Sols

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A sol can be defined as a colloidal dispersion in which a solid is the dispersed phase and liquid is the continuous phase. Gravy, stirred custard, and other thick sauces can be given as examples of sols. The proper ratio of the ingredients is necessary to achieve the desired viscosity of the sols at a certain temperature. If they are unacceptably thick, they can either be heated or more water can be added to reduce their viscosity.

Pectin is hydrophilic and attracts a layer of water that is bound tightly to the molecules by hydrogen bonds, like other carbohydrates and many proteins. Thus, water forms an insulating shield for the pectin or other hydrophilic colloid, providing a layer that inhibits bonding between the molecules of the colloidal substances (Fig. 6.7). Sols can be transformed into gels as a result of reduction in temperature. The solids in the discontinuous phase move with increasing difficulty through the continuous liquid phase and eventually start to associate with each other. In a pectin gel, the pectin molecules are the continuous phase and the liquid is the dispersed phase while in the pectin sol, the pectin molecules are the dispersed phase and the liquid is the continuous phase. Sols may be formed as a preliminary step in making a gel. Jams and jellies made with pectin are common examples that form a sol prior to the desired gel structure.

Pectin molecules . Continuous phase

Dispersed phase ,. Heating

Pectin molecules . Continuous phase

Dispersed phase ,. Heating

Pectin gel Figure 6.7 Pectin gel and sol.

Trapped liquid

Cooling

Free liquid

Pectin sol

6.7.2 Gels

A gel is the reverse of a sol in which a solid matrix is the continuous phase and a liquid is the discontinuous phase. The solid in the gel is sufficiently concentrated to provide the structure needed to prevent flow of the colloidal system.

Some of the free liquid may be released if the gel structure is cut. This phenomenon is known as syneresis. The type of solid and its concentration in the gel are important in determining the amount of syneresis. Syneresis may be undesired in some products such as jelly but may be useful in cheese production.

6.7.3 Emulsions

An emulsion is a colloidal system in which a liquid is dispersed as droplets in another liquid with which it is immiscible. Emulsions can be classified as oil-in-water (o/w) and water-in-oil emulsions (w/o). In oil-in-water emulsions, oil is dispersed in water as droplets. The typical example for o/w emulsions is mayonnaise. In water-in-oil emulsions, such as butter, droplets of water are dispersed in oil.

Viscosities of the emulsions are very high as compared to the viscosities of either of the liquids. For example, the viscosity of mayonnaise is higher than that of the vinegar and oil used in preparing mayonnaise. The stability of emulsions can be determined by the viscosity of the continuous phase, the presence and concentration of emulsifier, the size of the droplets, and the ratio of dispersed phase to the continuous phase.

When an emulsion is subjected to some stress such as freezing or centrifugation, it may break. That is, the two liquids separate into discrete phases.

In emulsion systems it is necessary to determine the difference between an emulsifier and a stabilizer (Dickinson, 1992). The emulsifiers aid emulsion formation and short-term stabilization by interfacial action. An emulsifier is a small molecule surfactant that is amphiphilic, having both polar or nonpolar parts, and thus has attraction toward both phases of the emulsion. The emulsifying agent collects around the surface of the dispersed spheres; as a result the droplets cannot touch each other directly and coalescence. Monoglycerides, polysorbates, sucrose esters, and lecithin are examples for emulsifiers. Egg yolk is the best common food naturally containing emulsifying agents. The primary emulsifying agent found in egg yolk is lecithin. Small molecules are not so effective for supplying long-term stability.

The amount of emulsifying agent present has a significant effect on the stability of the emulsion. Enough agent must be present to form a complete monomolecular layer around each droplet but there is no benefit of using more emulsifying agent than the required amount.

Stabilizers, on the other hand, are generally proteins or polysaccharides supplying long-term emulsion stability, possibly by an adsorption mechanism but not necessarily so. The main stabilizing action of food polysaccharides is by modifying viscosity or gelation in aqueous phase. Proteins act as both emulsifier and stabilizer since they have high tendency to adsorb at oil-water interfaces to form stabilizing layers around oil droplets (Dickinson, 2003).

An emulsifier should be surface active, meaning that it has an ability to reduce the surface tension at the oil-water interface. The lower the interfacial tension, the greater the extent to which droplets can be broken up during intense shear or turbulent flow (Walstra & Smulders, 1998).

An amphiphilic characteristic is required for a polymer to be surface-active. If it is a hydrocolloid, it must contain numerous hydrophobic groups. These groups will enable adsorbing molecules to adhere to and spread out at the interface which will protect the newly formed droplets. An ideal emulsifier is composed of species with relatively low molecular weight, with good solubility in aqueous continuous phase. For a biopolymer to be effective in stabilizing dispersed particles or emulsion droplets, it should have a strong adsorption characteristic, complete surface coverage, formation of a thick steric, and charged stabilizing layer (Dickinson, 2003). Strong adsorption implies that the amphiphilic polymer has a substantial degree of hydrophobic character to keep it permanently at the interface. Complete surface coverage means that the polymer is sufficient to fully saturate the surface. Formation of a thick steric stabilizing layer implies that the polymer is hydrophilic and has a high molecular weight (104-106 Da) within an aqueous medium with good solvent properties. Formation of a charged stabilizing layer implies the presence of charged groups on the polymer that contribute to the net repulsive electrostatic interaction between particle surfaces, especially at low ionic strength.

