Package good such as sardines, coke in can and juice in pouches may bulge or bloat through the course of its shelf life. Such maybe due to anaerobic gas producing bacteria or reaction of food components to container – the case of acid to metal reaction.
All cans for food use are lacquered or coated with non-reactive enamel. It prevent any unnecessary migration of metal to food and or any unwanted process.
Bulged aluminum pouch is a rare scene. I have encountered it only twice. As stated above, the cause maybe microbial or chemical. I setup a simple experiment to verify that acid to metal reaction may cause container bulging.
I secured three empty juice pouches. Cut off the top, cleaned and dried. Poured with equal amounts of vinegar. Added equal sizes aluminum strip on each container. Sealed and set aside for later observation.
Go and read the update, Pouch Bulging Experiment | The Update.