Functional Properties

Proteins contribute significantly to the physical properties of foodstuffs, especially ability to build or stabilize gels, foams, doughs, emulsions andfibrillar structures. Foaming, gelling, and emulsifying properties

Foaming Properties

Proteins act as foam-forming and foam-stabilizing agents in various foodstuffs, e.g., in baked products,sweets, desserts, and beer.

The properties of various proteins serum albumin foams ovalbumin does not. Mixtures of proteins,e.g., egg white, can be particularly effective. In that case, the globulins start the formation of foam; ovomucin is important for its stabilization, and ovalbumin and conalbumin are responsible for the heat-setting properties.

Foams are dispersions of gases in liquids.

Proteins stabilize such systems by forming flexible, cohesive films around the surface of the gas bubbles.

whipping, interface surface denaturation

decrease of the surface tension

interfaces gas bubbles

Film formation and foamability of a protein molecule depends on its diffusion rate and denatured.

These parameters in turn depend on

the molecular mass,

the surface hydrophobicity,

and the stability of the conformation

Foams collapse because large gas bubbles grow

stability of a foam depends on the stability of the protein film and its permeability for gases.

The stability of the film in turn depends on the amount of adsorbed protein and on the ability of the adsorbed protein to associate.

Surface denaturation exposes additional amino acid side-chains that can participate in intermolecular interactions.

The stronger the cross-linking, the more stable is the film.

stabilizing protein is characterized by a low molecular mass, high surface hydrophobicity, good solubility, a small net charge at the pH of the food, and easy denaturability.

Foams are destroyed by lipids and organic solvents such as higher alcohols

Egg yolk, prevents the whipping of egg white, at low concentrations.

The disturbance of protein association by the lecithins is responsible for that effect.

Enzymatic hydrolysis produces smaller molecules of a higher diffusion rate, better solubility, and higher surface hydrophobicity.

The disadvantages are the lower film stability and the loss of heat coagulability.

The introduction of charged or neutral groups or a partial thermal denaturation (e.g., of whey proteins) can also improve the desired properties.

strongly basic proteins (e.g., clupeines) increases the protein association within the films and allows the foaming of lipid-containing systems.

Emulsifying Properties.

Emulsions are disperse systems of two or more immiscible liquids.

interfacial films and prevent the dispersed phase from coalescing.

Proteins are able to stabilize emulsions due to their amphipathic nature.

The emulsifying properties of a protein adsorbability there, deformability of interfacial tension (surface denaturation).

The diffusion rate the temperature molecular mass of the protein, influenced by the pH and the ionic strength.

The adsorbability depends on exposure of hydrophilic and hydrophobic groups, the amino acid composition, pH, ionic strength, and temperature.

THE STABILITY OF THE CONFORMATION DEPENDS ON

the molecular mass, amino acid profile and number of intramolecular disulfide bonds of the protein.

PROTEIN QUALITIES AS AN EMULSIFIER

oil-in-water low molecular mass, a balanced amino acid composition in terms of charged, polar and apolar sidechains,

good solubility in water,

marked surface hydrophobicity, stable conformation

GEL FORMATIONGels are disperse systems of at least two components in which a solid phase (dispersed phase) forms a cohesive network in a liquid phase (continuous phase).

Gels are characterized by their lack of fluidity and their elastic deformability.

They are placed between solutions with repulsive forces between molecules of the dispersed phase predominating,

and precipitates with strong intermolecular forces predominating.

two types of gels, the ‘polymeric networks‘ a‘aggregated dispersions.formed by gelatin or by polysaccharides such asagarose or carrageenan.

Gels low concentration of polymer, transparency and fine texture.

Gel formation is started by adjusting to a suitable pH, by adding ions, or heating followed by cooling.

aggregation takes place mainly via hydrogen bonds.

Polymeric networks are thermo-reversible

Examples of aggregated dispersions are the gels formed by globular proteins after denaturation by heat.

The thermal unfolding of the protein exposes amino acid side-chains, which can take part in intermolecular interactions.

Interfacial properties

Hydrophilic and hydrophobic moieties, proteins can adsorb spontaneously at interfaces, and are often employed to stabilize multiple phase foods such as foams and emulsions.

The interfacial adsorption of the protein results in conformational changes which lead to a new free energy minimum and a reduction of surface tension

The ability of the protein to adopt a different structure at the interface depends on its molecular flexibility.

Flexible proteins,

β-casein, adsorb rapidly at the interface and adapt their structure,

Hydrophobic moieties are sheltered from the water phase, flexible, hydrophilic parts of the structure instead protrude into the water phase.

Protein structures caused by interactions with polysaccharides

Binding properties of food proteins

ability of proteins to bind to hydrophobic molecules. protein to assemble either spontaneously or to form aggregates during processing

structures are tunable with processing, and the encapsulated labile compounds (such as flavors, vitamins, drugs, polyphenols),

Bioactive molecules in particular areas of the gastrointestinal tract.

most proteins play a major role not only in structural scaffolding,

Protein Hydration

One of the essential components of food is water which effects the rheologicaland textural properties of foods

depending on its interaction with other food components such as proteins and polysaccharides.

The interaction of water with proteins may effect the functional properties of the proteins such as dispersibility, wettability, swelling, solubility, thickening/viscosity, water-holding capacity, gelation, coagulation, emulsification and foaming capacity

it is essential to analyze the hydration of proteins.

Water molecules bind to proteins through their charged groups (ion-dipole interactions);

backbone peptides groups: the amide groups of asparagine and glutamine;

hydroxyl groups of serine, threonine, tyrosine residues (dipole-dipole interactions);

nonpolar residues (dipole-induced dipole interaction, hydrophobic hydration).

dry protein interacts with water, the initial hydration occurs at the sites of ionizable groups of protein.

