Calcium in biopolymer-based food-packaging systems: Interfacial interactions, network regulation, and functional...

Biopolymer-based food-packaging films and related formats are widely investigated as alternatives to petroleum-derived plastics; however, their practical use remains limited by moisture sensitivity, weak wet integrity, insufficient barrier performance, and limited active functionality. Calcium functionalization provides a potential route to address some of these limitations through soluble Ca²⁺ salts or particulate Ca-based materials. This review critically examines Ca-functionalized biopolymer-based food-packaging systems using a structure-function-processing-safety framework. Natural-biopolymer matrices form the primary evidence base, while studies using synthetic matrices such as PLA, PVA, or EVOH are identified separately according to polymer class, packaging format, and evidentiary relevance. Soluble Ca salts, including Ca chloride, Ca lactate, and Ca gluconate, can form Ca²⁺-mediated junction zones in carboxylate-rich alginate and low-methoxyl pectin, where ionic crosslinking is comparatively well established. In chitosan-containing and protein-based systems, the reported effects are less uniform and may involve local coordination, charge screening, stabilization of anionic partners, aggregation, or double-network formation rather than an extended Ca-crosslinked network. Changes in swelling, solubility, or barrier performance are therefore treated as functional outcomes and not as direct proof of network densification unless supporting structural evidence was reported. Particulate Ca materials extend the role of Ca beyond ionic crosslinking. Nano-CaCO₃ and porous/vaterite CaCO₃ can contribute to reinforcement, barrier modification, UV shielding, and active-agent delivery, whereas CaO and CaO₂ provide reactive antimicrobial functions through alkalinity or moisture-triggered H₂O₂ release. Hydroxyapatite and related Ca phosphate phases offer opportunities for mineral reinforcement, adsorption, active-agent immobilization, and circular sourcing from Ca-rich biowastes. Across the reviewed systems, improvements in wet integrity, stiffness, barrier performance, or antimicrobial activity were frequently accompanied by lower extensibility, opacity, aggregation, altered interfacial pH, oxidative or sensory risks, or incomplete release control. However, many studies did not measure the corresponding paired property, limiting complete assessment of these trade-offs. Overall, Ca-functionalized biopolymer-based packaging systems should be designed not by maximizing Ca loading, but by matching Ca chemistry with polymer structure, processing conditions, release behavior, food-contact safety, and the intended preservation function.