Denaturation of whey proteins

Thermal denaturation is a traditional method for the recovery of proteins from whey as lactalbumin coagulation is optimal at pH 6 and about 90°C for 10 min (Chapter 4). 

The proteins can participate in sulphydryl-disulphide interchange reactions at temperatures above about 75°C at the pH of milk, but more rapidly at or above pH 7.5. Such interactions lead to the formation of disulphide-linked complexes of ]9-lg with K-casein, and probably a,2-casein and a-la, with profound effects on the functionality of the milk protein system, such as rennet coagulation and heat stability. 

The activated sulphydryls may decompose with the formation of H2S and H3C-S-CH3, which are responsible for the cooked flavour of severely heated milk, including UHT milk. These compounds are volatile and unstable and disappear within about 1 week after processing so that the flavour of UHT milk improves during the first few weeks after processing. 

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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. 

The viscosity of milk and milk products is reported to be important in the rate of creaming. The viscosity of milk increases with decrease in temperature because the increased voluminosity of casein micelles temperatures above 65°C increases viscosity due to the denaturation of whey proteins pH an increase or decrease in the pH of milk also causes an increase in casein micelle voluminosity. Fat globules that have undergone cold agglutination may be dispersed due to agitation, causing a decrease in viscosity. 

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).

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 ... 

Denaturation of whey proteins. Whey proteins are denatured very rapidly at 140°C as discussed in section 9.6.3, the denatured proteins associate with the casein micelles, via sulphydryl-disulphide interactions with K-casein, and probably with as2-casein, at pH values below 6.7. The whey proteins can be seen in electron photomicrographs as appendages on the casein micelles. 

P Resmini, L Pellegrino, JA Hogenboom, R Andreini. Thermal denaturation of whey protein in pasteurized milk. Fast evaluation by HPLC. Ital J Food Sci 3 51-62, 1989. 

Figure 3-3 Time-Temperature Relationships for the Heat Denaturation of Whey Proteins in Skim Milk. Source From H.A. Harland, S.T. Coulter, and R. Jenness, The Effects of Various Steps in the Manufacture on the Extent of Serum Protein Denaturation in Nonfat Dry Milk Solids. / Dairy Sci. 35 363-368, 1952.

Denaturation of whey proteins has been shown to unfold their polypeptide chains and convert thexx conformation from a globular to an extended form that facilitates their self interaction (aggregation) and interaction with K-casein by disulfide interchange, ionic and hydrophobic bonding (4). 

Dannenberg, F., and Kessler, H.G. (1988). Application of reaction-kinetics to the denaturation of whey proteins in heated miW.. Milchwissenschaft 3-7. 

Plock, J., Spiegel, T., and Kessler, H.G. (1997). Reaction kinetics of the thermal denaturation of whey protein in sweet whey concentrated by evaporation. Milchwissenschaft 53, 327-331. 

Schorsch et al. (2001) examined the effects of denaturation of whey proteins in the presence and absence of casein micelles on gel properties. Heat treatment sequence was found to influence the acid gelation properties of casein-whey mixtures. Denaturation of whey proteins in the absence of casein micelles induced more rapid gelation on addition of acid. Gels made from these milks had a more particulate gel structure than gels made from casein-whey mixtures which were heated without prior denaturation of the whey proteins. 

High pressure induces disruption of casein micelles and denaturation of whey proteins, increases pH of milk, reduces rennet coagulation time, and increases cheese yield, thereby possessing potential application in cheese making (O Reilly et al.,... 

High-pressure treatment of milk increased yield of cheese due to increased WHC as well as to denaturation of whey proteins and their association with casein. Drake et al. (1997) attributed the increased yield and higher moisture content of cheese made from high-pressure treated milk to the fact that casein molecules and fat globules might not aggregate closely and hence allow moisture to be trapped or held in cheese. Pandey and Ramaswamy (1998) reported that reduced hardness of Cheddar... 

Denaturation of whey proteins is of major technological significance and will be discussed in Chapter 9,... 

Anandharamakrishnan, C. RieUya, C.D. Stapleya, A.G.F. Effects of process variables on the denaturation of whey proteins during spray drying. Drying Technology, 2007, 25(5), 799-807. 

Haque, M.A. Aldred, P Chen, J. Barrow, C.J. Adhikari, B. Comparative study of denaturation of whey protein isolate (WPI) in convective air drying and isothermal heat treatment processes. Food Chemistry, 2013a, 141, 702-711. 

Skelte, G.A. Anthony B.M. Reaction kinetics of thermal denaturation of whey proteins in heated Reconstituted whole milk. Journal of Agricultural and Food Chemistry, 1996, 44,422-428. 

It should be noted that ultrasonic technology is a non-thermal process and hence very little changes to the physical and functional properties of dairy system is observed due to sonication. In most studies, the processing time required is a few seconds to less than a minute. In order to evaluate the effect of sonication on the constituents of milk, Chandrapala et al. [85] and Shanmugam et al. [86] carried out extensive research in recent years. Their studies have shown that sonication of a dairy system causes very little changes to the physical properties of whey proteins. They observed reversible changes to partial denaturation of whey proteins. In order to see the effect, the constituents of whey proteins, namely, pure- and 3 1 mixtures of p-Lactoglobulin (P-LG) and a-Lactalbumin (a-LA)... 

Fig. 10.17. Denaturation of whey proteins by heating at various temperatures for 30 min. 1 Total whey protein, 2 p-lactoglobulin, 3 a-lactalbumin, 4 proteose peptone, 5 immunoglobulin, 6 serum albumin...

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