*5.2.3. Plastic materials*

Plastic materials are partially or totally synthetic organic substances. Their principal components are represented by polymers constituted by carbon, hydrogen and, in some cases, oxygen, nitrogen, chloride, silica and sulphur. They have a relatively recent history but in the time gained a large number of applications for their versatility, low production cost, low weight, their good performance and their 'thermoplastic' nature which guarantees the recyclability and consequent low environmental impact.

Plastic containers are formed by blow-moulding (extrusion blow-moulding or injection blow-moulding). One of their disadvantages is the wall permeability to gases and vapours. If transparent, they also transmit light. Migration of small molecular weight substances (e.g. monomers, oligomers and additives) from plastic to food can also occur, thus affecting the quality of food (Tsimis & Karakasides, 2002). Oil contained in bottles with high air permeability (PE, PP, etc.) should be sold within 4 weeks, in contrast to polyvinyl chloride (PVC) bottles, which can hold olive oil for 3 months without appreciable quality loss.

PVC is a popular packaging material for edible oils in many countries, mainly due to its transparency, adaptability to all types of closure, total compatibility with existing packaging lines, and suitable for personalized design features (Kanavouras et al., 2004). PVC was used in food packaging without any doubts in 1970s. Nowadays for the dissolution of Vinyl Chloride Monomers (VCM) in oil during the storage time and for issues such as the environmental protection, the ample supply, plastic shaping, and its mechanical properties, PET has been supplanting PVC in the edible-oil market.

PET bottles are produced by bioriented extrusion and their advantages are: low water and oxygen permeability, high hardness and stiffness. PET is also more resistant to oil and fats, than the other plastic materials and for these properties it fits well for olive oil packaging. The disadvantages of this material are the processing condition (to control the humidity content), costs and the migration of acetaldehyde from bottle to food (Dipalma, 1986). As previously affirmed, plastic materials are, however, porous and thus permit the penetration of humidity and gases. Pristouri et al. (2010) demonstrated that between PET and PP, PET provided a better protection to olive oil than the other one due to its significantly lower oxygen transmission rate. Moreover Kiritsakis and Dugan (1984) reported that peroxide values were higher for olive oil packaged in plastic containers as compared to those packaged in glass bottles in the dark. Kanavouras et al. (2006) revealed instead that plastic containers had a particularly stronger protective role when oil was stored in the light, with respect to transparent glass. Besides in Italy the glass bottle is the most used, in other countries the olive oil packaging in plastic containers is developing. Contemporary trends in olive oil packaging include dark coloured glass bottles and PET bottles which have incorporated oxygen scavengers. These are used in order to prolong the shelf life of foods whose degradation kinetic depends on the partial pressure of oxygen inside the container. In particular, Del Nobile et al. (2003) confirmed that oxidation kinetics slower than that found with glass bottles can be obtained by bottling the olive oil into materials containing an oxygen scavenger. However, the slowest decay kinetics were obtained by bottling the oil in PET containers and by reducing the oxygen concentration prior to bottling to 10% of the equilibrium value. Also Gambacorta et al. (2004) studied the effect of several percentages of oxygen scavengers and of barrier resin included in PET on olive oil quality, and indicated that the materials having higher oxygen barrier properties can slow down the quality decay kinetics of extra virgin olive oil.
