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Octanol

0.99 x 10-3

0.09 x 10-3

Data compiled from Buttery et al. (1969, 1971, 1973) and Overbosch et al. (1991).

Data compiled from Buttery et al. (1969, 1971, 1973) and Overbosch et al. (1991).

from the vapor into the solvent.* Here, z is the coordination number of the flavor molecules, and wSS and wSF are the solvent-solvent and solvent-flavor interaction energies (Chapter 2). The overall change therefore depends on the relative strength of both the solvent-solvent and solvent-flavor interactions: Aw ~ zwSF - {■ zwSS.

The molecular characteristics of flavor molecules largely determine their partition coefficient in different solvents (Table 9.1). When a nonpolar flavor molecule is dispersed in a nonpolar solvent, or when a polar flavor molecule is dispersed in a polar solvent, there is a decrease in volatility with increasing molecular weight (Buttery et al. 1973). This is because the number of favorable attractive interactions between the flavor molecules and the solvent increases as the size of the flavor molecules increases. On the other hand, when a nonpolar flavor molecule is dispersed in a polar solvent, there is a decrease in volatility with increasing molecular weight (Buttery et al. 1969, 1971; Bomben et al. 1973; Franzen and Kinsella 1975). Nonpolar flavors are less volatile in nonpolar solvents than in polar solvents (Buttery et al. 1973, Bakker 1995), because a number of relatively strong hydrogen bonds have to be

* This simple analysis assumes that the sizes of the solvent and flavor molecules are approximately equal.

replaced by relatively weak van der Waals bonds when a nonpolar molecule is introduced into a polar solvent, which is energetically unfavorable because of the hydrophobic effect. In addition, polar flavors are less volatile in polar solvents than in nonpolar solvents, because the hydrogen bonds which form in polar solvents are more strongly attractive than the van der Waals bonds which form in nonpolar solvents.

9.2.1.2. Influence of Flavor Ionization

A number of water-soluble flavors have chemical groups which are capable of undergoing proton association-dissociation as a result of changes in pH (e.g., -COOH ^ -COO- + H+ or -NH+ ^ -NH2 + H+). The volatility and flavor characteristics of the different ionic forms of a molecule are different because of changes in their molecular interactions with the solvent (Baldwin et al. 1973, Wedzicha 1988, Guyot et al. 1996). For example, the ionized form of a flavor is less volatile in an aqueous solution than the nonionized form because of strong ion-dipole interactions between it and the surrounding water molecules. It is therefore important to take into account the effect of ionization on the partitioning of flavor molecules. The concentration of a specific ionic form at a certain pH can be determined using the Henderson-Hasselbach equation: pH = pKa - log(cAcid/cBase), where pKa = -logK) and Ka is the dissociation constant of the acidic group (Atkins 1994).

The volatility of the ionized form of a flavor is usually much lower than that of the nonionized form, and so the concentration of the flavor in the vapor phase is determined principally by the amount of nonionized flavor (cLN) present in the liquid phase. The partition coefficient is therefore given by:

In practice, it is more convenient to define an effective partition coefficient, which is equal to the concentration of the flavor in the vapor phase relative to the total amount of flavor in the liquid phase (cL = cL I + cLN), where cL I is the concentration of the ionized form of the flavor:

When the pH of the aqueous solution is well below the pKa value of the acid group, the flavor molecule is almost exclusively in the nonionized form (KeGL = KGL), and so the flavor in the vapor phase is at its most intense. As the pH is raised toward the pKa value of the acid group, the fraction of flavor molecules in the nonionized form decreases (KeGL < KGL), and so the flavor in the vapor phase becomes less intense.

It should be stressed that the ionization of a flavor molecule may also influence the partition coefficient because it alters its interactions with other charged molecules within the aqueous phase. For example, there may be attractive electrostatic interactions between an ionized flavor molecule and an oppositely charged biopolymer, which leads to flavor binding and therefore a reduction of its concentration in the vapor phase (Section 9.2.1.3).

9.2.1.3. Influence of Flavor Binding

Many proteins and carbohydrates are capable of binding flavor molecules and therefore altering their distribution within an emulsion (Franzen and Kinsella 1975, Bakker 1995,

O'Neill 1996, Hansen and Booker 1996, Hau et al. 1996). Flavor binding can cause a significant alteration in the perceived flavor of a food. This alteration is often detrimental to food quality because it changes the characteristic flavor profile, but it can also be beneficial when the bound molecules are off-flavors. A flavor chemist must therefore take binding effects into account when formulating the flavor of a particular product.

