Modified atmosphere cases and pads: an interplay worth testing

Gas-flushed trays do not need pads. That sentence gets said in packaging meetings often enough that it is worth testing, because it is only half true. A modified atmosphere pack holds its own humidity, and a pad inside the pack will pull moisture out of the headspace until it reaches equilibrium. Put the pad outside the pack, under the tray in a display case, and it is doing a completely different job with no effect on the gas mix at all. Confusing the two is where the money goes.

The useful question is not whether MAP and pads work together. It is which of the two jobs you are asking the pad to do, and whether you have measured the result rather than assumed it.

What the pad is doing outside the pack

In a case-ready operation the tray is the pack and the case is the environment. Drip that escapes the pack lands on the shelf, the deck, or the tray below. A pad under the tray catches that, keeps the deck dry, and stops one leaking pack from marking three others. Nothing about that interacts with the gas inside the film.

Inside the pack it is a different story. A pad under a flushed tray takes up purge that would otherwise pool against the film. Less free liquid means less fogging on the inside of the lidding film, and a cleaner top view, which is what the shopper actually buys. The cost is headspace humidity, and that is where the interaction lives. Watch the film, not the pad, for the first sign that the balance has moved.

There is a second effect that is easy to miss on a short test. A pad holding water is a slow release surface. In a pack that goes through a temperature swing, say from a 0 °C chiller into a 4 °C retail case, the pad will give some of that water back into the headspace. Over five days that is invisible. Over a fourteen-day pack it can be the difference between a clear film and a fogged one.

Three cells, five days, one measurement

A test that means something does not need a lab. Three cells, same product, same day, same chiller:

• Cell A: no pad, standard gas mix, current film.
• Cell B: pad inside the pack under the product.
• Cell C: pad outside the pack, under the tray.

Weigh the packs on day zero and day five. Photograph the film at the same angle each morning. Score free liquid, fogging and colour on a simple three-point scale. Five days is enough to see a difference on beef; chicken will show it in three.

Where the interaction goes wrong

A plant we worked with switched to a heavier sheet in the same week it changed film gauge. By day three the lidding film was condensing on the inside and a buyer read it as a temperature failure. The gas mix was fine. The pad was holding more water than the previous grade, the new film had a lower moisture transmission rate, and the two changes together pushed the headspace past saturation.

That is the pattern to watch for: a pad change and a film change in the same month. Either one alone is manageable. Together they produce a symptom that looks like a chill chain problem and is actually a moisture balance problem.

Test one variable at a time, keep four packs from each cell for a second look at day seven, and write down the film lot as well as the pad lot. Keep the whole thing to one page, because a test that needs a report will not be repeated next season.

If you are moving from a simple absorbent sheet to a polymer-based one, the SAP product range lists the grades by absorption under load, which is the property that decides whether the pack stays dry at the bottom of a stack.