Thermal conductivity of whey protein films undergoing swelling - Measurement by dynamic gauging

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 combination of 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, 20-50degreesC; and flow velocities, 0.03-0.30 m s(-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 W m(-1) K-1 to values at maximum swelling time of up to 0.43 W m(-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 100degreesC 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.