For many years, calorimetry has been recognized as a powerful and universal tool for monitoring chem. and biol. processes. A lab.-scale reaction calorimeter (RC1, Mettler-Toledo), initially developed for chem. reaction studies with a sensitivity of 100-150 mW/l, has been improved to enable the monitoring of very low heat prodn. rates (<10 mW/l). A major limitation to successful process control, has been the inability to achieve real-time quant. calorimetry. This is in part due to the operating principle of the RC1, in which the measured heat signal is calcd. from the temp. difference between the reaction mass and the jacket oil and the heat transfer coeff. (UA). The latter frequently varies during a reaction, particularly a bioreaction, due to changes in vol., viscosity and cell d., and is difficult to det. accurately during the process.In the present study, this problem has been solved by a tech. modification to the reactor vessel of the RC1. This involves forcing the heat transfer to occur through a well defined and const. area through the creation of a large resistance to heat transfer in the upper part of the reactor vessel. This was achieved by creating an air gap between the reactor contents and the reactor wall through the insertion of a PTFE sleeve. Control expts. undertaken with this modified system, in the absence of any reaction, showed that UA remained const. for vol. changes as large as 50% of the working vol. Similarly, a simulated fed-batch expt. with monitoring of the stirring power, showed that the baseline heat signal could be accurately and quant. cor. for large dynamic variations of the vol.Using monitoring of the oxygen uptake rate as a ref., this modified system was validated by application to fed-batch cultures of Bacillus sphaericus 1593M. This strictly aerobic bacterium produces parasporal insecticidal crystal proteins which are toxic to mosquito larvae. In these fed-batch cultures, the nutrient feed was controlled using measurement of the metabolic heat release, since the latter is proportional to the substrate uptake rates for a given metabolic state. A culture, composed of two repetitive fed-batch cycles followed by a batch cycle, demonstrated that real-time and quant. signals could be obtained, even for highly dynamic processes in which the vol. and agitation rate may vary significantly and where quick repetitive inoculations can be made. The result of this work is a modified RC1 (or Bio-RC1) which is as easy to use as any conventional bioreactor yet has the unique feature of being able to provide an accurate measurement of the energy dissipated as heat in chem. or biol. processes, over a wide range of operating conditions. [on SciFinder (R)]