Characterisation of the changes in the physicochemical properties of high-solids skim milk during thermal processing, and their relationship with fouling
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Date
2023
Authors
Murphy, Tara R.
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Publisher
University College Cork
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Abstract
Fouling of thermal processing equipment during production of dairy products and ingredients is a challenge for the dairy industry, with evaporator fouling during the production of skim milk powder being an area of particular focus. The build-up of foulant mass deposit within different regions of the evaporator technology has significant economic, operational, and environmental consequences for the dairy industry, as the decline in the heat performance of the equipment results in more frequent cleaning required and increased energy and water consumption, in addition to potential deterioration in product quality as the required product temperature may not be achieved. The objectives of the work reported in this thesis were to study the effect of pH on the heat stability and physicochemical properties of concentrated skim milk during thermal processing to assess their contributions to evaporator fouling. Heating concentrated skim milk (30%, w/w, total solids; pH 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7) using a high temperature short time thermal treatment (90 °C x 3 min), using a rheometer, showed that as the pH of the samples decreased, both viscosity and particle size distribution increased after heating, coinciding with an increase in sedimentation. Heat treatment (90 °C x 2 h) of concentrated skim milk at pH 6.2 on the fouling rig resulted in severe fouling. Moreover, when the concentrated skim milk was analysed for particle size and sedimentation after heat treatment using the fouling rig, the findings aligned with the initial analysis conducted during the rheometer based HTST thermal treatment. Further characterisation of evaporator fouling was carried out by heat treating concentrated skim milk samples at pH 6.1, 6.3, 6.5 and 6.7 on the fouling rig while in-line measurements of pH, conductivity and temperature were conducted, followed by analysis of the concentrated skim milk samples, cleaning-in-place chemicals and foulant material adhered to the heat exchanger after the fouling experiments. It was observed that samples of concentrated skim milk that caused fouling also displayed increased viscosity, particle size and sedimentation when compared to the samples that resulted in no fouling. The adhered foulant formed at pH 6.1 and 6.3 was found to consist predominantly of protein on a total solids basis, with protein profiling showing that the most prominent proteins were denatured β-lactoglobulin and α-lactalbumin, as well as a strong presence of caseins. It was found that by maintaining the pH of concentrated skim milk above 6.3, fouling during heat treatment could be successfully avoided. Furthermore, through pH adjustment to achieve no fouling to extensive fouling, it was possible to analyse physicochemical properties such as viscosity, particle size, and sedimentation, to provide a practical approach to predict the potential for fouling in concentrated skim milk. These findings contribute significantly to our understanding of the impact of protein stability on the formation of fouling deposits caused by concentrated skim milk, allow us to develop/validate predictive tools and to develop/test mitigation strategies for fouling.
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Keywords
Fouling
Citation
Murphy, T. R. 2023. Characterisation of the changes in the physicochemical properties of high-solids skim milk during thermal processing, and their relationship with fouling. MSc Thesis, University College Cork.