Role of aqueous phase composition and hydrophilic emulsifier type on the stability of W/O/W emulsions

Double W₁/O/W₂ emulsions can act as fat substitutes in food matrices, although synthetic emulsifiers are commonly used due to their inherent instability and susceptibility to coalescence. In order to guarantee the stability of the W/O interface, the synthetic emulsifier polyglycerol polyricinoleate (PGPR - 4.5% w/w) was used. However, the replacement of chemically synthesized ingredients by natural alternatives has been extensively pursued in food applications. In this sense, whey protein isolate (WPI) and sodium caseinate (SC) were used to stabilize the external aqueous phase (W₂) of water-in-oil-in-water double emulsions (W₁/O/W₂), in addition to Tween 80 that was used as a control. The composition of the internal aqueous phase and its effects on the double emulsion were studied by the addition of sodium chloride (0.2% w/w NaCl), gallic acid (0.5% w/w GA) or a GA/NaCl mixture (0.5% / 0.2% w/w). The effect of these different hydrophilic components was evaluated from measurements of droplet size, viscosity, ζ potential, interfacial tension and kinetic stability. SC-stabilized W/O/W emulsions showed better kinetic stability than WPI- and T80-systems. These results can be attributed to the initial droplet size (SC < T80 < WPI) and viscosity of the emulsions (SC < T80 < WPI). SC is a more flexible and unfolded protein that can quickly adsorb and rearrange at the interface, favoring the formation of smaller droplets and limiting the migration of inner water droplets to the outer phase. In addition to smaller droplets, the addition of SC (8% w/w) to the external aqueous phase promoted an increase in the viscosity of bulk systems, which reduced the destabilization rates by creaming and coalescence. All W/O/W systems containing NaCl in the inner aqueous phase presented greater kinetic stability during 7 days of storage. Although the addition GA was less efficient to stabilize double emulsions compared to NaCl, this phenolic compound reduced the interfacial tension, favoring the formation of WPI- and T80-droplets with smaller diameters. However, the use of GA/NaCl blend improved the stability and functionally of W/O/W double emulsions. We concluded that the type of hydrophilic emulsifier, the properties of the inner water droplets and the viscosity of phases influenced the droplet size, viscosity and kinetic stability of double emulsions. This work provides a better understanding of how composition influences the properties of double emulsions and how it can be used to design W/O/W emulsions as fat substitutes in more complex food systems.