Whey proteins heat denaturation

Figure 5.3. Schematic illustration of the relative effects of heating and homogenization on fat globules in milk. MFG = milk fat globule, CM = casein micelle, WP = whey protein, dWP = denatured whey protein.

In making yogurt, milk is initially heated at SO C for 15-30 min in order to denature the mhey proteins. This heating provokes the whey proteins to denature, and consequently associate with the casein micelles, resulting in a yogurt with improved viscosity and texture (Law and Leaver, 1997 Boyeef jiL, 1997 Ottecl ai, 1996). 

Casein. Milk contains proteins and essential amino acids lacking in many other foods. Casein is the principal protein in the skimmed milk (nonfat) portion of milk (3—4% of the weight). After it is removed from the Hquid portion of milk, whey remains. Whey can be denatured by heat treatment of 85°C for 15 minutes. Various protein fractions are identified as a-, P-, and y-casein, and 5-lactoglobulin and blood—semm albumin, each having specific characteristics for various uses. Table 21 gives the concentration and composition of milk proteins. 

Purely thermal denaturation of proteins requires much longer times collagen in moist heat below 120 °C needs 30 min to denature (Meyer et ah, 2005), wheat glutens must be subjected to 200-215 °C of dry heat for 72 min (Friedman et ah, 1987), and as mentioned above, whey proteins require at least 50 °C and 30 min for texturization without the use of extrusion processing. 

We have created structured networks in whey proteins using mild heat and shear, to create reversible TWPs. By understanding on a molecular basis, the effects of shear, ways of creating new functionality can be developed. This will enable development of extrusion parameters that permit controlled denaturation of whey proteins. 

Chemical reactions Polymerization of casein and whey proteins are due to some kind of chemical reactions. The different proteins as found in the supernatant of milk after precipitation at pH 4.6 are collectively called whey proteins. These globular proteins are more water soluble than caseins and are subject to heat dena-turation. Denaturation increases their water-binding capacity. The principal fractions are P-lactoglobulin, a-lactalbumin, bovine serum albumin (BSA), and immunoglobulins (Ig). 

Figure 3.2 Evolution of the microstructure of phase-separated biopolymer emulsion system containing pectin and 0.5 vt% heat-denatured (HD) whey protein isolate (WPI) stabilized oil droplets, (a) Composition 1U 3L (one-to-three mass ratio of upper and lower phases). The large circles are the water droplets (W), while the small circles are the oil droplets (O). This system forms a W2/W1-O/W1 emulsion, where O is oil, Wi is HD-WPI-rich and W2 is pectin-rich, (b) Composition 2U 2L. This system forms an 0/Wi/W2 emulsion, where O is oil, Wi is HD-WPI-rich and W2 is pectin-rich, (c) Composition 3U 1L. This system forms an 0/W]/W2 emulsion, where O is oil, Wi is HD-WPI-rich and W2 is pectin-rich. Reproduced from Kim et al. (2006) with permission.

Kim, H.-J., Decker, E.A., McClements, D.J. (2006). Preparation of multiple emulsions based on thermodynamic incompatibility of heat-denatured whey protein and pectin solutions. Food Hydrocolloids, 20, 586-595. 

Bryant, M.C., McClements, D.J. (1998). Molecular basis of protein functionality with special consideration of cold-set gels derived from heat-denatured whey. Trends in Food Science and Technology, 9, 143-151. 

Ikeda, S., Morris, V.J. (2002). Fine-stranded and particulate aggregates of heat-denatured whey proteins visualized by atomic force microscopy. Biomacromolecules, 3, 382-389. 

Proteins of egg white denature more rapidly than those of whey protein concentrate (13, 34). However, isolated p-lactoglobulin from the whey concentrate was more susceptible to surface denaturation than egg white ovalbumin. These data suggest that whey contains substances that protect the proteins from surface denaturation and may account for the lower stability of whey protein concentrate foams than those of egg white protein. A balance between the disaggregation effect of select pH values and the tendency toward greater aggregation of proteins at higher heating temperatures were correlated closely with maximum foam stability (13, 15). 

