Emulsifying and Emulsion Stabilizing Role of Proteins - Essay Example

Discuss how the primary, secondary and tertiary structure of a protein will influence the way in which it behaves when it adsorbs at an oil droplet surface in a food emulsion. Food emulsions are oil-water dispersion systems, in which proteins play the emulsifying and emulsion stabilizing role. The formation of an emulsion necessitates a high input of energy and the presence of at least one emulsifier in the system in order to aid in reducing the interfacial tension between water molecules and oil molecules. Then, following the molecular adsorption, an interfacial layer at the surface of droplets forms and protects the newly formed droplets from phase separation and coalescence. The stability of the system is controlled by the strength of the absorbed protein film around the dispersed droplets and the balance of repulsive and attractive forces of interactions among neighboring droplets and previous favoring droplet flocculation (Papadopoulos, 2008). The absorbed protein molecules on the surface of oil droplets, to some extent, influence the interactions among droplets and, thus, determine the physical stability and rheological properties. After adsorption on the interface, molecules exhibit a conformation that is influenced by molecular flexibility and environmental factors, including ionic strength, presence of low molecular weight emulsifiers or other proteins, and pH. These factors affect the strength of droplet-droplet interactions, protein hydrophobicity, or molecular charge. Protein can undergo irreversible changes during heat treatment. At 65 ?C, whey proteins unfold, exposing previously hidden hydrophobic groups into the surface of the molecule (Raikos, 2010). Such structural change can affect the functionality of the protein. When this change at molecular level is irreversible, the protein molecule has undergone denaturation. Heating whey proteins, at increasing heating temperature, results in the formation of small aggregates of ?-lactoglobulin. These small aggregates form bigger aggregates when ?-lactoglobulin denatures at increasing heating time. When heating time or temperature is raised, ?-lactalbumin denatures and forms complexes with the denatured ?-lactoglobulin. Both ?-lactalbumin and ?-lactoglobulin bind on the surface of casein micelles. During emulsification, milk proteins can rapidly adsorb on the surface of newly formed oil droplets. This adsorption decreases interfacial tension and leads to the formation of thick layers that prevent the flocculation or coalescence of oil droplets through electrostatic and steric stabilization mechanisms. Different proteins in a food system exhibit different degree of stabilizing and emulsifying properties due to their variation in their molecular structure, functionality, flexibility, hydrophobicity, charge, and molecular size. As well, the thermal stability of a protein-stabilized emulsion likely depends on the properties of proteins used in the preparing the emulsion. For instance, both caseinates and whey proteins are milk protein, but caseinates have globular structures that are sensitive to heat (Dalgleish, 2006). Hence, emulsions that contain whey protein may exhibit extensive destabilization due to heat treatment at temperatures above the protein’s denaturation point. The flexible and more disordered molecular structure, as compared with that of whey protein, of caseinates is not heavily altered by heat treatment. Thus, casein stabilized emulsions are less susceptible to heat aggregation. Caseins and whey proteins are commonly used as ingredients in food emulsions because they are excellent emulsifier and emulsion stabilizers. Whey protein concentrates and isolates are chiefly composed of ?-lactalbumin (?-la), bovine serum albumin, ?-lactoglobulin (?-lg), which are sensitive to heat and denature at relatively moderate temperatures. The denaturation of their structures, to a lesser extent, may also cause by their adsorption to the emulsion droplet surface. In whey protein stabilized emulsions, both protein in the continuous phase and protein at the droplet surface play a role in destabilization during heating process. After adsorption on the droplet surface, the molecular structure of ?-lg, the dominant whey protein, unfolds exposing a number of disulfide, non-polar, and thiol amino acids, which are originally inside the native protein structure, to the surface of the molecule (Dickinson, 2010). This exposure induces different thiol-disulfide and hydrophobic protein-protein interactions between protein molecules adsorbed on the same droplet surface. The repulsive forces between neighboring oil droplets prevent protein adsorbed on droplet surface from approaching each other. When these repulsive forces are overcome by attractive forces due to suitable ionic strength and pH conditions, the droplets approach one another. Then, the denaturation of protein molecules adsorbed on oil droplets induce interactions between protein molecules, resulting in emulsion destabilization. 