Fractionation of milk proteins.

No group of proteins has been as extensively investigated or as well characterized as bovine milk proteins. This can be attributed not only to the importance of milk as a food and as a raw material for a vast processing industry, but also to its convenience as an abundant source of relatively pure protein of varied structure and properties. In spite of the vast range of information available, however, new dairy protein products are a rarity, and the existing range of such products has seen little change in the last 15 years. This is not because of a scarcity of good product ideas, but because of technological and commercial constraints associated with their exploitation. The single most important factor to be considered in any process for separation of milk proteins is scale of operation. Invariably, the requirement is for bulk production and the technical challenges presented by a process involving tonnes of product or hundreds of thousands of litres of process stream are quite different from those familiar to the laboratory scientist. Many of the constraints and requirements are dictated by economic considerations given that, in most cases, the product value is closer to that of commodity protein than of a typical pharmaceutical product. Major factors, therefore, are such things as plant throughput, cost of downstream product recovery and capital cost of separation equipment. Adding to these constraints is the fact that protein damage, during separation, may be no more acceptable than in a conventional biochemical process. Taking these factors into consideration, it is not surprising that relatively few protein fractionation processes have been successfully introduced into the dairy industry. By far the most important are those based on selective precipitation, exploiting the unique properties of the most abundant class of milk proteins, the caseins, which are readily prepared from milk by acid or rennet coagulation. Casein is a major commodity product with world output at approx. 250000 tonnes and its production represents the only true protein-protein separation process in large-scale commercial use in the dairy industry. Other separation technologies in dairy protein manufacture, such as membrane filtration and adsorption chromatography are mainly employed for removal of non-protein components, principally lactose and minerals, from protein. Nonetheless, exciting possibilities for innovative protein products, based on these technologies, do exist. This paper therefore deals with new developments, currently under investigation or just recently introduced into commercial operation. The main milk protein classes, casein and whey protein, consist of several distinct components with potential in a wide variety of food and pharmaceutical outlets (Table 1). Most attention has been directed to whey proteins since, as globular proteins with unrelated primary structures, they present a wide diversity of functional and biological properties. Of particular interest in this respect are the antimicrobial whey proteins, immunoglobulins, lactoperoxidase (LP) and lactoferrin (LF) which account, respectively, for 15%, 1.5% and 0.5% of total whey proteins. Prospects for the use of these proteins in neonate feed and in health-care products, as protection against pathogenic organisms, have been reviewed [ 1, 21 and are the subject of several patents and commercial proposals