Many natural and processed foods consist either partly or wholly as emulsions or have been in an emulsified state at some time during their production; such foods include milk, cream, butter, margarine, fruit beverages, soups, cake batters, mayonnaise, cream liqueurs, sauces, desserts, salad cream, ice cream, and coffee whitener (Friberg and Larsson 1997, Krog et al. 1983, Jaynes 1983, Dickinson and Stainsby 1982, Dickinson 1992, Swaisgood 1996). Emulsion-based food products exhibit a wide variety of different physicochemical and organoleptic characteristics, such as appearance, aroma, texture, taste, and shelf life. For example, milk is a low-viscosity white fluid, strawberry yogurt is a pink viscoelastic gel, and margarine is a yellow semisolid. This diversity is the result of the different sorts of ingredients and processing conditions used to create each type of product. The manufacture of an emulsion-based food product with specific quality attributes depends on the selection of the most appropriate raw materials (e.g., water, oil, emulsifiers, thickening agents, minerals, acids, bases, vitamins, flavors, colorants, etc.) and processing conditions (e.g., mixing, homogeni-zation, pasteurization, sterilization, etc.).
Traditionally, the food industry largely relied on craft and tradition for the formulation of food products and the establishment of processing and storage conditions. This approach is unsuitable for the modern food industry, which must rapidly respond to changes in consumer preferences for a greater variety of cheaper, healthier, and more convenient foods (Sloan 1994, 1996; Katz 1997). In addition, the modern food industry relies increasingly on large-scale production operations to produce vast quantities of foods at relatively low cost. The development of new foods, the improvement of existing foods, and the efficient running of food-processing operations require a more systematic and rigorous approach than was used previously (Hollingsworth 1995).
Two areas which have been identified as being of particular importance to the improvement of food products are:
1. Enhanced scientific understanding of food properties. An improved understanding of the factors that determine the bulk physicochemical and organoleptic properties of emulsions will enable manufacturers to create low-cost high-quality food products in a more systematic and reliable fashion (Kokini et al. 1993, Rizvi et al. 1993).
2. Development of new analytical techniques to characterize food properties. The development and application of new analytical techniques to characterize the properties of emulsions are leading to considerable advances in research, development, and quality control (Dickinson 1995a,b; Gaonkar 1995). These techniques are used in the laboratory to enhance our understanding of the factors which determine the properties of foods and in the factory to monitor the properties of foods during processing in order to ensure that they meet the required quality specifications.
Emulsion science is a multidisciplinary subject that combines chemistry, physics, and engineering (Sherman 1968a; Becher 1957, 1983; Hiemenz 1986; Hunter 1986, 1989, 1993; Evans and Wennerstrom 1994). The aim of the emulsion scientist working in the food industry is to utilize the principles and techniques of emulsion science to enhance the quality of the food supply and the efficiency of food production. This book presents the conceptual and theoretical framework required by food scientists to understand and control the properties of emulsion-based food products.
1.2. GENERAL CHARACTERISTICS OF FOOD EMULSIONS 1.2.1. Definitions
An emulsion consists of two immiscible liquids (usually oil and water), with one of the liquids dispersed as small spherical droplets in the other (Figure 1.1). In most foods, the diameters of the droplets usually lie somewhere between 0.1 and 100 (Dickinson and Stainsby 1982; Dickinson 1992; Walstra 1996a,b). Emulsions can be conveniently classified according to the distribution of the oil and aqueous phases. A system which consists of oil droplets dispersed in an aqueous phase is called an oil-in-water or O/W emulsion (e.g., mayonnaise, milk, cream, soups, and sauces). A system which consists of water droplets dispersed in an oil phase is called a water-in-oil or W/O emulsion (e.g., margarine, butter, and
spreads). The substance that makes up the droplets in an emulsion is referred to as the dispersed or internal phase, whereas the substance that makes up the surrounding liquid is called the continuous or external phase. It is also possible to prepare multiple emulsions of the oil-in-water-in-oil (O/W/O) or water-in-oil-in-water (W/O/W) type (Dickinson and McClements 1995). For example, a W/O/W emulsion consists of water droplets dispersed within larger oil droplets, which are themselves dispersed in an aqueous continuous phase (Evison et al. 1995). Recently, research has been carried out to create stable multiple emulsions which can be used to control the release of certain ingredients, reduce the total fat content of emulsion-based food products, or isolate one ingredient from another (Dickinson and McClements 1995).
