Bag-in-box and canning

The bag is normally a laminate — a polyethylene inner for food contact, with a metallised or barrier layer such as ethylene vinyl alcohol to slow oxygen transmission — and the whole assembly is chosen for its barrier properties as much as for its strength. Bags are supplied sterile or are filled and sealed on a machine that does not expose the interior to air.

The great advantage is what happens after opening. Because the bag collapses, no headspace forms, so a broached box holds for weeks where a broached bottle holds for a day or two. That is why the format has become the standard for farm gate and festival cider, and it suits a still, unfiltered, traditional cider particularly well.

The limits are real. Oxygen crosses the film and, more importantly, crosses the tap, which is the weakest point in the barrier; unopened shelf life is measured in months rather than years and is shorter than glass. The format cannot hold pressure, so it is for still cider only. And a bag that has been punctured, creased hard or frozen may have lost its barrier without showing it.

A can is closed by rolling the flange of the can body and the curl of the lid together into a double seam: two operations, the first forming the interlocking hook and the second tightening it, with a sealing compound in the lid curl filling the remaining gap. Everything about a can’s integrity is in that seam, and canning operations measure it — seam thickness, seam height, countersink depth, body hook and cover hook length, and the overlap between them — as a routine quality check rather than as an occasional audit.

A seam that is out of specification leaks slowly, which for a carbonated cider means flat product and for any cider means spoilage. Seamer setup is a specialist job, and the checks are done with a micrometer and a seam saw or a projector on cans pulled from the run.

The lining matters as much. Bare aluminium is attacked by an acidic drink, so cans carry an internal lacquer, and the lacquer must be compatible with the product’s pH, its alcohol and its sulphite level. A damaged or unsuitable liner leads to metal pickup, a metallic taint, and eventually perforation of the can wall. Producers work with the can supplier on liner selection rather than assuming any can will do.

Total package oxygen is the dominant quality variable in canning, and cans are less forgiving than bottles because the headspace is small and the product is often lightly flavoured and lightly protected. Lines undercover-gas the can, jet the headspace with carbon dioxide or use a controlled foam-over immediately before the lid lands, and measure dissolved and headspace oxygen on the finished can.

This is one of the reasons small canning is harder than it looks. The equipment to seam a can reliably is not the difficult part; the equipment to get the oxygen out before the lid goes on, and to verify that it has gone, is. Mobile canning services exist largely because that combination is expensive to own for a small run.