Stability of epigallocatechin gallate and quercetin co‐loaded water‐in‐oil‐in‐water double emulsion: effects of whey protein isolate–flaxseed gum conjugate

BACKGROUND The practical applications of water‐in‐oil‐in‐water (W/O/W) double emulsions are limited by their instability. Whey protein isolate‐flaxseed gum (WPI–FSG) conjugates have demonstrated excellent emulsifying capabilities in previous studies; however, their effectiveness in stabilizing double emulsions remains largely unexplored. To address this knowledge gap, a W/O/W double emulsion co‐loaded with epigallocatechin gallate (EGCG) and quercetin was developed using a WPI–FSG conjugate as the stabilizer, and its stability was evaluated systematically under thermal, pH, and ionic strength stresses. RESULTS A W 1 /O/W 2 double emulsion co‐loaded with hydrophobic quercetin and hydrophilic EGCG was stabilized by incorporating WPI–FSG conjugates into both aqueous phases simultaneously. The addition of 10 g L −1 conjugate to the inner aqueous phase (W1) reduced the primary emulsion droplet size by 58.3%. Increasing the conjugate concentration to 50 g L −1 in the outer aqueous phase (W2) raised the encapsulation efficiency of quercetin to 74.14% and that of EGCG to 96.15%. Bioaccessibility increased by 0.6‐fold for quercetin and by 4.86‐fold for EGCG. The use of the WPI–FSG conjugate resulted in a higher encapsulation rate and greater reagent loading than the addition of the WPI–FSG mixture under ambient conditions (pH 4.0–10.0, NaCl concentration = 0–200 mM; storage temperature = 4–40 °C for 7 days; heating temperature = 25–100 °C). CONCLUSION The WPI–FSG conjugates showed significant potential for improving the stability and co‐delivery performance of W 1 /O/W 2 double emulsions. Their simultaneous use in the internal and external aqueous phases provides a robust strategy for encapsulating hydrophilic and hydrophobic bioactives, offering valuable insights for the development of functional food and pharmaceutical formulations. © 2026 Society of Chemical Industry.