In this work, we studied the antimicrobial activity of functionalized foils in in vitro conditions using the ISO 22196 standard method and tried to optimize the chosen protocol with an emphasis on approaching real food storage conditions. We prepared functionalized films based on polylactic acid (PLA) with coatings based on chitosan, juniper extract and their combinations in a two-layer coating. We also determined the wettability of the foils by measuring the surface free energy (SFE) and contact angle with water. The functionalized PLA films showed good antimicrobial activity against gram-positive bacteria S. aureus and gram-negative P. fragis and yeast S. cerevisiae, with a poor activity against gram-negative E. coli. The optimized protocol, where the samples were incubated at a low temperature, exhibited weak or no activity against S. aureus in various media and on the model food product (chicken salami). The best antimicrobial performance was demonstrated by PLA films with a juniper extract coating and a combination of juniper extract and chitosan. We also found that the addition of juniper extract increases the hydrophilicity of the film surface, which suggests that more hydrophilic films exhibit more effective inhibition of bacterial growth and have improved antimicrobial activity. The correlation is most clear in the case of test bacteria S. aureus and P. fragi. We conclude, that it is further necessary to develop a testing protocol for evaluating the antimicrobial activity of films in the presence of food products and in realistic storage conditions. In this way, we can anticipate an even more reliable insight into the antimicrobial action of functionalized foils for food packaging.
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