Films of whey protein were cleaned from stainless steel surfaces by treatment with alkaline solutions flowing through a duct. The films were monitored by the simultaneous use of a heat flux sensor and a fluid dynamic gauge, backed up by a protein assay. This combinationof techniques yields directly, for the first time, a quantitative account of the variation of a structure-sensitive property of the films, namely thermal conductivity, as their structures evolve during cleaning, particularly during the swelling stage of the cleaning process. For raw protein films—that is, for films that have not been subjected to ageing by steaming—the conditions studied fall in the ranges: NaOH concentrations, 0.3–2.0 wt%; bulk fluid temperatures, 20–50°C; and flow velocities, 0.03–0.30 ms −1 , corresponding to Reynolds numbers of 500–10,000. These resulted in swelling ratios of up to 4.4, for films which initially consisted of ca. 86% water, by volume. Thermal conductivities varied linearly with time during the swelling stage, from initial values of 0.26 Wm −1 K −1 to values at maximum swelling time of up to 0.43 Wm −1 K −1 . These latter values are substantially lower than one might expect for gels consisting mainly of water and differ substantially from estimates reported in the literature.A brief study is also reported of the effect of deposit ageing, which was simulated by holding plates in a steaming environment at 100°C for prolonged periods. In such cases, it proved possible to follow the evolution of thermal conductivity over the static and decay stages of the cleaning process.