Effect of concentration, pH and added chelating agents on the colloidal properties of heated reconstituted skim milk
thesisposted on 19.01.2017 by Chandrapala, Janage Jayani Sandamali
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The thermal processing of milk changes the composition and surface properties of the colloidal particles present and alters the physical properties of the milk. Whilst some changes such as those used to improve the texture of products such as yoghurt and are desirable, others such as gel formation during the manufacture of Ultra-High Temperature milk are highly undesirable. This work aims to characterize the effects of milk composition and pH on the chemical and physical changes that occur when milk is heated in order to understand and control the effect of thermal processing on the functional properties of the milk. Particularly important are: (i) the changes to the integrity of the casein micelles and the extent to which they are reversible on cooling of the heated milk, (ii) the changes to the speciation of the components of the serum as they re-equilibrate in response to the changed environment during heating and on cooling, (iii) the heat-induced denaturation of the whey proteins, (iv) the interaction between the components of the micelles and those in the milk serum, particularly those interactions that lead to aggregation or other changes that affect the functional properties of the milk on heating. This project includes thermal treatment (90°C/10 min) of control skim milk solutions (9% Milk Solids Non Fat) with or without addition of calcium chelating agents (orthophosphate (Pin) & Ethylenediaminetetraacetic acid (EDTA)) and concentrated skim milk solutions (up to 21% MSNF). The pH range chosen was 6.2 to 7.2. Almost all of the studies on heat stability to date have been carried out by heating the milk and determining the changes that have occurred after the milk is cooled. This project is focussed on the direct measurements in real time of the changes that occur at the exact temperature. The experimental techniques included pH, calcium activity and 31P NMR measurements at high temperatures to investigate the consequences to the change in mineral speciation, Size Exclusion Chromatography in combination with SDS-PAGE analysis for protein speciation during heating and Diffusing Wave Spectroscopy and viscosity measurements to determine the heat stability of milk systems. pH and calcium activity decreased with increase in temperature for all the milk systems studied. These changes were largely reversible as enough time was given for equilibration. pH and calcium activity changes during heating are a function of milk composition. The quantity, size and the composition of the protein aggregates present in the serum phase after mild centrifugation (~33,000g) of heated (90°C/10min) milk solutions were found to be a function of pH and milk composition (including the consequent differences in speciation of the components of milk). DWS and the viscosity measurements showed that pH at the temperature of heating is one of the primary determinants in influencing the aggregation of the proteins, which led to thermal stability of milk systems. Hence, changing the milk composition resulted in differences in pH at the temperature of heating, which led to different behaviours of heat stability of milk systems. Careful control of the composition of milk and thereby the pH at the temperature of heating allows a greater control of thermal stability of milk systems.