EMULSIFIER FUNCTION AND MECHANISM

III. EMULSIFIER FUNCTION AND MECHANISM

A. Emulsion Formation

The first step in the formation of a stable emulsion is dispersion of one liquid phase in The first step in the formation of a stable emulsion is dispersion of one liquid phase in

a substantial amount of energy or work must be supplied. Since emulsifiers reduce the surface tension, the addition of emulsifiers reduces the amount of work that must be done to form the emulsion. The most common method of emulsion formation is the application of mechanical energy via vigorous agitation.

The emulsifier is first dissolved in the aqueous or organic phase depending on the solubility of the emulsifier and on the type of emulsion desired. Next, sufficient agitation to cause surface deformation and large droplet formation is applied during the addition of one phase to the other. The next step is disruption of the droplets. To form a stable emulsion and prevent coalescence, sufficient emulsifier must be available to adsorb at the aqueous/organic interphase. The emulsifier lowers the Laplace pressure, which facilitates droplet deformation and disruption (Walstra, 1983). After droplet formation, the emulsifier partitions into the interphase of the aqueous/organic system stabilizing the emulsion. Droplet size, which is directly related to the emulsification procedure, is also dependent on the amount of emulsifier added, the type of emulsifier, and the emulsification temperature.

There are several possible methods for emulsion formation and a wide range of equipment is available for emulsion formation. These methods include shaking, stirring, and injection, and the use of colloid mills, homogenizers, and ultrasonics. An excellent summation on the mathematical evaluation of the process of emulsification and the rela- tionship of the factors involved is given by Walstra (1983).

One problem of key importance is the scale-up from laboratory to pilot plant to manufacturing (Lynch and Griffin, 1974). It is important to closely simulate manufacturing conditions during pilot plant preparation of the emulsion, particularly if this is not possible on a laboratory scale. On a laboratory scale, the use of a blender to prepare an emulsion may result in the application of more energy and a much faster rate than is possible in a manufacturing situation. If at all possible, laboratory equipment as well as pilot plant equipment should be of the same design as the production equipment. A motor-driven propeller or a hand-driven homogenizer would provide a much more realistic simulation of actual manufacturing conditions than a blender.

Equipment manufacturers can provide a wide range of equipment capable of emulsi- fication. The main types of equipment are stirrers (propeller and turbine types), colloid mills, homogenizers, and ultrasonic mixers. Stirrers are typically used to produce either coarse emulsions or as a premixer for some other type of emulsifying equipment. However, high-speed mixers are available that, according to one manufacturer (Arde Barinco, Mah- wah, NJ), work quite well for the production of stable food emulsions without the need for additional emulsification. Product examples included French dressing, margarine, but- ter sauce, and a flavor emulsion. Similar equipment, including laboratory scale equipment, is available from Greerco (Hudson, NH), among others.

Colloid mills emulsify based on the shearing action imparted to the liquid by a high- speed rotor moving within a fixed stator. The stator and rotor are separated by a very small gap (Becher, 1957). Clearance between the rotor and stator may be as small as 0.001 in. Many manufacturers, like Chemicolloid Laboratories (Garden City Park, NY), utilize grooved rotors and stators in their design, along with an adjustable clearance between the rotor and stator. Greeco offers a colloid mill with grooving and an adjustable gap that they recommend over their high speed mixer for emulsions of smaller droplet size, like Colloid mills emulsify based on the shearing action imparted to the liquid by a high- speed rotor moving within a fixed stator. The stator and rotor are separated by a very small gap (Becher, 1957). Clearance between the rotor and stator may be as small as 0.001 in. Many manufacturers, like Chemicolloid Laboratories (Garden City Park, NY), utilize grooved rotors and stators in their design, along with an adjustable clearance between the rotor and stator. Greeco offers a colloid mill with grooving and an adjustable gap that they recommend over their high speed mixer for emulsions of smaller droplet size, like

Another type of colloid mill with a rapidly spinning rotor, but minus the stator, is manufactured by Cornell (Springfield, NJ). Material enters a reduced pressure chamber and a film of product is impounded at the center of a rapidly spinning disc. The thin film formed on the surface of the spinning disc is forced outward by centrifugal force, which reduces the film thickness even further. Due to the characteristics of the product and spinning disc, substantial shear forces are developed that greatly reduce droplet size. An example of a food product that would best utilize this equipment design is salad dressings.

