10.4.1. Proximate Analysis
The concentration of droplets in an emulsion can be determined using many of the standard analytical methods developed to determine the composition of foods (Nielsen 1994, Pomeranz and Meloan 1994). A variety of solvent-extraction techniques are available for measuring fat content (Min 1994, Pal 1994). The sample to be analyzed is mixed with a nonpolar organic solvent which extracts the oil. The solvent is then physically separated from the aqueous phase, and the oil content is determined by evaporating the solvent and weighing the residual oil. A possible difficulty associated with applying this technique to oil-in-water emulsions is that the interfacial membrane may be resistant to rupture, and therefore all of the oil is not released. This problem has been overcome in a number of nonsolvent techniques developed to measure the fat content of dairy emulsions. In the Gerber and Babcock methods, an emulsion is placed in a specially designed bottle and then mixed with sulfuric acid, which digests the interfacial membrane surrounding the droplets and thus causes coalescence (Min 1994). The bottle is centrifuged to facilitate the separation of the oil and aqueous phases, and the percentage of oil in the emulsion is determined from the calibrated neck of the bottle. A similar procedure is involved in the detergent method, except that rather than sulfuric acid, a surfactant is added to promote droplet coalescence.
The water content of an emulsion can be determined using a variety of proximate analysis techniques (Mikula 1992, Pal 1994, Pomeranz and Meloan 1994, Bradley 1994). The simplest of these involves weighing an emulsion before and after the water has been evaporated, which may be achieved by conventional oven, vacuum oven, microwave oven, or infrared light. The moisture content can also be determined by distillation. The emulsion is placed in a specially designed flask which has a calibrated side arm. An organic solvent is mixed with the emulsion, and the flask is heated to cause the water to evaporate and collect in the side arm. This procedure is continued until all of the water has evaporated, and then its volume is determined from the calibrations on the side arm.
Many of these techniques are labor intensive, time consuming, and destructive and therefore are unsuitable for rapid quality assurance tests. Instruments based on infrared absorption are becoming increasingly popular for rapid and nondestructive analysis of food composition (Wehling 1994, Wilson 1995). Once calibrated, these instruments are capable of simultaneously determining the concentration of fat, water, protein, and carbohydrate. These instruments are likely to find widespread use in the food industry, particularly for online measurements.
One of the simplest methods of determining the dispersed-phase volume fraction of an emulsion is to measure its density (Pal 1994). The density of an emulsion (pg) is related to the densities of the continuous (pj) and dispersed phases (p2): pe= + (1 - ^)pi. The dispersed-phase volume fraction of an emulsion can therefore be determined by measuring its density and knowing the density of the oil and aqueous phases:
The densities of the dispersed and continuous phases in a food emulsion are appreciably different, about 900 kg m-3 for liquid oils and about 1000 kg m-3 for aqueous phases. It is possible to measure the density of an emulsion to about 0.2 kg m-3, and so the dispersed-phase volume fraction can be determined to within 0.002 (0.2%) using this technique. The fact that the density of liquid oils is lower than that of water means that the density of an emulsion usually decreases with increasing oil content (Figure 10.18).
Density Bottles. A liquid sample is poured into a glass bottle of known mass and volume (Pomeranz and Meloan 1994). The bottle and liquid are allowed to equilibrate to the measurement temperature and are then weighed using an accurate balance. The mass of emulsion (memulsion) required to completely fill the container at a given temperature is measured. The internal volume of the container is determined by measuring the mass of distilled water (a material whose density is known accurately) it takes to fill the bottle (Vbottle = -ffiwater/pwater). Thus, the density of the emulsion can be determined: pemulsion = fflemulsion/^bottle. The density of an emulsion is particularly sensitive to temperature, and so it is important to carefully control the temperature when accurate measurements are required. It is important to ensure that the container is clean and dry prior to weighing and that it contains no gas bubbles. Gas bubbles reduce the volume of liquid in the bottle without contributing to the mass and therefore lead to an underestimate of the density.
Hydrometers. Several methods for measuring the density of liquids are based on the Archimedes principle, which states that the upward buoyant force exerted on a body im mersed in a liquid is equal to the weight of the displaced liquid (Pomeranz and Meloan 1994). This principle is used in hydrometers, which are graduated hollow glass bodies that float on the liquid to be tested. The depth that the hydrometer sinks into a liquid depends on the density of the liquid. The hydrometer sinks to a point where the mass of the displaced liquid is equal to the mass of the hydrometer. Thus the depth to which a hydrometer sinks increases as the density of the liquid decreases. The density of a liquid is read directly from graduated calibrations on the neck of the hydrometer. Hydrometers are less accurate than density bottles, but much more rapid and convenient to use.
Oscillating U-tubes. The density of fluids can be measured rapidly and accurately using an instrument called an oscillating U-tube densitometer (Pal 1994). The sample to be analyzed is placed in a glass U-tube, which is forced to oscillate sinusoidally by the application of an alternating mechanical force. The resonant frequency of the U-tube is related to its mass and therefore depends on the density of the material contained within it. The density of a fluid is determined by measuring the resonant frequency of the U-tube and relating it to the density using an appropriate mathematical equation. The instrument must be calibrated with two fluids of accurately known density (usually distilled water and air). Density can be measured to within 0.1 kg m-3 in a few minutes using this technique. Recently, on-line versions of this technique have been developed for monitoring the density of fluids during processing. A small portion of a fluid flowing through a pipe is directed through an oscillating U-tube and its density is measured before being redirected into the main flow.
