Functionality has been defined as: "any property of a food or food ingredient, except its nutritional ones, that affects its utilization." For proteins then, there must be a large number of functions and functional properties listed in Table 1 are some of the most important ones to consider when discussing proteins.

Functions of proteins in foods

Protein Structure

A protein manifests functionality by interacting with other compnents within the food system. These interactions may involve solvent molecules, solute molecules, other protein molecules or substances that are dispersed in the solvent such as oil or air.

In order to describe the forces involved in these interactions, it is essential that the forces and energies involved in the achievement and maintenance of native protein structure be described. While a complete discussion of the forces involved is beyond the scope of this chapter, some observations on the nature of protein structure will be useful.

Proteins exist in the lowest kinetically attainable state of free energy. The free energy of the protein may not be the global minimum, but it will be the lowest that the protein can achieve in a reasonable period of time.

Protein structure is highly dependent upon the environment and the protein will assume different conformation as the environmental conditions change. Factors of importance include pH, temperature, dielectric constant, ionic strength and the presence of other molecules including air, fat, denaturants, etc.

One of the main ways that proteins lower their free energy involves the removal the removal of hydrophobic groups from the aqueous environment. This may provide the greatest single decrease in free energy of all the types of binding that occur within proteins. The strength of hydrophobic binding is, however, very sensitive to changes in temperature and dielectric constant and thus changes in these parameters can have large influences on protein structure.

Protein molecules may contain crosslinks of a non-covalent, eg. salt bridges or a covalent, eg. disulfide bonds, nature. These crosslinks lower the conformational entropy of the molecule which must be compensated for by a decrease in binding energy. The presence of crosslinks adds greatly to the stability of the native protein structure and makes the molecules resistant to unfolding or denaturation. For example, simple unfolding is inhibited sterically by the presence of crosslinking because portions of the molecules are held in place by the crosslinks. Denaturation is also less likely because one of the driving forces of denaturation, an increase in conformational entropy is greatly reduced. In a non-crosslinked protein, if unfolding to a random coil structure can be induced, there is a very large gain in the number of conformations the molecule can assume. This gain in conformational entropy is a large driving force for the maintenance of the denatured state when the denaturing agents are removed. In contrast, a highly crosslinked protein cannot assume the same degree of random conformations and thus the increase in entropy is much less. This helps explain why molecules that contain large numbers of disulfide bonds are often resistant to denaturation.

The structures attained by proteins are not rigid, but are very dynamic. There is rotational freedom about many of the bonds within the protein molecule and the entropy gain of this freedom lowers the total free energy of the native structure. There are also portions of the protein structure that are stabilized by rather weak secondary forces and these are often free to assume different conformations. These alternate conformations lead to structures of higher free energy and thus are not stable or long lived. A protein may be envisioned as a dynamic entity that is constantly sampling a variety of structures. These new structures are usually only slightly different from the native conformation and almost always lead to a situation where the free energy of this system increases. The increase in free energy causes the protein to spontaneously refold into the state of lowest free energy. Thus, the native structure of a protein is not the only structure it can assume, but rather the one of lowest free energy and hence of greatest probability. Slight changes in the environment can cause alternate structures to be of lowest free energy and thus lead to protein denaturation.

In order for a protein to exhibit functionality, it must interact with other components of the food system. These interactions may often require that the protein be free to either move throughout the system or to alter its structure in such a way to allow interactions with other components.

In some cases the simple presence of other molecules in the protein solution will allow interaction to occur, but more commonly, the interactions require an input of energy into the system to insure adequate mixing. This energy may alter the physical nature of the molecules being mixed, eg. decrease in average fat globule size and also alter the conformation of the protein molecule.

Flavor

In order for a compound to have flavor it must either interact with one of the four basic taste receptors on the tongue ( sweet, sour, bitter or salty ) or it must be volatile enough to interact with odor receptors in the olfactory organ. Most proteins can do neither of and so they have little direct impact on flavor. While free proteins as such have little affect, they may contribute to food flavor through one of the following:

bound molecules
absorption
break down products
chemical reactions.

Water Binding

Factors affecting water binding by food proteins
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Amino acid composition
Protein conformation
Surface polarity/ hydrophobicity
Ionic concentration
Ion species
pH
Temperature

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Types of water associated with proteins.
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Structural
Monolayer
Unfreezable
Hydrophobic Hydration
Capillary

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Factors that affect Viscosity

pH
temperature
concentration
ionic strength.
Gelation

Gelation
Definition by Ziegler and Foegeding:

A gel is a continuous netwrok of macroscopic dimensions immersed in a liquid medium exhibiting no steady-state flow.