Proteins are surface-active compounds and comparable with low molecular weight surfactants (emulsifiers). They result in lowering of the interfacial tension of fluid interfaces. Proteins emulsify an oil phase in water and stabilize the emulsion. The increased resistance of oil droplets to coalescence is associated with protein adsorption at the interfacial surface and the formation of interfacial layer having high viscosity. In comparison to the low molecular weight surfactants, proteins have the ability to form an adsorption layer with high strength and increase the viscosity of the medium which results in high stability of emulsion (Garti & Leser, 2001). Low molecular weight surfactants (LMW) commonly used in foods are mono- and diglycerides, polysorbates, sorbitan monostearate, polyoxyethylene sorbitan monostearate, and sucrose esters. They lower the interfacial tension to a greater extent than high molecular weight surfactants such as proteins and gums (Table 6.3). LMW surfactants have higher adsorption energies per unit area than proteins and gums but proteins and gums can adsorb at the interface with several segments. Although LMW surfactants are more effective than the others in reducing interfacial tensions, emulsions formed by them are less stable. This is due to the fact that steric repulsion between the protein-covered oil droplets is very effective against aggregation. In addition, since proteins have higher molecular weight, their adsorption and desorption are slower than the adsorption and desorption of LMW surfactants (Bos & van Vliet, 2001).

Surfactants can be classified as nonionic or ionic according to the charge of the head group. They are also categorized based on their ability to dissolve in oil or water. A measure of this is expressed by hydrophilic/lipophilic balance (HLB). HLB number can be calculated by using the

Table 6.3 General Characteristics of Interfacial Properties of LMW Surfactants and Proteins3

Interfacial Properties

LMW Surfactants

Proteins

Number of molecules Adsorbed amount Adsorption

Molecular size and shape Conformational changes upon adsorption Equilibrium interfacial tension at air-water interface Value of surface tension gradient

10-5 mol/m2 1.0-2.0 mg/m2 Reversible

40-22 mN/m High

10-7 mol/m2 2.0-3.0 mg/m2 Practically irreversible Often globular (4-5 nm) Yes

57-47 mN/m Low aFrom Bos, M. A., & van Vliet, T. Interfacial rheological properties of adsorbed protein layers and surfactants: A review. Advances in Colloid and Interface Science, 91, 437-471. Copyright © (2001) with permission from Elsevier.

Table 6.4 Ranges of HLB Values for Different Applications.3

HLB Value Application

3-6 Emulsifiers of w/o emulsions

7-9 Wetting agents

8-18 Emulsifiers of o/w emulsions 13-15 Detergents

15-18 Solubilizers aFrom Bos, M. A., & van Vliet, T. Interfacial rheological properties of adsorbed protein layers and surfactants: A review. Advances in Colloid and Interface Science, 91, 437471. Copyright © (2001) with permission from Elsevier.

relationship:

HLB = 7 + hydrophilic group numbers - ^ lipophilic group numbers (6.25)

HLB value is the rate of the weight percentage of hydrophilic groups to the weight percentage of hydrophobic groups in the emulsifier molecule. Emulsifiers with HLB values below 9 are lipophilic, those with HLB values between 8 and 11 are intermediate, and those with HLB values between 11 and 20 are hydrophilic (Lewis, i996). Table 6.4 shows the application of surfactants and their HLB values. Diacetyl tartaric acid ester of monoglycerides (DATEM), which is a common emulsifier used in baked products has a HLB value of 9.2 (Krog & Lauridsen, 1976). Sorbitan monostearate, with a HLB value of 5.7, can be used in cake mixes and cacao products.

Generally a combination of emulsifiers is necessary to achieve a stable emulsion. HLB value of a mixed emulsifier system (HLB)m containing emulsifier A and B can be calculated by using HLB values (HLBa and HLBb) and mass fraction (Xaa and XA) of individual emulsifiers:

Some food hydrocolloids are sufficiently surface active and act as emulsifying agents in oil-in-water emulsions. Their emulsifying properties come from the proteinaceous material covalently bound or physically associated with carbohydrate polymer. The emulsions produced are coarser than those with low molecular weight water soluble surfactants or milk proteins at the same emulsifier/oil ratio. After hydrocolloids are strongly adsorbed at the oil-water interface, hydrocolloids can be more effective than proteins or others. Polysaccharides affect emulsion stabilization by increasing the viscosity of the dispersing phase and surface adsorption. Surface tension values of several gums are given in Table 6.5.

Locust bean gum and guar gum were shown to decrease the surface tension of water at low concentrations (up to 0.5%) and surface tension of gum solutions was time dependent. Surface tensions decreased and adsorption rates increased significantly by increasing gum concentration (Garti & Leser, 2001).

Guar gum and locust bean gum are used for their thickening, water holding, and stabilizing properties. Although there are no hydrophobic groups in these gums, they function by modifying the rheological properties of aqueous phase.

Xanthan gum is used as a thickening agent in foods. Xanthan gum is used as a stabilizer for oil-in-water emulsions. It was proposed by some researchers that its stabilizing ability comes from the viscosity effect (Coia & Stauffer, 1987; Ikegami, Williams, & Phillips, 1992). The adsorption

Table 6.5 Surface Tension Values of Selected Gumsa

Tragacanth

Xanthan

Guar

Locust bean gum

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