Structural and functional properties of dried fish protein isolates

Dried fish (DF) are rich in protein and widely available worldwide; however, they have long been limited to being used as a traditional food. This study investigated the structural and functional properties of dried fish protein isolates (DFPIs) extracted from seven commonly consumed DF species in Bangladesh, including both sun-dried and fermented varieties. The isolates were prepared via isoelectric precipitation. Sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) analysis revealed that the DFPIs lacked intact muscle protein bands and were primarily composed of peptides of < 70 kDa. Circular dichroism spectroscopy showed extensive protein unfolding and hydrolysis, with only the Ganges River sprat DFPI retaining some ordered tertiary structure. The drying and fermentation processes significantly disrupted the secondary structure, resulting in low α-helix content and high proportions of β-sheets and random coils. Consequently, protein yield during extraction was relatively low, with a maximum of 36%. At neutral pH (7.0), DFPIs exhibited low heat-induced coagulation (maximum 23%) but showed excellent oil-holding capacity (up to 20 g/g), likely due to exposed hydrophobic groups. They also demonstrated good gelation abilities (minimum gelling concentration of 3–7%) and emulsifying properties, with Bombay duck DFPI forming stable emulsions with droplet sizes as small as 2 µm. These findings highlight the potential of DFPIs as functional ingredients in heat-processed food formulations, particularly as heat-stable emulsifiers.