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Molecular Aggregate

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Molecular Network

FIGURE 2.1 The molecules in food emulsions may adopt a variety of different structural arrangements depending on the nature of their interactions with their neighbors.

FIGURE 2.1 The molecules in food emulsions may adopt a variety of different structural arrangements depending on the nature of their interactions with their neighbors.

are insignificant at the molecular level because molecular masses are extremely small. Nevertheless, they do affect the behavior of food emulsions at the macroscopic level (e.g., sedimentation or creaming of droplets, the shape adopted by large droplets, meniscus formation, and capillary rise) (Israelachvili 1992). The forces that act at the molecular level are all electromagnetic in origin and can conveniently be divided into four types: covalent, electrostatic, van der Waals, and steric overlap (Hiemenz 1986, Israelachvili 1992, Atkins 1994). Despite acting over extremely short distances, often on the order of a few angstroms or less, intermolecular forces are ultimately responsible for the bulk physicochemical and organolep-tic properties of emulsions and other food materials.

2.3. ORIGIN AND NATURE OF MOLECULAR INTERACTIONS 2.3.1. Covalent Interactions

Covalent bonds involve the sharing of outer-shell electrons between two or more atoms, so that the individual atoms lose their discrete nature (Karplus and Porter 1970, Atkins 1994). The number of electrons in the outer shell of an atom governs its valency (i.e., the optimum number of covalent bonds it can form with other atoms). Covalent bonds may be saturated or unsaturated, depending on the number of electrons involved. Unsaturated bonds tend to be shorter, stronger, and more rigid than saturated bonds (Israelachvili 1992). The distribution of the electrons within a covalent bond determines its polarity. When the electrons are shared equally among the atoms, the bond has a nonpolar character, but when the electrons are shared unequally, the bond has a polar character. The polarity of a molecule depends on the symmetry of the various covalent bonds which it contains (see Section 2.3.2.). Covalent bonds are also characterized by their directionality (i.e., their tendency to be directed at clearly defined angles relative to each other). The valency, saturation, polarity, strength, and directionality of covalent bonds determine the three-dimensional structure, flexibility, chemical reactivity, and physical interactions of molecules.

Chemical reactions involve the breaking and formation of covalent bonds (Atkins 1994). The bulk physicochemical and organoleptic properties of food emulsions are altered by various types of chemical and biochemical reactions that occur during their production, storage, and consumption (Coultate 1996; Fennema 1996; Fennema and Tannenbaum 1996a,b). Some of these reactions are beneficial to food quality, while others are detrimental. It is therefore important for food scientists to be aware of the various types of chemical reaction that occur in food emulsions and to establish their influence on the overall properties of the system. The chemical reactions which occur in food emulsions are similar to those that occur in any other multicomponent heterogeneous food materials (e.g., oxidation of lipids [Nawar 1996], hydrolysis of proteins or polysaccharides [Damodaran 1996, BeMiller and Whistler 1996], cross-linking of proteins [Damodaran 1996], and Maillard reactions between reducing sugars and free amino groups [BeMiller and Whistler 1996]). Nevertheless, the rates and pathways of these reactions are often influenced by the physical environment of the molecules involved (e.g., whether they are located in the oil, water, or interfacial region) (Wedzicha

Until fairly recently, emulsion scientists were principally concerned with understanding the physical changes which occur in food emulsions, rather than the chemical changes. Nevertheless, there is currently great interest in establishing the relationship between emulsion properties and the mechanisms of various chemical reactions that occur within them (Wedzicha et al. 1991, Coupland and McClements 1996, Landy et al. 1996, Huang et al. 1997).

Despite the importance of chemical reactions in emulsion quality, it should be stressed that many of the most important changes in emulsion properties are a result of alterations in the spatial distribution of the molecules, rather than the result of alterations in their chemical structure (e.g., creaming, flocculation, coalescence, and phase inversion). The spatial distribution of molecules is governed principally by their noncovalent (or physical) interactions with their neighbors (e.g., electrostatic, van der Waals, and steric overlap). It is therefore particularly important to have a good understanding of the origin and nature of these interactions.

2.3.2. Electrostatic Interactions

Electrostatic interactions occur between molecular species that possess a permanent electrical charge, such as ions and polar molecules (Murrell and Boucher 1982, Reichardt 1988, Rogers

1989). An ion is an atom or molecule that has either lost or gained one or more outer-shell electrons so that it obtains a permanent positive or negative charge (Atkins 1994) (Figure 2.2). A polar molecule has no net charge (i.e., as a whole, the molecule is neutral), but it does have an electrical dipole because of an uneven distribution of the charges within it. Certain atoms are able to "pull" the electrons in the covalent bonds toward them more strongly than are other atoms (Atkins 1994). As a consequence, they acquire a partial negative charge (§-), and the other atom acquires a partial positive charge (§+). If the partial charges within a molecule are distributed symmetrically, they cancel each other and the molecule has no dipole (e.g., CCl4), but if they are distributed asymmetrically, the molecule will have a dipole

Ion-ion

Ion-dipole

Dipole-dipole

FIGURE 2.2 Schematic representation of the most important types of intermolecular electrostatic interactions that arise between molecules.

(Israelachvili 1992). For example, the chlorine atom in HCl pulls the electrons in the covalent bond more strongly than the hydrogen atom, and so a dipole is formed: Hs+Cls-. The strength of a dipole is characterized by the dipole moment || = ql, where l is the distance between two charges q+ and q-. The greater the magnitude of the partial charges, or the farther they are apart, the greater the dipole moment of a molecule.

The interaction between two molecular species is characterized by an intermolecular pair potential, w(s), which is the energy required to bring two molecules from an infinite distance apart to a separation s (Israelachvili 1992). There are a number of different types of electrostatic interactions that can occur between permanently charged molecular species (ion-ion, ion-dipole, and dipole-dipole), but they can all be described by a similar equation (Hiemenz 1986):

where Q1 and Q2 are the effective charges on the two species, e0 is the dielectric constant of a vacuum (8.85 x 10-12 C2 J-1 m-1), eR is the relative dielectric constant of the intervening medium, s is the center-to-center distance between the charges, and n is an integer that depends on the nature of the interaction. For ions, the value of Q is determined by their valency (z) and electrical charge (e) (1.602 x 10-19 C), whereas for dipoles, it is determined by their dipole moment || and orientation (Table 2.1). Numerical calculations of the intermolecular pair potential for representative ion-ion, ion-dipole, and dipole-dipole interactions are illustrated in Figure 2.3a.

Examination of Equation 2.1 and Figure 2.3a provides a number of valuable insights into the nature of intermolecular electrostatic interactions and the factors which influence them:

1. They may be either attractive or repulsive depending on the sign of the charges. If the charges have similar signs, wE(s) is positive and the interaction is repulsive, but if they have opposite signs, wE(s) is negative and the interaction is attractive.

2. Their strength depends on the magnitudes of the charges involved (Q1 and Q2). Thus, ion-ion interactions are stronger than ion-dipole interactions, which are in turn stronger than dipole-dipole interactions. In addition, the strength of interactions involving ions increases as their valency increases, whereas the strength of interactions involving polar species increases as their dipole moment increases.

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