RESUMEN
Droplet collision is shown to be important in the propagation of nucleation through supercooled oil-in-water emulsions by the use of a novel membrane technique. On the other hand, nucleation mediated by Tween 20 micelles is shown to be of relatively much less importance in both n-hexadecane and cocoa butter oil-in-water emulsions. The droplet collision phenomenon probably occurs via a surfactant bridge between the colliding droplets. When this process is taken into account we show that the Turnbull model for crystal nucleation kinetics explains very well nucleation in cocoa butter oil-in-water emulsions by seed crystals. On the basis of this model we characterized the seed crystals in cocoa butter via isothermal crystallization studies at 14.2, 15.0, 15.5, and 15.8 degrees C. We suggest that there are few seed crystals whose size exceeds 0.28 µm at 80 degrees C. In our cocoa butter samples there were between 10(16) and 10(17) seed crystals m(-3) whose average size we inferred to be less than 0.09 µm. A value of 0.133 mJ m(-2) is obtained for the Gibbs free energy of the nucleating surface in our West African cocoa butter samples. There is evidence that the alpha-polymorph of POS comprises the nucleating layer in the seed crystal. There is no evidence that surfactant influences the primary nucleation of oil crystals. However, the surfactant has a big effect on the kinetics of the secondary nucleation process, mediated by droplet collision. Copyright 2000 Academic Press.
RESUMEN
Ultrasound is a technique that is capable of tremendous accuracy and precision, whether in the laboratory or on-line in the factory. Data acquisition rates of the order of kHz transform process monitoring, providing unprecedented information of high statistical value. Ultrasound is complementary to other measurement modalities, probing the elastic and thermal properties of materials. In the absence of air bubbles, ultrasound travels through concentrated systems of particles, providing information about size, compressibility, structure, distribution, chemical and phase state. It is safe, can be non-invasive and can be economical. In these respects, it is unique.
RESUMEN
The creaming and rheology of oil-in-water emulsions (30 vol% n-tetradecane, pH 6.8) stabilized by a mixture of commercial sodium caseinate and the non-ionic emulsifier polyoxyethylene sorbitan monolaurate (Tween 20) has been investigated at 21 degrees C. The presence of sufficient Tween 20 to displace most of the protein from the emulsion droplet surface leads to greatly enhanced emulsion creaming (and strongly non-Newtonian rheology) which is indicative of depletion flocculation by nonadsorbed surface-active material (protein and emulsifier). In emulsions containing a constant amount of surface-active material, the replacement of a very small fraction of Tween 20 by caseinate in a stable pure Tween 20 emulsion leads to enhanced creaming for a small fraction of the droplets, and this fraction increases with increasing replacement of emulsifier by protein. This behavior is probably due to depletion flocculation, although an alternative bridging mechanism is also a possibility. The overall stability of these sets of emulsions can be represented in terms of a global stability diagram containing regions of bridging flocculation and coalescence (low content of surface-active material), stability (intermediate content), and depletion flocculation (high content). Copyright 1999 Academic Press.