Whey denaturation

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. 

There is a continuing interest to improve and extend the fimctional properties range of dairy proteins to provide both health benefits and their characteristic physical behaviors under different temperature, moisture, and pH conditions so that they may be included in foods that ordinarily do not contain them. One such research area is the extrusion texturization of whey proteins, which have resulted in dairy proteins with new characteristics imparted by a controlled texturization process, depending on the application desired (Hale et al., 2002 Manoi and Rizvi, 2008 Onwulata, 2009 Onwulata et al., 1998). Protein texturization is a two-step process that involves, first, the unfolding of the globular structure (denaturation) and, second, the alignments of the partially unfolded structures in the direction of mass flow in the extruder. The surface characteristics are imparted at the extruder die as the molten mass exits (Onwulata et al., 2003a). 

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. 

Denaturation and aggregation of whey proteins are affected by the pH of extrusion. When extruding WPI, alkaline conditions increase denaturation and solubility, decrease pasting properties, and produce more pronounced microstructural changes (Onwulata et ah, 2006). Denaturation in the extruder causes whey proteins to form small primary aggregates that combine to form large clusters. The clusters are then aligned by shear to form fibrous structures. 

WPI, whey protein isolates. Properties of nonextmded WPI pH 6.8, protein 88.9%, insoluble (denatured) 28.0%, and digestibility 87.7%. Means with different letters within a column are significantly (p < 0.05) different. 

Extrusion is an effective means of denaturing whey proteins to create texturized products. TWP may be used as an ingredient to improve the characteristics of many foods. The production of snack foods wifh... 

It is demonstrated here that extrusion is an effective tool for texturing whey proteins to create new functions for dairy proteins and that thermally denatured WPl is a unique ingredient that can be used in large amounts in nontraditional applications for non-TWPl. This review covers the use of extrusion texturized dairy ingredients in foods however, there are other examples of fhe successful use of this technique along with the product, TWPl in different types of nonfood applications, such as in biodegradable films, and bioplastics. 

Nakamura, T., (1993). Production of low antigenic whey protein hydrolysates by enzymatic hydrolysis and denaturation with high pressure, Milchwiss., 48, 141-147. 

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). 

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. 

In the case of cold-induced aggregation and gelation, two different types of gel microstructure, namely filamentous and particulate (Figure 2.1), have been obtained by adding different concentrations of a ferrous salt to solutions of pre-denatured p-lactoglobulin (the major whey protein). This substantial difference in microstructure turns out to have a major impact on the iron delivery, due to the different sensitivities of the structures to the relevant environmental conditions, such as pH and the presence of digestive enzymes. In particular, the filamentous gel micro-... 

Roff, C.F., Foegeding, E.A. (1996). Dicationic-induced gelation of pre-denatured whey protein isolate. Food Hydrocolloids, 10, 193-198. 

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. 

Whey protein concentrates (WPC) are produced by a variety of processing treatments to remove both lactose and minerals (20) as indicated in Figure 5. Even though it would be highly desireable to remove most of the lactose and minerals in these processes, it is not practical from an economic standpoint and thus most of these products only range in protein content from 35 to 50 %.The major objective of most of these processes is to produce a WPC with minimal protein denaturation in order to obtain a product with maximum protein solubility and functionality. However, from a practical consideration this objective is not readily obtainable, and thus most WPC products commercially available exhibit variable whey protein denaturation and functionality (20). 

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. 

Despite some conflicting evidence (Kinsella and Fox, 1986), it appears that denaturation has little influence on the amount of water bound by whey proteins. However, other factors which may accompany denaturation (e.g. Maillard browning, association or aggregation of proteins) may alter protein sorption behaviour. Drying technique affects the water sorption characteristics of WPC. Freeze-dried and spray-dried WPC preparations bind more water at the monolayer level than do roller-, air- or vacuum-dried samples, apparently due to larger surface areas in the former. As discussed above, temperature also influences water sorption by whey protein preparations. The sorption isotherm for /Mactoglobulin is typical of many globular proteins. 

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