*3.2. Isolation and Purification of T. flavidus Collagen*

*T. flavidus* skin (15 kg in dry weight, DW) was incubated in 150 L of 0.1 mol/L sodium bicarbonate for 3 h with continuous stirring and then rinsed with cold water until a neutral pH was reached. The resulting material was hydrolyzed in 120 L of 0.5 mol/L acetic acid for 24 h and then centrifuged at 10,000× *g* for 15 min to remove non-collagenous proteins and pigments. Skin collagen was salted out from the supernatant in the presence of 0.5 mol/L NaCl, followed by 20 min of 10,000× *g* centrifugation. The resulting precipitate was dissolved in 0.5 mol/L acetic acid to produce 100 L of crude collagen solution, which was then purified by electrodialysis. The samples were loaded to a DSA-Π electrodialyzer (Jiangsu Ritai Environmental Protection Engineering Co., Ltd., Yangzhou, China) equipped with both polyethylene cation-exchange membrane (361BW, Jiangsu Ritai Environmental Protection Engineering Co., Ltd., China) and polyethylene anion-exchange membrane (362BW, Jiangsu Ritai Environmental Protection Engineering Co., Ltd., China). The process conditions of electrodialysis were as follows: feed compartment, 150 L of 3% NaCl (*w*/*v*); concentrate compartment, 150 L of distilled water; electric potential, 80 V/cm; flow rate, ≤1 m3/h; electrodialysis duration, 2 h. The purification products were lyophilized and stored at −20 ◦C until analysis. Isolation and purification were all performed at 4 ◦C to prevent microbial growth (Figure 6). The extraction yield was calculated based on the following equation.

$$\text{Yield (g/100 g DW)} = \text{Mass of hydrochloride colllager (g)} / \text{Mass of dry fish skin (100 g)}\tag{1}$$

**Figure 6.** Isolation and purification of collagen from *T. flavidus* skin by electrodialysis.

#### *3.3. Quality Measurements*
