A considerable difference between two explicit finite difference heat transfer simulation approaches was described. Time step restrictions, which are often the basis for criticism of explicit methods, were shown to be less severe when the surface was not considered to have thermal mass. Thermal process design combines heat transfer and reaction kinetics considerations. Accuracy of lethality estimates depends on time step size. Guidelines were developed for applying discrete time-temperature data to thermal process calculations. For time steps small enough to permit a linear temperature change approximation, it was shown that for increasing (decreasing) temperatures, a value within 50 to 60% (40 to 50%) of total temperature change should be used.