Food as soft matter
A fantastic article about the soft matter aspects of food was recently written by Raffaele Mezzenga and his swiss colleagues in Nature Materials. The figure below concisely summarizes how many phenomena in the food sciences are the result of colloid interactions, with the example of casein proteins. These proteins can be treated as hard spheres or as the structural elements of a continuous network, depending on the interparticle interactions between them:
The top central figure shows the interaction energy (U/kT) as a function of volume fraction. At low volume fractions, but strong interactions between particles, fractal networks form. In the opposite case of high volume fractions with low interaction energies, dense suspensions are created. Three specific cases are illustrated around the central figure: A. Casein micelles in solution can be treated as hard spheres. The viscosity of the suspension increases until the system reaches a fully jammed state. The photo shows how the mixture can be turned upside down without separating. B. When a polysaccharide polymer is added to the system, various phases are possible through spinodal decomposition. Depending on the relative concentration, the system can form: (1) Xantham-rich droplets, (2) casein-rich droplets, or (3) a bicontinuous phase. C, D. The figures at the top show the similarities between ceramic materials (C) and casein networks in yogurt (D).
You know how it says homogenized on every container of milk and as a kid you never knew what it was. It essentially changes the size of solids (colloidal particles) of fat and proteins in solution. By decreasing the size of the particles it creates a more stable dispersion. This is an emulsion rather than a colloidal suspension but there are ions within the micelles. Overall a very complex system.
"Milk is an oil-in-water emulsion, with the fat globules dispersed in a continuous skim milk phase. If raw milk were left to stand, however, the fat would rise and form a cream layer. Homogenization is a mechanical treatment of the fat globules in milk brought about by passing milk under high pressure through a tiny orifice, which results in a decrease in the average diameter and an increase in number and surface area, of the fat globules. The net result, from a practical view, is a much reduced tendency for creaming of fat globules. Three factors contribute to this enhanced stability of homogenized milk: a decrease in the mean diameter of the fat globules (a factor in Stokes Law), a decrease in the size distribution of the fat globules (causing the speed of rise to be similar for the majority of globules such that they don't tend to cluster during creaming), and an increase in density of the globules (bringing them closer to the continuous phase) owing to the adsorption of a protein membrane. In addition, heat pasteurization breaks down the cryo-globulin complex, which tends to cluster fat globules causing them to rise."
Cool movie of homogenization valve: