Grain damage is any degradation in the quality of grain. In the current grain trade, this damage can affect price, feed quality, food product quality, and susceptibility to pest contamination. Between the field and the end use, grain may go through any number of handling operations which can each contribute to grain damage. For example, grain might encounter free fall, conveyors, spouts, grain throwers, elevators, hoppers, dryers, and many more. Overall, these handling methods can be evaluated as to what effect they have on the grain. Damaged grain can often be characterized by the extent to which it reduces storage time. For example, cracked or broken kernels are more susceptible to insect or bacteria as well as chemical degradation. The damage to the actual grain is only one example of losses incurred after harvest. To quantify grain damage, one must also understand grain quality. Grain quality is a very broad term and can relate to many topics such as foreign material, chemical compositions, mechanical damage, insect infestations, and many more. These references to quality are highly dependent on the end use of the grain. Certain types of damage may be acceptable to specific industries, whereas others cannot use grain with these issues.
Definition Grain damage is such a broad term that it can be difficult to pinpoint all factors that can be considered damage. In addition, these factors are not easily measured. Many common methods for determining grain damage levels include some type of visible inspection, which can carry with it a large amount of error. The United States Department of Agriculture(USDA) has for many years listed standards for many types of grains. In those standards, they have identified a normalized grading scale based on factors such as test weight and limits of damaged kernels and foreign material. Throughout these standards, the definitions listed for damage can be open to interpretation by the person evaluating the grain. For example, "damaged kernels" for maize (corn) refers to kernels that are "badly ground-damaged, badly weather-damaged, diseased, frost-damaged, germ-damaged, heat-damaged, insect-bored, mold-damaged, sprout-damaged, or otherwise materially damaged." These characteristics are largely subjective and not easily measured. In the USDA's scale, as the grade number decreases, the quality of grain increases. A summary of these standards for maize, soybeans, and wheat are listed in the tables below (as effective September 1996). The USDA also lists these standards for many less popular grains such as barley, canola, flaxseed, oat, rye, sorghum, and sunflower seed.
Types
Broken/Cracked Kernels
One of the most common form of grain damage is broken or cracked kernels. This is likely to happen during handling processes and transportation. The USDA lists the cutoffs for broken maize as anything that falls through a 12/64 round-hole sieve, and broken soybeans as anything that falls through an 8/64 round-hole sieve. These standards to do not directly account for any cracks that are present in the seed coat. These cracks in kernels are known to cause deterioration of the grain more quickly through accelerated insect and fungal infestation and a quicker susceptibility to breaking during further handling. The allowable storage time for maize was found to decrease with an increase in mechanical damage (broken/cracked kernels). The storage time was found to decrease as mechanical damage increased from zero to forty percent. With this research, it is necessary to attempt to minimize the amount of mechanical damage in grains. In addition to exterior cracks, stress cracks can form inside of the kernel. These stress cracks usually come from the combination of thermal and mechanical handling processes, such as drying. Kernels with internal stress cracks are found to have a higher breakage susceptibility and can break more quickly during further handling.
Measuring Broken/Cracked Kernels There are many methods available for determining mechanical grain damage. The simplest method involves visible inspection of the grain. The USDA lists the threshold for broken grain to be anything that will fit through a 12/64 and 8/64 inch round-hole sieve for maize and soybeans respectively as listed above. This does not account for any cracks or defects in the leftover grain itself. In 1976, Chowdhury and Buchele developed a Numerical Damage Index for maize. They proposed a system that characterized the damage to each kernel on a scale from D1 (no defects) to D5 (very severely damaged). The procedure then takes a weighted average of the number of kernels assigned to each level to obtain a standardized measure of mechanical damage. This approach is based on visual observations, which carry with them a large amount of variability due to the individual conducting the test. Other researchers have attempted to measure mechanical damage in more concrete ways. One method is to use a dye that will adhere only to the broken areas of the kernel. From there, the dye is washed off with some type of solvent and measured using a colorimetric technique. A more recently developed technique uses the dielectric properties of the grain to measure the damage level; this method proved to very accurately measure the number of damaged kernels.
Fungal and insect infestation
Fungal and insect infestations can cause dry matter losses which in turn make the grain less valuable. Insect infestation and growth of fungi (including mold) go hand in hand, as many insects actually feed on the storage molds created through the deterioration of grain. Insect infestations can result in not only grain damage as understood by shorter storage times, but can also affect the actual weight of the grain, leading to lower prices at market. In addition, these problems can reduce the nutritional or chemical value of the grain, which can be very important based on the end use. Mold growth can be triggered by moisture, mechanical damage, storage temperature, and other factors. Mycotoxins refers to the toxic fungal chemicals that grow on crops. Of these, the most researched of these is aflatoxin, which have the potential to be carcinogenic (cancer-causing).
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