The equilibrium partition coefficient of a flavor in the presence of a biopolymer is given by:

where cLF is the concentration of free (unbound) flavor present in the liquid. The concentration of free flavor depends on the nature of the binding between the flavor and biopolymer (Overbosch et al. 1991). Binding may be the result of covalent bond formation or physical interactions, such as electrostatic, hydrophobic, van der Waals, or hydrogen bonds (Chapter 2). It may take place at specific sites on the surface of a biopolymer molecule or nonspecifically at any location on the surface. It may be reversible or irreversible.

The extent of flavor binding to a biopolymer can be conveniently characterized by a binding coefficient:

where cLB is the concentration of bound flavor in the liquid phase (cL = cLF + cLB). The stronger the binding between the flavor and a biopolymer, the greater the value of K*.

It is convenient to define an effective partition coefficient, which relates the concentration of flavor in the gas phase to the total amount of flavor in the liquid phase (Overbosch et al. 1991):

K^e CG KGL

This equation indicates that the concentration of flavor in the vapor phase is not influenced by the biopolymer when the binding coefficient is small (K* << 1), but that there is a large reduction in the concentration in the vapor phase when the binding is strong (K* > 1).

Binding constants are frequently measured experimentally by equilibrium dialysis. A biopolymer solution is placed inside a semipermeable dialysis bag, which is then suspended in a solution of the flavor molecules (Figure 9.2). The large biopolymer molecules are restricted to the inside of the dialysis bag, while the small flavor molecules can move through it. After the system has reached equilibrium, the amount of flavor bound to the biopolymer molecules is determined by measuring the concentration of flavor in the bag above that which would be expected in the absence of biopolymer. An alternative technique which is also widely used to measure flavor binding is head space analysis (Section 9.2.3). In this technique, the partition coefficient is determined by measuring the equilibrium concentration of volatiles in the head space above a solution. By measuring the partition coefficient at different biopolymer concentrations, it is possible to determine the binding constant.

Flavor molecules, such as alkanes, aldehydes, and ketones, have been shown to be capable of specifically binding to various different types of protein (e.g., casein, P-lactoglobulin,

FIGURE 9.2 The principles of equilibrium dialysis. A biopolymer solution is placed in a dialysis bag which is then suspended in a flavor solution. The amount of flavor bound to the biopolymer molecules is determined by analyzing the flavor concentration in the cell.

bovine serum albumin, and soy protein) (Langourieux and Crouzet 1995, O'Neill 1996, Hansen and Booker 1996, Boudaud and Dumont 1996). These proteins have little flavor themselves, but can cause significant changes in the flavor profile of an emulsion by binding either desirable or undesirable flavors. The extent of flavor binding depends on the molecular structure of the flavor and protein molecules. Nonpolar flavors are believed to bind to nonpolar patches on the surfaces of proteins through hydrophobic attraction. An increase in binding of flavors to P-lactoglobulin has been observed in the presence of urea or on heating, because these treatments cause partial unfolding of the proteins, which increases their surface hydrophobicity (O'Neill 1996). Flavors may also bind to polysaccharides, but in this case the molecular interactions involved are more likely to be van der Waals, electrostatic, or hydrogen bonds (Hau et al. 1996). In some systems, there may be irreversible binding due to chemical reactions between biopolymer and flavor molecules.

9.2.1.4. Influence of Surfactant Micelles

Surfactants are normally used to physically stabilize emulsion droplets against aggregation by providing a protective membrane around the droplet (Chapter 7). Nevertheless, there is often enough free surfactant present in an aqueous phase to form surfactant micelles (Section 4.5). These surfactant micelles are capable of solubilizing nonpolar molecules in their hydrophobic interior, which increases the affinity of nonpolar flavors for the aqueous phase and therefore decreases their partition coefficient (KGL). By a similar argument, reverse micelles in an oil phase are capable of solubilizing polar flavor molecules and therefore decreasing their partition coefficient (KGL).

The partitioning of flavor compounds between a surfactant solution and the gas above it can be described by the following equation:

kgl kgm kgc where is the volume fraction of the micelles, is the volume fraction of the continuous phase surrounding the micelles ($M + = 1), KGM (= cG/cM) is the gas-micelle partition coefficient, and KGC (= cG/q) is the gas-continuous phase partition coefficient. It is difficult to directly measure the partitioning of a flavor between gas and micelles, and so it is more convenient to rewrite the above equation in the following form:

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