Stages in heat induced gelation
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Protein unfolding
Water binding
Protein-protein interactions
Water immobilization

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Bound Water

A small portion of the water is tightly bound to proteins
The majority of water in a gel is capillary water
A three dimensional netwrok must be formed to entrain water

Factors affecting gel formation.
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Temperature
Protein concentration
pH
Salt concentration
Calcium concentration
Free sulfhydryl concentration

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pH

pH affects the strength and nature of the gel
At lower pH values more coagulated gels are formed
The effect of sulfhydryl reagents is greater at higher pH values

Calcium Concentration

Calcium forms crosslinks between proteins and adds structure to the gel
Approximately 20 mM Ca is optimal for WPC gelation
At higher levels, calcium may promote protein aggregation
In WPC, increased calcium decreases gel strength

Sulfhydryl Groups

Sulfhydryl groups can form effective crosslinks in protein gels
Excessive sulfhydryl groups can inhibit gel formation
The effect of sulfhydryl groups are most noted above pH 7.5
In WPC, increases in sulfhydryl groups increase gel strength

Protein Hydrophobicity

Hydrophobic groups can form crosslinks in protein gels
Excessive hydrophobicity can cause aggregation prior to proper crosslink formation
In normal WPC, increased hydrophobicity increases gel strength

Emulsification

Emulsions are thermodynamically unstable mixtures of immiscible liquids . If energy is applied the systems may be dispersed, but increased surface energy causes the phases to coalesce unless an energy barrier to coalescence is established. The forces responsible for phase separation include:

Phase Separation

Lipid-lipid interactions are predominantly due to London forces.
Water-Water interactions are predominantly due to H bonds.
Water-Lipid Interactions:

DG is positive
DH is negative
DS is negative

Factors Important to Protein Stabilized Emulsions

Rate of diffusion
Solubility
Viscosity
Protein Flexibility
Net Charge
Protein Hydrophobicity

Protein Stabilized Emulsions
In order to form and stabilize an emulsion, a protein must:

Diffuse to the interface
Unfold
Expose hydrophobic groups
Interact with lipid

Temperature

Emulsion formation favored around 60 C
Lower viscosity
Favors hydrophobic interactions

Lower temperature decreases energy barrier to lipid-water interactions
Freezing can cause physical damage to the interface

Diffusion

Protein must diffuse to the interface
Rate of diffusion affected by presence of salts
Rate of diffusion affected by viscosity
Most studies indicate that diffusion from the bulk phase to the interface is rate limiting

Emulsified droplets can be stabilized by the addition of molecules that are partially soluble in both phases. In foods a number of small emulsifier molecules can serve this function. Proteins capable of unfolding at the interface may also serve this function. Protein coats the lipid droplet and provides an energy barrier to particle association and phase separation.

Stoke's law:

Where v equals the velocity of the fat globule, r is the radius of the fat globule, g is the force of gravity, Dp is the density difference between the two phases and u is the viscosity of the continuous phase.

Foaming Foaming

Similar in some respects to emulsification.
Denaturation of protein at an air-liquid interface.
Hydrophobic groups inter the air, hydrophilic amino acids remain in the water.

Lipids

Presence of lipid interferes with formation of foams.
For high fat products, must have one type of protein to form the emulsion and another to incorporate air.
Low fat foams are often made with egg white, while high fat foams are often produced with casein.

Foam Stability

Factors that decrease foam stability:
Gravitational Drainage
Capillary Pressure Drainage
Mechanical Disturbances
Factors that increase foam stability:
Surface viscosity
Gibbs-Maragoni effect
Electric double layers

Fiber Spinning
Mechanism:

Protein generally made up to 10 - 50% protein at pH values in excess of 10.
High pH causes protein to unfold and mixture to become viscous.
Protein is extruded through a die with openings of from 0.002 to 0.006 inches in diameter
Causes an alignment of protein chains which then are placed into an acid-calcium bath.

Extrusion

Method of protein modification - simpler than spinning.
Does not give well defined fibers, but rather, fibrous particles
Goal is to achieve mouthfeel similar to meet.
Can use either defatted or full fat meals.

Extrusion

Protein is dispersed at high temperature and pressure.
Extruded from high pressure to atmospheric.
Water flashes off and product swells creating large voids.
Often other proteins utilized to give better texture.

Dough Formation
Dough - An extensible, viscoelastic protein network formed upon the mixing of an appropriate amount of water to cereal proteins:
Wheat
Rye
Barley

Proteins may be added as a source of of the enzyme, lipoxygenase, used to bleach flower and to "age" it.
Some flours of low protein content will produce a better product if protein is added.
If NFDM, must be high heat product.
Generally, addition of oil seed protein will decrease loaf volume and give poor crumb structure.

Table 1. Functional requirements of food protein ingredients.

Table 2. Food Protein requirements for application in different food products.

Table 3: Functional characteristics of some common food proteins.