Fumaric Acid as a Physical Cross-Linking Agent for Alginate-Based Active Films Incorporating Essential Oils: Structure–Property Relationships and Antifungal Performance

Abstract

Sodium alginate is a widely studied polysaccharide for the development of biodegradable films; however, the mechanical performance and functionality of alginate materials strongly depend on the nature of the employed cross-linking agents. In this study, free-standing sodium alginate films were developed using fumaric acid as an alternative organic diacid for physical cross-linking, together with glycerol as a plasticizer. Unlike the commonly used citric acid, fumaric acid possesses a linear unsaturated diacid structure that may promote distinct intermolecular interactions within the alginate network. Active films were further obtained by incorporating volatile bioactive compounds, including geraniol, citral, and Pelargonium graveolens essential oil. The influence of formulation parameters on film structure and properties was investigated through mechanical characterization, optical analysis, moisture measurements, and low-field NMR relaxometry. The results showed that fumaric acid effectively modulates the alginate network, providing improved flexibility compared with conventional citric-acid-based systems while maintaining film integrity. The incorporation of essential oil components further altered the film microstructure, leading to reduced Young’s modulus, increased elongation at break, and enhanced UV-blocking capacity. In addition, the films exhibited antifungal activity against Colletotrichum acutatum, a major phytopathogenic fungus responsible for anthracnose disease in strawberries. Postharvest assays on strawberries showed a trend toward reduced disease incidence and severity during storage. These findings show that fumaric-acid-modified alginate films incorporating volatile bioactive compounds represent promising biodegradable materials for active packaging, while providing further insights into how organic diacids modulate intermolecular interactions and network organization in alginate-based polymer systems.