The formation of an emulsion may involve a single step or a number of consecutive steps, depending on the nature of the starting material and the method used to create it. Prior to converting separate oil and aqueous phases into an emulsion, it is usually necessary to disperse the various ingredients into the phase in which they are most soluble. Oil-soluble ingredients, such as vitamins, colors, antioxidants, and surfactants, are usually mixed with the oil, whereas water-soluble ingredients, such as proteins, polysaccharides, sugars, salts, vitamins, colors, antioxidants, and surfactants, are usually mixed with the water. The intensity and duration of the mixing process depend on the time required to solvate and uniformly distribute the ingredients. Adequate solvation is important for the functionality of a number of food components (e.g., the emulsifying properties of proteins are often improved by allowing them to hydrate in water for a few minutes or hours prior to homogenization) (Kinsella and Whitehead 1989). If the lipid phase contains any crystalline material, it is necessary to warm it to a temperature where all the fat melts prior to homogenization; otherwise it is difficult, if not impossible, to create a stable emulsion (Mulder and Walstra 1974, Phipps 1985).
The process of converting two immiscible liquids into an emulsion is known as homogenization, and a mechanical device designed to carry out this process is called a homogenizer
(Loncin and Merson 1979). To distinguish between the nature of the starting materials, it is convenient to divide homogenization into two categories. The creation of an emulsion directly from two separate liquids will be defined as primary homogenization, whereas the reduction in size of the droplets in an existing emulsion will be defined as secondary homogenization (Figure 6.1). The creation of a particular type of food emulsion may involve the use of either of these types of homogenization or a combination of both. For example, the preparation of a salad dressing in the kitchen is carried out by direct homogenization of the aqueous and oil phases and is therefore an example of primary homogenization, whereas homogenized milk is manufactured by reducing the size of the fat globules in raw milk and so is an example of secondary homogenization. In many food-processing operations and laboratory studies, it is more efficient to prepare an emulsion using two steps (Dickinson and Stainsby 1982). The separate oil and water phases are converted to a coarse emulsion which contains fairly large droplets using one type of homogenizer (e.g., a high-speed blender), and then the size of the droplets is reduced using another type of homogenizer (e.g., a high-pressure valve homogenizer). Many of the same physical processes occur during primary and secondary homogenization (e.g., mixing, droplet disruption, and droplet coalescence), and so there is no clear distinction between the two.
Emulsions which have undergone secondary homogenization usually contain smaller droplets than those which have undergone primary homogenization, although this is not always the case. Some homogenizers are capable of producing emulsions with small droplet sizes directly from the separate oil and water phases (e.g., ultrasound, microfluidizers, or membrane homogenizers) (see Section 6.4).
The physical processes which occur during homogenization can be highlighted by considering the formation of an emulsion from pure oil and pure water. When the two liquids are placed in a container, they tend to adopt their thermodynamically most stable state, which consists of a layer of oil on top of a layer of water (Figure 6.1). This arrangement is adopted because it minimizes the contact area between the two immiscible liquids and because oil has a lower density than water (Section 7.2). To create an emulsion, it is necessary to supply energy in order to disrupt and intermingle the oil and water phases, which is usually achieved by mechanical agitation (Walstra 1993b). The type of emulsion formed in the absence of an emulsifier depends primarily on the initial concentration of the two liquids: at high oil concentrations, a water-in-oil emulsion tends to be formed, but at low oil concentrations, an oil-in-water emulsion tends to be formed.* In this example, we assume that the oil concentration is so low that an oil-in-water emulsion is formed. Mechanical agitation can be applied
* In the presence of an emulsifier, the type of emulsion formed is governed mainly by the properties of the emulsifier (i.e., the HLB number and optimum curvature) (Chapter 4).
in a variety of different ways (Section 6.4), the simplest being to vigorously shake the oil and water together in a sealed container. Immediately after shaking, an emulsion is formed which appears optically opaque because light is scattered by the emulsion droplets (Farinato and Rowell 1983). The oil droplets formed during the application of the mechanical agitation are constantly moving around and frequently collide and coalesce with neighboring droplets (Walstra 1993b). As this process continues, the large droplets formed move to the top of the container due to gravity and merge together to form a separate layer. As a consequence, the system reverts back to its initial state — a layer of oil sitting on top of a layer of water (Figure 6.1). The thermodynamic driving forces for this process are the hydrophobic effect, which favors minimization of the contact area between the oil and water, and gravity, which favors the upward movement of the oil (Section 7.2).
To form an emulsion that is (kinetically) stable for a reasonable period of time, one must prevent the droplets from merging together after they have been formed (Walstra 1983, 1993b). This is achieved by having a sufficiently high concentration of emulsifier present during the homogenization process. The emulsifier adsorbs to the surface of the droplets during homogenization, forming a protective membrane which prevents them from coming close enough together to coalesce. The size of the droplets produced during homogenization depends on two processes: (1) the initial generation of droplets of small size and (2) the rapid stabilization of these droplets against coalescence once they are formed (Section 6.3).
Many of the bulk physicochemical and organoleptic properties of food emulsions depend on the size of the droplets they contain, including their stability, texture, appearance, and taste (Chapters 7 to 9). One of the major objectives of homogenization is therefore to create an emulsion in which the majority of droplets fall within an optimum range which has previously been established by the food manufacturer. It is therefore important for food scientists to appreciate the major factors which determine the size of the droplets produced during homogenization.
This brief introduction to homogenization has highlighted some of the most important aspects of emulsion formation, including the necessity to mechanically agitate the system, the competing processes of droplet formation and droplet coalescence, and the role of the emulsifier. These topics will be considered in more detail in the rest of the chapter.
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