The concentration of droplets in an emulsion is usually described in terms of the dispersed-phase volume fraction (Section 1.3.1). The process of converting two separate immiscible liquids into an emulsion, or of reducing the size of the droplets in a preexisting emulsion, is known as homogenization. In the food industry, this process is usually carried out using mechanical devices known as homogenizers, which subject the liquids to intense mechanical agitation (Chapter 6).
It is possible to form an emulsion by homogenizing pure oil and pure water together, but the two phases rapidly separate into a system which consists of a layer of oil (lower density) on top of a layer of water (higher density). This is because droplets tend to merge with their neighbors when they collide with them, which eventually leads to complete phase separation. The driving force for this process is the fact that the contact between oil and water molecules is energetically unfavorable (Israelachvili 1992), so that emulsions are thermodynamically unstable systems (Chapter 7). It is possible to form emulsions that are kinetically stable (metastable) for a reasonable period of time (a few days, weeks, months, or years) by including substances known as emulsifiers and/or thickening agents prior to homogenization (Chapter 4). Emulsifiers are surface-active molecules which absorb to the surface of freshly formed droplets during homogenization, forming a protective membrane which prevents the droplets from coming close enough together to aggregate (Chapters 6 and 7). Most emulsifiers are amphiphilic molecules (i.e., they have polar and nonpolar regions on the same molecule). The most common emulsifiers used in the food industry are amphiphilic proteins, small-molecule surfactants, and phospholipids (Chapter 4). Thickening agents are ingredients which are used to increase the viscosity of the continuous phase of emulsions, and they enhance emulsion stability by retarding the movement of the droplets. The most common thickening agents used in the food industry are polysaccharides (Chapter 4). A stabilizer is any ingredient that can be used to enhance the stability of an emulsion and may therefore be either an emulsifier or a thickening agent.
An appreciation of the difference between the thermodynamic stability of a system and its kinetic stability is crucial for an understanding of the properties of food emulsions (Dickinson 1992). Consider a system which consists of a large number of molecules that can occupy two different states: Elow and Ehigh (Figure 1.2). The state with the lowest free energy is the one which is thermodynamically favorable and therefore the one that the molecules are most likely to occupy. At thermodynamic equilibrium, the two states are populated according to the Boltzmann distribution (Atkins 1994):
where ^ is the fraction of molecules that occupies the energy level E, k is Boltzmann's constant (k = 1.38 x 10-23 J K-1), and T is the absolute temperature. The larger the difference
between the two energy levels compared to the thermal energy of the system (kT), the greater the fraction of molecules in the lower energy state. In practice, a system may not be able to reach equilibrium during the time scale of an observation because of the presence of an energy barrier (AE*) between the two states (Figure 1.2). A system in the high energy state must acquire an energy greater than AE* before it can move into the low energy state. The rate at which a transformation from a high to a low energy state occurs therefore decreases as the height of the energy barrier increases. When the energy barrier is sufficiently large, the system may remain in a thermodynamically unstable state for a considerable length of time, in which case it is said to be kinetically stable or metastable (Atkins 1994). In food emulsions, there are actually a large number of intermediate metastable states between the initial emulsion and the separated phases, and there is an energy barrier associated with a transition between each of these states. Nevertheless, it is often possible to identify a single energy barrier, which is associated with a particular physicochemical process, that is the most important factor in determining the overall kinetic stability of an emulsion (Chapter 7).
