Food dispersions includes emulsions such as milk, cream, sauces, etc. The main characteristic of these foods is the presence of small particles, and the consequent high interfacial area between the particles and the continuous phase. The properties of food colloids are defined by the interactions among the particles.
Food emulsions consist of an oil phase containing hydrophobic compounds and an aqueous phase containing water-soluble compounds. One phase is dispersed into the other, defined as oil-in-water emulsions or water-in-oil emulsions, dependently if water or oil are the continuous phase, respectively. Emulsions are thermodynamically unstable, and phase separation can be deaccelerated or even prevented through kinetic factors. The origin of destabilization is based on gravitational force, attractive and repulsive forces among the particles, etc.
The destabilization can then be seen by creaming, flocculation, and coalescence. In addition to these, emulsion phase inversion and Ostwald ripening are phenomena that can happen in emulsions. Creaming is a phase separation caused by the upward migration of droplets due to density difference between phases. Flocculation is the aggregation of droplets due attractive forces. Coalescence is the merging of droplets.
Stokes Law and phase stability
Even in apparently stable systems, with a shelf life of several years, the number and size of droplets change with time. Stokes’ Law gives the creaming / sedimentation rate for an isolated, rigid, uncharged droplet: U=2/9 R2dρg/η. R stands for the radius, dρ for the density difference, g for gravity and η for viscosity. Creaming may be considered as negligible compared with Brownian motion when U is less than 1 mm/day. Stokes’ Law shows how to prevent or minimize creaming: i) Reduction of droplet size, for instance by the addition of considerable amounts of amphiphiles such as surfactants, or by the use of homogenizers at high operating pressure. ii) Reduction of density differences between the phases. Density difference between the oil phase and water phase is, depending on other factors, about 50 kgm-3. While the density of large droplets is similar to the oil phase, very small droplets have a density closer to that of the aqueous phase. iii) Tuning the viscosity of the continuous phase, by adding polymeric thickeners, for instance gums. iv) in the moon.
Expert in food chemistry. PhD in Physical Chemistry. Professor at University of Coimbra, Portugal, consulting for international and national companies. CEO at DBox Portugal. Professional experience at Procter & Gamble (USA) and BASF (Germany). Head of 32 funded scientific projects in food coatings, nano and micro encapsulation, green chemistry and materials. Principal investigator at the COLLING group of the University of Coimbra. Holder of 19 international awards.
