The size of the droplets produced during homogenization is important because it determinesthe stability, appearance, and texture of the final product. To create a product with specificproperties, it is therefore necessary to ensure that the majority of the droplets fall within somepreestablished size range. For this reason, it is important for food scientists to be aware ofthe major factors which influence droplet size.
6.5.1. Emulsifier Type and Concentration
For a fixed concentration of oil, water, and emulsifier, there is a maximum interfacial areawhich can be completely covered by the emulsifier. As homogenization proceeds, the size ofthe droplets decreases and the interfacial area increases. Once the droplets fall below a certainsize, there is insufficient emulsifier present to completely cover their surface, and so they tendto coalesce with their neighbors. The minimum size of stable droplets that can be producedduring homogenization (assuming monodisperse droplets*) is therefore governed by the typeand concentration of emulsifier present:
rcSmin =⋅ ⋅3 Γsat φ(6.7)* For a polydisperse emulsion, the radius used in Equation 6.6 should be the volume–surface mean radius
where Γsat is the excess surface concentration of the emulsifier at saturation (in kg m–2), φ is
the dispersed-phase volume fraction, and cS is the concentration of emulsifier in the emulsion
(in kg m–3). Equation 6.7 indicates that the minimum droplet size can be decreased by
increasing the emulsifier concentration, decreasing the droplet concentration, or using anemulsifier with a smaller Γsat. For a 10% oil-in-water emulsion containing 1% of emulsifier,
the minimum droplet radius is about 60 nm (assuming Γsat = 2 × 10–6 kg m–2). In practice,
there are a number of factors which mean that the droplet size produced during homogeni-zation is greater than the theoretical minimum.
In order to attain the theoretical minimum droplet size, a homogenizer must be capableof generating a pressure gradient that is large enough to disrupt any droplets that are greaterthan rmin (i.e., >2γ/rmin2) (Section 6.4.7). Some types of homogenizer are not capable of
generating such high pressure gradients and are therefore not suitable for producing emul-sions with small droplet sizes, even though there may be sufficient emulsifier present(Walstra 1983). The emulsion must also spend sufficient time within the homogenizationzone for all of the droplets to be completely disrupted. If an emulsion passes through ahomogenizer too rapidly, some of the droplets may not be disrupted. Even if a homogenizeris capable of producing small droplets, the emulsifier molecules must adsorb rapidly enoughto form a protective interfacial layer around the droplets which prevents them from coalesc-ing with their neighbors.
The emulsifier also influences the droplet size by reducing the interfacial tension betweenthe oil and aqueous phases and therefore facilitating droplet disruption. Consequently, themore rapidly an emulsifier adsorbs, and the greater the reduction of the interfacial tension,the smaller the droplets that can be produced at a certain energy input.
Many different types of emulsifier can be used in the food industry, and each of theseexhibits different characteristics during homogenization (e.g., the speed at which they adsorb,the maximum reduction in interfacial tension, and the effectiveness of the interfacial mem-brane in preventing droplet coalescence). Factors which influence the adsorption kinetics ofemulsifiers, and their effectiveness at reducing interfacial tension, were discussed in Chapter5, and factors which affect the stability of droplets against coalescence will be covered inChapter 7. A food manufacturer must select the most appropriate emulsifier for each type offood product, taking into account its performance during homogenization, solution condi-tions, cost, availability, legal status, ability to provide long-term stability, and the desiredphysicochemical properties of the product.
6.5.2. Energy Input
The size of the droplets in an emulsion can be reduced by increasing the amount of energysupplied during homogenization (as long as there is sufficient emulsifier to cover the surfacesof the droplets formed). The energy input can be increased in a number of different waysdepending on the nature of the homogenizer. In a high-speed blender, the energy input canbe enhanced by increasing the rotation speed or the length of time that the sample is blended.In a high-pressure valve homogenizer, it can be enhanced by increasing the homogenizationpressure or recirculating the emulsion through the device. In a colloid mill, it can be enhancedby using a narrower gap between the stator and rotator, increasing the rotation speed, by usingdisks with roughened surfaces, or by passing the emulsion through the device a number oftimes. In an ultrasonic homogenizer, the energy input can be enhanced by increasing theintensity of the ultrasonic wave or by sonicating for a longer time. In a microfluidizer, theenergy input can be enhanced by increasing the velocity at which the liquids are brought intocontact with each other or by recirculating the emulsion. In a membrane homogenizer, theenergy input can be enhanced by increasing the pressure at which the liquid is forced throughthe membrane. Under a given set of homogenization conditions (energy input, temperature,
composition), there is a certain size below which the emulsion droplets cannot be reducedwith repeated homogenization, and therefore homogenizing the system any longer would beinefficient.
