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The packaging industry has dramatically increased the number of packaging systems and designs made of plastics over recent decades. Plastics, in contrast with more traditional packaging materials such as glass and metals, are permeable systems that permit the exchange of low molecular weight compounds, e.g. gases and vapours, between the inner and the outer atmosphere. Despite this drawback, the availability of shapes and forms in which they can be manufactured, their ease of processing and handling, low price, excellent chemical resistance, etc., have made them very attractive in many packaging elds. Consequently, much industrial and academic research has been devoted to understanding the mechanisms of mass transport in polymers, to enable design of materials with improved barrier properties. The present paper reviews some of these developments, and highlights the structural factors that cause polymers to behave as high barrier materials, taking as benchmark the properties of one of the most widely used family of high barrier materials, the ethylene -vinyl alcohol copolymers.MST/5992

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Ethylene vinyl alcohol copolymer (EVOH) films containing green tea extract were successfully produced by extrusion. The films were brown and translucent, and the addition of the extract increased the water and oxygen barrier at low relative humidity but increased the water sensitivity, the glass transition temperature, and the crystallinity of the films and improved their thermal resistance. An analysis by HPLC revealed that the antioxidant components of the extract suffered partial degradation during extrusion, reducing the content of catechin gallates and increasing the concentration of free gallic acid. Exposure of the films to various food simulants showed that the liquid simulants increased their capacity to reduce DPPH(•) and ABTS(•+) radicals. The release of green tea extract components into the simulant monitored by HPLC showed that all compounds present in the green tea extract were partially released, although the extent and kinetics of release were dependent on the type of food. In aqueous food simulants, gallic acid was the main antioxidant component released with partition coefficient values ca. 200. In 95% ethanol (fatty food simulant) the K value for gallic acid decreased to 8 and there was a substantial contribution of catechins (K in the 1000 range) to a greatly increased antioxidant efficiency. Kinetically, gallic acid was released more quickly than catechins, owing to its faster diffusivity in the polymer matrix as a consequence of its smaller molecular size, although the most relevant effect is the plasticization of the matrix by alcohol, increasing the diffusion coefficient >10-fold. Therefore, the materials here developed with the combination of antioxidant substances that constitute the green tea extract could be used in the design of antioxidant active packaging for all type of foods, from aqueous to fatty products, the compounds responsible for the protection being those with the higher compatibility with the packaged product.