Find water inside a palletised box and the first assumption is a leak. Usually there is no leak. The water came out of the air, and it will keep coming until the temperature difference behind it is dealt with.
Why it happens
Warm air holds more moisture than cold air. Take produce from a warm pack house into a cold room and the air inside and around the box cools. When it reaches its dew point, the moisture it can no longer hold comes out as liquid on the nearest cold surface – which is often the inside of the box or the product itself.
Then the box is moved out again, warms up, and the cycle repeats in reverse. Each cycle deposits more water.
Why plastic boxes show it more
A plastic box is a vapour barrier and a cold surface, so condensation forms on it readily and does not get absorbed. A fibreboard carton soaks it up and hides it, which is why the same problem looks worse in plastic even when it is not. This is one of the genuine trade-offs against cartons and it is worth knowing before someone reports it as a defect.
What it causes
- Wet product surfaces, which accelerates decay and encourages mould
- Labels and print lifting from the inside out
- Water pooling in the base of the box and sitting against the product
- Slip hazards and wet floors in the cold store
- Weight gain that shows up as unexplained freight variance
What helps
Reduce the temperature difference. Pre-cool before packing where you can, and avoid moving cold pallets into warm humid air. This is the actual fix and the others are mitigation.
Let the product dry after washing before it goes into a cold room.
Allow air movement around and through the pallet so moisture is carried away rather than settling.
Drainage in the box base, so that water which does form has somewhere to go instead of pooling.
Avoid sealing the pallet immediately after it comes out of the cold room – give it time to come up to temperature, or wrap it while it is still cold and accept that condensation will form outside the wrap instead.
What does not help
Sealing the box more tightly. Condensation forms from moisture already inside, so a better seal keeps the water in rather than keeping it out. Adding vent area helps a little by allowing exchange, but it does not address the cause.
How to work out if this is your problem
Measure the product temperature and the air dew point at the point where the box is closed. If the product is below the dew point of the surrounding air, condensation is guaranteed. It is a two-minute check with a hygrometer and it settles the question.
Wrapping, films and what they do
Stretch wrap and pallet films change where condensation appears rather than whether it appears. Wrapping a cold pallet before it has come up to temperature means the condensation forms on the outside of the wrap instead of on the product, which is usually what you want.
Wrapping a warm pallet and then cooling it traps the moisture inside, where it condenses on the coldest surface available – which will be the product.
Perforated films let some moisture escape and reduce pooling, at the cost of slower cooling and more moisture exchange generally. They are a reasonable compromise where condensation is unavoidable, and they are not a substitute for managing the temperature difference.
What we can do
We can supply produce boxes with drained bases and vent patterns that help, but the honest answer is that condensation is a temperature management issue. If you are seeing it, look at your cooling sequence before you look at the box.