PFG-NMR on Double Emulsions: a Detailed Look into Molecular Processes

Abstract

Single emulsions are of high interest in different fields and therefore well investigated. They provide more advantages when an emulsion is part of another emulsion, as doubleor multiple emulsion. Multiple emulsions, as novel food systems, reduce the amount of fat in food or encapsulate active substances. They are not only of interest for food, but also for pharmaceutical or cosmetic products. As in case of single emulsions, multi-modality and droplet size distribution (DSD) of multiple emulsions are important parameters for product quality and physical properties (e.g. viscosity and shelf life). These parameters are also important to gain insight into mechanisms taking place in the second step of the two-step production of double emulsions, which still is not understood in detail. Currently, common measurement techniques for the determination of DSD and its modality of single emulsions are Laser light scattering, Laser diffraction, Ultra sonic attenuation and PFG-NMR. However, acoustic and optical methods do not work properly or not even at all, when aiming for information about structure properties of the inner emulsions. Additionally, results on the outer emulsion are not reliable anymore when using the optical methods, due to multiple diffraction inside the double emulsion. PFG-NMR is a well established method to determine the droplet size distribution of single emulsions. It is known to work non-destructive with a small workload and a high selectivity. In this context, high selectivity means, that the optical properties of the media do not play a role. Using 1 H-NMR for example, only molecules containing hydrogen can be detected. These molecules can be differentiated by their specific chemical shift in high resolution NMR, or by relaxation filters as applied in low filed NMR. Therefore, the PFG-NMR method is in principle suitable to determine structural parameters like DSD in double emulsions. However, the current restrictions have to be exploited, and new models have to be created to analyze the signal decays measured on double emulsions. Molecular processes like diffusion phenomena, relaxation and spectral properties have to be understood in order to obtain a correct interpretation of the data and quantitative meaningful double emulsion‟s DSD. First published in: Magnetic Resonance in Food Science. An Exciting Future. Ed.: J.-P. Renou. 2011. ISBN 978-1-84973-233-8