What a barrier actually stops, and how to specify one

Five problems that get treated as one

A barrier is not a single property. Oxygen turns fats rancid and fades color. Water vapor softens dry goods and lets microbes back in. Light degrades pigments, oils and vitamins. Grease migrates out of the product and stains the print. Aroma escapes, or worse, arrives from the shelf next door.

A material can be excellent against one and useless against another. Clear PET stops moisture reasonably and oxygen poorly. Kraft paper stops light and nothing else. Aluminum foil stops all five and cannot be seen through. Asking "is this a good barrier" has no answer until you say against what.

Thickness is not barrier, and mil is the wrong number

Every flexible bag is sold on its mil — thousandths of an inch of total thickness. It is the figure quoted because it is easy to measure and it sounds like strength. It describes the polyester and polyethylene wrapped around the barrier, not the barrier itself.

The barrier in a bag is the aluminum layer, and there are two kinds. Real foil is a rolled sheet of metal laminated into the structure and it stops oxygen almost completely. Metallized film is aluminum vapor-deposited onto plastic in a layer thousands of times thinner, and it stops a useful fraction. A 4 mil foil bag outperforms a 7 mil metallized bag on oxygen by orders of magnitude and costs less. We set that comparison out in full on our guide to mylar bag gauges.

If a supplier will not say whether a bag is foil or metallized, assume metallized. The mil figure is often quoted alone because it is the flattering one.

Specify from shelf life, not from feel

The right order is: decide how long the product must last, then choose the barrier, then choose the thickness for handling. Weeks to a few months is metallized territory and it is what most retail snacks use. A year and beyond needs real foil. Ten to twenty-five years — emergency stores, freeze-dried food — needs foil plus an oxygen absorber, because the bag stops oxygen getting in and the absorber removes what was sealed inside with the product.

That last distinction catches people out. A perfect bag with no absorber still contains the air it was sealed with, and that air is enough to stale a product over years. The sizing and absorber rules are on our guide to bags for freeze-dried storage.

When the product attacks the packaging

Some products are hard on a barrier from the inside. Fat is the main one: kibble, treats, nut butters and fried snacks all migrate oil through a thin inner layer, delaminate the print and show as a dark patch within weeks. The fix is a fat-resistant inner layer, not a thicker outer one.

Essential oils and strong fragrances do something similar to adhesives and seals. Acidic products attack aluminum where the foil is exposed at a cut edge. None of this shows in a two-week sample, which is why a shelf-life trial with the real product matters more than any spec sheet. The fat case is worked through on our bags built for high-fat pet food.

Paper barriers, and the fluorine question

Paper can be made grease-resistant two ways: by refining the pulp until the fiber itself resists oil, which is what greaseproof paper is, or by coating it. Coatings used to be fluorinated, and those are now restricted in a growing list of states and countries. True greaseproof needs no coating at all, which makes it both the older technology and the cleaner one.

For a food business the practical rule is to ask directly whether a grease-resistant paper is fluorine-free and to get the answer in writing, because "grease-resistant" on its own does not answer it. The grades and where each belongs are on our food wrapping paper in greaseproof grades.

Every window is a hole in the barrier

A window sells the product and weakens the pack, and both are true at once. On a snack bag with a six-week shelf life that trade is easy. On a storage bag rated for years it is not a trade at all — the window is the shelf life. Between those, the honest question is how much of the rated life you are willing to spend on being seen.

The same logic applies to a clear lid over a paperboard base, where the film is doing the selling and the base is doing the protecting. That construction, and what the clear part costs in practice, is on our clear lid boxes with a transparent top.

Light is a barrier problem too. A window over an oil, a spice or a pigmented cosmetic shortens its life even when the film is a perfect moisture barrier.