Many studies demonstrate the suitability of the heat production rate for analysing and controlling aerobic growth and production processes ranging from the mu L to the m(3) range. Recently anaerobic bioprocesses leading to biofuels (e.g. ethanol, n-butanol), biogas, solvents (e.g. acetone, 1,2 butandiol) and small organic acids (e.g. acetate, butyrate) have attracted industrial attention. However, calorimetric monitoring of such processes challenges particularly the sensitivity of the applied sensors. A simple and cost-efficient solution for large scale reaction calorimeters is needed. We show that the sensitivity of a standard heat flux reaction calorimeter (here tested for RC1e) can be improved by the factor of 10 to (1.886 +/- 0.012) KW-1 by simply adding an additional internal thermal shield. Signals of about 5 mW L-1 can be detected this way and the steady-state response time is about 10 min. Both criteria of relevance (sensitivity and steady-state response times) are sufficient for anaerobic growth processes. This was successfully demonstrated at the example of the acetone-butanol-ethanol (ABE) fermentation using Clostridium acetobutylicum. Here the calorimetric.signal matches perfectly other simultaneously measured on-line signals and parallel measurements with a high-sensitive microcalorimeter. (C) 2013 Elsevier B.V. All rights reserved.