Structural and functional characterisation of hydrogels prepared from Porphyridium purpureum under acidic conditions

The red microalgae Porphyridium purpureum exhibits exceptional nutritional properties due to rich protein content, extracellular polysaccharides, polyunsaturated fatty acids, vitamins, and minerals. The coloured and bioactive phycobiliproteins make this microalga a valuable source for developing innovative food products. Herein, we developed a simple procedure to induce the formation of coloured hydrogels (POR) from P. purpureum under acidic conditions (pH 2) by inhibiting the repulsion of charged groups and facilitating polysaccharide chain association. We further investigated the effects of adding alginate at a 0.5 % concentration on the gel structure (POR ALG) and techno-functional properties. The resulting vividly coloured hydrogels were characterised in terms of microstructure (via SEM and confocal microscopy), functional groups (FTIR), rheological behaviour, water uptake and water-holding capacity, digestibility, and antioxidant activity. Alginate addition significantly improved the gel consistency (POR ALG), decreased porosity, and increased the storage modulus by one order of magnitude compared to POR gel. Confocal microscopy revealed that alginate inhibited phycobiliprotein agglomeration, reduced fluorescence, and provided more uniform protein distribution. The water uptake capacity was notably higher in POR ALG hydrogel at pH 2, whereas POR hydrogel had the highest capacity at neutral pH. In vitro digestion studies demonstrated that the hydrogels resisted gastric digestion, while bioactive (chromo)peptides are released in the intestinal phase, thereby preserving their antioxidant activity. Lyophilisation emerged as the preferred drying method, maintaining rehydration potential and structural integrity. The developed P. purpureum-based hydrogels demonstrate significant potential as functional food ingredients, offering bioactive benefits, vibrant colour stability, and protection for sensitive molecules during digestion.