Enhancing Protein Extraction from Porphyridium cruentum using Ultrasound-Assisted Isoelectric Solubilisation and Precipitation

Abstract This study aimed to optimise a multistep biorefinery process integrating ultrasound-assisted cell wall disruption with sequential pH-driven isoelectric solubilisation/precipitation to maximise compound recovery from Porphyridium cruentum . The optimal conditions for the first extraction were pH 8.0 and 100 g·L −1 , which achieved the highest release of B-phycoerythrin (4.2 g·100 g −1 ). Subsequent alkaline solubilisation (pH 12.0) enabled efficient recovery of non-pigment proteins, with precipitation yields maximised at pH 3.8. Reusing the liquid fraction allowed process sustainability to be enhanced without compromising compound recovery. Although an additional acidic extraction step (pH 2.0–2.5) was evaluated, the protein recovery was limited. The resulting fractions displayed distinct functional properties. The B-phycoerythrin-rich fraction imparted colour effectively to model food matrices, obtaining a minimal colour difference of 3.6 for commercial pink gin. The second protein-rich fraction exhibited a high emulsifying capacity and thermal stability at pH 8.0. The third extracted fraction proved to be rich in a wide variety of macro and micronutrients, and the leftovers from these products proved to have potential as plant biostimulants, increasing the seedling vigour index in cucumber ( Cucumis sativus ) seeds by 54–58% compared to water. This integrated approach demonstrates the potential for complete valorisation of P. cruentum biomass through a resource-efficient, low-energy process aligned with circular bioeconomy principles.