Then, water clusters form near the polar and charged protein surfaces and hydration at the polar surfaces is completed. The hydrophobic hydration of nonpolar

Water binding capacity (also called hydration capacity) of proteins defined as grams of water bound per gram of protein when a dry protein powder is equilibrated with water vapor at 90-95 % relative humidity.

Solubility

Solubility of proteins depends on the equilibrium between protein-protein andprotein-solvent interactions.

High solubility of a protein increases its functionality and usage in the food production.

Hydrophobic and ionic characteristics of the proteins are the major factors that affect the solubility.

Hydrophobic interactions decrease the solubility because of the promotion of protein-protein interactions while ionic interactions increase the solubility by promoting protein-water interactions

Proteins are classified into four categories

according to their solubility as, (1) albumins; soluble in water at pH 6,6 (e.g. serum albumin, ovalbumin);

(2) Globulins;soluble in dilute salt solution at pH 7,0 (e.g. glycinin, phaseolin); Glutelins; soluble only in acid (pH 2,0) and alkaline (pH 12,0) solutions (e.g. wheat glutelins);

(3) Prolamins; soluble in 70% ethanol (e.g. zein, gliadins).

Flavor Binding

Flavor is one of the important characteristics of the sensory properties of thefoods.

Although proteins are odorless they can bind flavor compounds.

Proteins bind flavor compounds tightly, retain them during processing of foods,

In dry conditions proteins bind flavors with van der Waals interactions,hydrogen bonding, and electrostatic interactions.

In liquid or high moisture products, proteins bind flavor through hydrophobic regions on the protein surface.

Oilseed proteins and whey proteins carry undesirable flavors and this limits their foodApplications.

Viscosity

The viscosity of a solution is related to its resistance to flow under an appliedforce (or shear stress).

Viscosity or consistency of the products is very important for the consumer acceptance of several liquid and semisolid-type foods (e.g. soups, beverages).

High-molecular-weight polymers such as proteins greatly increase viscosity.

The viscosity behavior of proteins is affected by several variables including size, shape,protein-solvent interactions, hydrodynamic volume and flexibility in the hydrated state.

Dough FormationFood proteins, especially wheat proteins, have ability to form a viscoelastic dough suitable for making bread and other bakery products.

The formation of dough and its characteristics stem form proteins.

Gluten in cereals is the major protein for dough formation.

The dough structure is based on extensive three-dimensional network of gluten protein sub-units joined together by disulfide cross-links

Gluten is a mixture of gliadin and glutenins proteins and its amino acid composition affects the functionality of gluten in the dough.

The high glutamine and hydroxyl amino acid residues responsible for the gluten water binding properties whereas cysteine and cystine residues have functions in the polymerization of gluten proteins due to sulfhydryl-disulfide interchange reactions

Antioxidant Properties of ProteinsDue to the health concerns related to the use of synthetic antioxidants, extensivestudies have been carried out to find or develop safe and natural antioxidants. Manyproteins including casein, soy proteins, ovalbumin, oilseed proteins, gliadin, zein,bovine serum albumin, yam dioscorin, lactoferrin, sericin, carnosine, etc have beenreported to have an antioxidant activity (Rajalakshmi and Narasimhan 1996, Kouoh et35al. 1999, Kim et al. 2001, Hou et al. 2001, Hu et al. 2003). It was reported that aminoacids show their antioxidative properties both as primary antioxidants or secondaryantioxidants (Sakanaka et al. 2004). The proteins owe their antioxidant activity to theirconstituent amino acids. The antioxidant activity of aromatic amino acids such astyrosine, phenylalanine and tryptophan and sulfur containing amino acids such ascysteine is due to their ability to donate protons to free radicals (Hu et al, 2003,Rajapakse et al. 2005, Je et al. 2004). On the other hand, the basic amino acids such ashistidine, lysine and arginine and acidic amino acids such as aspartate and glutamateshow their antioxidant activity by chelating metal ions (Saiga et al. 2003, Rajapakse etal. 2005). The reports of different workers show that histidine may behave as both aradical scavenger and a metal chelator due to its imidazole ring (Chen et al. 1996,Rajapakse et al. 2005). Thus, this amino acid may have a critical importance for theantioxidant activity of proteins. It is also reported that there is a close relationshipbetween the hydrophobicity and antioxidant activity of peptides (Chen et al. 1995,Rajapakse et al. 2005, Saıga et al. 2003). In fact, many antioxidative peptides containhydrophobic amino acid residues such as valine and leucine at the N-terminus (Kim etal. 2001). It seems that the hydrophobicity is important since it increases the interactionof protein with the lipids. Moreover, Hu et al. (2003) reported that the cationiccharacteristics of protein inhibit lipid oxidation due to the electrostatic repulsion oftransition metals away from the lipid droplets.The presence of some antioxidant amino acids is not the only factor thatdetermines the antioxidative properties of proteins or peptides. The correct positioningin the peptide sequence is also a very important factor effective on antioxidant activity(Rajapakse et al. 2005, Chen et al. 1996). It was reported that the position of histidine,proline, leucine, and glutamic acid in the chains of antioxidative peptides is effective ontheir radical scavenging activities. For example, the peptides having proline at the Nterminusmore effectively prevents oxidation of linoleic acid than peptides havingproline at the C-terminus (Chen et al. 1996). Also, peptides having histidine residues atthe N-terminus show higher metal chelating activity than peptides having histidine atthe C-terminus (Chen et al. 1998).By modification, it is possible to enhance the antioxidant activity of proteins.For example, it was reported that Maillard reaction with polysaccharides may increasethe antioxidant activity of proteins by improving their hydrophilic/hydrophobic balance(Nakamura et al. 1998). The antioxidant activity