Gc

where <D + <C = 1. Thus the partition coefficient between an emulsion and its vapor can be predicted from a knowledge of KGD and KGC. Experiments with flavor compounds dispersed

FIGURE 9.3 In a two-liquid system, the flavor partitions between the oil, water, and vapor phases according to the partition coefficients.

Cq/CW iTGW CQ/CW KqQ CQ/CQ

FIGURE 9.3 In a two-liquid system, the flavor partitions between the oil, water, and vapor phases according to the partition coefficients.

FIGURE 9.4 Influence of dispersed-phase volume fraction on the concentration of a polar (KDC = 0.01) and a nonpolar (K^c = 100) flavor in the vapor phase of an oil-in-water emulsion (VG = 10 cm3, VE = 100 cm3).

in oil-in-water emulsions have shown that this equation gives a good description of the behavior of emulsions, provided the flavor does not interact with the interface or any free emulsifier in the aqueous phase (Guyot et al. 1996).

Predictions of the mass fraction of flavor in the vapor phase of oil-in-water emulsions with the same overall flavor concentration but different dispersed-phase volume fractions are shown in Figure 9.4. There is a decrease in the fraction of a nonpolar flavor in the vapor phase as the oil content increases, whereas the amount of a polar flavor is relatively unaffected. Thus the nonpolar flavors in an emulsion become more odorous as the fat content is decreased, whereas the polar flavors remain relatively unchanged. This has important consequences when deciding the type and concentration of flavors to use in low-fat analogs of existing emulsion-based food products.

One possible limitation of Equation 9.13 is that it does not take into account the droplet size. The assumption that the partitioning of additives is independent of particle size is likely to be valid for emulsions which contain fairly large droplets (i.e., d > 1 |im), because the influence of the curvature of a droplet on the solubility of the material within it is not significant (Hunter 1986). However, when the droplet diameter falls below a critical size, there is a large increase in the solubility of the material within it because of the increased Laplace pressure. Thus one would expect the flavor concentration in the continuous phase and in the vapor phase to increase as the size of the droplets in an emulsion decreased. These predictions are supported by experimental evidence which indicates that the partition coefficient of a nonpolar additive in an emulsion is less than in a macroscopic two-phase system with the same overall composition (Matsubara and Texter 1986, Texter et al. 1987, Wedzicha 1988).

9.2.1.6. Partitioning in Emulsions in the Presence of an Interfacial Membrane

Even though the interfacial region constitutes only a small fraction of the total volume of an emulsion, it can have a pronounced influence on the partitioning of surface-active molecules, especially when they are present at low concentrations, which is usually the case for food flavors. The influence of the interfacial membrane can be highlighted by a simple calculation of the amount of a surface-active additive which can associate with it (Wedzicha 1988). Assume that the additive occupies an interfacial area of 1 m2/mg, which is typical of many surface-active components (Dickinson 1992). The interfacial area per unit volume of an emulsion is given by the following relationship: AS = 6Q/dVS, where dVS is the volume-surface mean diameter (McClements and Dungan 1993). If we assume that the additive is present in 100 cm3 of an emulsion with a dispersed-phase volume fraction of 0.1 and a mean droplet diameter of 1 |im, then the total interfacial area of the droplets is 60 m2. It would therefore take about 60 mg of additive to completely saturate the interface, which corresponds to a concentration of approximately 0.1 wt%. Many flavors are used at concentrations which are considerably less than this value, and therefore their ability to accumulate at an interface has a large influence on their partitioning within an emulsion.

The accumulation of a flavor at an interface reduces its concentration in the oil, water, and gaseous phases by an amount which depends on the interfacial area, the flavor concentration, and the affinity of the flavor for the interface.

Reversible Binding. When the binding between the flavor and the interface is reversible, we can define a number of additional partition coefficients:

cD cC cG

In this case, the partition coefficient between the gas and the emulsion is given by:

kge kgd kgc kgi

In practice, it is difficult to directly measure the partition coefficient between the gas and interfacial region (KGI), and so it is better to express the equation in terms of properties which are simpler to measure (i.e., KGI = Kgc/KicC). In addition, the properties of the interface are usually better expressed in terms of the interfacial area rather than the volume fraction. Equation 9.15 can therefore be expressed in the following manner:

kge kgd kgc kgc where K*C = r I/cC is the partition coefficient between the interface and the continuous phase and rI is the mass of the additive per unit interfacial area. Thus the partition coefficient of an emulsion (KGE) can be predicted from experimental measurements which are all relatively simple to carry out (i.e., KGC, KGD, and KIC). This equation assumes that the concentration of flavor at the interface is well below the saturation level. Once the interfacial region becomes saturated with flavor, the remainder will be distributed between the bulk phases.

The influence of the interface on the volatility of a surface-active flavor molecule is shown in Figure 9.5. As the size of the droplets in the emulsion is decreased, the interfacial area increases, and therefore a greater amount of flavor associates with the interface, thereby reducing its volatility. Nevertheless, it should be stated that this type of behavior is only likely to occur when there is no free emulsifier in the aqueous phase, either as individual molecules or micelles. In practice, there is often free emulsifier in the aqueous phase and the flavor molecules may bind to it as well as to the interfacial region. In these systems, the flavor volatility is more likely to depend on the total concentration of emulsifier in the system rather than on the droplet size. Another factor which must be considered is that the concentration

FIGURE 9.5 Influence of droplet size on the volatility of surface-active flavor molecules which associate with the interfacial membrane.

of a flavor in the continuous and gas phases may actually increase with decreasing droplet size because of the influence of the droplet curvature on solubility (Section 9.2.1.5).

Irreversible Binding. When the binding of the flavor to the interface is irreversible, then its concentration in the vapor phase is only determined by the amount of free flavor in the emulsion (KGE = cG/cEJ). Under these circumstances, it is usually more convenient to use an effective partition coefficient, which is equal to the concentration of flavor in the vapor phase relative to the total amount of flavor in the emulsion (KGE = cG/cE):

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