The most common myth people have about the PET carboys is that it is a plastic container which will allow oxygen into the beer or wine. But in reality, minuscule amount of Oxygen actually creeps in, in some cases. So for a glass carboy that has a rating of 100 out of 100 for protecting against oxygen, the PET is a 99.9 out of 100. That is almost unnoticeable amount of oxidation.
A type of polyester resin which is widely tested to meet or exceed FDA food contact requirements is PET (polyethylene terephthalate). It is quite common to see PET containers at home. Many food and cosmetic products utilize them for packaging. One can find easily find these in their homes anytime.
There is a recycle symbol at the bottom or the details side of the product and can be easily spotted. There is a number right in the middle of the symbol and if it is the number 1 then it is definitely a PET container. Brewers use the PET carboys for fermentation- primary as well as secondary.
Though they come in various size options, while brewing 3, 5 and 6 sized gallon carboys are commonly used. People prefer size 5 gallon PET carboy for secondary fermentation, whereas the 6 gallon for primary beer fermentation or secondary wine fermentation.
One cannot blame the PET carboy for the bad beer. It is something else that caused the wrong brewing. Many brewers have shared the same experience with the PET carboy. Some have not even noticed a hint of oxidation and said the 100 is the right number. Some have found the PET carboy to be actually 100/100 and some 99.9/100. In any case PET wins as a brewing apparatus for fermentation.