A. Dispersed Phase Volume Fraction

Table 2 Experimental Techniques for Characterizing the Physicochemical Properties of

Food Emulsions [1a]

Dispersed phase volume

fraction

Droplet size distribution



Microstructure

Creaming and sedimentation

Droplet charge

Droplet cyrstallization

Emulsion rheology

Interfacial tension

Interfacial thickness



Proximate analysis, density, electrical conductivity, light

scattering, NMR, ultrasound

Light scattering (static and dynamic), electrical

conductivity, optical microscopy, electron

microscopy, ultrasound, NMR

Optical microscopy, electron microscopy, atomic force

microscopy

Light scattering, ultrasound, NMR, visual observation

Electrokinetic techniques, electroacoustic techniques

Density, NMR, ultrasound, differential scanning

calorimetry, polarized optical microscopy

Viscometers, dynamic shear rheometers

Interfacial tensiometers (static and dynamic)

Ellipsometry, neutron reflection, neutron scattering, light

scattering, surface force apparatus



volume fraction of emulsions are outlined in Table 2. Traditional proximate analysis

techniques, such as solvent extraction to determine oil content and oven drying to

determine moisture content, can be used to analyze the dispersed phase volume

fraction of emulsions. Nevertheless, proximate analysis techniques are often destructive and quite time-consuming to carry out, and are therefore unsuitable for rapid

quality control or on-line measurements. If the densities of the separate oil and

aqueous phases are known, the dispersed phase volume fraction of an emulsion can

simply be determined from a measurement of its density:



␾ = ( ␳ emulsion Ϫ ␳ continuous phase)( ␳ droplet Ϫ ␳ continuous phase)



(15)



The electrical conductivity of an emulsion decreases as the concentration of oil

within it increases, and so instruments based on electrical conductivity can also be

used to determine ␾. Light scattering techniques can be used to measure the dispersed

phase volume fraction of dilute emulsions (␾ < 0.001), whereas NMR and ultrasound

spectroscopy can be used to rapidly and nondestructively determine ␾ of concentrated and optically opaque emulsions. A number of these experimental techniques

(e.g., ultrasound, NMR, electrical conductivity, density measurements) are particularly suitable for on-line determination of the composition of food emulsions during

processing.

B.



Droplet Size Distribution



The size of the droplets in an emulsion influences many of their sensory and bulk

physicochemical properties, including rheology, appearance, mouthfeel, and stability

[3,5]. It is therefore important for food manufacturers to carefully control the size

of the droplets in a food product and to have analytical techniques to measure droplet

size. Typically, the droplets in a food emulsion are somewhere in the size range of

0.1–50 ␮m in diameter.

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.



Food emulsions always contain droplets that have a range of sizes, and so it

is usually important to characterize both the average size and the size distribution

of the droplets. The droplet size distribution is usually represented by a plot of droplet

frequency (number or volume) versus droplet size (radius or diameter). Some of the

most important experimental techniques for measuring droplet size distributions are

included in Table 2.*

Light-scattering and electrical conductivity techniques are capable of providing

a full particle size distribution of a sample in a few minutes. Since, however, these

techniques usually require that the droplet concentration be very low (␾ < 0.001),

samples must be diluted considerably before analysis. Optical and electron microscopy techniques, which provide the most direct measurement of droplet size distribution, are often time-consuming and laborious to operate, and sample preparation

can cause considerable artifacts in the results. In contrast, recently developed techniques based on NMR and ultrasonic spectroscopy can be used to rapidly and nondestructively measure the droplet size distribution of concentrated and optically

opaque emulsions [1a]. These techniques are particularly useful for on-line characterization of emulsion properties.

C.



Microstructure



The structural organization and interactions of the droplets in an emulsion often play

an important role in determining the properties of a food. For example, two emulsions

may have the same droplet concentration and size distribution, but very different

properties, because of differences in the degree of droplet flocculation. Various forms

of microscopy are available for providing information about the microstructure of

food emulsions. The unaided human eye can resolve objects that are farther apart

than about 0.1 mm (100 ␮m). Most of the structural components in food emulsions

(e.g., emulsion droplets, surfactant micelles, fat crystals, ice crystals, small air cells,

protein aggregates) are much smaller than this lower limit and cannot therefore be

observed directly by the eye.

Optical microscopy can be used to study components of size between about

0.5 and 100 ␮m. The characteristics of specific components can be highlighted by

selectively staining certain ingredients or by using special lenses. Electron microscopy can be used to study components that have sizes down to about 0.5 nm. Atomic

force microscopy can be used to provide information about the arrangements and

interactions of single atoms or molecules. All these techniques are burdened by

sample preparation steps that often are laborious and time-consuming, and subject

to alter the properties of the material being examined. Nevertheless, when carried

out correctly the advanced microscopic techniques provide extremely valuable information about the arrangement and interactions of emulsion droplets with each

other and with the other structural entities found in food emulsions.

D.



Physical State



The physical state of the components in a food emulsion often has a pronounced

influence on its overall properties [1a]. For example, oil-in-water emulsions are par*A comprehensive review of analytical methods for measuring particle size in emulsions has recently

been published [31].



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ticularly prone to partial coalescence when the droplets contain a certain percentage

of crystalline fat (Sec. VI.B). Partial coalescence leads to extensive droplet aggregation, which decreases the stability of emulsions to creaming and greatly increases

their viscosity. In water-in-oil emulsions, such as margarine or butter, the formation

of a network of aggregated fat crystals provides the characteristic rheological properties. The most important data for food scientists are the temperature at which

melting or crystallization begins, the temperature range over which the phase transition occurs, and the value of the solid fat content at any particular temperature.

