Ultrasound-assisted preparation of double nano-emulsions loaded with glycyrrhizic acid in the internal aqueous phase and skim milk as the external aqueous phase

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In this study, response surface methodology (RSM) based on central composite rotatable design applied to encapsulate glycyrrhizic acid (GA), within the ultrasound-assisted double emulsions. Three independent variables considered as the 10–30% weight of the internal aqueous phase (W1) in the primary-emulsion (W1/O), 6–10% of polyglycerol-polyricinoleate (PGPR) as the surfactant in the oil phase, and 4–6 min sonication-time; dependent variables for optimal W1/O emulsion fabrication were droplet-size, poly-dispersity-index (PDI), and sedimentation-index. Then, 5–25% wt. of the W1/O emulsion dispersed into the skim milk as an external aqueous phase (W2) to form double-emulsions (W1/O/W2) through sonication for 1–5 min. The lowest droplet-size and sedimentation-index obtained for the W1/O emulsions formulated by 19% W1 in the W1/O emulsion, 6% PGPR and a sonication time of 6 min. Two optimal-conditions for preparation of double-emulsions were determined based on the results of encapsulation efficiency, droplet-size, and viscosity as follows: the first optimum treatment (15% W1/O and 3 min sonication) and the second optimal conditions (24% W1/O and 4 min sonication). The encapsulation efficiency of GA in these double-emulsions was 92%. The emulsions stored for 7 days at 37 °C showed instability (2-phase separation). In addition, droplet-size and encapsulation-efficiency increased and decreased during storage, respectively.