Proteins stability 3-casein behavior

The presence of both acidic and basic side chains has led to protein such as casein acting as amphoteric electrolytes and their physical behavior will depend on the pH of the environment in which the molecules exist. The isoelectric point for casein is about pH = 4.6 and at this point colloidal stability is at a minimum. This fact is utilized in the acid coagulatimi techniques for separating casein from skimmed milk. 

There have been a limited number of studies on the effects of enzymic modification of protein concentrates on functional properties other than solubility. Studies on functional properties, as modified by enzymic treatments, emphasize foam formation and emulsifying characteristics of the hydrolysates. Treatment of chicken egg albumen alters the functional properties of the egg proteins in terms of foam volume and stability and the behavior of the proteins in angel food cakes (25). Various proteolytic enzymes were used to degrade the egg albumen partially. However, proteolytic enzyme inhibitors indigenous to the egg proteins repressed hydrolysis of the egg proteins compared with casein. 

Functional properties of some enzymatically modified and EPM-treated products of milk proteins [136] were determined as follows. An enzymatically prehydrolyzed commercial milk protein concentrate (SR) without further hydrolysis, and casein hydrolyzed by alcalase, a-chymotrypsin, and papain, respectively, were used as substrates in the EPM reaction. The concentration of the hydrolysates was 20% w/ v in the EPM reactions. A methionine methyl ester hydrochloride/ substrate ratio of 1 5 was used for incorporating this amino acid. After incubation, the products with methionine incorporation were simultaneously dialyzed for 2 days through a cellophane membrane against distilled water. The nondialyzable fractions and the EPM products without amino acid enrichment were freeze-dried. Covalent methionine incorporation in the EPM products with amino acid enrichment was verified by exopeptidase hydrolysis of the protein chains. The functional properties of the different EPM products are summarized in Table 1. An important functional property of proteins and/or peptide mixtures is their emulsifying behavior. This is highly influenced by the molecular structure, the position and ratio of hydrophobic-hydrophilic amino acids. Emulsion activity was found to be low (34.0) for casein, and the values determined for enzyme hydrolyzed and modified products were in general even lower. The papain hydrolysate, sample H3, showed here a different behavior as well this was the one of the sample series that had the highest EAI value (43.0). The emulsion stability of the enzymatically modified products displayed tendencies quite opposite to the values of emul-... 

The interfacial behavior of protein-surfactant complexes is important in several areas such as the stability of emulsions and foams and the adsorption of proteins and surfactants from their binary solutions onto solid surfaces. Of particular interest is the adsorption of the milk proteins /3-lactoglobulin and /3-casein at the oil-water interface in the presence of nonionic surfactants in relation to food emulsions [56-58] and foam stability [59]. The adsorption of gelatin at the air-water [52,53,60], oil-water [6], and solid-water [62] interfaces in the presence of surfactants has also been studied. Other studies reported include adsorption from aqueous solutions of lysozyme plus ionic surfactants at solid surfaces [63,64], /3-lactoglobulin plus SDS onto... 

The stabilizing properties of individual caseins, sodium caseinate and casein micelles are described with respect to the formation and behavior of emulsions. In particular, attempts are made to relate the properties of the emulsions (or rather the interfacial proteins) to the properties of the proteins and protein coitplexes dien they are in their solution or suspended state. In this, the stabilizing action of K-casein in the different emulsions is described an inpDrtant factor being its susceptibility to atack by rennet, which may serve as an indicator of its conformation on the interface. 

With a typical size ranging from nanometric (<100 nm) to submicrometric (<1 pm), biopolymeric particles and nanoparticles, made of proteins or polysaccharides, thanks to their excellent compatibility with foods, are able to efficiently encapsulate, protect and deliver bioactive compounds, forming different structures, such as random coils, sheets, or rods around the bioactive molecules. The most suitable biopolymers for the incorporation into foods include (1) proteins, such as whey proteins, casein, gelatin, soy protein, zein, and (2) polysaccharides, such as starch, cellulose, and other hydrocolloids, with the particle formulation depending on the desired particle functionality (size, morphology, charge, permeability, environmental stability), on end product compatibility and in general in product behavior, as well as on release properties and in body behavior. 

It is possible to calculate from statistical mechanical principles the approximate conformations of the adsorbed caseins, by assuming that they are flexible, and composed of chains of hydrophilic and hydrophobic amino acids (91). The calculations of these model systems show many of the features of the actual measured properties, especially the tendency of the adsorbed 6-casein to protrude further from the inter face than the Ogj-casein (92). These calculations have in turn been used to explain the differing stability of the two different types of emulsions (93). These calculations have considerable success in explaining both the structure and stability of casein-coated emulsions, but are less adaptable to explain the behavior of more rigid protein surfactants. However, the same principles have been used to explain the apparently anomalous adsorption of phosvitin (16). 

Firstly, the stability of the foam formed by P-casein and whole casein appears very different, the former being more stable. In order to further investigate this issne, we evalnate several surface properties of these two proteins. The surface tension and surface rheology do not seem to be accurate enough to account for this large difference in foam stability, since they show very similar values. However, the thickness of the foam films stabilized by the two proteins respectively seems to determine the ultimate behavior of the foam. Hence, the thicker foam film measured for P-casein probably prevents coalesce of air bubbles resulting in more stable foam formed by this protein as compared to whole casein. 

Whereas Tween 20 displaces completely p-casein from the surface, whole casein appears to be more resistant to the displacement. This feature is reflected in the foam stability and in the drainage of thin liquid films of whole casein/Tween 20 mixtures. The reason for this might be related to the more compact structure of K-casein and its resistance against displacement (Maldonado-Valderrama and Langevin, 2008), also suggesting that this fraction governs the foam stability of whole casein. This section illustrates the important effect of the nature of the components on foam stability of mixed protein/surfactants systems and the important relationship with surface behavior. 

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