While PET has a very low permeability when it comes to carbon dioxide, it readily allows oxygen to pass through. And oxygen is the enemy of wine! When we talk about storing soft drinks, permeability to carbon dioxide is the critical factor. A beverage that loses carbonation loses its appeal. In this case oxygen permeability is not an issue. While oxygen passing into a plastic soft drink bottle from the air may react with some of the flavor components, the effect would be minor given that we don鈥檛 store soft drinks for extended periods. But of course we do store wine to age it. And this is where oxygen becomes a problem.
Grape juice contains a variety of compounds called polyphenols which can react with oxygen and produce a variety of colors and flavors. This really is the same chemistry that occurs when an apple is cut and exposed to the air. Reaction between polyphenols and oxygen produces the brown discoloration. Not only will the apple slices look different, they will also taste different. The same thing can happen with wine. White wine is more susceptible to such changes because it lacks some of red wines colored compounds, the anthocyanins, which can act as antioxidants. Sulfur dioxide is also an effective antioxidant, which explains why compounds such as sodium bisulfite are used to preserve wine. Burning sulfur inside wine barrels to produce sulfur dioxide is an age-old method of preservation.
Now back to our plastic bottles. As we have seen, empty soft drink bottles are too permeable to oxygen and are not appropriate for storing wine. But wine can be purchased in plastic containers, although I suspect a true eonophile would look warily upon this method of marketing. So how do the marketers solve the problem of oxygen permeability? By sandwiching a layer of an oxygen-impermeable plastic between layers of food-grade polyethylene or polypropylene. Ethylene-vinyl alcohol copolymer is ideal for this purpose since it allows very little oxygen to pass through. A bit of ingenious chemistry. So while it is not a good idea to store your wine in old soda bottles, it is quite acceptable to purchase wine in plastic containers.
