*Article* **Optimization of Extraction Conditions and Characterization of Pepsin-Solubilised Collagen from Skin of Giant Croaker (***Nibea japonica***)**

**Fangmiao Yu, Chuhong Zong †, Shujie Jin †, Jiawen Zheng, Nan Chen, Ju Huang, Yan Chen, Fangfang Huang, Zuisu Yang, Yunping Tang \* and Guofang Ding**

Zhejiang Provincial Engineering Technology Research Center of Marine Biomedical Products, School of Food and Pharmacy, Zhejiang Ocean University, Zhoushan 316022, China; fmyu@zjou.edu.cn (F.Y.); zongchuhong1997@163.com (C.Z.); m18868006087@163.com (S.J.); jwzheng1996@163.com (J.Z.); chennan\_0576@163.com (N.C.); qiuqiu20130621@163.com (J.H.); cyancy@zjou.edu.cn (Y.C.); gracegang@126.com (F.H.); abc1967@126.com (Z.Y.); dinggf2007@163.com (G.D.)

**\*** Correspondence: tangyunping1985@163.com; Tel.: +86-580-229-9809 † The authors contributed equally to this study and share first authorship.

Received: 27 December 2017; Accepted: 10 January 2018; Published: 14 January 2018

**Abstract:** In the present study, response surface methodology was performed to investigate the effects of extraction parameters on pepsin-solubilised collagen (PSC) from the skin of the giant croaker *Nibea japonica*. The optimum extraction conditions of PSC were as follows: concentration of pepsin was 1389 U/g, solid-liquid ratio was 1:57 and hydrolysis time was 8.67 h. Under these conditions, the extraction yield of PSC was up to 84.85%, which is well agreement with the predict value of 85.03%. The PSC from *Nibea japonica* skin was then characterized as type I collagen by using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The fourier transforms infrared spetroscopy (FTIR) analysis revealed that PSC maintains its triple-helical structure by the hydrogen bond. All PSCs were soluble in the pH range of 1.0–4.0 and decreases in solubility were observed at neutral or alkaline conditions. All PSCs had a decrease in solubility in the presence of sodium chloride, especially with a concentration above 2%. So, the *Nibea japonica* skin could serve as another potential source of collagen.

**Keywords:** marine collagen; *Nibea japonica*; response surface methodology; optimization; characterization
