Permeability — Of all the types of plastics that can be made into bottles and carboys, only special types of polyethylene terephthalate (PET) have been successfully used to make containers for carbonated beverages. The reason is simple – only these special types of PET are sufficiently impermeable to keep carbonation (i.e., carbon dioxide) in and the oxygen out. BetterBottle fermentation carboys are made of a unique, scientifically tested PET and they are specially manufactured so that oxygen permeability is negligible. Use the Technical tab at the top of page to access an informative discussion of Permeability.
BetterBottle PET carboys are slightly more permeable than glass; however, it would be a mistake to assume that using a glass carboy will guarantee superior results. It is important to think of a glass carboy as just one part of complete fermenting system that also includes an air lock, a stopper, some hose, a racking cane etc. In typical use, more oxygen diffuses through, or leaks by, liquid-filled air locks and many types of stoppers (most notably silicone rubber stoppers) than diffuses through the walls of BetterBottle carboys and fittings. Moreover, every time a stopper is removed from a glass carboy in order to perform a test or to make an adjustment, a considerable amount of oxygen enters the wine or beer. Air contains about 20% oxygen. If the head space (ullage) in a carboy is just 200 ml, 40 ml of oxygen will enter every time the stopper is removed. In fact, much more oxygen will enter. Oxygen reacts very rapidly with wine and beer, which essentially sucks up oxygen. Racking from an open glass carboy with a siphon tube or pump will also expose the wine or beer to a large amount of oxygen. In actual practice, BetterBottle carboys are equal to, or better than, glass carboys, because BetterBottle fittings make it convenient to control the extent to which wine or beer are exposed to oxygen. (Use the Product Information tab at the top of page to access Tips on Usage.)
