3.1 Physicochemical and thermal characterization
In order to confirm previous results obtained from Santos et al. (2021) [26], results of UV-vis spectroscopy analysis of annatto extract, CA nanofibers, and CA@A nanofibers are presented in Figure 1. The annatto extract spectrum revealed a strong absorption band at 410, which can be attributed to bixin and norbixin [29,30,31]; according to Calogero et al. (2015) [30], these two substances exhibit absorption bands above 400 nm due to an electronic-dipole transition oriented along the axis of the molecule. For carotenoids such as bixin and norbixin, the p/p* transition is relevant; in this conjugated system, the p-electrons are highly delocalized and the excited state is of comparatively low energy. As a result, relatively little energy is required to cause the transition, which corresponds to absorption in the visible region and can be attributed to the peak obtained in the spectrum. No peaks attributed to other annatto compounds were observed. This result agrees with previous work demonstrating that annatto seed extraction with low polarity solvents produces highly concentrated extracts mainly comprised of cis-bixin (70–80%) along with lesser quantities of other carotenoids as trans-bixin, cis-norbixin, and trans-norbixin [30,31,32,33,34,35,36]. The UV spectrum of CA@A nanofiber samples revealed a low-intensity absorption band centered at 410 nm, confirming the presence of annatto extract.