3.5.3. Water Solubility

The film water solubility was tested using the procedure of Gómez Estaca, et al. [86] with slight modification. Film sections of 1 × 1 cm2 (*<sup>n</sup>* = 6, in each case) were placed in an oven at 105 ◦<sup>C</sup> until constant weights were reached (Wi). The films were then immersed in water for 24 h at room temperature with gentle shaken. After filtration, the residual films were dried again at 105 ◦C for 24 h (Wf). The solubility of the films was calculated as:

$$\text{Solubility (\%)} = \frac{\text{Wi} - \text{Wf}}{\text{Wi}} \times 100\%$$

where, Wi = Initial weight of the film, Wf = Weight of the un-dissolved dried films residue.

#### 3.5.4. Light Barrier Properties

Using an UV-Visible Spectrophotometer (LLG Labware, model unispec, Meckenheim, Germany), the light barrier properties of the different polymer films were measured by exposing them to light absorption at wavelengths ranging from 200 nm to 800 nm.

#### 3.5.5. Colour Properties

The colour of the films was determined using a CIE colorimeter (Konica minolta Sensing, CR 410, Japan), and was expressed as: L\*: luminance/brightness, a\*: red/green and b\*: yellow/blue. DE\* (total difference in colour) was calculated using the equation below [87]:

$$
\Delta \mathbf{E} \star = \sqrt{\left(\Delta \mathbf{L} \ast \right)^2 + \left(\Delta \mathbf{a} \ast \right)^2 + \left(\Delta \mathbf{b} \ast \right)^2}
$$

where ΔL\*, Δa\* and Δb\* are the differences between the corresponding colour parameter of the sample and the white standard.

#### 3.5.6. Fourier Transform Infrared Spectroscopy

The FTIR analyses of collagens ASC and PSC extracted from smoothhound skin and of different films were performed using a Cary 630 FTIR spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) within the wavenumber ranging between 4000 and 400 cm−1. In each case, the sample was placed directly on the FTIR spectrometer fitted with an Agilent diamond ATR sample.

#### 3.5.7. 1,1-Diphenyl-2-picrylhydrazyl Radical Scavenging Ability

The free radical-scavenging ability of films was measured as reported by Shimada, et al. [88] with slight modification. Films were cut in small pieces and dissolved in acetic acid solution (0.5 M) at 5 mg/mL, then 500 μL 0.1 mM DPPH solution was added to 500 μL of each film sample and kept in the dark for 30 min. Absorbance was measured at 571 nm and DPPH radical scavenging ability was calculated with the following equation:

$$\text{DPPH}(\%) = \frac{(\text{A control} - \text{A sample})}{\text{A control}} \times 100\%$$

where Asample and Acontrol were the absorbencies of sample and control group, respectively.
