Grain moisture content is composed of chemically bound water and absorbed and adsorbed water. The difference between these two (one has a letter B and the other a letter D) 'types' of water goes to their state – normally thought of as solid, liquid or gas. Absorbed water is in the liquid state and is evidenced by the milk in growing grain. It is somewhat like when a sponge absorbs water and it can be squeezed out.
Adsorbed water is a quasi-state in that it is neither liquid nor solid nor gas. It is water molecules sticking to a surface – in this case the constituents within seeds (grain) and especially on starch. The surface is not the exterior of the grain kernel, rather the large surface area of matter inside the kernels.
Water does not adsorb on oil like it does on starch so grains that are high in oil (e.g. soybeans, canola) do not have as much capacity to adsorb water as grains with less oil content (e.g. corn, sorghum). A similar difference applies to protein content. The MC at a given temperature and RH combination will vary dependent to some extent on the ratio of constituents. Less water is needed to re-wet/dry high oil grains each percentage point of MC for this reason.
Excerpt from Wikipedia under Adsorption:
Adsorption is the adhesion of atoms, ions,or molecules from a gas, liquid, or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. This process differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid (the absorbent). Note that adsorption is a surface-based process while absorption involves the whole volume of the material. The term sorption encompasses both processes, while desorption is the reverse of adsorption. It is a surface phenomenon
Chemically bound water does not move into and out of grain like adsorbed water and composes a small percentage of total water; ~ 20% in corn or 3 – 4% of total weight. In mature dormant grain there is no absorbed water (liquid) remaining. Protein has water chemically bound and does not give up or take on water during storage. However, artificial drying (high heat) can force water from protein (denaturation) which changes its attributes and in general lowers value.
Managing MC in stored grain involves only the adsorbed water after dormancy is reached. During curing that leads to dormancy seeds will release water from chemical activity and it is necessary to carry this away as it is released to prevent build up of moisture that can support mold and insect activity. No liquid (absorbed) nor chemically bound water should be involved. This requires managing the water vapor in the air surrounding the grain. The most efficient method is precise selection for fan run times synchronized with the weather.