Structure and functional behaviour of whey proteins

Structure and functional properties of the whey proteins are interrelated and are governed by changes in the globular conformation of the molecule. In particular the heat sensitivity of the whey proteins appears to be a characteristic property that affects their structure and physico-chemical properties. After a brief analysis of some of the physico-chemical properties of the major whey proteins, attention is paid to their thermal behaviour as observed by differential scanning calorimetry (DSC) in a temperature range up to 150 deg C. Two distinct heat effects are observed; the first, near 70 deg C, is denoted as the denaturation heat and the second, near 130 deg C, is ascribed to unfolding of residual protein structure. Environmental effects such as pH, lactose and calcium salts are discussed in relation to heat denaturation and protein solubility. It has been shown that heat denaturation of the whey proteins will not necessarily result in an impaired protein solubility when the heating is conducted under the proper conditions. Special attention is paid to the relation between heat stability in milk and the thermal behaviour of beta-lactoglobulin at different pH values and calcium concentrations. Both the heat stability is milk and the thermal behaviour of beta-lactoglobulin change quickly in a very narrow pH range between pH 6.7 and 7.0. The importance of sulphydryl/disulphide groups has been stressed in this relationship.