The author has developed new calculation methods for serially connected flow-through type food reactor units, working on the macro-mixing principle and being independent regarding the residence time of macro-elements. Constant temperatures and ’closed behaviour’ (a concept from the dispersion models) are also conditioned for the units. Quality attributes include concentration of surviving microbes, activity of enzymes, concentration of vitamins or other constituents, sensory attributes and related physical properties. Their variations at constant temperature, however, should be dependent only on the product of a rate constant and time, and moreover based on previous knowledge of the residence time distributions (RTDs) in the units. A rather simple convolution integral has been deduced for the calculation of the overall change of a quality attribute. Further simplifications are possible for first order changes. As an example, details of treatment of three units at different temperatures and figures illustrating some basic ideas and the difference between original and modified RTD functions are given.