Hygroscopic goods have the characteristic of a variable water content. They are capable of absorbing water vapor from the air or releasing it back again. In a first approximation, it may be stated that:
In ambient air of low relative humidity, they release water vapor, while in ambient air of high relative humidity, they absorb water vapor. They are thus capable of modifying the proportion of their mass constituted by water, i.e. the water content of the product.
A product is referred to as "container dry" when its water content is such that, under normal weather conditions during packing and transport of the container, it results in no impairment of quality. In the case of organic products, an excessively high water content may result in mold, rot and biochemical changes, such as with cocoa and coffee beans. For some products, these phenomena are also associated with self-heating, which may go as far as spontaneous combustion, for example oil-bearing seeds and feedstuffs containing residual oil.
Critical water content means a water content which, if exceeded during container transport or storage, must be expected to cause the onset of the listed disadvantageous changes.
For safe transport, it is therefore important for the water content of a product to match the required values on acceptance in the container and for this water content to be maintained in transit, which must be ensured by the storage conditions.