NFDM, which retains casein micelles similar to those in fresh milk, is produced by pasteurization of sklmmllk, vacuum concentration and spray drying under processing conditions that result in either "low heat" or "high heat" product. Low heat NFDM is required for most applications that depend upon a highly soluble protein, as the case for most emulsification applications, since it is manufactured under mild temperature conditions to minimize whey protein denaturation and complexation with casein micelles. 

Lactalbumin is an insoluble whey protein product produced by heating whey to high temperatures ( > 90 C) to denature and render the proteins insoluble when adjusted to isoelectric conditions by the addition of acid. These proteins offer little functionality in emulsification applications. 

Co-preclpltate is an insoluble milk protein product that is produced by heating skinimllk to high temperatures ( > 90 C) to denature the whey proteins and complex them with the casein micelles. The heated system is subsequently adjusted to isoelectric point conditions of pH 4.5-5 to precipitate the complexed whey protein-casein micelles, centrifuged or filtered to recover the precipitate, washed and dryed. The resulting product, which is virtually insoluble, exhibits only minor functionality in most typical emulsification applications. 

Whey protein concentrates (WPC), which are relatively new forms of milk protein products available for emulsification uses, have also been studied (4,28,29). WPC products prepared by gel filtration, ultrafiltration, metaphosphate precipitation and carboxymethyl cellulose precipitation all exhibited inferior emulsification properties compared to caseinate, both in model systems and in a simulated whipped topping formulation (2. However, additional work is proceeding on this topic and it is expected that WPC will be found to be capable of providing reasonable functionality in the emulsification area, especially if proper processing conditions are followed to minimize protein denaturation during their production. Such adverse effects on the functionality of WPC are undoubtedly due to their Irreversible interaction during heating processes which impair their ability to dissociate and unfold at the emulsion interface in order to function as an emulsifier (22). 

Addition of CaCl2 to about 0.2 M causes aggregation of the casein such that it can be readily removed by low-speed centrifugation. If calcium is added at 90°C, the casein forms coarse aggregates which precipitate readily. This principle is used in the commercial production of some casein co-precipitates in which the whey proteins, denatured on heating milk at 90°C for 10 min, co-precipitate with the casein. Such products have a very high ash content. 

Casein can be precipitated from solution by any of several salts. Addition of (NH4)2S04 to milk to a concentration of 260 g 1 1 causes complete precipitation of the casein together with some whey proteins (immunoglobulins, Ig). MgS04 may also be used. Saturation of milk with NaCl at 37°C precipitates the casein and Igs while the major whey proteins remain soluble, provided they are undenatured. This characteristic is the basis of a commercial test used for the heat classification of milk powders which contain variable levels of denatured whey proteins. 

The whey proteins, which represent about 20% of the proteins of bovine milk, are typical globular proteins with high levels of secondary and tertiary structures, and are, therefore, susceptible to denaturation by various agents, including heat. The denaturation kinetics of whey proteins, as measured by loss of solubility in saturated NaCl at pH 4.6, are summarized in Figure... 

Figure 9.13 Heat denaturation of whey proteins on heating skim milk at various temperatures (°C) as measured by precipitability with saturated NaCl (from Jenness and Patton, 1959).

Figure 9.14 The denaturation of the total ( ) and individual whey proteins in milk, heated at various temperatures for 30 min /l-lactoglobulin ( ), a-lactalbumin (O). proteose peptone ( ), immunoglobulins (A), and serum albumin ( ) (from Webb and Johnson, 1965).

Some of the most important consequences of the heat denaturation of whey proteins are due to the fact that these proteins contain sulphydryl and/or disulphide residues which are exposed on heating (Figure 9.17). They are important for at least the following reasons ... 

The viscosity of milk and creams tends to increase slightly with age, due in part to changes in ionic equilibria. Heating skim milk to an extent that denatures most of the whey proteins increases its viscosity by about 10%. Homogenization of whole milk has little effect on its viscosity. The increase in the volume fraction of fat on homogenization is compensated by a decrease in the volume fractions of casein and whey proteins because some skim milk proteins are adsorbed at the fat-oil interface. Pasteurization has no significant effect on the rheology of whole milk. 

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