Effects of Various Heat Treatments on Structure and Solubility of Whey Proteins

The heat stability of milk has been the subject of a considerable amount of research for about a century. This research has been aimed mainly at understanding the effects of compositional and processing factors on heat stability and elucidating the mechanisms of protein coagulation. This paper provides an overview of the factors that influence the pH dependence of the heat stability of normal and concentrated milks. The principal heat-induced changes in the milk system that contribute to coagulation are discussed. Current knowledge of the mechanisms of heat coagulation in normal and concentrated milks is also reviewed.

Heat stability of milk

During the last decade, marked progress has been made in the study of the fine details of the structures of milk proteins such as caseins, beta-lactoglobulin, alpha-lactalbumin, and lactotransferrin. Many of the functional properties of the individual milk proteins, as well as the milk protein products, may be described at the molecular level. This article is an attempt to thoroughly review the three-dimensional structures of major milk proteins, and to correlate them with the functional aspects of these proteins as food ingredients.

Structures and functionalities of milk proteins

Present distribution and milk production. Main components (proteins, casein fractions, size distribution of casein, whey protein fractions, lipids, fatty acids, phospholipids, physical state and properties of fat globules, lactose, minerals and vitamines). Enzymic coagulation. Effects of heat on milk. Antibacterial activity

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Composition and characteristics of camel milk

In this chapter, methods for assessing the heat stability of unconcentrated and concentrated milk are discussed. The effects of compositional factors, processing conditions and additives on the heat stability of milk are reviewed and the changes that occur on heating are considered in relation to the heat stability of milk. Three possible explanations for the pH-dependence and mechanism of thermal coagulation of milk, which incorporate the research on the heat stability of milk over the last century, are presented.

Heat-Induced Coagulation of Milk

Reduction of the level of colloidal calcium phosphate (CCP) progressively increased the heat stability of milk at pH values 7·2, an effect not offset by a 4-fold increase in κ-casein concentration. Doubling CCP had a slight destabilizing effect in the pH range 6·5–7·5. Addition of β-lactoglobulin to serum protein-free casein micelles had a marked destabilizing effect at pH values > ~6·8, but increased stability in the pH range 6·4–6·8. β-Lactoglobulin had a similar and more apparent effect on the heat stability of Na caseinate dissolved in milk diffusate. It is suggested that rather than being a stabilizing factor responsible for the maximum in the heat stability-pH curve, the true effect of β-lactoglobulin is to shift the curve to more acid pH values (reason unknown) and to sensitize the caseinate system to heat-induced Ca phosphate precipitation at pH values > ~7·0. Low stability at ~pH 7·0 introduces an apparent maximum in the heat stability-pH curve at ~pH 6·8, but this has no independent existence. At pH values >7·2, increased protein charge more than off-sets the influence of heat-precipitated CCP and stability again increases in micellar but not in soluble casein systems.

Heat stability of milk: influence of colloidal calcium phosphate and β-lactoglobulin

Ovine and caprine milks showed a marked stability maximum at ~ pH7 in their heat-stability/pH curves, but became very unstable at higher pH values. A low level of κ-casein appears to be responsible for this low stability of ovine milk; β-lactoglobulin offsets the stabilizing influence of κ-casein at lower pH values. Removal of colloidal calcium phosphate from ovine or caprine milks had very little influence on their heat stabilities and it was necessary to reduce the concentration of soluble salts to a very low level before an effect was observed. At the pH of maximum stability ovine and caprine milks, although quite variable, had stabilities in the same range as bovine milks. Equine milk was very unstable to heat and the shape of the heat-stability/pH curve of most individual samples was similar to ovine and caprine milks.

M. C. T. Hoynes

Papers

Heat stability of milk: influence of colloidal calcium phosphate and β-lactoglobulin

Heat stability characteristics of ovine, caprine and equine milks