The term "emulsion stability" is broadly used to describe the ability of an emulsion to resist changes in its properties with time (Chapter 7). Nevertheless, there are a variety of physico-chemical mechanisms which may be responsible for alterations in the properties of an emulsion, and it is crucial to be clear about which of these mechanisms are important in the system under consideration. A number of the most important physical mechanisms responsible for the instability of emulsions are shown schematically in Figure 1.3. Creaming and sedimentation are both forms of gravitational separation. Creaming describes the upward movement of droplets due to the fact that they have a lower density than the surrounding liquid, whereas sedimentation describes the downward movement of droplets due to the fact that they have a higher density than the surrounding liquid. Flocculation and coalescence are both types of droplet aggregation. Flocculation occurs when two or more droplets come together to form an aggregate in which the droplets retain their individual integrity, whereas coalescence is the process where two or more droplets merge together to form a single larger droplet. Extensive droplet coalescence can eventually lead to the formation of a separate layer of oil on top of a sample, which is known as "oiling off." Phase inversion is the process whereby an oil-in-water emulsion is converted into a water-in-oil emulsion or vice versa. The
factors which determine these and the other major forms of emulsion instability are discussed in Chapter 7, along with methods of controlling and monitoring them. In addition to the physical processes mentioned above, it should be noted that there are also various chemical, biochemical, and microbiological processes that occur in food emulsions which can also affect their shelf life and quality.
Most food emulsions can conveniently be considered to consist of three regions which have different physicochemical properties: the interior of the droplets, the continuous phase, and the interface (Figure 1.4). The molecules in an emulsion distribute themselves among these three regions according to their concentration and polarity (Wedzicha 1988). Nonpolar molecules tend to be located primarily in the oil phase, polar molecules in the aqueous phase, and amphiphilic molecules at the interface. It should be noted that even at equilibrium, there is a continuous exchange of molecules between the different regions, which occurs at a rate that depends on the mass transport of the molecules through the system. Molecules may also move from one region to another when there is some alteration in the environmental conditions of an emulsion (e.g., a change in temperature or dilution within the mouth). The location and mass transport of the molecules within an emulsion have a significant influence on the aroma, flavor release, texture, and physicochemical stability of food products (Dickinson and Stainsby 1982, Wedzicha et al. 1991, Coupland and McClements 1996, Landy et al. 1996).
FIGURE 1.4 The ingredients in an emulsion partition themselves between the oil, water, and interfacial regions according to their concentration and interactions with the local environment.
Many of the properties of emulsions can only be understood with reference to their dynamic nature. The formation of emulsions by homogenization is a highly dynamic process which involves the violent disruption of droplets and the rapid movement of surface-active molecules from the bulk liquids to the interfacial region (Chapter 6). Even after their formation, the droplets in an emulsion are in continual motion and frequently collide with one another because of their Brownian motion, gravity, or applied mechanical forces (Melik and Fogler 1988, Dukhin and Sjoblom 1996, Lips et al. 1993). The continual movement and interactions of droplets cause the properties of emulsions to evolve over time due to the various desta-bilization mechanisms mentioned in Section 1.2.2. An appreciation of the dynamic processes that occur in food emulsions is therefore extremely important for a thorough understanding of their bulk physicochemical and organoleptic properties.
Most food emulsions are much more complex than the simple three-component (oil, water, and emulsifier) systems described in Section 1.2.1. The aqueous phase may contain a variety of water-soluble ingredients, including sugars, salts, acids, bases, surfactants, proteins, and carbohydrates. The oil phase usually contains a complex mixture of lipid-soluble components, such as triacylglycerols, diacylglycerols, monoacylglycerols, free fatty acids, sterols, and vitamins. The interfacial region may contain a mixture of various surface-active components, including proteins, phospholipids, surfactants, alcohols, and solid particles. In addition, these components may form various types of structural entities in the oil, water, or interfacial regions, such as fat crystals, ice crystals, protein aggregates, air bubbles, liquid crystals, and surfactant micelles. A further complicating factor is that foods are subjected to variations in their temperature, pressure, and mechanical agitation during their production, storage, and handling, which can cause significant alterations in their overall properties.
It is clear from the above discussion that food emulsions are compositionally, structurally, and dynamically complex materials and that many factors contribute to their overall properties. One of the major objectives of this book is to present the conceptual framework needed by food scientists to understand these complex systems in a more systematic and rigorous fashion. Much of our knowledge about these complex systems has come from studies of simple model systems (Section 1.5). Nevertheless, there is an increasing awareness of the need to elucidate the factors that determine the properties of actual emulsion-based food products. For this reason, many researchers are now focusing on the complex issues that need to be addressed, such as ingredient interactions, effects of processing conditions, and phase transitions (Dickinson 1992, 1995b; Dickinson and McClements 1995; Dalgleish 1996a; Hunt and Dalgleish 1994, 1995; Demetriades et al. 1997a,b).
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