Increasing the energy input usually leads to an increase in manufacturing costs, andtherefore a food manufacturer must establish the optimum compromise between droplet size,time, and cost. The energy input required to produce an emulsion containing droplets of agiven size depends on the energy efficiency of the homogenizer used (Walstra 1983).
Under most circumstances, there is a decrease in droplet size as the energy input isincreased. Nevertheless, there may be occasions when increasing the energy actually leadsto an increase in droplet size because the effectiveness of the emulsifier is reduced byexcessive heating or exposure to high pressures. This could be particularly important forprotein-stabilized emulsions, because the molecular structure and functional properties ofproteins are particularly sensitive to changes in their environmental conditions. For example,globular proteins, such as β-lactoglobulin, are known to unfold and aggregate when they areheated above a certain temperature, which reduces their ability to stabilize emulsions (Sec-tion 4.6).
6.5.3. Properties of Component Phases
The composition and physicochemical properties of both the oil and aqueous phases influ-ence the size of the droplets produced during homogenization (Phipps 1985). Variations inthe type of oil or aqueous phase will alter the viscosity ratio (ηD/ηC) which determines the
minimum size that can be produced under steady-state conditions (Section 6.3). Different oilshave different interfacial tensions when placed in contact with water because they havedifferent molecular structures or because they contain different amounts of surface-activeimpurities, such as free fatty acids, monoacylglycerols, or diacylglycerols. These surface-active lipid components tend to accumulate at the oil–water interface and lower the interfacialtension, thus lowering the amount of energy required to disrupt a droplet.
The aqueous phase of an emulsion may contain a wide variety of components, includingminerals, acids, bases, biopolymers, sugars, alcohols, ice crystals, and gas bubbles. Many ofthese components will alter the size of the droplets produced during homogenizationbecause of their influence on rheology, interfacial tension, coalescence stability, or ad-sorption kinetics. For example, the presence of low concentrations of short-chain alcoholsin the aqueous phase of an emulsion reduces the size of the droplets produced duringhomogenization because of the reduction in interfacial tension (Banks and Muir 1988). Thepresence of biopolymers in an aqueous phase has been shown to increase the droplet sizeproduced during homogenization due to their ability to suppress the formation of smalleddies during turbulence (Walstra 1983). Protein-stabilized emulsions cannot be producedclose to the isoelectric point of a protein or at high electrolyte concentrations because theproteins are susceptible to aggregation. A knowledge of the composition of both the oil andaqueous phases of an emulsion and the role that each component plays during homogeni-zation is therefore important when optimizing the size of the droplets produced by ahomogenizer.
Experiments have shown that the smallest droplet size that can be achieved using a high-pressure valve homogenizer increases as the dispersed-phase volume fraction increases (Phipps1985). There are a number of possible reasons for this: (1) increasing the viscosity of anemulsion may suppress the formation of eddies responsible for breaking up droplets; (2) ifthe emulsifier concentration is kept constant, there may be an insufficient amount present tocompletely cover the droplets; and (3) the rate of droplet coalescence is increased.
6.5.4. Temperature
Temperature influences the size of the droplets produced during homogenization in a numberof ways. The viscosity of both the oil and aqueous phases is temperature dependent, andtherefore the minimum droplet size that can be produced may be altered because of avariation in the viscosity ratio (ηD/ηC) (Section 6.3). Heating an emulsion usually causes a
slight reduction in the interfacial tension between the oil and water phases, which would beexpected to facilitate the production of small droplets (Equation 6.1). Certain types ofemulsifiers lose their ability to stabilize emulsion droplets against aggregation when they areheated above a certain temperature. For example, when small-molecule surfactants are heatedclose to their phase inversion temperature, they are no longer effective in preventing dropletcoalescence, or when globular proteins are heated above a critical temperature, they unfoldand aggregate (Chapter 4). Alterations in temperature also influence the competitive adsorp-tion of surface-active components, thereby altering interfacial composition (Dickinson andHong 1994).
