Protein concentrates globular

Protein Concentration. For a given type of protein, a critical concentration Is required for the formation of a gel and the type of gel varies with the protein concentration. For example, gelatin and polysaccharide solutions will form gels at relatively low concentrations of the gelling material. Considerably higher protein concentration Is usually required for the gelation of globular proteins. 

The sample concentration affects the observed, v values for example, when protein concentration increases, lower s values are observed. A number of experiments with different protein concentrations are necessary to correct this concentration-dependent bias, to allow extrapolation of the, v value to a protein concentration of zero. This new value extrapolated to zero concentration is known as, r20w, and is used in the calculation of MW (M) using Eq. 13.15. But if is known that the protein or macromolecule has a simple globular shape, instead of calculating D it is possible to use the following empirical formula ... 

A rapid FTIR method for the direct determination of the casein/whey ratio in milk has also been developed [26]. This method is unique because it does not require any physical separation of the casein and whey fractions, but rather makes use of the information contained in the whole spectrum to differentiate between these proteins. Proteins exhibit three characteristic absorption bands in the mid-infrared spectrum, designated as the amide I (1695-1600 cm-i), amide II (1560-1520 cm-i) and amide III (1300-1230 cm >) bands, and the positions of these bands are sensitive to protein secondary structure. From a structural viewpoint, caseins and whey proteins differ substantially, as the whey proteins are globular proteins whereas the caseins have little secondary structure. These structural differences make it possible to differentiate these proteins by FTIR spectroscopy. In addition to their different conformations, other differences between caseins and whey proteins, such as their differences in amino acid compositions and the presence of phosphate ester linkages in caseins but not whey proteins, are also reflected in their FTIR spectra. These spectroscopic differences are illustrated in Figure 15, which shows the so-called fingerprint region in the FTIR spectra of sodium caseinate and whey protein concentrate. Thus, FTIR spectroscopy can provide a means for quantitative determination of casein and whey proteins in the presence of each other. 

When a protein solution is heated, a gel may be formed. This occurs with well-soluble globular proteins, if the protein concentration is above a critical value c0. The gel is only formed after at least part of the protein has been heat denatured—e.g., as inferred from a change in the DSC curve—and the gel formation is irreversible upon cooling see Figure 17.14d. Gel formation is a relatively slow process, taking at least several minutes and possibly hours. 

A more advanced model was suggested very recently by [78] based on the adsorption isotherm for proteins given by Eq. (2.124). In addition to diffusion of the molecules in the bulk, a kinetic process was assumed equivalent to the mechanism used in the mixed kinetic model. The configuration changes, i.e. orientation of a globular protein molecule to the surface, were characterised by one rate constant k. The following Fig. 4.7 shows model calculations where the following parameters were used coi = 2.5-10 m /mol, W2 = 5.010 m /mol (i.e. coj/ ] = 2), a i = 200. These parameters correspond to those for HSA adsorbed at the water/air interface [79]. The diffusion coefficient was taken to be D = lO cmVs and the protein concentration as 10 mol/I. The equilibrium surface pressure of the protein solutions was taken to be 20 mN/m, typical for HSA at this concentration. It should be noted first that the time required for an experimentally observable decrease of the surface tension, say by 0.5 mN/m, is about 3100 s... 

The quadrupolar contribution is mostly expected in membranes with a high protein concentration, where ordered arrays of integral proteins exist. Examples of this type include the purple membranes of Halobacterium halo-hium, the inner mitochondrial membrane, etc. The presence in biomembranes of extended domains of tightly packed globular proteins in a doubletiered pattern is a basic idea in the structure-function unitization model of biomembranes. The estimated flexocoefScient of an array of identical double-tiered quadrupolar proteins is substantial > 4.5 X 10- ° C as... 

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