Towards an online milk concentration sensor using ERT: Correlation of conductivity, temperature and composition

The milk powder production process is a multistage process in which the function of each stage affects the final product quality. Having accurate and multidimensional knowledge of the milk solution at different stages of the process is crucial in order to maintain satisfactory control of the process. Electrical resistance tomography (ERT) is a novel, robust and high speed method of process imaging. It has the potential to overcome the lack of online concentration sensors and provide multidimensional concentration information at various stages of the milk powder production process. In order to achieve this goal an accurate model is required to correlate the ERT's multidimensional conductivity measurements to milk concentration/total solids content. This work focuses on obtaining a correlation of milk conductivity, temperature and composition (including powder protein, lactose and fat content on a dry basis) to the total solids content for whole and skim milk in the process to produce milk powder. For this purpose, the Design of Experiments (DOE) methodology was applied to plan and conduct a Central Composite Design (CCD) experiment. The resulting Response Surface, correlating the factors to whole and skim milk concentration, provided an accuracy of over 93%. The results were also compared to the results of a multiple linear regression model correlating only milk conductivity and temperature to its total solids content. Multiple linear regression models presented better accuracy for lower concentrations while the CCD model presented better accuracy for higher concentrations. (C) 2012 Elsevier Ltd. All rights reserved.