Novel composite films based on sodium alginate and gallnut extract with enhanced antioxidant, antimicrobial, barrier and mechanical properties

Abstract

Composite films made of sodium alginate (NaAlg), glycerol, and gallnut extract (GE) at different NaAlg/GE weight ratios of 100/2.5, 100/25 and 100/50 were prepared and characterized in terms of antioxidant activity, water vapor permeability, mechanical, thermal and optical properties. Likewise, the antimicrobial activity of the produced films was evaluated against two common pathogenic bacteria including Staphylococcus aureus (ATCC 6538) and Escherichia coli (ATCC 11775) and two pathogenic fungi namely Aspergillus niger and Penicillium digitatum. Our results showed that increasing GE content from 0 to 25 wt% (based on NaAlg weight) increased tensile strength and elongation at break of NaAlg films respectively in the range of 48–103% and 135–185% without affecting their thermal stability. Moreover, increasing GE from 2.5 to 50 wt% reduced water vapor permeability of composite films in the range of 28.5–50.1% compared to the neat NaAlg films and increased their light barrier properties. As revealed by scanning electron microscopy and Fourier-transform infrared analysis, such improvements in functional and physical properties of NaAlg films can be attributed to the good compatibility between the polymeric matrix and the incorporated GE along with the increase in interactions between NaAlg molecules and GE polyphenolic compounds. Moreover, the incorporation of GE significantly improved the antioxidant capacity of NaAlg/GE composite films most likely due to the high total phenolic content of the incorporated GE. Furthermore, the resulting films showed good antibacterial activity against Gram-positive and Gram-negative pathogenic bacteria, suggesting their significant contribution, as antibacterial packaging materials, towards shelf life extension and food safety preservation.