## 1 Introduction  
The growing demand for minimally processed and safe food has driven research into active packaging materials that improve safety and quality or increase shelf life through the controlled release of active components. Active packaging materials may include oxygen-binding agents, odor-absorbing or odor-releasing agents, antioxidants, and antimicrobials [1]. An active package with antimicrobial activity is designed to prevent the growth of or kill pathogenic and/or spoilage microorganisms [2]. This packaging system consists of a polymer membrane with incorporated antimicrobial agents or membranes formed from polymers, which themselves have antimicrobial activity [3, 4]. Antimicrobial agents incorporated into polymers could be organic compounds, such as spice and plant extracts [5], or inorganic compounds, such as metal oxide nanoparticles [6] or chlorine dioxide (ClO\({}_{2}\)). Several different metals and their oxides have been researched as antimicrobial agents for food contact surfaces and active packaging, such as silver, copper, zinc, and titanium [7]. Research on cobalt in this application is, however, very limited. The polymers used for packaging films can be either synthetic or biosynthetic and differ in strength, stiffness, and permeability. Biosynthetic polymers are based on biological macromolecules, can be produced from renewable plant sources, and are usually biodegradable [8]. Examples of biosynthetic polymers include polyvinyl alcohol, succinic polybutylene, poly(lactic acid), and polyglycyclic acid. Poly(lactic acid) (PLA) is a polyester obtained either through the processing of monomeric lactic acid or the fermentation of agricultural products, such as maize and sugar beet. PLA is biodegradable and does not contribute to long-term waste disposal problems [9]. It is on the US Food and Drug Administration's list of substances generally recognized as safe (GRAS) [10]. The high heat treatment capacity of PLA allows a variety of processing methods to be used, including casting and extrusion [11]. However, a major disadvantage of PLA for many food applications is its high moisture and gas permeability. Overall, PLA is an environmentally friendly polymer and very promising for use in food packaging materials, primarily for fresh or short shelf-life foods [10; 11]. The search for new antimicrobial compounds that are suitable for food use is also ongoing, and one group of compounds that shows potential is the metal-organic group. One such compound is a cobalt citrate complex [12]. It is an aqueous, soluble complex of cobalt(II) and citric acid (hereafter abbreviated as Co-cit), bearing the molecular formula (NH4)4[Co(C6H5O)72]. Its synthesis and isolation were performed by cobalt and citric acid reactions with a molecular ratio of one to two at pH 8 (Figure 1). For the assessment of the antimicrobial activity of the complex for its potential application in foodstuffs, due consideration must be given to the biological significance of its components, i.e., citric acid and cobalt. Citric acid (2-hydroxypropane-1,2,3-tricarboxylic acid, C6H8O7) is a common food additive. It is highly soluble in water and soluble in ethanol [13; 14]. It plays an important role in energy metabolism in both prokaryotic and eukaryotic cells [12]. Prominent among its properties is the ability to bind metal ions (Figure 1A), which affects the solubility and mobility of metals and, therefore, their bioavailability in biochemical processes [13]. One biologically significant metal ion is cobalt (Co(II)). Although an adult human body contains only about 1 mg, cobalt is a key trace element in human physiology [15]. It is a component of cobalamin, the vitamin B12 cofactor, and plays an indirect role in DNA synthesis. Cobalt deficiency can lead to anemia, while an excess can cause cardiac,  
Figure 1: (**A**) Structure of \(\alpha\)-hydroxycarboxylic acid citric acid as a metal ion binder; (**B**) Structure of the crystallographically characterized anionic complex in (NH4)4[Co(C6H5O7)2] (Co-cit).