Ultrasound-Assisted Fucoidan Double Emulsions in Yogurt: In Vitro Gastrointestinal Digestion and Lactic Acid Bacteria Viability

Background: Fucoidan is a sulphated polysaccharide from brown seaweed with reported antioxidant activity, but its high molecular weight and hydrophilicity limit its incorporation into food matrices. Method: To address this, fucoidan was encapsulated in a water-in-oil-in-water (W1/O/W2) double emulsion (FuD) prepared by ultrasound, characterized, and added to yogurt at 3.0% and 5.0% (v/v). Its performance was then evaluated during simulated gastrointestinal digestion and 21-day cold storage. Results: FuD had a mean particle size of 119 nm at Day 0, an apparent encapsulation efficiency (Folin–Ciocâlteu [FC]-reactive material basis) of about 80%, and DPPH radical scavenging activity of about 87%. Adding FuD to yogurt significantly improved viscosity and FC-reactive (phenolic-associated) content compared with plain yogurt. During simulated digestion, FC-reactive material in FuD and fortified yogurt changed through the gastric and intestinal phases; because the FC assay is not specific to fucoidan and is confounded by protein-derived reducing residues released during proteolysis, these changes should not be interpreted as fucoidan release or bioaccessibility without further fucoidan-specific validation. Over 21 days of storage, fucoidan-fortified yogurts maintained lactic acid bacteria (LAB) counts above 6.0 log CFU/g, showed lower yeast/mold counts than the control (3.20 vs. 4.20 log CFU/g), and had no detectable coliforms. Conclusions: Under the conditions tested, the fucoidan-loaded double emulsion was incorporated into set yogurt at up to 5.0% (v/v) the milk coagulated within 6 h at 42 °C in every formulation, and the fortified yogurts showed higher apparent viscosity, higher FC-reactive content, higher LAB counts and lower yeast and mold counts than the plain yogurt control over 21 days. No lipid oxidation assay was performed, and no fucoidan-specific assay was applied; claims of oxidative stability, fucoidan bioaccessibility or probiotic functionality are therefore not supported by the present data.