Phlorotannin–Alginate Extract from Nizimuddinia zanardinii for Melanosis Inhibition and Quality Preservation of Pacific White Shrimp

Phlorotannin–alginate extracts from brown seaweeds offer promising natural solutions for food preservation. This study investigated the extraction, characterization, and application of phlorotannins and alginate from two brown seaweed species, Sargassum cristaefolium and Nizimuddinia zanardinii, for inhibiting melanosis and preserving quality in Pacific white shrimp during ice storage. Preliminary screening identified N. zanardinii methanol extract as superior, yielding the highest phlorotannin content (19.14 ± 0.65 mg Phloroglucinol/g) with potent antioxidant (98.95 ± 0.74% DPPH inhibition) and copper-chelating (73.44 ± 1.64%) activities. Consequently, N. zanardinii was selected for subsequent extraction and application studies. Alginate extraction efficiency was 4.73 ± 0.38 g/100 g seaweed, demonstrating moderate antioxidant properties. The extracts effectively inhibited shrimp polyphenol oxidase, with 2% phlorotannins + 1% alginate showing 84.51% inhibition. When applied to shrimp, this combination significantly delayed melanosis development, suppressed microbial growth, and maintained lower pH, total volatile basic nitrogen (TVB-N), and lipid oxidation values during 16 days of ice storage compared to untreated controls. Sensory evaluation confirmed better retention of quality attributes in treated shrimp. These findings demonstrate the potential of N. zanardinii phlorotannin–alginate extracts as effective natural preservatives for maintaining shrimp quality during cold storage, offering a sustainable alternative to synthetic additives in seafood processing.