ABSTRACT Transfer function approach to predict thermal histories of conductive foodstuffs when surroundings present different forcing functions during start-up of processes is reported. Food is assimilated to a delayed first-order system with unit gain and characterized by two parameters, the dead time (L) and the time constant (τ). The comparison of experimental data with the proposed models for cooking as well as for retorting processes presented very satisfactory results when the equipment evolved as first-order, second-order and bounded integrating systems. PRACTICAL APPLICATIONS Transfer functions appear as relevant mathematical tools because they are relatively simple models that need only dynamics input–output data to represent more complex systems related to food process engineering, generally nonlinear ones. Besides, their potential application allows the performance of optimal process control in real-time modality. In particular, this paper deals with the problem of processes start-up, which represents a transient operation to be overcome, and also with the ability of the transfer function methodology to estimate both food and equipment temperature evolution during different food operations when changes in the surrounding temperature take place.