An accurate measurement of emulsion density can be carried out using inexpensive equipment that is available in many laboratories. The technique is nondestructive and can be used to analyze emulsions which are concentrated and optically opaque. One possible problem with the technique is that the physical state of the emulsion constituents may alter the accuracy of a measurement. For example, the density of solid fat is greater than that of liquid oil, and therefore the density of an emulsion depends on the solid fat content, as well as the total fat content. In these situations, it is necessary to heat the emulsion to a temperature where it is known that all of the fat crystals have melted and then measure its density.
10.4.3. Electrical Conductivity 10.4.3.1. Principles
The dispersed-phase volume fraction of an emulsion can be conveniently determined by measuring its electrical conductivity (e) (Clausse 1983, Robin et al. 1994, Asami 1995). The electrical conductivity of water is much higher than that of oil, and so there is a decrease in e as the oil content of an emulsion increases (Figure 10.18). In dilute emulsions, the dis-persed-phase volume fraction is related to the electrical conductivity by the following equation (Clausse 1983):
where the subscripts 1, 2, and e refer to the continuous phase, dispersed phase, and emulsion, respectively. More complex expressions have been derived to relate the dispersed-phase volume fraction of concentrated emulsions to their electrical properties.
The electrical conductivity of an emulsion can simply be determined using a conductivity cell (Siano 1998). This cell consists of a couple of electrodes which are connected to electrical circuitry that is capable of measuring the electrical conductivity of the sample contained between the electrodes. The electrical conductivity of an aqueous phase depends on the concentration of electrolytes present, and so it is important to properly characterize the properties of the component phases.
The electrical conductivity technique can be used to determine the dispersed-phase volume fraction of concentrated and optically opaque emulsions without the need for any sample preparation. Measurements are independent of the size of the emulsion droplets, which is an advantage when the droplet size distribution is unknown (Robin et al. 1994). The electrical conductivity of an emulsion is dependent on the physical state of its constituents, and therefore it may be necessary to heat an emulsion to a temperature where all of the crystals have melted before making a measurement. The electrical conductivity is also sensitive to the ionic strength of the aqueous phase, and therefore it is necessary to take this into account when carrying out the analysis (Skodvin et al. 1994).
The dispersed-phase volume fraction can be measured using many of the techniques used to determine droplet size distributions (Section 10.3). Light-scattering and electrical pulse counting techniques can be used to determine dispersed-phase volume fractions in dilute emulsions (ty < 0.1%), whereas Doppler shift spectroscopy, ultrasonic, electroacoustic, dielectric, neutron-scattering, and NMR techniques can be used to analyze much more concentrated emulsions. All of these techniques rely on there being a measurable change in some physicochemical property of an emulsion as its droplet concentration increases (e.g., the intensity of scattered or transmitted light, the attenuation or velocity of an ultrasonic wave, the amplitude or decay time of an NMR signal) (Figure 10.18). Some of these techniques can be used to simultaneously determine the droplet size distribution and dispersed-phase volume fraction, whereas others can be used to determine ty independently of a knowledge of the droplet size.
10.5. DROPLET CRYSTALLINITY 10.5.1. Dilatometry 10.5.1.1. Principles
Dilatometry has been used for many years to monitor the crystallinity of both the dispersed and continuous phases in emulsions (Turnbull and Cormia 1961, Skoda and van den Tempel 1963, Phipps 1964). The technique is based on measurements of the density change which occurs when a material melts or crystallizes. The density of the solid state of a material is usually greater than that of the liquid state because the molecules are able to pack more efficiently.* Consequently, there is a decrease in the density of a material when it melts and an increase when it crystallizes. The fraction of crystalline droplets in an emulsion can be determined from the following equation:
* With the important exception of water near its freezing point.
P eS peL
where, pe is the density of an emulsion that contains partially crystalline droplets, and peL and peS are the densities of the same emulsion when the droplets are either completely liquid or completely solid, respectively. The values of peL and peS are usually determined by extrapolating density measurements from higher and lower temperatures into the region where the droplets are partially crystalline.
In principle, dilatometry can be carried out using any experimental technique that is capable of measuring density changes. In practice, dilatometry is often performed using a specially designed piece of apparatus (Figure 10.19). A known mass of sample is placed into a glass bulb which is connected to a calibrated capillary tube. A liquid, such as mercury or colored water, is poured into the capillary tube above the fat. The change in volume of the sample when it crystallizes or melts is then determined by observing the change in height of the liquid in the capillary tube. These measurements can be carried out either as the temperature of the sample is varied in a controlled way or as the sample is held at a constant temperature over time (Turnbull and Cormia 1961). The data can be presented as a volume change or as a density change, depending on which is most convenient.
The temperature dependence of the density of an oil-in-water emulsion that contains droplets which undergo a phase transition is shown in Figure 10.20. When the emulsion is heated from a temperature where the droplets are completely solid, there is a sharp decrease in density when the droplets melt. Conversely, when an emulsion is cooled from a temperature where the droplets are completely liquid, there is a sudden increase in density when the droplets crystallize. The crystallization temperature is considerably lower than the melting temperature because of supercooling (Section 4.2). In food emulsions, oil droplets normally melt over a much wider temperature range than that shown for a pure oil in Figure 10.20 because they contain a mixture of different triacylglycerols, each with its own melting point.
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