation, the neutral ization heat from alkaline feeding contributes to the total heat produ ction measured. When the carbon dioxide partial pressure was maintaine d constant and the volume of the exhaust gas produced and the alkaline consumption were known, monitoring of the small biological heat produ ction rates under anaerobic conditions was possible with reasonable ac curacy.