Milk Proteins

Milk proteins can be divided into two major groups: caseins and whey (or serum) proteins. Of these two, whey proteins, which are produced by heat precipitation and are used extensively as food ingredients, are extremely thermal labile (60). Thermal denaturation of whey proteins in fluid milk is manifested by the development of cooked flavor (61). Efficient use of whey as a functional food ingredient requires a knowledge of the denaturation behavior of individual whey proteins (62). Whey protein denaturation is considered a two-stage process: (1) disruption of tertiary and secondary structure, followed by (2) aggregation and coagulation, a phenomenon often associated with gel formation (5). Disulfide interchange reactions are largely responsible for aggregation and coagulation of /Mactoglobulin (a major component of whey) (63). The ability of whey proteins to form heat-induced gels is important in many food systems (eg, yogurt). Factors important in gelation include temperature, duration of thermal treatment, type and concentration of ions, state of the sulfur-containing amino acids, type of acidulant, and total solids concentration. In general, increases in total solids offer a protective effect by decreasing the rate and extent of whey protein denaturation (64). The presence of ions affects the thermal aggregation of /Mactoglobulin variably: phosphate and citrate inhibit, whereas calcium enhances, aggregation (65). Thermal denaturation of whey proteins modifies the course of milk coagulation and the rheological properties of the curd formed by acid or enzymes. In milk, thermally induced formation of intermolecular disulfide linkages occurs between unfolded /Mactoglobulin molecules and between unfolded /Mactoglobulin and casein micelles (ie, K-casein and possibly aal-casein) (66). At temperatures in excess of 100°C, whey proteins may be extensively bound to casein micelles, thereby altering their surface properties.

Nonfat dry milk (NFDM) is used extensively in formulated foods and is produced from pasteurized skim milk, which is vacuum concentrated and spray-dried under conditions that result in either a low-heat or high-heat product. Low-heat NFDM is required for most applications that depend on a highly soluble protein (eg, emulsification) because it is processed under conditions that minimize whey protein denaturation and complexation with casein micelles (67). The casein proteins (ie, a-, /?-, and K-caseins) are generally not heat coagulable. In normal fluid milk, caseins resist coagulation for as long as 14 h at boiling temperatures or 1 h at 130°C. Thermocoagulation of casein in milk can occur as a result of compositional changes within the milk itself induced by sustained exposure to high temperatures (eg, increased acidity, a shift from soluble to colloidal forms of calcium and phosphates, denaturation, and hydrolysis of other milk proteins). Coagulation of milk is often attributed to the destabilization of casein micelles; however, this phenomenon may be the summation of many changes in the colloidal system (68).

Nutrition Essentials

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