into two groups, depending on the degree of chemical heterogeneity between the phases. The first of these, chemically heterogeneous systems, are those where the dispersed phase consists of components with different solubility properties and a different chemical character to that of the continuous phase. The second type comprises chemically homogeneous systems, where the dispersed phase is obtained by crystallization or precipitation of a major part of the continuous phase.
2.1 Chemically homogeneous dispersions
Chemically homogeneous dispersions are defined here as dispersions where the dispersed phase and the contin-uous phase have chemical compositions with important similarities. Examples of such systems are honey, where we have saccharide crystals dispersed in a concentrated syrup, or semi-solid fat, which consists of solid ceride crystals dispersed in a solution of liquid trigly-cerides.
Major characteristics of homogeneous dispersions are the dynamic characters of their phase boundaries.
There is a continuos flux of material from the dissolved state into the solid state, and vice versa. The inter-faces are dynamic and changing, with the process of ostwald ripening rapidly eliminating the smaller parti-cles. Surface-active components may influence both the crystal growth and the nucleation.
Important examples of homogeneous dispersions are ice crystals in concentrated protein and salt solutions (frozen animal tissue materials and other frozen low-carbohydrate foods), or ice crystals dispersed in a concentrated syrup (frozen desserts, ice cream, etc.).
2.2 Chemically heterogeneous dispersions
The most characteristic food model colloid in this class is that of triglycerides dispersed in water. Triglycerides
display an extremely low solubility in water, suppressing the ripening processes, and thus make instability mech-anisms far more important. Suspensions of insoluble materials, such as fibre particles (for instance, cloud-ing materials in soft drinks), or protein and carbohydrate particles in oil (for instance, chocolate systems which normally consist of about 70% of sucrose dispersed in cacao liquid) are also examples of heterogeneous disper-sions. Such colloidal systems might be described as being either emulsions, suspensions or foams. The various instability mechanisms occurring in homogeneous and heterogeneous dispersions are compared in Table 2.1.
2.2.1 Emulsions
Emulsions are created by applying mechanical agitation to an oil phase, thus dispersing it into droplets. In order to stabilize the droplets, they need to be protected by an adsorbed layer of emulsifiers or protecting colloids.
The emulsifying material creates repulsive and attracting interaction patterns, which determines the properties of the final food system.
Such destabilising factors are different between oil-in-water and water-in-oil systems, as shown in Table 2.2.
Water-in-oil emulsions, are commonly exemplified by systems such as butter, spreads and margarine. These consist of water droplets formed in an oil phase of a liquid oil or a semi-solid partially crystalline oil phase.
These systems are typically stabilized by oil-soluble emulsifiers and by the presence of solid particles. The water solubility in a triglyceride, in contrast to the triglyceride solubility in water, is about 0.5%, thus making the ripening processes fast unless the droplets are stabilized by the inclusion of a totally oil-insoluble material such as salt.
A second factor that differs between oil-in-water and water-in-oil systems is the role of fat crystals. In oil-in-water systems, such crystals induce coalescence. How-ever, the finalization of the droplet fusion is also slowed Table 2.1. Instability mechanisms in homogeneous and heterogeneous dispersions
Mechanism Sedimentation Flocculation Coalescence Ostwald ripening Nucleation Material bridges
Homogeneous Less important due to the small density
difference and the high volume fraction Unimportant due to the large particle size Most systems contain solid particles
Very important for small particles ( < l - 1 0 um).
Important during the formation of the dispersed system
Important
Heterogeneous
Usually very important unless the particle size is very small
Important if the particle size is below 1 um Important in liquid/liquid and liquid/gas systems Important in foams
Might be important during foam formation Only important for water bridges in
oil-continuous dispersions (sucrose in oil)
down in the presence of a high-fraction solid fat. Hence, partial coalescence is a typical state for an emulsified semi-solid fat (1, 2). In water-in-oil emulsions, the role of the fat crystals is in the opposite direction, i.e. it contributes to the stability of the system.
2,2.2 Suspensions
Suspensions of insoluble material are commonly found in many food and feed systems. Some of their prop-erties are summarized in Table 2.3. The particles are almost always obtained from the grinding operation of a biological tissue or of a pure solid food material (typically sucrose or protein). These processes gener-ate particles with a typical size range of 5-500 urn. The sedimentation of particles of these sizes is rapid under dilute conditions, which thus makes such dilute sus-pensions unstable. More concentrated systems produce stable suspensions by the formation of a concentrated network, which then only slowly change due to a con-solidation process.
The strength of this particle network is of large tech-nical importance. For example, it gives rise to various rheological characteristics such as the yield values of more or less plastic dispersions, and determines the sensitivity to consolidation. A developed consolidation process in a suspension gives a layer of clear liquid on top of the suspension.
Feed suspensions are usually prepared by dispers-ing a powdered, agglomerated or pelleted material in
water just before feeding to animals. These suspensions display a rapid sedimentation, which is mainly deter-mined by the size of the particles and their effective densities. A rapid sedimentation during the distribution of the feed slurry could commonly lead to problems with the feed composition.
2.2.3 Foams
Foams consist of gases dispersed in a liquid. The nature of such foams varies depending on the situation. Some foams are transient with a short lifetime, for instance a Champagne foam, while other foams are more or less permanent, e.g. the foam formed in bread.
A typical feature of foams is that diffusion between the phases (Ostwald ripening or disproportionation) is of critical importance rapidly eliminating all particles in the colloidal range (3). The density difference between the two phases is always large, and consequently a densely packed layer of comparatively large bubbles is always rapidly formed (typically 90-99% of the dispersed phase) if the viscosity is low (4). The stability of these foams is largely determined by the film rupture characteristics (of the film separating the bubbles) and by film drainage. The surface-active components present in the liquid form two-dimensional layers along the air-water interfaces, display-ing surface rheological properties which contribute to the stability of the film via a range of different mechanisms, as illustrated in Table 2.4.
However, a range of very important food foams are stabilized by a three-dimensional gel through the foam Table 2.2. Characteristics of water-in-oil and oil-in-water food emulsions
Oil-in-water Water-in-oil
Stabilizing components Proteins, hydrophilic surface-active Hydrophobic surface-active lipids, fat lipids crystals
Destabilizing mechanisms Flocculation, coalescence, fat crystals Ostwald ripening, coalescence (partial coalescence)
Particle size Typically 0.5-5.0 \im Typically 5.0-50 um Examples Mayonnaise, cream Butter, spreads
Table 2.3. Properties of food and feed suspensions
Colloidal parameter Rheological properties Consolidation properties Particle size Smaller particles-more connections and Smaller particles-faster consolidation
higher strength
Interparticle interaction Attractive interaction-higher strength Attractive interactions-slower consolidation.
Volume fraction High volume fraction-high strength High volume fraction-slow consolidation Density difference between No effect Larger density difference-faster
continuous and dispersed phases. consolidation
Container height No effect High container-large consolidation stress and faster process
lamellae. Such foams generally have very thick lamellae and a less dispersed phase (typically 50-85% - which corresponds to overruns of 100-500%). The structure of the foam is viscous to solid, and the foam may be permanent.
The stabilizing mechanisms displayed under these two conditions are different, and thus when dealing with food foams one has to recognize the role of the solidifying structure of the foam lamella in many important technical systems.