The temperature is also important because it determines the physical state of the lipid phase(Section 4.2). It is practically impossible to homogenize a fat that is either completely orsubstantially solid because it will not flow through a homogenizer or because of the hugeamount of energy required to break up the fat crystals into small particles. There are alsoproblems associated with the homogenization of oils that contain even small amounts of fatcrystals because of partial coalescence (Chapter 7). The crystals from one droplet penetrateanother droplet, leading to the formation of a “clump.” Extensive clump formation leads tothe generation of large particles and to a dramatic increase in the viscosity, which wouldcause a homogenizer to become blocked. For this reason, it is usually necessary to warm asample prior to homogenization to ensure that the lipid phase is completely liquid. Forexample, milk fat is usually heated to about 40°C to melt all the crystals prior to homogeni-zation (Phipps 1985).
6.6. DEMULSIFICATION
Demulsification is the process whereby an emulsion is converted into the separate oil andaqueous phases from which it was comprised and is therefore the opposite of homogeniza-tion (Lissant 1983, Menon and Wasan 1985, Hunter 1989). Demulsification is important ina number of technological processes in the food industry (e.g., oil recovery or the separationof lipid and aqueous phases). Demulsification is also important in research and developmentbecause it is often necessary to divide an emulsion into the separate oil and aqueous phasesso that their composition or properties can be characterized. For example, the oil phase mustoften be extracted from an emulsion in order to determine the extent of lipid oxidation(Coupland et al. 1996) or to measure the partition coefficient of a food additive (Huang etal. 1997). Demulsification is achieved by causing the droplets to come into close contactwith each other and then to coalesce. As this process continues, it eventually leads to thecomplete separation of the oil and aqueous phases. A knowledge of the physical principlesof demulsification requires an understanding of the factors that determine the stability ofemulsions to flocculation and coalescence (Chapter 7).
A variety of different types of emulsifier are used in the food industry to stabilize dropletsagainst flocculation and coalescence (Chapter 4). Each type of emulsifier relies on differentphysicochemical mechanisms to prevent droplet aggregation, including electrostatic, steric,hydration, and thermal fluctuation interactions (Chapter 3). The selection of the most appro-priate demulsification technique for a given emulsion therefore depends on a knowledge of
the type of emulsifier used to stabilize the system and the mechanisms by which it providesstability.
6.6.1. Nonionic Surfactants
Nonionic surfactants usually stabilize emulsion droplets against flocculation through a com-bination of polymeric steric, hydration, and thermal fluctuation interactions (Section 4.5).Nevertheless, the interfacial membranes formed by nonionic surfactants are often unstable torupture when the droplets are brought into close contact. Demulsification can therefore beachieved by altering the properties of an emulsion so that the droplets come into close contactfor prolonged periods. This can often be achieved by heating an emulsion so that the polarhead groups of the surfactant molecules become dehydrated, because this reduces the hydra-tion repulsion between the droplets and allows them to come closer together. In addition, theoptimum curvature of the surfactant monolayer tends toward zero as the size of the headgroup decreases, which increases the likelihood of coalescence (Section 4.5). Thisdemulsification technique cannot be used for some emulsions stabilized by nonionic surfac-tants because the phase inversion temperature is much greater than 100°C or because heatingmay cause degradation or loss of one of the components being analyzed. In these cases, it isnecessary to induce demulsification using alternative methods.
The addition of medium-chain alcohols has also been found to be effective in promotingdemulsification in some systems (Menon and Wasan 1985). There are two possible explana-tions for this behavior: (1) the alcohol displaces some of the surfactant molecules from theinterface and forms an interfacial membrane which provides little protection against dropletaggregation or (2) the alcohol molecules are able to get between the tails of the surfactantmolecules at the interface, thereby increasing the optimum curvature of the interface towardzero and increasing the likelihood of coalescence (Section 4.5). In some emulsions, it ispossible to promote droplet coalescence by adding a strong acid which cleaves the headgroups of the surfactants from their tails so that the polar head group moves into the aqueousphase and the nonpolar tail moves into the droplet, thereby providing little protection againstdroplet coalescence.
