Most foods are dispersed systems. A few homogenous solution, like coking oil and some drinks, but even beer – as consumed - has a foam layer.
The properties of a dispersed system cannot be fully derive from its chemical composition, since they also depend on physical structure.
Manufactured foods, as well as some natural foods, may have a somewhat simpler structure: Beer foam is a solution containing gas bubbles; milk is a solution containing fat droplets and protein aggregates (casein micelles); plastic fats consists of an oil containing aggregated triacylglycerol crystals; a salad dressing may be just an emulsion; several; gels consist of a network of polysaccharide molecules that immobilized a solution.
But other manufactured foods are structurally complicated in that they contain several different structural elements of widely varying size and state of aggregation: filled gels, gelled foams, material obtained by extrusion or spinning, powders, margarine, dough, bread and so forth.
The existence of a dispersed state has some important consequences:
- Since different components are in different compartments, there is no thermodynamic equilibrium. To be sure, even a homogenous food may not be in equilibrium, but for dispersed systems this is a much more important aspect.
- Flavor components may be in separate compartments, which may slow down their release during eating. Probably more important compartmentalization of flavor components may lead to fluctuation in flavor release during eating, thereby enhancing flavor because it offsets to some extent adaptation of the senses to flavor components. Most ‘compartmentalization’ food taste quite different from the same food that has been homogenized before eating.
- If, as is often the case, attractive forces act between structural elements. The system has a certain consistency, which is denied as its resistance against permanent deformation. This may be an important functional property as it is related to attributes such as stand-up, spreadability, or ease of cutting. Moreover, consistency affects mouth feel, as does any physical inhomogeneity of the food; food scientist often lump these properties under the word texture.
- If the products has significant consistency, any solvent present - most foods, water - will be immobilized against bulk flow. Transport of mass (and mostly of heat also) then has to to occur by diffusion rather than convection. This may have a considerable effect on reaction rates.
- The visual appearance of the system may be greatly affected. This due to the scattering of light by structural elements, provided they are larger than about 50 nm. Large inhomogeities are visible as such and give rise to what is the dictionary meaning of texture.
- Since the system is physically inhomogeneous, at least at a microscopic scale, it may be physically unstable. Several kinds of change may occur during storage, which may be perceived as the development of macroscopic inhomogeneity, such as separation into layers.
