Double Emulsions Relevant to Food Systems: Preparation, Stability, and Applications.

This review describes advances in the preparation of food-relevant double emulsions (DEs) of the water-in-oil-in-water (W/O/W) and oil-in-water-in-oil (O/W/O) types with emphasis on research published within the last decade. The information is assembled and critically evaluated according to the following aspects: the food application area, the range of encapsulated components and emulsion composition, the emulsification preparation methods, the balancing of the osmotic pressure, the stabilization by increased viscosity or gelation, the role of protein-polysaccharide interactions, and the techniques used to estimate DE yield and emulsification efficiency. Particular focus is directed toward the control of encapsulation and release behavior, including strategies that have been employed to improve the retention ability of the inner phase droplets by modifying the outer oil-water interface through mixed ingredient interactions, Pickering stabilization by particles, and biopolymer gelation. We also briefly consider the incorporation of DEs into dried microcapsules and the stability of W/O/W emulsions during eating and digestion. It would appear that 2 outstanding issues are currently preventing full realization of the potential of DEs in food applications: (i) the lack of availability of large-scale production equipment to ensure efficient nondestructive 2nd-stage emulsification, and (ii) the limited range of food-grade ingredients available to successfully replace polyglycerol polyricinoleate as the primary emulsifier in W/O/W formulations.

Three-dimensional molecular mapping of a multiple emulsion by means of CARS microscopy.

Multiple emulsions consisting of water droplets dispersed in an oil phase containing emulsifier which is emulsified in an outer water phase (W/O/W) are of great interest in pharmacology for developing new drugs, in the nutrition sciences for designing functional food, and in biology as model systems for cell organelles such as liposomes. In the food industry multiple emulsions with high sugar content in the aqueous phase can be used for the production of sweets, because the high sugar content prevents deterioration. However, for these emulsions the refractive indexes of oil and aqueous phase are very similar. This seriously impedes the analysis of these emulsions, e.g., for process monitoring, because microscopic techniques based on transmission or reflection do not provide sufficient contrast. We have characterized the inner dispersed phase of concentrated W/O/W emulsions with the same refractive index of the three phases by micro Raman spectroscopy and investigated the composition and molecular distribution in water-oil-water emulsions by means of three-dimensional laser scanning CARS (coherent anti-Stokes Raman scattering) microscopy. CARS microscopy has been used to study water droplets dispersed in oil droplets at different Raman resonances to visualize different molecular species. Water droplets with a diameter of about 700 nm could clearly be visualized. The advantages of CARS microscopy for studying this particular system are emphasized by comparing this microscopic technique with conventional confocal reflection and transmission microscopies.