If your bags develop dark patches on the shelf, this is what is happening and it is entirely preventable.

Fat migrates

Oils in a treat do not stay put. Over weeks they work into and through the inner layer of a bag that was never specified to resist them, and they show as a translucent patch that spreads.

Warmth accelerates it, so a bag near a shop window or on a summer delivery run shows it first. That is why the problem often looks random.

An oxygen barrier is a different job

Barrier films are usually specified against oxygen and moisture ingress, which protects against staling and is worth having.

It says nothing about grease resistance, and the two are not the same property. A pouch can have an excellent oxygen barrier and no grease resistance at all.

What it does to the print

Once oil reaches the print layer, ink softens and colours shift. On a matte laminate it shows as a wet-looking patch; on kraft it darkens the paper permanently.

In the worst cases the laminate delaminates at a seam, which fails the seal as well as the appearance.

The fix, and what it costs

A grease-resistant inner layer, specified up front. It is a modest cost per bag and it is the difference between a shelf life and a complaint.

If you want kraft, take laminated kraft rather than unlined — and accept that it is no longer recyclable as paper, which is a real trade-off worth deciding with your eyes open.