Protein stabilized rheological properties

Rheological measurements were carried out to investigate the rheological properties of emulsions stabilized by different fat-water interfaces and the influence of fat droplets on the formation of the protein networks during a process of gelation. 

T.D. Dimitrova and F. Leal Calderon Rheological Properties of Highly Concentrated Protein-Stabilized Emulsions. Adv. Colloid Interface Sci. 108-109, 49 (2004). 

The term food colloids can be applied to all edible multi-phase systems such as foams, gels, dispersions and emulsions. Therefore, most manufactured foodstuffs can be classified as food colloids, and some natural ones also (notably milk). One of the key features of such systems is that they require the addition of a combination of surface-active molecules and thickeners for control of their texture and shelf-life. To achieve the requirements of consumers and food technologists, various combinations of proteins and polysaccharides are routinely used. The structures formed by these biopolymers in the bulk aqueous phase and at the surface of droplets and bubbles determine the long-term stability and rheological properties of food colloids. These structures are determined by the nature of the various kinds of biopolymer-biopolymer interactions, as well as by the interactions of the biopolymers with other food ingredients such as low-molecular-weight surfactants (emulsifiers). 

We tentatively suggest that the frequently observed increase in stabilizing efficiency of proteins after they have been denatured in solution could be because they are aggregated and adsorb as aggregates, resulting in an interfacial layer that exhibits very quickly highly viscoelastic and solidlike rheological properties. 

Rheological properties. Viscosity, an important physicochemical property of many foods, can be modified by proteins or polysaccharides. The caseins form rather viscous solutions, a reflection of their rather open structure and relatively high water-binding capacity. While the high viscosity of caseinate may be of some importance in casein-stabilized emulsions, it causes production problems for example, due to very high viscosity, not more than about 20% protein can be dissolved even at a high temperature. The low protein content of caseinate solution increases the cost of drying and results in low-density powders which are difficult to handle. 

The behavior of proteins at interfaces influences the formation of foams and emulsions (32). Stabilization of foams and emulsions depends, to a great extent, on the formation, rheological, and mechanical properties of the interfacial film ( ). Factors which ensure optimum film properties in simple systems may retard film formation or cause destabilization in foams or emulsions (3 ) for example, many rheological properties of films are maximum in the isoelectric pH range of specific proteins, yet most proteins have minimum solubility in this pH range (34). Thus, environmental and processing factors which... 

Because protein-ba sed foams depend upon the intrinsic molecular properties (extent and nature of protein-protein interactions) of the protein, foaming properties (formation and stabilization) can vary immensely between different proteins. The intrinsic properties of the protein together with extrinsic factors (temperature, pH, salts, and viscosity of the continuous phase) determine the physical stability of the film. Films with enhanced mechanical strength (greater protein-protein interactions), and better rheological and viscoelastic properties (flexible residual tertiary structure) are more stable (12,15), and this is reflected in more stable foams/emulsions (14,33). Such films have better viscoelastic properties (dilatational modulus) ( ) and can adapt to physical perturbations without rupture. This is illustrated by -lactoglobulin which forms strong viscous films while casein films show limited viscosity due to diminished protein-protein (electrostatic) interactions and lack of bulky structure (steric effects) which apparently improves interactions at the interface (7,13 19). 

In the case of the major cytoplasmic protein of leaves, ribulose 1,5-biphosphate carboxylase (RUBISCO), the surface rheological properties and foam stability were maximum at pH 5.5, close to the isoelectric point (pH 4.8) and all parameters measured were greater than any other protein studied (2 ). This may be related to the large molecular size of RUBISCO, i.e. 560 000 daltons, and its disulfide stabilized globular structure. 

Y. Hemar, D. S. Home. Dynamic rheological properties of highly concentrated protein-stabilized emulsions. Langmuir. 2000 16 3050-3057. 

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