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The demand for natural antioxidant active packaging is increasing due to its unquestionable advantages compared with the addition of antioxidants directly to the food. Therefore, the search for antioxidants perceived as natural, namely those that naturally occur in herbs and spices, is a field attracting great interest. In line with this, in the last few years, natural antioxidants such as α-tocopherol, caffeic acid, catechin, quercetin, carvacrol and plant extracts (e.g. rosemary extract) have been incorporated into food packaging. On the other hand, consumers and the food industry are also interested in active biodegradable/compostable packaging and edible films to reduce environmental impact, minimise food loss and minimise contaminants from industrial production and reutilisation by-products. The present review focuses on the natural antioxidants already applied in active food packaging, and it reviews the methods used to determine the oxidation protection effect of antioxidant active films and the methods used to quantify natural antioxidants in food matrices or food simulants. Lastly consumers' demands and industry trends are also addressed.

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A new antioxidant active packaging film has been developed based on polylactic acid (PLA) in which green tea extract (GTE) has been immobilized by extrusion. Two GTE concentrations were tested, 1% (w/w) and 2% (w/ w). Four GT samples were compared regarding their antioxidant capacity (by DPPH radical scavenging method and β-carotene bleaching method), total phenolics content (TPC) and total flavonoids content (TFC). The commercial GTE presented with highest antioxidant capacity by the DPPH radical assay (EC 50 = 0.12 ± 0.00 mg/mL) and the highest TPC (416 ± 9.95 mg gallic acid equivalents (GAE)/g extract) and it was selected to be incorporated in the active film. Films were evaluated regarding their mechanical properties (e.g. tensile strength decreased 12% with the incorporation of GTE in the PLA matrix and strain at break increased 9.6 and 36% with the addition of 1 and 2% of GTE) and water vapour transmission rates (watervapour barrier properties improved with the addition of higher amounts of GTE).The study of the effectiveness of PLA/GTE films against lipid oxidation was performed by the following methods: peroxide value, p-anisidine value, thiobarbituric acid reactive substances (TBARS) assay and hexanal monitoring, after packaging smoked salmon slices during different storage times (0, 7, 15, 30, 45 and 60 days). The results showed that the incorporation of GTE in the PLA films protects the smoked salmon from lipid oxidation in the different storage times tested. However, additional studies should be performed to better understand the GTE mechanism of action as the results suggest a pro-oxidant effect of PLA/GTE 2% after 60 days of storage and to evaluate the potential antimicrobial activity of active films.

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Soymilk is a water extract of soybeans, closely resembling dairy milk in physical appearance and composition. Most fatty acids in soybean and its derivates are unsaturated, and therefore susceptible to oxidation. The aim of this study was to evaluate the possible effect of the thermal conditions during the elaboration process of soymilk on its fatty acid profile. For this, the fatty acid composition of soymilk, okara (soymilk residue), and soybean were studied by gas chromatography with flame ionization detection (GC-FID). No major differences in the fatty acid patterns were found.

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