From agro-industrial waste to active packaging. Pomegranate peel as a functional filler in starch-based films

In this study, starch-based films, incorporating 10, 20, and 30% of w/w pomegranate (Punica granatum L.) peel extract (E) obtained by subcritical water extraction or the whole pomegranate peel powder (P), were developed and characterized in terms of microstructure, mechanical, barrier, optical together with the thermal properties and stability and antioxidant capacity. Furthermore, in vitro and in vivo antimicrobial analysis of the films incorporating the highest filler content (E30 and P30) were carried out to evaluate their antimicrobial efficiency against E. coli and L. innocua. The incorporation of pomegranate-derived fillers significantly (p < 0.05) affected the functional performance of starch films. Thus, the presence of both fillers provided complete UV-blocking capacity to the films in the 200–400 nm range, which could be of interest for foodstuff sensible to oxidative degradative reactions. The addition of the extract significantly increased the stretchability of the films (by approximately 37–95%), in agreement with the plasticizing effect of the low molecular compounds present in the extract. This effect was greater when the extract content in the film rose, the stretchability of the E30 films being 95% higher than that observed in the control. On the contrary, the peel powder led to a remarkable reinforcement effect, increasing the elastic modulus and tensile strength values by 156–367% and 63–146%, respectively, and reducing the film elongation by 57% (p < 0.05), regardless of the peel content. Filler-rich films exhibited both antioxidant and antimicrobial activities against E. coli and L. innocua bacteria. The films incorporating 30% of peel powder exhibited the highest antibacterial potential, although the antimicrobial effectiveness depended on the type of food matrix used. When applied to inoculated salmon stored at 10 °C for 6 days, these films reduced E. coli and L. innocua populations by approximately 1.6 log CFU.