A familiar piece of emulsifying equipment is the homogenizer due to its widespread use for the homogenization of milk. Homogenizers emulsify by forcing the product through a small orifice under substantial pressure. The most likely mechanism for droplet size reduction is the dual effect of cavitation and turbulence. As the orifice size is decreased and the pressure is increased, the size of the dispersed phase droplets is decreased. In order to utilize the homogenizer most effectively, one manufacturer suggests that the prod- uct entering the homogenizer should be premixed so the particle size is less than 20 µm in diameter. Another possibility is the use of either multistage homogenizers or multiple passes through a single homogenizer to insure small, uniform droplets. The homogenizer is, of course, recommended for milk homogenization. It is also recommended for pro- cessing dispersions such as catsup and tomato sauce, for producing flavor and beverage emulsions, and for the production of frozen whipped toppings.

One piece of equipment of somewhat unusual design is the Hydroshear manufac- tured by APV Gaulin. The equipment utilizes a unique chamber design to subject the product to high shear. The chamber has a double conical shape. The fluid is forced tangen- tially into the middle of the chamber. The fluid moves in a spiral motion from the outside to the inside of the chamber. Due to the conical shape, the velocity of the product layers increases as the radius decreases and the chamber height increases. Differences in veloci- ties between concentric layers generates regions of very high shear. The emulsified product is then discharged through small openings in each cone apex. Droplet size, although smaller on the average than that obtained with colloid mills, is greater than that possible with homogenizers. It is recommended for reduction of droplet size in preparation for a single-pass homogenizer. Energy input for this equipment is substantially less than for a homogenizer.

Ultrasonic emulsification is the treatment of liquids with high-frequency vibrations to produce high-intensity cavitation. Sound waves move through the liquid, compressing and stretching it, which results in cavity or bubble formation within the liquid. Upon collapse of the bubbles tremendous shear forces are generated. The ultrasonic process can

be influenced by controlling static pressure, temperature, amplitude of vibration and flow rates. According to Branson (Danbury, CT) and Sonic (Stratford, CT), two of several ultrasonic equipment manufacturers, there are both limitations and advantages to ultra- sonic emulsifying equipment. The equipment is not particularly effective for large volumes or for highly viscous products. Advantages include lower capital and operating costs, no

B. Emulsion Stability

The following discussion on emulsion stability is rather abbreviated and the reader inter- ested in a comprehensive treatment is referred to the review by Tadros and Vincent (1983) and Friberg et al. (1990). They provide an excellent theoretical treatment of both the destabilization and stabilization mechanisms involved with emulsions.

There are several phenomena that can cause emulsion destabilization. Each is af- fected by the presence of emulsifiers. To best understand the mechanism of emulsion formation and ultimately the forces stabilizing emulsions, the mechanisms of destabiliza- tion should be understood.

1. Mechanisms of Destabilization Emulsion destabilization can be due to one or all of five possible mechanisms; flocculation,

coalescence, sedimentation or creaming, Ostwald ripening, and phase inversion (Tadros and Vincent, 1983).

a. Flocculation. The adherence of droplets to form aggregates or clusters and the buildup of these aggregates is referred to as flocculation. It occurs when the attractive forces between the droplets exceeds that of the repulsive forces, without a breakdown in the structural integrity of the interfacial film surrounding the droplets. These attractive forces are primarily long-range London–van der Waals forces and electrostatic forces. Once flocculated, the droplets sediment faster to the bottom (or rise faster to the top, cream) than drops of the original size (Friberg et al., 1990).

b. Coalescence. When aggregates or flocculates of the dispersed phase combine to form a single, larger drop the phenomena is referred to as coalescence. Coalescence is really a reflection of the nature of the interfacial film on the surface of the droplet. A strong, stable film on the surface of the droplet, due to addition of the correct concentration of the appropriate emulsifier, will minimize this type of destabilization. When coalescence occurs, the integrity of the interfacial film is lost and droplets in close contact combine, with the result of a reduction in the number of droplets. The ultimate effect is the formation of a single ‘‘drop,’’ the sedimentation becomes faster, and the emulsion separates into two layers.

c. Changes in Droplet Concentration. Droplet concentration can increase preferen- tially in either the top or bottom portion of the emulsion depending upon the relative density of the two phases. Sedimentation and creaming are two examples of this phenom- ena. Reducing the average droplet size and adding an emulsifier will substantially reduce the rate at which this occurs.

