Neutral protease

A sample swim bladder of Acipenser schrencki weighing 1000 g (frozen) was supplied by Hubei Qingjiang Sturgeon Valley Special Fisheries Co. (Yichang, China). Alkaline protease (200 U/L) was provided by Wuxi UPG Bio-technology Co., Ltd. (Nanjing, China). Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) supplied papain (6000 U/mg). Neutral protease (50 U/mg), nitro blue tetrazolium (NBT), b-nicotinamide adenine dinucleotide (NADH), 2,2-diphenyl-1-picrylhydrazyl (DPPH), potassium ferrocyanide, phenazine methosulfate (PMS), and trifluoroacetic acid (TFA) were provided by Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). All reagents and chemicals were of analytical grade.

The raw material used for fish oil extraction was whole dotted gizzard shad (Konosirus punctatus), which was purchased and stored at −20 °C until use. Alkaline protease with a potency of 200,000 U/g was obtained from Shenzhen Kangchu Yuan Co., Ltd. (Shenzhen, China). Trypsin (250,000 U/g) and neutral protease (50,000 U/g) were procured from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). All other chemicals utilized in this experiment were analytically pure and purchased from Tianjin Komeo Chemical Reagents Co. (Tianjin, China), Beijing Tongguang Fine Chemical Co. (Beijing, China), and Tianjin Dongli District Tianda Chemical Reagent Factory (Tianjin, China).

Silver carp scales were supported by Qianjiang Changui Aquatic Food Co., Ltd. (Qianjiang, China). Pepsin (1200 U/g), H₂O₂ and FeSO₄·7H₂O were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Alkaline protease (200 U/μL) was provided by Wuxi Youpuke Biotechnology Co., Ltd. (Wuxi, China). Neutral protease (50 U/mg), reduced glutathione, trichloroacetic acid (TCA), L-tyrosine, tyrosinase, DPPH and other reagents were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Commercial tilapia SCPs (CAPs) (MW = 600–1500 Da) were purchased from Hainan Huayan Collagen Technology Co., Ltd. (Hainan, China). All reagents were analytically pure.