Food foams are dispersions of gas bubbles in a continuous liquid or semisolid phase.
Foaming is responsible for the desirable rheological properties of many foods, such as the texture of bread, cakes, whipped cream, ice cream, and beer froth. In most of these products, proteins are the main surface-active agents that help in the formation and stabilization of the dispersed gas phase. Thus foam stability may be an important food quality criterion.
However, foams are often a nuisance for the food processor, such as in the production of potato starch or sugar.
In most cases, the disperse phase is air or carbon dioxide and the continuous phase is composed of two films of proteins adsorbed on the interface between a pair of air bubbles, with a thin layer of liquid between them. The diameters of bubbles might range from 1μm to several centimeters, depending on the surface tension and viscosity of the aqueous phase and energy input. Evenly distributed small bubbles can impart foods with desired consistency, exquisite texture, and softness and improve the dispersivity and flavor. The formation of bubbles is shown in Figure 5-12.
Figure 5-12. Formation of bubbles [15].
Bubbles can also be formed by whipping or violently shaking a protein-containing solution in the presence of gas. Whipping is the most common measure used to produce bubbles. Compared with the first method mentioned above, whipping can produce stronger mechanical stress and shearing force and hence gases are dispersed more uniformly.
However, too strong mechanical stress might affect the aggregation and formation of bubbles by hindering the absorption of protein at the interface. Besides, the amount of proteins required for stabilizing bubbles is also increased by 1%~40% (w/v). The volumes of materials can increase by 300%~2000% after whipping.
The third method of producing bubbles is to suddenly release the pressure of a pressured solution. This method has been used in the production of whipped butter.
The foaming power of proteins and the stability of bubbles foams are affected by many factors, such as pH, salts, saccharides, lipids, and protein concentration. Table5-9 lists the foaming power of some proteins.
Table 5-9. Foaming capability of protein [16]
Protein type Foaming power at 0.5%
protein concentration (W/V)
Protein type Foaming power at 0.5% protein concentration (W/V) Bovine serum
albumin 280 β-Lactoglobulin 480
Whey protein isolate
600 Fibrinogen 360
Ovalbumin 40 Soybean protein (enzyme
hydrolyzed)
500
Egg albumen 240 Gelatin (acid-processed
pig skin produce)
760
The pH is an important factor that affects the foaming ability and foam stability of proteins. Generally, high solubility is a prerequisite for good foaming ability and foam stability. However, insoluble protein granules, such as fibrillin in its isoelectric point, may
LIQUID GAS BUBBLE
impart good foam stability. Some proteins give rise to small to foam volume in their isoelectric points, but the produced foams have excellent stability, such as globulins in pH 5~5, glutelin in pH 6.5~7.5, and lactalbumin in pH 4~5. Some protein-stabilized foams are more stable in extreme pH values, possible due to increased viscosity of involved proteins.
The foams in most foods are formed in other pH than the isoelectric points of contained proteins.
Sugars suppress the expansion of protein-stabilized foams, but increase the foam stability. This is because sugars can increase the viscosity of the bulk phase and reduce the drainage of the lamella fluid. Sugars can stabilize the conformation of proteins and hinder their adsorption and unfolding on the interface. Hence, proteins cannot produce large interfacial area and foam volume in the presence of sugars. This is why sugars are often added after the foam expansion has already been completed in the manufacture of meringue and other sugar-containing foods.
Lipids in low concentrations (about 0.1%) seriously impair the foam ability of proteins.
This is because lipids are more surface-active than proteins. Lipids readily adsorb at the air-water interface and inhibit adsorption of proteins during foam formation. Besides, lipid films lack the cohesive and viscoelastic properties necessary to withstand the internal pressure of the foam bubbles, the bubbles rapidly expand, then collapse during whipping. Thus, lipid-free whey protein concentrates and isolates, soy proteins, and egg proteins without egg yolk display better foaming properties than do lipid-contaminated preparations.
Partially denatured proteins have improved foaming abilities than their nature forms.
Moderate heat treatment can enhance the foaming ability of soybean protein (70~80°C) and lactalbumin (40~60°C). Although moderate heat treatment increases the expansion capacity, it decreases the foam stabilization. Excessive heat treatment might impair the foaming ability of proteins.