d. Ostwald Ripening. If the two phases forming the emulsion are not totally immiscible and there are differences in droplet size within the emulsion, larger droplets will form at the expense of smaller droplets due to a process known as Ostwald ripening. Ostwald ripening is always a factor since variation in initial droplet size always occurs in macro- emulsions and both phases are never completely immiscible. The driving force for Ostwald ripening is the difference in chemical potential between droplets of different sizes. Equilib- rium will only exist when all droplets are the same size, which really means a single d. Ostwald Ripening. If the two phases forming the emulsion are not totally immiscible and there are differences in droplet size within the emulsion, larger droplets will form at the expense of smaller droplets due to a process known as Ostwald ripening. Ostwald ripening is always a factor since variation in initial droplet size always occurs in macro- emulsions and both phases are never completely immiscible. The driving force for Ostwald ripening is the difference in chemical potential between droplets of different sizes. Equilib- rium will only exist when all droplets are the same size, which really means a single

2. Mechanisms of Stabilization by Emulsifiers There are several factors, some of which are dependent on the emulsifiers and stabilizers

added, involved in emulsion stabilization (Nawar, 1986). The first is the reduction of interfacial tension by the emulsifiers. Next is repulsion between droplets due to similar electrical charges on the surface of the droplets. A third is the formation of mesophases or liquid-crystalline phases which will provide the most stable configuration for a specified set of conditions. A fourth is the addition of macromolecules or particulate material which can substantially increase emulsion viscosity and stability. An increase in viscosity of the continuous phase adds to the kinetic stability. However, without a concurrent energy bar- rier, viscosity will have a small effect on stabilization. Viscosity enhancers increase the stability of the energy barrier (Friberg et al., 1990).

Emulsion stability is also dependent upon the conditions under which the emulsion is formed. This includes not only the constituents of the emulsion, but the emulsifier concentration, the emulsion temperature, and the physical state (crystalline versus fluid) of the emulsifier (Friberg and Mandell, 1969). Even the order of addition of the constituents is an important factor. Addition of lecithin to the lipid phase prior to the addition of the aqueous phase can substantially alter droplet size, liquid crystal formation, and emulsion stability (Friberg et al., 1976). Another contributing factor is the nature of the internal and continuous phases. Both affect emulsion stability. Two types of emulsions, those prepared with unsaturated emulsifiers and unsaturated oil and those prepared with satu- rated emulsifiers and saturated oil, were more stable that those prepared with emulsifiers and oil of intermediate or mixed saturation (Garti and Remon, 1984).

a. Interfacial Tension. The reduction of interfacial tension through the addition of emulsifiers is a key factor in emulsion formation. It allows emulsion formation with con- siderably less energy input than would be required without the presence of an emulsifier (Becher, 1983). Once the interfacial film consisting of emulsifier is formed, it acts as an effective barrier to droplet coalescence (Schuster and Adams, 1984). It has been found that, in the presence of emulsifiers, the droplet interface may acquire viscoelastic properties which are important in the prevention of coalescence (Joanne et al., 1994, Williams et al., 1997). A strong interaction between the hydrophilic portion of the emulsifier and the aqueous phase leads to a large reduction in the surface tension of the water (Boyle, 1997). According to Schuster and Adams (1984), this also effects the type of emulsion formed.

A weak interaction between water and the hydrophilic portion of the emulsifier molecule will favor a W/O emulsion, while a strong interaction will favor an O/W emulsion.

b. Electrical Charge. Ionic emulsifiers provide an additional mechanism for emulsion stabilization relative to nonionic emulsifiers, through ion–ion and ion–solvent interactions (Gunnarsson et al., 1980). In addition, the introduction of charged groups on the surface of the emulsion droplets increases the repulsive forces between droplets (Larsson and Krog, 1973). Ionic emulsifiers will form an electrically charged double layer in the aqueous solution surrounding each oil droplet (Nawar, 1986). The explanation for the stability of emulsions due to charge separation relies heavily on the DLVO theory (Friberg et al., b. Electrical Charge. Ionic emulsifiers provide an additional mechanism for emulsion stabilization relative to nonionic emulsifiers, through ion–ion and ion–solvent interactions (Gunnarsson et al., 1980). In addition, the introduction of charged groups on the surface of the emulsion droplets increases the repulsive forces between droplets (Larsson and Krog, 1973). Ionic emulsifiers will form an electrically charged double layer in the aqueous solution surrounding each oil droplet (Nawar, 1986). The explanation for the stability of emulsions due to charge separation relies heavily on the DLVO theory (Friberg et al.,

