132 Int. J. Exergy, Vol. 25, No. 2, 2018 Copyright © 2018 Inderscience Enterprises Ltd. The mode of interaction of the constituents of a microbial system determines the attainable exergy utilisation Bahar Değerli* and Mustafa Özilgen Department of Food Engineering, Yeditepe University, Istanbul, 34755, Turkey Email: bahardegerli@gmail.com Email: mozilgen@yeditepe.edu.tr *Corresponding author Abstract: The typical text book definition of exergy is the “maximum work that this system can produce if it is brought to thermal, mechanical and chemical equilibrium with its surroundings via reversible processes”. In microbial systems, this ability depends on the interaction, e.g., collaboration, competition or antagonism, between the constituents of the system. Leavening of dough with mixed cultures of microorganisms is a typical example, where the microbial constituents of a system may present different modes of interaction depending on the temperature and their relative amounts. Data pertinent to leavening at four different temperatures and six different inoculum ratio of Saccharomyces cerevisiae and Lactobacillus plantarum were processed. The maximum carbon dioxide production (42.7 g CO 2 /kg dough), expansion work (0.334 kJ/kg dough) and dough volume increase (3,300 cm 3 /kg dough) were obtained at 35°C with the inoculum consisting of 80% S. cerevisiae and 20% L. plantarum. The system achieved the highest exergy utilisation (2000 kJ/kg dough) under these conditions, demonstrating that the microbiological and the exergy optima were not independent and achieved under the same conditions. Keywords: exergy efficiency; constituents of the system; ability of resource utilisation; sour dough leavening. Reference to this paper should be made as follows: Değerli, B. and Özilgen, M. (2018) ‘The mode of interaction of the constituents of a microbial system determines the attainable exergy utilisation’, Int. J. Exergy, Vol. 25, No. 2, pp.132–151. Biographical notes: Bahar Değerli received BSc in Chemical Engineering from Yeditepe University, in Turkey, and MSc in Molecular Biology-Genetics and Biotechnology from Istanbul Technical University, in Turkey. She is currently a PhD student in the Biotechnology Institute at Yeditepe University. She is working on the kinetic modelling and thermodynamic assessment of the biosystems. Mustafa Özilgen is a Chemical Engineer and a Professor at Yeditepe University in Turkey. He received BS and MS from the Middle East Technical University (Turkey) and PhD from the University of California at Davis (USA). He taught classes at the Middle East Technical University, Massey University (New Zealand) and University of California at Davis (USA) and worked as a consultant at Marmara research Center (Turkey). He is an author of five