To obtain enough quantity of foam, the whipping operation must last long enough in suitable strength for sufficient unfolding and adsorption. However, excessive whipping can reduce the expansion volume and foam stability. For example, ovalbumin is very sensitive to excessive whipping. Whipping of ovalbumin for 6~8min can lead to coagulation-flocculation in the air-water interface. These insoluble proteins could not be completely absorbed on the interface and the viscosity of the formed liquid sheet cannot meet the requirement of high stability of foams.
Most proteins contain more than one subunit. Hence, the foaming ability of a protein is also affected by the interaction between the subunits absorbed on the interface. For example, the excellent foaming ability of egg whit has been attributed to its protein composition.
Acidic proteins can get improved foaming ability when complexing with an alkaline protein.
No close correlation between emulsifying ability and foaming ability has been found for protein.
Salts affect the foaming of proteins by influencing the solubility, viscosity, unfolding and aggregation of proteins. The foaming ability and foam stability of most globular proteins, such as bovine serum albumin, egg albumin, gluten and soy proteins increase with increasing concentration of NaCl. This behavior is usually attributed to neutralization of charges by salt ions. However, some proteins, such as whey proteins, exhibit the opposite effect: Both foam ability and foam stability decrease with increasing concentration of NaCl (Table 5-10).
Bivalent cation ions, such as Ca2+ and Mg2+, in concentration 0.02~0.04mol/L could bridge
with the carboxyl group of proteins to form protein films with good viscoelasticity. Hence, the presence of these ions can increase the foam stability.
5.2. Viscoelasticity
Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation.
Table 5-10. Effect of NaCl on the foaming ability and foam stability of whey protein isolate [17]
Concentration of NaCl(mol/L)
Total interfacial area (cm2/mL of foam)
Times of 50% collapse of initial area (second) both viscous and elastic properties. When a mixture of wheat flour and water (about 3:1 ratio) is kneaded, a dough with viscoelastic properties forms that is suitable for making bread and other bakery products.
These unusual dough characteristics are mainly attributable to the proteins in wheat flour.
Wheat flour contains 20% soluble and 80% insoluble protein fractions. The soluble proteins are primarily albumin, globulin-type enzymes and minor glycoproteins. These proteins do not contribute to the dough-forming properties of wheat flour. The major storage protein of wheat is gluten. Gluten is a heterogeneous mixture of proteins, mainly gliadins (soluble in 70%~90% ethanol) and glutenins (insoluble in water or ethanol, but soluble in acidic and alkaline solutions). Gluten can entrap gas during fermentation to produce the viscoelastic property. Besides, the starch granules, pentosan, polar and non-polar lipids, and soluble proteins in wheat flour facilitate the formation of the viscoelastic protein network and dough texture. Gluten is rich in Gln (more than 33% of the amino acid residues) and hydroxyl-containing amino acids. Hence, the amino acids can form hydrogen bonding readily and this property contributes to the water adsorption and cohesion-adhesion property of gluten. About 30% of gluten's amino acid residues are hydrophobic, and the residues contribute greatly to its ability to form protein aggregates by hydrophobic interactions and to bind lipids and other nonpolar substances.
Cysteine and cystine residues account for 2~3% of gluten's total amino acid residues.
During formation of the dough, these residues undergo sulfhydryl-disulfide interchange reactions resulting in extensive polymerization of gluten proteins.
Several physical and chemical transformations occur during mixing and kneading of a mixture of wheat flour and water. Under the applied shear and tensile forces, gluten proteins absorb water and partially unfold. The partial unfolding of protein molecules facilitates hydrophobic interactions and sulfhydryl-disulfide interchange reactions, which result in
formation of thread-like polymers. These linear polymers in turn are believed to interact with each other, presumably via hydrogen bonding, hydrophobic associations, and disulfide cross-linking, to form a sheet-like film capable of entrapping gas.
Because of these transformations in gluten, the resistance of the dough increases with time until a maximum is reached, and this is followed by a decrease in resistance, indicative of a breakdown in the network structure. The breakdown involves alignment of polymers in the direction of shear and some scission of disulfide cross-links, which reduces the polymer size. The time it takes to reach maximum dough strength during kneading is used as a measure of wheat quality for bread making—a longer time indicating better quality.
