*3.3. Optical Properties*

Figure 3 shows the visual aspect of the electrospun PHBV films to evaluate their contact transparency. The effects of the addition of OEO and the NEs on the color coordinates (*L*\*, *a*\*, *b*\*) and the values of Δ*E*, *T*, and *O* of the electrospun PHBV films are shown in Table 2. One can observe that all the here-prepared PHBV films presented a high contact transparency, but they also developed a slightly yellow appearance when the active substances were incorporated. The Δ*E* values of the active PHBV films with respect to the neat PHBV film were 8.36, 7.52, and 15.82 for the films with OEO, RE, and GTE, respectively. Therefore, the highest color change was observed for the GTE-containing PHBV film. The main changes observed were based on a decrease in brightness (*L*\*) and an increase in the *b*\* coordinate, that is, a yellower material, which was related to the intrinsic color of the added active substances.

**Figure 3.** Visual aspect of the electrospun films of: (**A**) Neat poly(3-hydroxybutyrate-*co*-3-hydroxyvalerate) (PHBV); (**B**) Oregano essential oil (OEO)-containing PHBV; (**C**) Rosemary extract (RE)-containing PHBV; (**D**) Green tea tree extract (GTE)-containing PHBV. Films are 1.5 × 1.5 cm2.

**Table 2.** Color parameters (Δ*E*\*, *a*\*, *b*\*, and *L*\*) and transparency characteristics of the electrospun films of poly(3-hydroxybutyrate-*co*-3-hydroxyvalerate) (PHBV) containing oregano essential oil (OEO), rosemary extract (RE), and green tea tree extract (GTE).


*a***\***: red/green coordinates (+a red, −a green); *b***\***: yellow/blue coordinates (+b yellow, −b blue); *L***\***: Luminosity (+L luminous, −L dark); **Δ***E*\*: color differences; *T*: transparency; *O*: opacity. a–d Different letters in the same column indicate a significant difference (*p* < 0.05).

One can also observe that the OEO-containing PHBV film presented a transparency similar to that of the neat PHBV film, both having *T* values in the range of 3–4, which indicated a greater passage of visible light through the material. However, the incorporation of RE and, particularly, of GTE resulted in an increase of *T* up to values of 6.4 and 16.4, respectively. Therefore, the capacity of transmission of visible and UV light of the films was significantly reduced by the addition of RE and GTE (*p* < 0.05), causing a phenomenon of light scattering due to the characteristic tones of the active substances. Similarly, whereas opacity was kept relatively low for the neat PHBV film and the OEO-containing PHBV films, which both had *O* values in the 0.015–0.02 range, these values increased up to 0.026 and 0.067 for the RE- and GTE-containing PHBV films, respectively. Then, the presence of the latter active substances, particularly GTE, reduced the transparency properties by blocking the passage of UV-Vis light and it increased the opacity of the films, caused by the scattering of light. However, as other authors have previously stated, this property can be also a desired characteristic in some packaging materials for the protection of foodstuff from light, especially UV radiation, which can cause lipid oxidation in the food products [1,40]. In this sense, the work reported by Gómez-Estaca et al. [52] also

concluded that the addition of certain NEs to fish gelatin films decreased the transparency of the films and increased the opacity of the final material.
