Characterization of oil droplet size distributions in spray-dried emulsions by PFG-NMR

Single emulsions are liquid mixtures, containing a lipophilic and a hydrophilic phase. In general, one of the phases is dispersed in the other one. Therefore, there are two main types of single emulsions, namely water in oil (W/O) and oil in water (O/W) emulsions.
Due to their structure, emulsions possess a wide range of applications e.g. in pharmaceutical, cosmetic and food industry. Especially in the dairy industry O/W emulsions can be found as basic or end products. In order to obtain storable and convenient products a commonly used process step is the production of instant products (e.g. milk powder) by spray-drying of O/W emulsions (e.g. milk). These spray-dried powders can be defined as "solid emulsions". Moreover, in the case of sprayed-dried milk-powder this "solid emulsion" possesses another outstanding property: The melting temperature of the dispersed phase is above the typical storage temperature.
Important parameters to characterize these products are geometric properties and size distribution of the oil droplets. It is hardly possible to gain information about these parameters using state-of-the art-analysis, like laser diffraction, as this cannot be applied due to the solid state of the continuous phase.
It is well-known that the use of Pulsed Field Gradient-Nuclear Magnetic Resonance (PFG-NMR) offers the possibility to determine the droplet size distribution of emulsions. Mathematical algorithms are used to correlate the detected decay of local diffusion of the oil molecules to the droplet size distribution.
In the present investigations the physical approach of Murday and Cotts is used for modeling the decay of the measured signal in a variety of solid milk powders. As the self diffusion of the oil enters the equations, the dependence of the diffusion coefficient on the measurement temperature was investigated to determine the optimum measurement temperature for the expected range of droplet sizes.