c. Liquid Crystal Stabilization. Macroemulsions, although thermodynamically unsta- ble, can attain rather long-term stability, strongly suggesting an intermediate stability level. This was attributed to the formation of a liquid-crystalline state by the emulsifier (Friberg and Mandell, 1969; Friberg et al., 1969). Jansson and Friberg (1976) indicated the presence of a liquid-crystalline state reduces the rate of coalescence even if droplet flocculation occurs. Nakama et al. (1997) obtained a stable W/O emulsion without coalescence of the water droplets that contained a substantial amount of water (90%) using a lauroamphogly- cinate (LG) and oleic acid (OA) mixture. The X-ray diffraction patterns and the strong hydrophobicity showed that the equimolar complex composed of LG, OA, and water was

a liquid crystal with a reversed phase hexagonal structure. The reversed hexagonal liquid crystal was capable of solubilizing a certain amount of liquid paraffin in its alkyl group parts while maintaining its hexagonal structure.

An emulsifier film can actually exist in several different mesophases or liquid- crystalline states (Schuster and Adams, 1984). Conversion between physical states by the emulsifier at the oil/water interface is referred to as lyotropic mesomorphism. This phenomenon occurs when an emulsifier/water mixture is heated to a sufficient temperature so that the hydrocarbon chains liquify and, simultaneously, water penetrates between lay- ers of the polar groups (Flack, 1983). The result is the formation of liquid-crystalline structures referred to as either lamellar, cubic, or hexagonal. Various structural arrays can exist, including alternating films, spheres, and cylinders. The resultant structure is depen- dent upon several factors including emulsifier molecular structure, the concentration ratio of the emulsifier to water, temperature, ionic strength, and pH (Schuster and Adams, 1984). Each parameter affecting stability is closely related to the others. Altering one, even slightly, may alter the type of mesophase formed and the emulsion stability. Kwon and Rhee (1996) investigated the emulsifying capacity of coconut protein as a function of salt, phosphate, and temperature. They reported that between pH 4.0 and 5.0, protein nitrogen solubilities of coconut flour (CF) and coconut protein concentrate (CPC) in water were lower than those in salt solutions. In salt solutions, the nitrogen solubility was lowest at pH 1, and increased steadily as the pH was increased from 3 to 6. Increased phosphate addition increased emulsifying capacity, while an increase in temperature decreased emul- sifying capacity. Other workers (Taiwo et al., 1997) studied the influence of temperature and additives on the adsorption kinetics of food emulsifiers. They found that the presence of salt and greater temperatures reduced the interfacial tension of egg york solutions, while the interfacial tension of whey protein concentrate solutions increased with salt addition. Increased temperature caused the equilibrium interfacial tension of both emulsifiers to decrease and attain equilibrium more quickly.

Another concentration-dependent phenomenon that can occur is critical micelle for- mation. At low concentrations, strongly hydrophilic emulsifiers will disperse or dissolve completely in water. If sufficient emulsifier is added to an aqueous solution, strongly hydrophilic emulsifiers will form micelles, a type of liquid-crystalline microstructure (Schuster and Adams, 1984). This entropy-driven association occurs to minimize water interaction with the hydrophobic portion of the emulsifier. The emulsifier concentration at which the micelle formation occurs is the critical micelle forming concentration (CMC).

and, to a lesser degree, the temperature, ionic strength, and pH. Micellar structures are very small, 0.005 to 0.01 µm in diameter.

d. Stabilization by Macromolecules and Finely Divided Solids. Emulsion stability can

be increased by the addition of macromolecules like gums and protein. Tharp (1982) found that colloids, such as xanthan gum, carboxy methyl cellulose, and guar gum, significantly increased emulsion stability. At a constant emulsifier and colloid concentration, emulsion stability was enhanced by increased emulsification temperature, increased degree of shear, and increased pH, in the pH range of 3–6. Colloids act by either increasing the viscosity or by partitioning into the oil/water interface and providing a physical barrier to coalescence (Nawar, 1986).

Tadros and Vincent (1983) have provided a detailed theoretical discussion on emul- sion stability due to added macromolecules and finely divided solids. Small, finely divided solids can adhere to the surface of a lipid droplet, stabilizing the emulsion by forming a physical barrier to coalescence, through steric hindrance. The type of emulsion formed, as well as the emulsion stability, is dependent upon the relative ability of the two phases to wet the particles (Nawar, 1986). This can be greatly influenced by the addition of emulsifiers. An example of stabilization due to finely divided solids is the stabilization effects of amorphous silica. Addition of amorphous silica to an O/W emulsion containing an emulsifier substantially increased both emulsion stability and viscosity (Villota, 1985).

