The moisture content (MC) level of newly harvested grain impacts the allowable storage time of grain (seeds) more than any other factor. It is generally accepted that dryer grain will keep longer; wetter grain will go out of condition faster.
Because of this the timing of harvest and post harvest handling practices are driven mostly by the MC. This in turn has led to various methods to artificially dry grain when the weather does not cooperate.
One practice is high heat drying prior to binning for storage. However, high heat drying does do damage to seeds in several ways which reduces value. One is stress cracks that increase breakage, another is denaturing of protein and loss of nutrients like vitamins and enzymes if certain temperatures are reached. Generally it is recognized that artificially dried grain is of lower value than field dried grain or methods that mimic field drying.
Why does the title compare Curing with Artificial Drying? Shouldn’t it be field drying compared with artificial or high heat drying?
It is true that drying occurs in the field – but not in a forced manner. The dry down that occurs in the field is a natural progression in seed life that commences at fertilization, continues through growth, filling, and ripening. In the case of grain this process is not complete until dormancy is reached. Dormancy is a state after all the bio-chemical actions within the seed are complete and it is ready to winter until spring to sprout. If this process is allowed to complete the seed has a natural resistance to spoilage. If it is interrupted by premature water removal with high heat some of this resistance is lost.
What Is the Distinction Between Drying and Curing?
Curing includes drying but at a natural pace; a pace we see in nature. Field dry down is natural curing and is controlled primarily by temperature because the bio-chemical changes within seeds occur faster at a higher temperature up to a point – just like plants grow faster at at higher temperature up to a point. High heat drying (artificial drying) interrupts and diminishes the value from natural curing.
Field dry down is natural curing. Crops reach a point when no more sugar is being produced by absorption of sun energy. This is defined loosely as physiological maturity. A common view is harvest can begin at this point to prevent field losses and spread out the investment in harvesting equipment because there will not be a further increase in yield potential.
However the seeds (grain) are not dormant at this point. Biochemical processes continue within a seed until the simple sugars are converted into starch and other components such as oil and protein. Water is released during some of these processes. The rate is governed primarily by temperature and little by the surrounding level of RH. This release of water (not necessarily from the kernel but from a bio-chemical process) continues even during a rain at a steady rate mostly dependent on the temperature.
Adsorbed ('D') water moves quickly into and out of seeds depending on the surrounding air conditions. This is different from the release of water from a bio-chemical process during curing. Higher MC does not cause starch to switch back to sugar by combining with a water molecule. Moisture release from curing is not reversible but MC increases of the seed can occur with increases of air humidity. One part of kernel MC can be going down while another part is going up.
[Water exists within seeds in three different manners: chemically bound as part of a molecule — like protein, absorbed (liquid) water — like a sponge soaks up and adsorbed water which is neither liquid nor gas but water molecules that stick to the surface of material inside the seed. At dormancy only chemically bound water and adsorbed water remain — all the absorbed (liquid) water is gone. Normally, the solid state of water — ice — is not associated with grain. References to freezing grain mean to lower it below the freezing point of water. There is no liquid water in mature grain to freeze.]
A clear, warm, dry day speeds up field dry down. The rate of curing would be about the same on a warm, wet day. The difference is the desorption/sorption of water — adsorbed water. Once curing is finished the seeds are dormant and then the influence on MC is only from changes in adsorbed water. The chemistry is stable until sprouting is triggered.
Curing could be thought of as ripening. The chemically bound water released during curing and any liquid water remaining in the early stages should be viewed as resources that will increase the value at full dormancy if the biological processes are not interrupted by heat stress. Removing this water prematurely with high heat lowers quality and value.
An excerpt from Purdue University:
The rate of field drydown of corn grain and the resulting grain moisture content at harvest obviously influences growers' cost of artificially drying the grain after harvest. An early drydown of the crop also facilitates early or at least timely harvest of the crop prior to the colder and, often, wetter conditions of late fall.
Kernel moisture content decreases as the kernel develops through the blister stage (~ 85% moisture), milk stage (~ 80% moisture), dough stage (~ 70% moisture), dent stage (~ 55% moisture), and finally physiological maturity (~ 30% moisture). Prior to physiological maturity, decreases in kernel moisture occur from a combination of actual water loss (evaporation) from the kernel plus the continued accumulation of kernel dry matter via the grain filling process. After physiological maturity (identified by presence of the kernel black layer), percent kernel moisture continues to decrease primarily due to water loss from the kernel.