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where, r is now the amount of flavor that is irreversibly bound to the interface per unit surface area. If the flavor does not interact with the interface, and the interfacial region has a negligible volume, then this equation reduces to Equation 9.13, but if the flavor binds to the interface, its concentration in the vapor phase is reduced.

Recent studies using oil-in-water emulsions that contain different types of flavor compounds have indicated that amphiphilic flavors, such as butyric acid, bind strongly to the interface of a droplet and thus reduce the partition coefficient (KGL) (Guyot et al. 1996). Nevertheless, a great deal of systematic research is still needed to determine the factors which influence the volatility of different flavor compounds in food emulsions. Special emphasis should be placed on establishing the molecular basis of this process so that predictions about the flavor profile of a food can be made from a knowledge of its composition and the type of flavor components present. This type of information could then be used by flavor chemists to formulate foods with specific flavor profiles.

9.2.2. Flavor Release

Flavor release is the process whereby flavor molecules move out of a food and into the surrounding saliva or vapor phase during mastication (McNulty 1987, Overbosch et al. 1991). The release of the flavors from a food material occurs under extremely complex and dynamic conditions (Land 1996). A food usually spends a relatively short period (typically 1 to 30 s) in the mouth before being swallowed. During this period, it is diluted with saliva, experiences temperature changes, and is subjected to a variety of mechanical forces. Mastication may therefore cause dramatic changes in the structural characteristics of a food.

During mastication, nonvolatile flavor molecules must move from within the food, through the saliva, to the taste receptors on the tongue and the inside of the mouth, whereas volatile flavor molecules must move from the food, through the saliva, and into the gas phase, where they are carried to the aroma receptors in the nasal cavity (Thomson 1986). The two major factors which determine the rate at which these processes occur are the equilibrium partition coefficient (because this determines the magnitude of the flavor concentration gradients at the various boundaries) and the mass transfer coefficient (because this determines the speed at which the molecules move from one location to another). In this section, we examine some of the simple models which have been developed to describe the complex processes which occur during the release of both nonvolatile and volatile flavor components from foods.

9.2.2.1. Release of Nonvolatile Compounds (Taste)

Ideally, we would like to know the maximum amount of flavor which can be released from a food and the time taken for this release to occur.

Maximum Amount Released. A relatively simple model, based on the equilibrium partition coefficient of the flavor between oil and water, has been used to describe the maximum amount of flavor which can be released by an oil-in-water emulsion when it is placed in the mouth (McNulty 1987). The model assumes that the food is initially at equilibrium, so that the distribution of the flavor between the droplets and continuous phase is given by the equilibrium partition coefficient (Kow). When the food is placed in the mouth, it is diluted by saliva (Figure 9.6). Immediately after dilution, the concentration of flavor in the aqueous phase is reduced, and so there is a thermodynamic driving force which favors the release of flavor from the droplets until the equilibrium flavor distribution is restored.

The potential extent of the flavor release can be characterized by the ratio of the flavor in the aqueous phase once equilibrium has been reestablished to that immediately after dilution:

WL = [<(Kow - 1) + 1](DF -W [<(Kow - 1) + DF](1 -

where DF is the dilution factor of the emulsion (= VJ V), V and V are the emulsion volume before and after dilution, ^ is the dispersed-phase volume fraction of the initial emulsion, cWd is the concentration of flavor in the aqueous phase immediately after dilution, and cWe is the concentration in the aqueous phase once equilibrium has been reestablished. The higher the

FIGURE 9.6 The flavor in a food is initially distributed according to the partition coefficients. When it is diluted with saliva, the equilibrium is upset, and flavor is released from the droplets.

value of Ef, the greater the potential for flavor release. Despite its simplicity, this model can be used to make some valuable predictions about the factors which determine the flavor release from foods (McNulty 1987) (e.g., the extent of flavor release on dilution increases as either ^ or Kow increases). The major limitation of this model is that it provides no information about the rate at which the flavor is released from the droplets.

Kinetics of Flavor Release. The taste of an emulsion depends on the rate at which the flavor molecules move from the food to the receptors on the tongue and inside of the mouth. Flavor molecules may be located in either the oil or water phase, although it is widely believed that taste perception is principally a result of those molecules which are present in the water phase (McNulty 1987), because the flavor must cross an aqueous membrane before reaching the taste receptors (Thomson 1986). An indication of the kinetics of flavor release can therefore be obtained from a knowledge of the time dependence of the flavor concentration in the aqueous phase.

When an oil-in-water emulsion is diluted with saliva, some of the flavor molecules in the droplets move into the aqueous phase. A mathematical model has been developed to describe the rate at which a solute is released from a spherical droplet surrounded by a finite volume of a well-stirred liquid (Crank 1975):

where Mt is the total amount of solute which has left the sphere by time t, M^ is the total amount of solute which has left the sphere once equilibrium has been established, D is the translational diffusion coefficient of the flavor within the droplets, t is the time, r is the droplet radius, a = 3 V/(4nr3)KDC, V is the volume of the continuous phase, and qn are the nonzero roots of the relation, tan qn = 3qn/(1+aqn). This equation assumes that the concentration of solute (flavor) in the aqueous phase is initially zero, and therefore this equation is only strictly applicable to emulsions which are diluted with high concentrations of saliva. Nevertheless, it does provide some useful insights into the rate of flavor release from oil droplets.

The influence of droplet radius on the flavor release rate from droplets in a typical oil-in-water emulsion is shown in Figure 9.7. The release rate increases as the size of the droplets decreases. For the system shown in Figure 9.7, the time required for half of the flavor to leave the emulsion droplets is given by t1/2 = (0.162r)2/D. The variation of t1/2 with droplet radius

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