To evaluate emulsion stability and thereby characterize the potential of an emulsifier, the rate at which the combined destabilization phenomena occur must be determined. These rates can be determined from the changes in the size and distribution of the oil droplets with time. There are several methods available for this determination. According to Trumbetas et al. (1978), nuclear magnetic resonance (NMR) analysis can provide a better indication of stability than HLB values. Samples were evaluated that contained a mixture of polyoxyethylene 20 sorbitan monostearate, sorbitan monostearate, lipid, and water. Another method, used to evaluate the effects of processing on emulsion stability, was centrifugation (Tornberg and Hermansson, 1977). Frenkel et al. (1982) evaluated the stability of W/O emulsion by monitoring turbidity at 400 and 800 nm. They stated that the method was suitable for determination of the required HLB, the amount and type of emulsifier, and the fraction of water. Tung and Jones (1981) examined the microstructures of mayonnaise and salad dressing with light microscopy, transmission electron micros- copy, and scanning electron microscopy (SEM), while SEM was used to determine lipid droplet sizes. Chemical fixation and critical point dehydration were effective in providing

a method for observation of undiluted samples. Sherman (1971) recommended that before an accelerated method for the testing of emulsion stability was used, a rigorous evaluation to determine the correlation between the accelerated method and actual storage conditions must be done. Those procedures, based on assumptions about the effects of either alterations in temperature or centrifugation rather than empirical analysis, may not be valid. He recommends, for simple emulsions, either a determination of the rate of change in particle size after a limited time under actual storage conditions or the phase inversion temperature analysis developed by Shino- da’s group (Shinoda, 1969; Shinoda and Sagitani, 1978; and Shinoda et al., 1980).

C. Emulsifier Selection

Perhaps the most important factor in preparing an emulsion is the selection of the appro-

(Shinoda and Kunieda, 1983). These include the HLB system of Griffin (1949, 1954), the H/L numbers (Moore and Bell, 1956), the water number of Greenwald et al. (1956), the phase inversion temperature (PIT) of Shinoda (1967), and the emulsion inversion point (EIP) (Marszall, 1975). Even with the best of methods selection can be very difficult, except perhaps for the few foods that are relatively straightforward emulsions, such as mayonnaise and margarine. Often one of the best sources of information for the food technologist will be the emulsifier manufacturer. There are several companies that have considerable expertise and can provide excellent advice in this area.

Several parameters should be considered during emulsifier selection (Nash and Brickman, 1972). These parameters include (1) approval of the emulsifier by the appro- priate government agency, (2) desired functional properties, (3) end product application, (4) processing parameters, (5) synergistic effect of other ingredients, (6) home preparation, and finally (7) cost.

Obviously, before an emulsifier can be used in a food product it must be approved by the appropriate regulatory agency. Assuming this criterion is met, the most important considerations would be both the required functional properties of the selected emulsifier and the application. Delineating the required functional properties such as emulsification, starch complexation, and crystallization control, and the specific end product application are the two major factors in emulsifier selection. An exact determination of these two parameters should focus attention on a limited number of emulsifiers. The processing methodology and equipment available in the processing facility could further limit the range of emulsifiers that are of potential use. It is at this stage that an ingredient supplier(s) could begin to provide helpful assistance.

By far, the most widely used rule for the selection of food emulsifiers is the HLB number, published by Griffin (1949,1954). The HLB index, called the hydrophile– lipophile balance, is based upon the relative percentage of hydrophilic to lipophilic groups within the emulsifier molecule. Griffin assigned values ranging from 1 to 20 (Waginaire, 1997). Lower HLB values indicate a more lipophilic emulsifier, while higher values indi- cate a more hydrophilic emulsifier. Emulsifiers with HLB numbers in the 3–6 range are best for W/O emulsions, whereas emulsifiers with HLB numbers in the range of 8–18 are best for O/W emulsions. Depending upon the application and the types of oils to

be emulsified there is an optimal HLB. An equation developed by Griffin (1954) can be used to determine the HLB number for several types of nonionic emulsifiers, particularly the ethoxylated alcohols and the polyhydric fatty acid esters (Tadros and Vincent, 1983). To determine the HLB for the fatty acid ester type emulsifiers, Griffin (1949,1954) used the equation

HLB ⫽ 20 S 1⫺

where A is the acid number and S is the saponification number of the ester. For the polysorbate type of emulsifier the HLB value can be determined from the equation