A quantitatively correct kinetic model for the temperature-induced denaturation and aggregation of P-lactoglobulin is presented. The model recognizes an initiation, a propagation and a termination step by analogy with polymer radical chemistry. The decrease in native P-lactoglobulin is predicted to follow order 312, in agreement with experimental results. The size of the protein polymer particles is predicted to be proportional to the square root of the initial P-lactoglobulin concentration. The scattered light intensity is proportional to the product of concentration and size of the protein polymer particles. The initial increase in scattering intensity of the particles therefore scales with the initial squared P-lactoglobulin concentration. The influence of other reaction conditions, e.g. ionic strength and pH, can be incorporated via the reaction constants of the reaction kinetic pathway.Like many other proteins, the milk serum proteins P-lactoglobulin and a-lactalbumin are sensitive to heating at 60-100 "C. During heating, intramolecular and intermolecular changes and reactions occur, which are generally denoted as denaturation and aggregation. There is a vast amount of literature on this topic, much of which is heuristic in nature.Knowledge of the precise mechanisms of the processes is still inadequate, and there is no clear physical picture on which to base predictions. Such predictions are of great interest for industrial (food) processing of these proteins, because they can aid in the understanding and control of the functionality and quality of food products.Although a quantitative model for denaturation and aggregation is still lacking, much information on milk serum proteins has been obtained. Upon heating, the proteins undergo conformational changes, which are thought to result in an unfolding of the molecules. The whole process is described as denaturation. These changes are reversible in principle, but experimentally denaturation often appears to be irreversible [ 1, 21. Renaturation may be hampered by secondary effects like electrostatic and hydrophobic interactions between newly exposed parts of the molecules, which lead to aggregation of denatured molecules. Native molecules are inert as long as solvent conditions are non-critical and aggregation does not seem to occur before denaturation has taken place.The appearance of heated P-lactoglobulin dispersions is a very intriguing feature. It can vary from a transparant appearance to an opalescent and milk-white turbid appearance, and not only dispersions but also firm gels may be formed The final appearance of heated protein solutions strongly depends on the medium conditions such as ionic strength, type of ions (especially Caz+) and pH. Transparent dispersions and gels are found at low ionic strength and a pH distinct from the isoelectric pH (e.g. above pH 6.5). In a rheological characterization, transparent gels exhibit elastic properties [2, 31 quite similar to those of polymer gels. In a recent study on ovalbumin [6], similar features were obse...

Heat-induced aggregation of β-lactoglobulin was investigated as a function of pH, heating temperature, and NaCl concentration by measurements of reaction kinetics, differential scanning calorimetry, and light scattering. The aggregation can be well interpreted using a reaction scheme consisting of two steps:  a denaturation equilibrium, with a first-order unfolding reaction, followed by second-order aggregation reactions. Denaturation becomes rate limiting at high heating temperature, pH values close to the isoelectric point of the protein, and high NaCl concentration. At neutral pH a maximum is seen in the overall reaction rate as a function of NaCl concentration, which is explained by a stabilizing, salting-out effect of the salt in combination with an increase in the rate of successive aggregation reactions. At high NaCl concentrations physical bonding becomes increasingly important; large aggregates that continue to grow in time are formed, and two phases are distinguished in the aggregation step. The onset time of the secondary aggregation is related to a critical concentration of primary (denatured or small, aggregated) particles. Keywords: Whey; β-lactoglobulin; aggregation; kinetics; light scattering

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SummarySkim milk was heated at 85 °C for different holding times. As a result of such heating, whey proteins, in particular β-lactoglobulin, denatured and associated with casein micelles. This led to an increase in size of the casein micelles but also to a different interaction between them. Both these changes could be described by using a quantitative model which was developed for the viscosity of so-called adhesive hard spheres. We applied the model successfully to skim milk and were able to describe on a quantitative basis the changes due to the heat treatment of milk. It was shown that after heating the casein micelles became larger and acquired a mutual attraction. The unfolding of the whey proteins and their subsequent association with the casein micelles appeared to be responsible for these changes. How this reaction influences the fouling of heat exchangers is discussed.