Development of stable liquid water-in-oil emulsions by modifying emulsifier-aqueous phase interactions

Abstract

This research investigated the stabilization mechanism of liquid water-in-oil (W/O) emulsions by modifying emulsifiers, aqueous and continuous phase compositions, and interactions. At first, liquid W/O emulsions were developed to resist multiple thermal cycles for their application in droplet digital polymerase chain reaction (ddPCR), which significantly improved the detection limit of conventional PCR. Thermally stable coarse W/O emulsions as DNA microreactors were developed with polyglycerol polyricinoleate (PGPR) and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) as emulsifiers dissolved in a mixture of light and heavy mineral oil, with a range of viscosities. Coarse emulsions, formed by vortex mixing, were subjected to PCR thermal-cycling, after which AOT-stabilized water droplets remained stable; however, PGPR-stabilized water droplets size significantly increased. Higher AOT molecular packing at the interface was proposed as the mechanism of thermal stability. Next, the (de)-stabilization mechanism of glycerol monooleate (GMO)-stabilized liquid W/O emulsions in mineral (MO) and canola oil (CO) was investigated. It was hypothesized that hydroxyl group donating agents in the aqueous phase would prevent GMO's desorption from the oil-water interface by forming stronger hydrogen bonding. W/O emulsions with 20% aqueous phase were formed by high-pressure homogenizer. Of the three agents, emulsions with low methoxyl pectin (LMP) showed the highest stability in both oils after 7-day storage compared to citric or ascorbic acid with or without sodium chloride. Water and GMO melting behaviour, determined by differential scanning calorimetry, and intermolecular interaction by Fourier transform infrared spectroscopy revealed stronger H-bonding between GMO and LMP, thereby improving emulsion stability. Finally, the viscoelastic behaviour of GMO-stabilized W/O emulsion was improved such that an elastic gel could be formed by increasing the water content (20 to 50 wt%) and incorporating specific ingredients in the continuous and dispersed phases for application in food-grade low-fat tablespreads. Fully hydrogenated soybean oil was incorporated in CO to create a fat crystal network in the continuous phase. Emulsions with LMP in the aqueous phase exhibited self-supported structure without phase-separation while control emulsions, without LMP, showed flow or water separation. All emulsions exhibited strong gel-like properties; however, control emulsions showed structure breakup after 30-day storage. Overall, the studies in liquid W/O emulsions were the basis to improve our understanding of the molecular assembly and interactions at the W-O interface, which subsequently supported the research to enhance emulsion elasticity.