Within the cardboard box, the crux of today’s bag usually consists of five layers of various metallic and/or plastic films (Fradique et al., 2011; Reeves, 144 Alternative packaging 2010). Individually, they provide the barriers to oxygen and/or strengthening membranes for the bag itself (Reeves, 2010).
The results of a 2011 study by the University of Bordeaux found that
multi-layer PET (Polyethylene Terephthalate) plastic materials combined
with layers of a Gas Barrier Resin (an example being an ethylene-vinyl
alcohol copolymer and MXD6 nylon mix) provided significantly improved protection from oxygen ingress, as opposed to a single layer PET container, over the generally short shelf life of bag-in-box wines (Yeamans-Irwin2011).
Some packaging suppliers make a bag which utilizes a metalized layer to provide a greater restriction of oxygen. This material is, however, more susceptible to the minute fracturing during shipment and handling, thus creating the possibility of oxygen ingress (Reeves, 2010).
The shelf life of wine in a bag-in-box is dependent upon a number
of factors: rate of O2 ingress, storage temperature, humidity, wine
composition, bag size, film fracturing, and the rate of use (Reeves,
2010). In general, however, the recommendation is three to six months
(Cravero, 2016).
Besides oxygen entering through the bag layers, two other possible
sources of ingress are the plastic tap or valve and the seams. The potential for leaking air into the wine comes through the seam to which the tap connects to the bag and to the tap itself. Depending upon the storage conditions and the type of wine, researchers have found that volatile compounds in some wines are reduced, apparently caused by both absorption of the plastic material and the tap seal. Further, tasters in one study found the aroma of plastic after only 30 days of storage in bag-in-box (Cravero, 2016).
Another study found that free SO2, which acts as a preservative, was halved after three months of storage and reduced by 75% after six months (Cravero,2016). Taste tests comparing wine in bag-in-box to that in bottles are mixed at best.