Molecular size reduction and functional properties of Atlantic salmon waste protein treated by subcritical water

Atlantic salmon waste protein (AsWP) was hydrolyzed using subcritical water hydrolysis at 135-210 °C and 30-60 bar for 20 min, to reduce protein molecular size and produce hydrolysates with functional properties. At 195 °C and 50 bar, hydrolysis yield and degree of hydrolysis were highest at 88.47 % and 22.59 %, respectively. SDS-PAGE and ESI-MS confirmed the reduction in protein molecular size and formation of smaller peptides and amino acids. The mean molecular weight of the hydrolysates decreased with increasing temperature and pressure, reaching a low of 1053 Da at 210 °C and 60 bar. The zeta potential of the control sample was -6.33 mV, decreasing to -13.57 mV under the highest hydrolysis conditions. Particle size was smallest and most homogeneous (2000-4000 nm) at 210 °C and 60 bar. Notable Fourier-transform infrared spectroscopy bands associated with proteolytic reactions included amide I (~1626 cm^-1), amide II (~1511 cm^-1), and amide III (~1388 cm^-1). The relative crystallinity of the hydrolysates ranged from 12.45 % to 58.76 %, with the lowest value at 210 °C and 60 bar. Fluorescence intensity decreased as temperature and pressure increased. The emulsification and functional activities of the hydrolysates indicated their suitability for applications in food, cosmetics, and pharmaceutical industries.