Control moisture in a cache by drying the food before it goes in, because the water that spoils a sealed container is in the food, not in the air above it; a desiccant then handles the small amount of vapour in the headspace, and the lid, gasket and wax handle liquid water from outside. A sealed container of damp food is a spoilage chamber, and when the food is low in acid it can also be a botulism hazard.
Part of Hidden Food Caches
Cache moisture control is the combined use of dry food, desiccants, liners and seals to keep stored food below the water level at which mould, bacteria and insects grow.
The Water Is In The Food
The parent article names moisture as the primary reason caches fail and lists the tools: double-bagging, desiccants (silica gel at one packet per litre of container volume, or rice dried in an oven or over a fire for an hour), vacuum sealing and wax on the seams. It treats them as a set. They are not equal, and the order in which they matter is the opposite of the order in which people reach for them.
Moisture Barriers states the thresholds that decide which defence a store needs: mould needs a water activity above 0.85, bacteria need above 0.95 and insects survive down to 0.70. Water activity is a measure of how much of the water in a food is free for organisms to use; it is not the same as the percentage of water. A food held below the thresholds cannot grow what the thresholds govern, however long it sits sealed. A food above them grows it. That is the whole logic of cache moisture control: keep the food under the line, and keep outside water from lifting it over the line.
What You Need
- Food dried to its stated moisture level — for grain see Grain Storage, which gives the moisture figures by storage time
- A rigid container and liner as in Container Selection
- Desiccant — silica gel, or rice baked dry
- Wax for seams
- A way to check for water on opening — look for condensation on the lid and a damp smell
Method: Dry, Pack Cool, Seal, Check
Step 1: Dry the food first. For grain use the bite test and the moisture limits in the grain article; for dried meat and fish use the snap test in Moisture Testing. The desiccant cannot rescue damp food.
Step 2: Pack when the food and the container are cool and dry. Warm food packed in a cool container and sealed releases its heat and vapour, and the vapour condenses on the cooler lid and walls.
Step 3: Pack full. Minimise the air space, as the parent article says; less air holds less vapour.
Step 4: Put the desiccant at the top, bagged. The desiccant adsorbs vapour from the air space and should not touch the food directly in a way that contaminates it.
Step 5: Seal. Gasket lid closed fully, seam waxed, liner twisted and tied.
Step 6: Open on a dry day and check. Look under the lid for drops, check the desiccant, and sniff for mustiness. Replace or dry desiccant that has taken up water.
Worked Example: Food Water Against Headspace Water
Take the container from Container Selection: about 15 kg of rice in an 18.9 L bucket, with about 3 L of air left above it (an assumption for the example).
- Water in the rice: a difference of just one percentage point of moisture content is 15 kg × 0.01 = 0.15 kg = 150 g of water.
- Water the headspace air can hold: air at 25 °C holds at most about 23 g of water vapour per cubic metre (recalled from standard psychrometric tables, not from this corpus). The headspace is 3 L = 0.003 m³, so it holds at most 23 × 0.003 = 0.069 g, about 0.07 g.
- Ratio: 150 ÷ 0.069 = 2,170, so the one point of extra moisture in the food holds about two thousand times as much water as the air above it.
If the container is cooled to 10 °C, where air holds about 9.4 g per cubic metre (recalled), the headspace can hold 9.4 × 0.003 = 0.028 g and the surplus that condenses is 0.069 − 0.028 = 0.041 g, a few hundredths of a gram. Condensation from the air is therefore not what spoils a sealed cache of properly dry food. Damp food is, along with water that gets in around a failed seal, which is why those two get the effort, and the desiccant only mops up the headspace.
Common Mistakes
- Reaching for silica gel to cure damp rice. The ratio above shows it cannot.
- Sealing food warm. The heat moves moisture around inside the container.
- A desiccant that is already saturated. Dry it again or replace it.
- Opening the cache in rain or fog. You add water for the next year.
- Ignoring a faint musty smell. It is the first sign of mould, and a sign to inspect the whole container.
Safety
Damp, sealed and low in acid is where botulinum toxin forms
A sealed container of moist food kept cool and without air can grow Clostridium botulinum, whose toxin is not destroyed by a warm spell and may not smell or look wrong. The bacteria need a water activity above 0.95 according to the moisture-barriers article, so dry staples are safe from this, and moist or home-canned low-acid food is not. Treat swollen, gassy or foul-smelling containers as lethal, do not taste, and dispose of them sealed; see Food Safety. Silica gel is not food: keep it bagged and out of children’s reach, and discard rice used as a desiccant once it has taken up water. Handle mouldy grain with a mask, since the spores harm lungs.
What’s Next
- Learn next: Pest Prevention — the other half of a failed cache
- Combine with: Inventory and Rotation — checking cached food on a schedule