## 3 Results and Discussion
### Antioxidant Activities of FBP Hydrolysates  
Antioxidant compounds play an important role in the prevention of free radical-induced tissue damage [48]. Compared with some synthetic compounds, such as butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT), food-derived antioxidant peptides are potential antioxidants with fewer negative side effects [49]. Recently, a series of protein hydrolyses, with in vitro antioxidant activity, has been discovered from fish byproducts, such as yak (_Bos grununiens_) bone [41], _Cyprius carpio_ skin [50], tilapia (_Oreochromis nilotica_ L.) skin [51], salmon (_Salmo salar_) trimmings [52], and rainbow trout (_Oncorhynchus mykiss_) byproducts [53]. Figure 4: Heatmap of FAAs\({}^{\prime}\) composition (mg/100 mL) and major taste components of FBP hydrolyses. SFBP: fish bone protein prepared through steam explosion-assisted extraction; HFBP: fish bone protein prepared through hot-pressure extraction; SFBP-H: SFBP hydrolyses; HFBP-H: HFBP hydrolyses. \({}^{*}\) Bitter AA: calculated from the sum of valine, isoleucine, leucine, phenylalanine, histidine, tryptophan and tyrosine; Sweet AA: calculated from the sum of threonine, serine, glycine and alanine; Umami AA: calculated from the sum of aspartic acid and glutamic acid. DPPH radical scavenging activities are one of the most used methods to evaluate the ability of antioxidants to scavenge free radicals, while an ABTS radical elimination assay measures the antioxidant activity of both hydrophilic and lipophilic compounds [13]. The scavenging effect of bone hydrolyses on DPPH and ABTS radicals is shown in Figure 5. The SFBP samples showed higher scavenging activities for the DPPH and ABTS radicals (42.56% and 59.94%) before enzymatic hydrolysis than that of the HFBP samples (36.24% and 55.71%). This indicated that some peptides with high antioxidant activity were released under hydrothermal conditions, even without enzymatic hydrolysis. Similar phenomena were also found in our previous study about gelatin hydrolyses that were prepared from tilapia (_Oreochromis niloticus_) scale, through hydrothermal pretreatment [17]. As shown in Figure 5a, the DPPH radical scavenging activity of SFBP hydrolyses increased with hydrolysis time, and the same trend was also found in HFBP hydrolyses. The scavenging activities of both the bone hydrolyses reached maximum values of 59.34% and 53.58% at 3 h, respectively. The HFBP hydrolyses presented a lower DPPH value  
Figure 5: Antioxidant activities of SFBP-H and HFBP-H. DPPH radicals scavenging activity (**a**) and ABTS radicals scavenging activity (**c**) of SFBP-H and HFBP-H treated with Flavourzyme for various hydrolysis times (0–3 h); the concentration of all hydrolyses used in the experiments was 6 mg/mL. DPPH radicals scavenging activity (**b**) and ABTS radicals scavenging activity (**d**) of SFBP-H and HFBP-H at various concentrations after hydrolysis for 3 h. SFBP: fish bone protein prepared through steam explosion-assisted extraction; HFBP: fish bone protein prepared through hot-pressure extraction; SFBP-H: SFBP hydrolyses; HFBP-H: HFBP hydrolyses. Different letters indicate that the results of SFBP-H and HFBP-H differ significantly (\(p<0.05\)). than that of the SFBP hydrolysates. All the samples exhibited DPPH radical scavenging activity in a dose-dependent manner (Figure 5b). The IC\({}_{50}\) values of SFBP-H and HFBP-H were 4.24 and 5.33 mg/mL, respectively. The IC\({}_{50}\) values of the protein hydrolysates were comparable to those of the hydrolysates obtained from _Cyprimus carpio haematopterus_ scale gelatin (IC\({}_{50}\) 4.46 and 6.97 mg/mL) [54], but lower than that of the hydrolysates from shark skin gelatin (IC\({}_{50}\) 27.39 mg/mL) [55]. As shown in Figure 5c, the ABTS radical scavenging activity of SFBP-H and HFBP-H also increased with hydrolysis time, during the first 1 h, which finally reached about 90.02%, while it changed insignificantly (\(p>0.05\)) at a longer hydrolysis time.