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Bone hydrolysates from bighead carp (Aristichthys nobilis) were prepared using Protamex and Alcalase with degrees of hydrolysis (DH) of 5%, 10% and 15%. The antioxidant activity of bone hydrolysates was evaluated in vitro and then the hydrolysates with better antioxidant activity were used to immerse bighead carp fillets through a vacuum impregnation process at concentrations of 1% and 2%. Among the six hydrolysates, fish bone hydrolyzed with Protamex at DH 10% exhibited the highest ability to scavenge 1, 1-diphenyl-2-picrylhydrazyl (DPPH) (88.79%), 2, 2′-azino-bis-3-ethylbenzthiazoline-6-sulphonic acid (ABTS) (57.76%) and hydroxyl radicals (62.72%), as well as to chelate ferrous ions (91.46%). The hydrolysates effectively postponed freezing- and thawing-induced protein/lipid oxidation. Compared with the fillets without treatment, the impregnated fillets had higher sulfhydryl contents, greater Ca2+-ATPase activity, lower carbonyls and lower thiobarbituric acid-reactive substances (TBARS). Bone hydrolysates also have a positive effect on the texture and water-holding ability of freeze-thawed fish fillets. Fish bone hydrolysates of Protamex could serve as potential antioxidants to preserve fish fillets.

This study aimed to isolate and characterize pepsin-solubilized collagen (PSC) from marine and freshwater fish swim bladders. The physicochemical properties, protein pattern, amino acid composition, structure, thermal denaturation temperature, and antioxidant activity of PSC from four different swim bladder sources were investigated and compared. The results demonstrated that the four types of collagen extracted were all type I collagen. The yield of PSC extracted from grass carp (GCSB-PSC), bighead carp (BCSB-PSC), grouper (GSB-PSC), and monkfish swim bladders (MSB-PSC) were 38.98, 27.97, 18.16, and 10.35%, respectively. Compared to the other three PSCs, BCSB-PSC has the highest thermal denaturation temperature (38.60 °C). Based on FTIR spectroscopy and circular dichroism (CD) analysis, the extracted PSCs retained the triple helix and secondary structure well. Antioxidant studies showed that in the swim bladders of four species the swim bladder PSC could scavenge DPPH and ABTS radicals. Overall, swim bladders from marine and freshwater fish can be utilized as raw materials for collagen extraction, and the extracted collagen has potential commercial applications.

The development of an efficient pretreatment, prior to enzymatic hydrolysis, is a good strategy for the sustainable use of refractory fish byproducts. This study compared hydrothermal pretreatments at 159 °C for 2 min, followed by water extraction (steam explosion-assisted extraction, SE) and 121 °C for 70 min (hot-pressure extraction, HPE), for the recovery of proteins from fish backbones. The effect of enzymatic hydrolysis on the properties of the obtained fish bone protein (FBP) was also evaluated. The results demonstrated that FBP had high contents of protein (81.09–84.88 g/100 g) and hydroxyproline (70–82 residues/1000 residues). After hydrolysis with Flavourzyme, for 3 h, the FBP hydrolysates that were pretreated with SE (SFBP-H) exhibited a better degree of hydrolysis (DH) and nitrogen recovery (NR), and a higher level of umami taste free amino acids (151.50 mg/100 mL), compared with the HPE-treated samples. The obtained SFBP-H mainly distributed below 3000 Da and had strong scavenging effects on 1,1-diphenyl-2-picrylhydrazy (DPPH) (IC50 = 4.24 mg/mL) and 2,2-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) (IC50 = 1.93 mg/mL) radicals. Steam explosion-assisted extraction is a promising route for recovering proteins from native fish bone materials, and improving the flavor and antioxidant activity of the hydrolysates.

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