R. Bernewitz

NMR on emulsions: characterisation of liquid dispersed systems†

Pulsed field gradient NMR (PFG‐NMR) is an important method for the characterisation of emulsions. Apart from its application in quality control and process development, especially high‐field NMR methods can be applied to investigate emulsions properties on the molecular level. Meanwhile, complex emulsion structures such as double emulsions have been developed and require analytical tools especially for the determination of droplet size distributions. This contribution provides an overview on the possibilities and methods of PFG‐NMR referring to measurement, data processing and interpretation of droplet size distributions. Comparison of techniques and measurements on double emulsions are presented. Copyright

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

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

Imaging of Double Emulsions

Double emulsions are dispersed liquid systems which provide a variety of applications for example in fat reduction and encapsulation of active agents. Both double emulsion types, water in oil in water (WOW) and oil in water in oil (OWO), are relevant for foods. From the point of view of characterisation, they challenge the measuring techniques classically known and established for dispersed systems. Alternative techniques as diverse imaging technologies and the corresponding data processing strategies are increasingly important in emulsion science. X-rays, electronic beams and nuclear magnetic resonance are major imaging tools which differ in terms of energy scales as well as beam length and time scales that further have an impact on the structural information. Imaging techniques provide a manifold of possibilities but also need an in-depth and thorough data treatment and interpretation. This chapter gives an overview of imaging techniques and data processing currently applied on double emulsions. The focus is, however, not on the technical details but rather on the question of information which can be obtained from the corresponding techniques. Possibilities and limits are discussed based on the physical principles and known applications. Currently applied data analysis strategies are presented, which leads in the last section to the discussion about the information which can be extracted from the data regarding structure, chemical composition and time-dependent processes in double emulsions.

Structures in Food: Possibilities of Imaging and Diffusometry

Apart from chemical composition, structure is an important parameter which influences not only food processing schemes, but also product characteristics as taste/mouth feeling and finally shelf life. Structure analytics should be non-destructive and non-invasive, but also capable to determine structure of fluids as well as solids over several orders of magnitude of length and time scales. NMR imaging has been applied for almost all foods, and structural parameters as size distributions have been determined in wet and dry foods. As relaxation properties of foods vary, a variety of imaging techniques were explored. They range from single point imaging for short T2-materials over conventional spin and gradient echo methods to spectroscopic imaging. The limiting factors are voxel size (minimally 15 µm×15 µm×15 µm), with respect to the accessible length scale, and the measurement time with respect to the time resolution of temporally changing products. Below the length scale of imaging, down to about 1 µm, diffusion properties can be exploited in liquid foods. The most prominent example is the measurement of size distribution functions of emulsions. Whereas this approach is already rather old, it is still further developed and applied on various emulsion types. An overview over imaging and diffusion methods is given with respect to their advantages and limitations.

Double Emulsion Character with PFG-NMR- Methods: WOW and OWO

Double emulsions are of high scientific interest, especially in the field of functional foods. In a first approach, these dispersed systems are described by structural parameters, chemical composition and diffusion parameters. Progress has been made in the characterisation of double emulsions by means of diverse NMR methods with respect to droplet size distribution (DSD), disperse phase ratios (DPR) and molecular exchange (ME) of WOW- and OWO-double emulsions. However, both double emulsion types exhibit different NMR-properties, such that the PFG-NMR methods have to be adapted for the special needs of these systems. Different NMR sequences and data processing steps are compared for OWO and WOW double emulsions. Methods to determine DSD, DPR and ME – relevant for description und understanding of release – are presented and compared with findings from alternative measuring techniques. In this work, diverse NMR methods were summarised to characterise both double emulsion types, WOW and OWO. PFG-NMR is a powerful tool to investigate the structural parameters of double emulsions. The inner DSD and DPR can be determined after the production. NMR relaxometry, using paramagnetic relaxation agents, delivers information about diffusion mechanisms on the molecular scale, which reveals deep insights into molecular diffusion phenomena.