Characterization of weakened Haematococcus pluvialis encapsulated in alginate-based hydrogel

Haematococcus pluvialis (Hp), a freshwater chlorophyte microalga, is a major natural source of astaxanthin (ASX), a potent antioxidant with anti-inflammatory, anticarcinogenic and muscle pigmentation properties. However, ASX bioavailability is limited by the rigid cyst wall and, although cell wall rupture improves bioavailability, the free form is unstable under high temperatures, pH extremes, light or oxygen. Encapsulation techniques improve ASX stability, making it suitable for functional foods and aquaculture, especially in salmonid feeds where natural pigments are preferred. The present study evaluates the stability of weakened Hp (Hp_w) biomass encapsulated in alginate (ALG) via ionic gelation.

Encapsulation utilizing ALG achieved high efficiency (97 ± 2.63%) and loading capacity (32 ± 0.90%), confirming its suitability as a wall material. ALG-Hp_w hydrogels displayed significant color intensity, enhancing potential feed or food hues. Low bulk density (0.59 ± 0.01 g cm^-3), moisture content (11.97 ± 0.20%) and water activity (0.28 ± 0.00) suggest minimized oxidation processes. Hydrogels measured 1.30 ± 0.06 mm with a uniform sphericity factor of 0.058 ± 0.03. Confocal laser scanning microscopy confirmed uniform Hp_w distribution and scanning electron microscopy revealed fissure-free surfaces, ensuring minimal permeability. DPPH (i.e. 2,2-diphenyl-1-picrylhydrazyl) scavenging activity was similar between Hp_w extract (38.32 ± 2.30% to 96.32 ± 0.88%) and ALG- Hp_w hydrogels (33.20 ± 1.55% to 93.30 ± 0.44%). ALG Increased Hp_w decomposition temperature by 40.97 °C. Encapsulation of Hp_w in ALG significantly enhanced the bioaccessibility of ASX. The ALG-based encapsulation effectively preserved ASX stability, retaining over 90% of its content under storage conditions.

ALG is a suitable biopolymer for encapsulating Hp_w, preserving antioxidant activity, and enhancing thermal properties, making it valuable for broader applications. © 2025 Society of Chemical Industry.