The development of the viscoelastic dough is thought to be related to the extent of sulfhydryl-disulfide interchange reactions. This is supported by the fact that when reductants, such as cysteine, or sulfhydryl blocking agents, such as N-ethylmaleimide are added to dough, viscoelasticity decreases greatly. On the other hand, addition of oxidizing agents, such as iodate and bromate, increase the elasticity of the dough. This implies that wheat varieties that are rich in SH and S-S groups might possess superior bread-making qualities, but this relationship is unreliable. Thus, the role of sulfhydryl-disulfide interchange reactions in the development of viscoelastic dough is poorly understood.
The strength of dough is related to the contents of glutenin and completely insoluble residue proteins. Experiments on wheat flour supplemented with glutenins and gliadins of different proportions indicate that glutenins determine the elasticity, cohesiveness, mixture tolerance, extensibility, the expansion of dough and the mobility of gliadins. Suitable proportions of the two categories of proteins are important for bread making.
High glutenins contents can increase the cohesiveness of dough and suppress the expansion of entrapped CO2, while high gliadins contents increase the elongation and make the gluten film fragile and permeable, leading to poor CO2 retention and bread collapse.
The different effects of glutenins and gliadins on bread-making may be related to the differences in their structure and composition. In gluten these exist as single polypeptides with molecular weight ranging from 30,000 to 80,000. Although gliadins contain about 2~3%
half-cystine residues, they apparently do not undergo extensive polymerization via sulfhydryl-disulfide interchange reactions. The disulfide bonds appear to remain as intramolecular disulfides during dough making. Dough made from isolated gliadins and starch is viscous but not viscoelastic.
Glutenins are heterogeneous polypeptides with molecular weight ranging from 12,000 to 130,000. These are further classified into high-molecular-weight (M.W. >90,000, HMW) and low-molecular-weight (M.W. <90,000, LMW) glutenins. In gluten, glutenin polypeptides are present as polymers joined by disulfide cross-links, with molecular weights ranging into the millions. Because of their ability to polymerize extensively via sulfhydryl-disulfide interchange reactions, glutenins contribute greatly to the elasticity of dough. Therefore, an optimum ratio of gliadins and glutenins seems to be necessary to form viscoelastic dough.
Some studies have shown a significant positive correlation between HMW glutenin content and bread quality in some wheat varieties, but not in others. Available information indicates that a specific pattern of disulfide crosslink association among LMW and HMW glutenins in gluten structure may be far more important to bread quality than the amount of this protein.
For example, association/polymerization among LMW glutenins which have the similar structure form the HMW gliadin. This type of structure contributes to viscosity of the dough, but not to elasticity. In contrast, if LMW glutenins link to HMW glutenins via disulfide
cross-links (in gluten), then this is believed to contribute to dough elasticity. It is possible that in good-quality wheat varieties, more of the LMW glutenins may polymerize with HMW, whereas in poor-quality wheat varieties, most of the LMW glutenins may polymerize among themselves. These differences in associated states of glutenins in gluten of various wheat varieties may be related to differences in their conformational properties, such as surface hydrophobicity, and reactivity of sulfhydryl and disulfide groups.
Baking does not lead to the re-denaturation of gluten, because glutenins and gliadins occur naturally in partially unfolded form and are further unfolded during kneading. When heated in 70~80°C, gluten loses water and the water is absorbed by gelatinized starch granules. Hence, gluten can maintain the softness and water content (40%~50%) of bread in baking.
Soluble protein fractions, including albumin- and globulin-type proteins, are denatured and coagulated during baking. This partial gelling is beneficial for the cohesion of crumbs.
Hence, supplementation of external proteins is good for bakery foods quality, such as nutrition fortification. However, not all proteins are beneficial for gluten network formation.
For example, supplementation of wheat flour with albumin- and globulin-type proteins, such as whey proteins and soy proteins, adversely affects the viscoelastic properties and baking quality of the dough. These proteins decrease bread volume by interfering with formation of the gluten network. Addition of phospholipids or other surfactants to dough counters the adverse effects of foreign proteins on loaf volume. In this case, the surfactant/protein film compensates for the impaired gluten film. Although this approach results in acceptable loaf volume, the textural and sensory qualities of the bread are less desirable than normal.
5.3. Gelation
Gelation is the process of aggregation and ordered network formation of denatured proteins. The gelation of proteins is very important for some foods, including various dietary products, jelly, gelatin gel, soybean protein gel, and textured vegetable proteins. Tofu, which is a favorite food in China, is a typical food prepared by protein gelation.