Phase transitions can be monitored by measuring changes in any property (e.g.,

density, compressibility, heat capacity, absorption or scattering of radiation) that is

altered upon conversion of an ingredient from a solid to a liquid (Table 2). The

density of a component often changes when it undergoes a phase transition, and so

melting or crystallization can be monitored by measuring changes in the density of

a sample with temperature or time.

Phase transitions can also be monitored by measuring the amount of heat absorbed or released when a solid melts or a liquid crystallizes, respectively. This type

of measurement can be carried out by means of differential thermal analysis or

differential scanning calorimetry. These techniques also provide valuable information

about the polymorphic form of the fat crystals in an emulsion. More recently, rapid

instrumental methods based on NMR and ultrasound have been developed to measure

solid fat contents [1a]. These instruments are capable of nondestructively determining

the solid fat content of a sample in a few seconds and are extremely valuable analytical tools for rapid quality control and on-line procedures. Phase transitions can

be observed in a more direct manner by means of polarized optical microscopy.

E.



Creaming and Sedimentation Profiles



Over the past decade, a number of instruments have been developed to quantify the

creaming or sedimentation of the droplets in emulsions. Basically the same light

scattering, NMR, and ultrasound techniques used to measure the dispersed phase

volume fraction or droplet size distributions of emulsions are applied to creaming or

sedimentation, but the measurements are carried out as a function of sample height

to permit the acquisition of a profile of droplet concentrations or sizes. Techniques

based on the scattering of light can be used to study creaming and sedimentation in

fairly dilute emulsions. A light beam is passed through a sample at a number of

different heights, and the reflection and transmission coefficients are measured and

related to the droplet concentration and size. By measuring the ultrasonic velocity

or attenuation as a function of sample height and time, it is possible to quantify the

rate and extent of creaming in concentrated and optically opaque emulsions. This

technique can be fully automated and has the two additional advantages: creaming

can be detected before it is visible to the eye, and a detailed creaming profile can

be determined rather than a single boundary. By measuring the ultrasound properties

as a function of frequency, it is possible to determine both the concentration and

size of the droplets as a function of sample height. Thus a detailed analysis of

creaming and sedimentation in complex food systems can be monitored noninvasively. Recently developed NMR imaging techniques can also measure the concentration and size of droplets in any region in an emulsion [9]. These ultrasound and

NMR techniques will prove particularly useful for understanding the kinetics of



Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.



creaming and sedimentation in emulsions and for predicting the long-term stability

of food emulsions.

F.



Emulsion Rheology



The rheology of an emulsion is one of its most important overall physical attributes

because it largely determines the mouthfeel, flowability, and stability of emulsions

[3]. A variety of experimental techniques are available for measuring the rheological

properties of food emulsions. The rheology of emulsions that have low viscosities

and act like ideal liquids can be characterized by capillary viscometers. For nonideal

liquids or viscoelastic emulsions, more sophisticated instrumental techniques called

dynamic shear rheometers are available to measure the relationship between the stress

applied to an emulsion and the resulting strain, or vice versa. As well as providing

valuable information about the bulk physicochemical properties of emulsions (e.g.,

texture, flow through pipes), rheological measurements can provide information

about droplet–droplet interactions and the properties of any flocs formed in an

emulsion.

G.



Interfacial Properties



Despite comprising only a small fraction of the total volume of an emulsion, the

interfacial region that separates the oil from the aqueous phase plays a major role in

determining stability, rheology, chemical reactivity, flavor release, and other overall

physicochemical properties of emulsions. The most important properties of the interface are the concentration of emulsifier molecules present (the surface load), the

packing of the emulsifier molecules, and the thickness, viscoelasticity, electrical

charge, and (interfacial) tension of the interface.

A variety of experimental techniques are available for characterizing the properties of oil–water interfaces (Table 2). The surface load is determined by measuring

the amount of emulsifier that adsorbs per unit area of oil–water interface. The thickness of an interfacial membrane can be determined by light scattering, neutron scattering, neutron reflection, surface force, and ellipsometry techniques. The rheological

properties of the interfacial membrane can be determined by means of the twodimensional analog of normal rheological techniques. The electrical charge of the

droplets in an emulsion determines their susceptibility to aggregation. Experimental

techniques based on electrokinetic and electroacoustic techniques are available for

determining the charge on emulsion droplets. The dynamic or equilibrium interfacial

tension of an oil–water interface can be determined by means of a number of interfacial tension meters, including the Wilhelmy plate, Du Nouy ring, maximum bubble

pressure, and pendant drop methods.

REFERENCES

1a.

1b.

2.

3.

4.



D. J. McClements. Food Emulsions: Principles, Practice and Techniques. CRC, Boca

Raton, FL, 1999.

S. Friberg and K. Larsson. Food Emulsions. 3rd ed., Dekker, New York, 1997.

E. Dickinson and G. Stainsby. Colloids in Foods. Applied Science, London, 1982.

E. Dickinson. Introduction to Food Colloids. Oxford University Press, Oxford, 1992.

D. G. Dalgleish. Food emulsions. In: Emulsions and Emulsion Stability (J. Sjoblom,

ed.). Dekker, New York, 1996.



Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.