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...

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SummaryIn situ light scattering, where light scattered from a sample is measured directly while the sample is heated in the instrument, is presented as a simple and effective technique for studying the heat-induced aggregation of β-lactoglobulin. This technique was shown to be applicable not only to monitoring the initial aggregation steps, but to following the overall aggregation process with time. The experiments gave results similar to measurements carried out after a heat-quench treatment, but were more informative. From experiments on a standard NIZO β-lactoglobulin sample, a strongly desalted standard NIZO sample, different genetic variants of β-lactoglobulin and a mixture of these, we concluded that the standard NIZO sample was suitable for studying heat-induced aggregation. This sample has been investigated more extensively. Results with β-lactoglobulin (10–100 g/1) at 65 °C fitted a kinetic model for the denaturation and aggregation of β-lactoglobulin. This model, which held for β-lactoglobulin dissolved in water at near neutral pH and at 60–75 °C, recognizes an initiation, propagation and termination reaction, by analogy with polymer radical chemistry. It gave a quantitatively correct description of the dependence of the scattering intensity on the initial β-lactoglobulin concentration. Salt composition, pH and temperature strongly influenced the aggregation of β-lactoglobulin. Particle size increased with salt concentration in the range studied (up to 20 mM-NaCl and 1·0 mM-CaCl2). When the pH increased from 6°9 to 8·0 particle size was strongly reduced, whereas it strongly increased when pH was lowered to 6·2. Between 61·5 and 70 °C temperature did not affect particle size, whereas aggregation rate strongly increased. These effects could be incorporated in the kinetic model via the reaction constants of the reaction kinetic pathway.

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Whey proteins are globular, heat-sensitive proteins. The gel structure, the formation of this structure, and the rheological properties of particulate whey protein isolate (WPI) gels have been investigated. On increasing the NaCl concentration, the permeability of the WPI gels increased, indicating a coarsening of the gel structure, confirmed by confocal scanning laser microscopy pictures. Only a part of the total amount of protein present contributed to the gel network at the gel point (the primary spatial structure). Large variations were observed in the amount of aggregated material at the gel point (and thus the primary spatial structure) as a function of NaCl concentration, due to differences in the kinetics of the denaturation/aggregation process. After the gel point more protein is incorporated in the gel network by "thickening" the strands in the gel and "decorating" the pores in the gel, apparently without changing the gross spatial structure. Power law behavior was found for the permeability dependence of aged gels on the amount of aggregated material at the gel point. For various salt concentrations the curves coincided to one master curve. This power law behavior is consistent with a primary spatial structure of fractal flocs with a fractal dimensionality of 2.4. The elastic modulus is remarkably related (via a power law) with the total amount of protein contributing to the gel network, in contrast to permeability.