The release of ZnO from PBS films to the food simulants took
place over four steps. Firstly, Zn2+ dissociated from ZnO and diffused
to the food simulant [25] through PBS matrix. Zn2+ ions were
then leached by the food simulant thereby transferring mass to the
PBS film surface. Finally, the dissolved Zn2+ ions completely
migrated to the food simulants until the thermodynamic equilibrium
reached. The release of Zn2+ from PBS films was conducted
in three food simulants namely distilled water, acetic acid 3%
and ethanol 10% and graphically presented in Fig. 4. Fig. 4(a)
informed that the release of Zn2+ from the films with 10 wt% ZnO
higher than those from 6 and 8 wt%. The release of Zn2+ concentration
was strongly influenced by ZnO content. Over duration period
studied, Zn2+ was found to release slowly and reached the maximum
value at less than 10 ppm. The same trend was also observed
when ethanol was used as food simulant. In Fig. 4(c), Zn2+ released
rapidly when the films were immersed in acetic acid. The release of
Zn2+ ions into solvent has been proposed by many researchers.
Kasemets et al. [27] found that the antimicrobial activity of ZnO
nanoparticles occurred from the solubility of Zn2+ ions in the medium.
This activity directly depended on the concentration and time
[28]. At low ZnO concentrations (0.01–1 mM), Padmavathy and
Vijayaraghavan [29] concluded that ZnO had low antimicrobial
performance against E. coli, and the presence of soluble Zn2+ ions
may act as nutrients for this microorganism.