Alkaline extraction of collagen and calcium phosphates from fish bones: influence of source and extraction conditions

Calcium phosphates and collagen are compounds with valuable functional properties and applications in sectors such as cosmetics and medicine. Both are present in fish bones, a by-product of the food industry, which can be recovered. In this work, we report an alkaline extraction process based on NH4(HCO3) solution that enables the simultaneous recovery of collagen and calcium phosphates. Extractions were performed at 4o C to preserve the collagen structure, and the addition of a protease inhibitor cocktail was also considered. Bones from two fishes – salmon (Salmo salar) and tuna (Thunnus thynnus) – were processed to assess the effect of species on the composition and characteristics of the extracts. Results showed that single-phase hydroxyapatite (Ca10(PO4)(OH)2, HAp) was obtained from tuna bones (yield >50 %, crystallinity 64–77 %), whereas biphasic material consisting of HAp and β-tricalcium phosphate (β-Ca3(PO4)2) was obtained from salmon bones (yield about 40–45 %, crystallinity 30 %). Regarding the protein fraction, extracts from salmon bones (yield around 2 %) were those with the best properties, as they retained part of the supramolecular structure of the native collagen (i.e. inter-helical lateral packing). These extracts also exhibited antioxidant activity (IC50 = 0.16 mg/mL) and biocompatibility due to the presence of different collagen peptides. Indeed, cell growth assays performed with fibroblasts and osteoblast-like lines showed viability consistently above (130 % compared to the control). Overall, these findings demonstrate that this alkaline extraction process provides an effective valorisation strategy for fish bones, yielding compounds with promising functional properties.