Moisture is the least interesting figure on a spice specification. It is also the one most likely to ruin a shipment.
Almost every complaint about mould on arrival traces back to it, and in most of those cases nobody did anything wrong. This page explains the mechanism, and what actually prevents it.
Moisture content and water activity are not the same thing
Two terms get used as if they mean the same thing. They do not.
Moisture content is the total amount of water in the material, as a percentage by weight. If a 100 kg bag contains 10 kg of water, moisture content is 10 per cent.
Water activity, written aw, measures how much of that water is available. Some water is chemically bound to the material and cannot support life. The rest is free.
Mould does not care about total water. It cares about available water.
Most moulds need a water activity above roughly 0.6 to grow. Below that they stay dormant.
This is why two lots with the same stated moisture can behave completely differently in a container. In one, the water is bound. In the other, it is free.
For most spice trade, moisture percentage is what gets contracted, because it is cheap and quick to measure. That is fine. But if you are buying something marginal, or something that has caused you trouble before, it is worth asking for water activity as well.
Container rain: the mechanism behind most arrival complaints
This is the part worth understanding properly, because once you see it you will design around it.
A container is a sealed steel box. Steel conducts heat very well.
The container is loaded in an Indian port. It is warm and humid. Warm air holds a lot of water vapour, and the goods themselves contain moisture.
The ship sails. It moves through cooler latitudes, or simply through a normal day and night cycle.
During the day the sun heats the container roof. The air inside warms. Moisture evaporates out of the cargo into the air.
At night the roof cools quickly, faster than the cargo does. The warm, moist air touches the cold steel. The water condenses on the underside of the roof.
Then it drips back down onto the top layer of bags.
Repeat this for twenty or thirty nights. The result is a consignment that was within specification when it was packed and arrives with a mouldy top tier.
Nobody falsified a certificate. The physics did it.
Why it is always the top tier
Because that is where the water lands. If you open a container and find damage on the top bags while the bags below are perfect, you are almost certainly looking at container rain rather than a quality problem at origin.
It is worth knowing that, because the two have completely different remedies.
What actually reduces the risk
Five things, in rough order of how much difference they make.
1. Pack below the ceiling, not at it
If your specification says 12 per cent maximum, and the supplier packs at 11.8 per cent, they have met the specification and left you no margin at all.
Ask for a target rather than only a maximum. “12 per cent maximum, 10 to 11 per cent target” costs the supplier a little more drying time and saves you containers.
This is the single most effective line you can add.
2. Container liners and desiccant
A liner is a large bag that sits inside the container, separating the cargo from the steel. Desiccant bags hang from the walls or the roof and absorb moisture from the air.
Both are cheap relative to a claim. For any long route through the tropics, they should be standard rather than an upsell.
Decide who pays for them and write it down. That single conversation prevents most of the arrival disputes in this trade.
3. Kraft paper over the top tier
A layer of heavy paper over the top bags catches condensation before it reaches the product. It is the cheapest protection available and it is often skipped.
4. Do not stuff a hot container
A container that has been standing closed in the sun is full of hot air. Seal that in with your cargo and you have loaded the problem along with the goods.
Open it and let it cool before loading. It costs an hour.
5. Know which products are worst
Some goods pull water out of the air far faster than others. These are called hygroscopic. Watch them closely:
- Garlic powder and onion powder. Among the worst.
- Chaat masala and any blend built on salt. Salt attracts water strongly.
- Ground spices generally, because grinding exposes more surface.
- Jaggery and anything with sugar.
For these, a liner is not optional and the moisture target should sit well under the ceiling.
Measuring moisture properly
The method matters, as it does with every other figure.
Oven drying is the reference method. A weighed sample is dried at a set temperature for a set time, then weighed again. The weight lost is the water. It is accurate and it takes hours.
Handheld meters give a reading in seconds. They are useful for a quick check at a warehouse. They are affected by temperature, by how tightly the probe is pushed in, and by where in the bag you test.
Contract on the oven method. Use meters for monitoring, not for certificates.
And specify that the measurement is taken on the packed lot, not on material before packing. Things change during handling.
What to put on the contract
- Moisture as a maximum, with a target range as well.
- The measurement method.
- That the measurement is on the packed lot.
- Water activity, for marginal or high risk products.
- Whether a liner and desiccant are used, and who pays.
- Kraft paper over the top tier, if you want it.
- Who is responsible if container rain damage occurs.
That last line is the one people leave out, and it is the one that matters when something goes wrong. Under most Incoterms rules, risk passes to the buyer when the goods are handed over for carriage, which means transit damage is an insurance matter rather than a supplier fault.
Our claims policy sets out how that is handled, and shipping and documentation covers loading and stuffing in more detail. Writing a specification covers how to set the moisture clause out so it is enforceable.
If damage does occur
Move quickly. Photograph the container interior before unloading, including the roof. Note the seal number. Record which tiers are affected.
Notify the carrier within the time limit in the bill of lading, which is usually short, and open a file with your insurer immediately.
Evidence gathered on the day is worth far more than evidence gathered a week later.
The short version
Moisture content and water activity are different measures, and mould responds to the second one.
Container rain causes most arrival complaints, and it is physics rather than fault. Pack below the ceiling, use a liner and desiccant on tropical routes, put paper over the top tier, and agree in advance who pays for what.
Do that and the least interesting line on your specification stops costing you money.