2. Describe the molecular characteristics of globular proteins that allow food manufacturers to exploit the heat-induced denaturation of globular proteins to increase the viscosity of a fluid food. Food emulsion products are usually subjected to heat-treatment for cooking or sterilization purposes. The length and degree of heat processing may result in the destabilization of emulsion. The type and extent of destabilization largely depends on extrinsic factors, like the presence of polysaccharides or emulsifiers, pH, or ionic strength, and intrinsic parameters, such as surface hydrophobicity and molecular stability of involved proteins (Papadopoulos, 2008). In some food products, like gels, milk, and egg-based creams, the destabilization of emulsion by heat and droplet interaction is purposively intended to create a filled gel network structure that possesses the desired textural and rheological properties. However, in most food emulsion products, such changes are undesirable because droplet interaction leads to the destabilization of emulsion and decrease of the product’s commercial life. Hence, these undesirable changes should be prevented or minimized. In some food products, low molecular weight emulsifiers are mixed, while the stability of the fairly liquid emulsion is improved through the addition of polysaccharides like starch or carrageenan, which also enhance the textural and rheological properties of the products (Papadopoulos, 2008). During heat processing, emulsifiers, polysaccharide stabilizers, and additives in food products may also negatively or positively affect the stability of the emulsion system, depending on the type of the additive or on the system’s characteristics. The adsorption of protein at the oil-water interface is supported by the system’s thermodynamics. During adsorption, the protein’s hydrophobic residues move from the bulk aqueous phase and orient toward the oil phase, after a structural rearrangement at the interface. This means that, the adsorbed globular protein possesses a structure that lies somewhere intermediate between denatured and native states. This structure is sometimes called as the molten globule state. A number of studies reported that, although the protein undergoes conformational changes after its adsorption on the surface of oil droplets, its large proportion of well-organized structure is maintained (Raikos, 2010). Globular proteins are a typical ingredient of food products due to their physicochemical properties, including emulsion stabilization, surface activity, gel formation, and foaming capacity. They are commonly used to enhance the stability and texture of food products like baked products, dairy emulsions, and processed meats. Their capability of giving desirable properties in food applications depends on the composition of the mixture they are dispersed in, their concentration, and their molecular structure (Dalgleish, 2006). By understanding how these factors affect the functionality of globular proteins, means of controlling these factors can be determined in order to enhance the quality of protein-based food products. For instance, ?-lactoglobulin, a globular protein from whey proteins, can be used as a gelling agent because of its capability of forming a rigid 3-D network that traps water and other components of a mixture. At neutral pH, native globular proteins in a dispersion medium can hardly form gels because the intermolecular repulsion is greater than the system’s intermolecular attraction. Native globular proteins can form gels if they are heated above their thermal denaturation temperature, in which electrostatic repulsion among proteins is lessened (Chanasattru, Decker, and McClements, 2006). The aggregation of denatured proteins is driven by high hydrophobic attraction and formation of disulfide bonds among them. Then, the microstructure of the resulting gel network is dependent on the electrostatic attraction among the protein molecules and the mixture’s pH and ionic strength. Under low ionic strength and pH that is far from the protein’s isoelectric point, the electrostatic repulsion between proteins becomes relatively strong. Thus, the proteins are likely to form filamentous aggregates, leading to a transparent elastic gels with capability of holding water. On the other hand, at high ionic strength and pH that is near the protein’s isoelectric point, the electrostatic repulsion between protein molecules becomes relatively weak. Hence, the proteins are likely to form particulate aggregates, resulting in rubbery opaque gels that poorly hold water molecules. Water-holding capacity is the protein’s capability of absorbing water molecules and holding them against gravity within a matrix. In food applications, water-holding capacity is more important than water binding property since the latter is unstable at raised temperatures because hydrogen bonding is broken at such condition. The high solubility of whey proteins even at their isoelectric point is attributed to the large proportion of surface hydrophilic to hydrophobic residues at the protein’s backbone. Whey proteins are also soluble in a pretty wide range of pH, between 2 to 9, making them a good ingredient of a variety of food products, including acidic beverages (Dickinson, 2010). They also possess an excellent thickening capability, which is important in designing, developing, and processing of food products. In particular, ?-lactoglobulin increases its viscosity with shearing time and at very high concentrations due likely to the partial protein denaturation. The dispersion of denatured, randomly coiled molecules tends to be more viscous than the aggregates of compact folded globular proteins of the same molecular mass. References Chanasattru, W., Decker, E.A., and McClements, J.D. (2006) ‘Modulation of thermal stability and heat-induced gelation of -lactoglobulin by high glycerol and sorbitol levels.’ Food Chemistry 103, 512–520 Dalgleish, D.G. (2006) ‘Food emulsions: their structures and structure-forming properties.’ Food Hydrocolloids 20, 415–422 Dickinson, E. (2010) ‘Food emulsions and foams: Stabilization by particles.’ Current Opinion in Colloid & Interface Science 15, 40–49 Papadopoulos, K. N. (2008) Food chemistry research developments. New York: Nova Science Publishers Raikos, V. (2010) ‘Effect of heat treatment on milk protein functionality at emulsion interfaces.’ Food Hydrocolloids 24, 259–265 Read More