Do Molded Pulp Egg Cartons Absorb Water in a Cold Store?

A cold store is not wet, but it is humid, and a moulded pulp carton left in one for a week will pick up moisture from the air. The consequence is not a soggy carton; it is a softer carton, which means the stack settles and the eggs at the bottom take load they were not designed to take. This note explains how much water a carton can pick up, what it does to strength, and the two controls that keep a cold-chain route safe.

How much moisture does a pulp carton absorb?

At 50 percent relative humidity, a moulded pulp carton holds roughly 8 percent moisture by weight. At 85 percent it can reach 13 to 15 percent. The change happens over hours to days, not minutes, which is why a short exposure is less harmful than a long storage.

The absolute number matters less than the change during storage. A carton that enters dry and leaves wet has lost stiffness even though nothing spilled on it.

What does that do to stacking strength?

It reduces it, substantially. Between 50 and 85 percent relative humidity, the compression strength of a pulp structure can fall by 30 to 50 percent, and creep accelerates at the same time.

The practical consequence: a stack height validated in a dry warehouse is not valid in a cold store. Reduce the layers or shorten the storage, or both.

Is condensation a problem?

Yes, and it is the faster route. A carton moved from a cold store into a warm packing area condenses water on its surface. That water enters the pulp directly, not through the air, and the local moisture can exceed what humidity alone would produce.

The fix is a tempering step: let the pallet equalise in a buffer area before opening or handling it. The same rule applies to any paper packaging moved between climates.

What are the two controls?

Control one: a moisture-resistant treatment. A water-repellent treatment on the carton slows absorption and reduces the condensation effect. It does not make the carton waterproof, and it changes the repulpability profile, so the claim and the treatment must be checked together.

Control two: storage discipline. Keep the pallet off the floor, away from the door, and under a cover that breathes rather than seals. A sealed cover traps the moisture that the carton is trying to release.

How should a cold-chain carton be specified?

Four items: the moisture treatment and its target, the compression requirement at the storage humidity, the maximum storage duration at full stack, and the tempering step in the handling procedure.

A specification that states only "cold store suitable" has stated an intention, not a requirement.

What should be checked on arrival?

Measure the carton moisture with a meter at three points, inspect the stack for lean, and check the bottom layer for compression set. A carton that shows a visible set on the bottom layer has already lost the margin.

For the stacking test itself, see egg carton stacking and breakage, and for compostability, compostable egg packaging certification.

At what temperature and humidity should a cold store run?

Table eggs are normally held at 2–5 °C and 85–90 percent relative humidity, which is the worst combination for a paper carton: cold enough to condense on transfer, humid enough to push the board toward 13–15 percent moisture. The band is not negotiable for the eggs, so the carton has to be specified for it. Some operations run 10–12 °C to reduce condensation, trading shelf life for carton life. Either way, record the actual band and give it to the carton supplier, because a carton specified for 50 percent relative humidity was specified for a different warehouse.

How long can a pallet stay in the cold store before the carton loses margin?

Moisture uptake reaches equilibrium in roughly 48–72 hours at 85 percent relative humidity, and most of the strength loss happens inside that window. After it the carton is stable but weaker, so the danger is not the seventh day, it is the third. Practical guidance: treat 72 hours as the point where the stack must be re-assessed, and never validate a layer count from a pallet that has stood a week. If the route needs longer storage, drop two layers and re-measure instead of assuming the original figure still holds.

Does a water-repellent treatment change recyclability?

Yes, and the change must be declared. A fluorocarbon-free water repellent applied at 2–4 g/m² keeps the fibre repulpable in most mills, but a wax or PE dispersion at a higher add-on can fail the repulpability screen used in Europe, where a furnish is expected to yield under 10 percent rejects in a standard hydropulper test. Ask for the treatment chemistry, the add-on rate and a repulpability statement. A carton that meets the cold-chain requirement and loses its recycling claim has solved one problem by creating a larger one, and the larger one is printed on the retail pack.

Should a freezer store use the same carton as a chiller?

No. At −18 °C moulded pulp becomes brittle and its impact resistance falls sharply, so a carton that survives a chiller drop can crack at the corner in a freezer. Frozen product also condenses heavily on transfer, and the ice bonds the carton to its neighbour, which tears the surface when the pallet is broken. For frozen routes, specify a heavier grade or a laminated liner, allow a tempering step of at least 12 hours in a buffer area, and run the drop test at −18 °C rather than at room temperature. The test temperature is part of the specification.