6.6.2. Ionic Surfactants
Ionic surfactants stabilize droplets against coalescence principally by electrostatic repulsion(Chapter 3). Like nonionic surfactants, the membranes formed by ionic surfactants are notparticularly resistant to rupture once the droplets are brought into close contact (Evans andWennerstrom 1994). The most effective method of inducing droplet coalescence in thesesystems is therefore to reduce the magnitude of the electrostatic repulsion between thedroplets (Menon and Wasan 1985). This can be achieved by adding electrolyte to theaqueous phase of the emulsion so as to screen the electrostatic interactions. Sufficientelectrolyte must be added so that the energy barrier between the droplets becomes of theorder of the thermal energy or less (Chapter 3). This process can most easily be achievedusing multivalent ions, because they are more effective at screening electrostatic interactionsat low concentrations than monovalent ions. Alternatively, the pH may be altered so that thesurfactant loses its charge, which depends on the dissociation constant of the ionizablegroups. Electromechanical methods can also be used to promote demulsification. An electricfield is applied across an emulsion, which causes the charged droplets to move toward theoppositely charged electrode. A semipermeable membrane is placed across the path of thedroplets, which captures the droplets but allows the continuous phase to pass through. Thedroplets are therefore forced against the membrane until their interfacial membranes areruptured and they coalesce.
6.6.3. Biopolymer Emulsifiers
Biopolymers principally stabilize droplets against coalescence through a combination ofelectrostatic and polymeric steric interactions. In addition, they tend to form thick viscoelasticmembranes that are highly resistant to rupture. There are two different strategies that can beuse to induce droplet coalescence in this type of system:
1. The biopolymer can be digested by strong acids or enzymes so that it is broken intosmall fragments that are either not surface active or do not form a sufficientlystrong membrane.
2. The biopolymers are displaced from the interface by small-molecule surfactants,and then the droplets are destabilized using one of the methods described above.Some proteins are capable of forming an interfacial membrane in which the mol-ecules are covalently bound to each other through disulfide bonds. In order todisplace these proteins, it may be necessary to cleave the disulfide bonds prior todisplacing the proteins (e.g., by adding mercaptoethanol).
The Gerber and Babcock methods of determining the total fat content of milk are examplesof the first of these strategies, while the detergent method is an example of the second (Pike1994).
6.6.4. General Methods of Demulsification
A variety of physical techniques are available which can be used to promote demulsificationin most types of emulsions. In all of the demulsification processes mentioned above, theseparation of the oil phase from the aqueous phase can be facilitated by centrifuging theemulsion after the coalescence process has been initiated. In some emulsions, it is alsopossible to separate the phases directly by centrifugation at high speeds, without the need forany pretreatment (Menon and Wasan 1985). Centrifugation forces the droplets to one end ofthe container, which causes their interfacial membranes to become ruptured and thereforeleads to phase separation.
Demulsification can also be achieved using various types of filtration devices (Menon andWasan 1985). The emulsion is passed through a filter which adsorbs emulsion droplets. Whena number of these adsorbed droplets come into close contact, they merge together to form asingle large droplet which is released back into the aqueous phase. As the emulsion passesthrough the filter, this process continues until eventually the oil and water phases are com-pletely separated from each another.
6.6.5. Selection of Most Appropriate Demulsification Technique
In addition to depending on the type of emulsifier present, the choice of an appropriatedemulsification technique also depends on the sensitivity of the other components in thesystem to the separation process. For example, if one is monitoring lipid oxidation or tryingto determine the concentration of an oil-soluble volatile component, it is inadvisable to usea demulsification technique that requires excessive heating. On the other hand, if the samplecontains a lipid phase that is crystalline, it is usually necessary to warm the sample to atemperature where all the fat melts prior to carrying out the demulsification procedure.
