The modern consumer is faced with a huge variety of different types of food products from which to choose, and within each category there are a number of different brand names. The initial choice of a particular product is governed by many factors, including its cost, quality, packaging, ease of preparation, and nutritional value. Once a consumer has made a decision to purchase a certain brand name, the manufacturer wants to ensure that he or she is satisfied with the product and will purchase it again. It is therefore important to make certain that the product has desirable and reproducible quality attributes (i.e., appearance, taste, texture, and shelf life). In previous chapters, we considered emulsion rheology, which is related to texture (Chapter 8), and emulsion stability, which is related to shelf life (Chapter 7). In this chapter, focus is on the factors which influence the flavor and appearance of food emulsions.
The term "flavor" refers to those volatile components in foods which are sensed by receptors in the nose (aroma) and those nonvolatile components which are sensed by receptors in the tongue and the inside of the mouth (taste) (Thomson 1986). In addition, certain components in foods may also contribute to flavor because of their influence on the perceived texture (mouthfeel) (Kokini 1987). The flavor of a food is therefore a combination of aroma, taste, and mouthfeel, with the former usually the most important (Taylor and Linforth 1996). Flavor perception is an extremely complicated process which depends on a combination of physicochemical, biological, and psychological phenomena (Thomson 1986, Bell 1996). Before a food is placed in the mouth, its flavor is perceived principally through those volatile components which are inhaled directly into the nasal cavity. After the food is placed in the mouth, the flavor is determined by nonvolatile molecules which leave the food and are sensed by receptors on the tongue and the inside of the mouth, as well as by those volatile molecules which are drawn into the nasal cavity through the pharynx at the back of the mouth (Thomson 1986). The interactions between flavor molecules and human receptors which lead to the perceived flavor of a food are extremely complicated and are still fairly poorly understood (Thomson 1986). In addition, expectations and eating habits vary from individual to individual, so that the same food may be perceived as tasting differently by two separate individuals or by a single individual at different times. This section focuses on the physicochemical aspects of flavor partitioning and release in foods, because these are the most relevant topics to emulsion science. Although the physiological and psychological aspects of flavor are extremely important, they are beyond the scope of this book.
It is widely recognized that the perceived flavor of a food is not simply determined by the type and concentration of the flavor molecules present (Taylor and Linforth 1996), but by a number of other physicochemical factors as well, including:
1. The environment (matrix) in which the flavor molecules are located (e.g., the lipid, aqueous, or interfacial regions)
2. The physical state of the environment (e.g., gas, liquid or solid)
3. The structural organization of the components (e.g., emulsion versus nonemulsion)
4. The chemical state of the flavor molecules (e.g., degree of ionization or self-association)
5. The physical and chemical interaction of flavors with other molecules (e.g., proteins, carbohydrates, surfactants, or minerals)
6. The rate at which flavor molecules move from one environment to another
Given the large number of factors which contribute to food flavor, it is extremely difficult to accurately predict the flavor of a final product from a knowledge of its physicochemical characteristics. For this reason, the formulation of food flavors is often the result of art and craft, rather than the application of fundamental scientific principles. Even so, a more rigorous scientific approach to this topic would have great benefits for the food industry because it would enable manufacturers to design foods in a more systematic and cost-effective manner. In this section, some of the more fundamental aspects of the science of emulsion flavor are reviewed.
The perception of a flavor depends on the precise location of the flavor molecules within an emulsion. Most flavors are perceived more intensely when they are present in the aqueous phase rather than the oil phase (McNulty 1987, Kinsella 1989). The aroma is determined by the presence of volatile molecules in the vapor phase above an emulsion (Overbosch et al. 1991, Taylor and Linforth 1996). Certain flavor molecules may associate with the interfacial region, which alters their concentration in the vapor and aqueous phases (Wedzicha 1988). It is therefore important to establish the factors which determine the partitioning of flavor molecules within an emulsion. An emulsion system can be conveniently divided into four phases between which the flavor molecules distribute themselves: the interior of the droplets, the continuous phase, the oil-water interfacial region, and the vapor phase above the emulsion.* The relative concentration of the flavor molecules in each of these regions depends on their molecular structure and the properties of each of the phases (Baker 1987, Bakker 1995). In this section, we start by examining flavor partitioning in some simple model systems and then move on to some more complicated and realistic model systems. It should be stressed that most of the physical principles of flavor partitioning in emulsions are also applicable to the partitioning of other types of food ingredients, such as antioxidants, colors, preservatives or vitamins (Wedzicha 1988, Coupland and McClements 1996, Huang et al. 1997).
The simplest situation to consider is the partitioning of a flavor between a homogeneous liquid and the vapor above it (Figure 9.1). At thermodynamic equilibrium, the flavor distributes itself between the liquid and vapor according to the equilibrium partition coefficient (Wedzicha 1988):
* Flavors could also be present at the air-emulsion interface, but the interfacial area of this region is usually so small that the amount of flavor involved is negligible.
where aG and aL are the activity coefficients of the flavor in the gas and liquid phases, respectively. The concentration of flavors in foods is usually very low, and so the activity coefficients can be replaced by concentrations, since interactions between flavor molecules are insignificant (Wedzicha 1988, Overbosch et al. 1991):
where cG and cL are the concentrations of the flavor in the gas and liquid phases, respectively. It is often more convenient to represent the partitioning of a flavor as the mass fraction of the total amount of flavor in the system which is in the vapor phase:
where VG and VL are the volumes of the gas and liquid phases, respectively.
The magnitude of the partition coefficient depends on the relative strength of the interactions between a flavor molecule and its surroundings in the gas and liquid phases (Tinoco et al. 1985, Israelachvili 1992):
where AGGL is the difference in free energy per mole of the solute in the gas and liquid phases.
The free energy term depends on the change in molecular interactions and configurational entropy which occurs when a flavor molecule moves from the vapor phase into the liquid phase. The configurational entropy favors the random distribution of the flavor molecules throughout the whole volume of the system, rather than their confinement to just the liquid phase, and therefore it favors volatilization. The change in energy associated with the molecular interactions is determined by the formation of solvent-flavor bonds (~zwSF) and the disruption of solvent-solvent bonds (~ y zwSS), which occurs when a flavor molecule moves
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