Heating is necessary for the formation of most protein gels. The addition of salts, especially those containing Ca2+, improves the gelation rate and gel strength. Proteins can also interact with polysaccharides to form gels. For example, the positively-charged gelatin can bind to negatively-charged sodium alginate through nonspecific ionic interaction to form gel with a high melting point (80°C).
Many protein gels are highly hydrated and each gram of protein can hold up to more than 10 grams of water. Many food components can be trapped in the network of protein gels.
The water contents of some protein gels might reach up to 98%. Although the trapped water has properties similar to the water in dilute solutions, the water cannot be squeezed out easily.
5.4. Hydration
The conformation of each protein in a solution is affected by their interactions with water. The interaction between proteins and water also occurs in solid foods. In addition, dry concentrated proteins or protein isolates must be hydrated before use. Hence, the hydration
and rehydration properties of proteins markedly affect their applications in the food industry.
Figure 5-13 illustrates the sequence of interactions occurred between dry protein and water.
Figure 5-13. Sequence of the interactions between dry protein and water [10].
The protein-protein and protein-water interactions are affected by environmental conditions, including protein concentration, pH, temperature, time, ionic strength and the presence of other components. The pH alternation affects the ionization and net charge of proteins and thus changes the attraction and repulsion between proteins and their water binding capacity. The water binding capability of proteins decreases with the increase of temperature. High temperature decreases the bonding force between hydrogen bonds and leads to protein denaturation and aggregation. Protein denaturation or aggregation reduces the surface area of the protein and consequently the hydration of polar amino acids. When proteins with compact strength are heated, the proteins are unfolded and the peptide chains and polar chains that are originally located inside the structure are exposed to water and the hydration capability is increased as a result. The gelation of some proteins, such as lactoalbumin, is irreversible. If the gels of these proteins are dried, the gel network is transformed to the capillary of the protein powder. The dried proteins thus have significantly increased hydration capability. However, the rate and degree of hydration are also affected by the size and the holes on the surface and inner of proteins. Ions might compete for the side chains of amino acids with water and subsequently influence the hydration capability, swelling property, and solubility of proteins. Salts in low concentrations enhance the hydration capability of proteins, but salts in high concentrations can lead to dehydration due to the excessive water-salt interaction over the water-protein interaction.
5.5. Solubility
It is a long process for proteins to reach their equilibrium solubility and the protein solubility varies with the pH, ionic strength, and temperature of the environment and protein concentration. The solubility of most proteins decreases irreversibly when heated. However, heat treatment is unavoidable during food processing and protein denaturation might occur during extraction and purification even in mild conditions. Hence, the nitrogen solubility index (NSI) of soybean protein powder, concentrated soybean protein, and soybean protein isolate vary in the range 10%~90%.
Dry
The solubility of proteins is essential for determining the optimum conditions of protein extraction, purification and separation and is an important indicator of its application range.
The degree of insolubility can be used as a measure to evaluate the denaturation and agglutination of proteins. It is reported that proteins with greater initial solubility can disperse quickly and yield well dispersed hydrocolloid system with macrostructure and smooth texture. In addition, high initial solubility facilitates the diffusion between of proteins to the gas/water and oil/water interface for enhanced their surface activity.
5.4. Viscosity
The viscosity or consistency of a liquid reflects its resistance to flow under an applied force or shear stress and can be represented in viscosity coefficient η, which is defined as the ratio of shear stress to shear rate (or flow rate). The viscosity of protein fluids is largely related to the apparent diameter of dispersed protein molecules or protein particles. The apparent diameter of proteins is decided by the following parameters: (1) the inherent nature of protein molecules, such as molar concentration, molecular size, molecular volume, molecular structure and charge; (2) protein-solvent interaction, which affects the swelling and solubility of proteins and the hydrokinetics of the fluid surrounding protein molecules; (3) protein-protein interaction. The interaction determines the size of protein aggregates and the interaction opportunity increases in high protein concentrations.
The viscosity coefficient of protein-containing solutions, such as homogenates, emulsions, pastes and gels, might decrease with the increase of shearing stress. This behavior is known as shear thinning. During shearing, protein molecules are redirected to the flow
The viscosity coefficient of protein-containing solutions, such as homogenates, emulsions, pastes and gels, might decrease with the increase of shearing stress. This behavior is known as shear thinning. During shearing, protein molecules are redirected to the flow