There are tests for other physical factors that are quality attributes but not grading factors or chemical composition. These tests provide additional information about the processability of corn for various uses, as well as its storability and potential for breakage in handling. The processability, storability and ability to withstand handling of corn are influenced by corn’s morphology or parts. Corn kernels are made up of four parts, the germ or embryo, the tip cap, the pericarp or outer covering, and the endosperm. The endosperm represents about 82% of the kernel, but consists of soft (also referred to as floury or opaque) endosperm and of horneous (also called hard or vitreous) endosperm as shown to the right. The endosperm contains primarily starch and protein, while the germ contains oil and some proteins, and the pericarp and tip cap are mostly fiber.
The following tests reflect these intrinsic parts of the corn kernels, in addition to the growing and handling conditions that affect corn quality.
Stress cracks are internal fssures in the horneous (hard) endosperm of a corn kernel. The pericarp of a stress-cracked kernel is typically not damaged, so the outward appearance of the kernel may appear unaffected at first glance.
The cause of stress cracks is pressure buildup due to large moisture gradients and temperature gradients within the kernel’s horneous endosperm. This can be likened to the internal cracks that appear when an ice cube is dropped into a lukewarm beverage. The internal stresses cannot build up as much in the soft, floury endosperm as in the horneous endosperm; therefore, corn with a higher percent of horneous endosperm is more susceptible to stress cracking than softer grain with a lower percent of hard endosperm. A kernel may have one, two, or multiple cracks. High-temperature drying is the most common cause of stress cracks. The impact of high levels of stress cracks on various uses includes:
General Increased susceptibility to breakage during handling, leading to increased broken corn needing to be removed during cleaning operations for processors, and possible reduced grade/value.
Wet Milling Lower starch yield because the starch and protein are more difficult to separate. Stress cracks may also alter steeping requirements
Dry Milling Lower yield of large flaking grits (the prime product of many dry milling operatons).
Alkaline Cooking Non-uniform water absorption leading to overcooking or undercooking, which affects the process balance.
Growing conditions will greatly affect the need for artificial drying, thus influencing the degree of stress cracking found from region to region. For example, late maturity and late harvest due to factors such as rain-delayed planting or cool temperatures tend to increase the occurrence of stress cracks due to the need for artificial drying.
Measurements of stress cracks include Stress Crack Percent (the percentage of kernels with at least one crack) and Stress Crack Index (SCI) which is the weighted average of single, double and multiple stress cracks. The Stress Crack Percent reports only the number of kernels with stress cracks whereas SCI shows the severity of cracking. For example, if half the kernels have only single stress cracks, the SC% is 50 and the SCI is 50. However, if all the cracks are multiple stress cracks, indicating higher potential for handling issues, the SC% remains at 50 but the SCI becomes 250. Lower numbers for the percentages and index are always better. In years with very high stress crack percentages, the SCI is valuable because high SCI numbers (perhaps 300 to 500) indicate the sample had a very high percentage of multiple stress cracks. Multiple stress cracks are somewhat more detrimental to quality changes than single stress cracks.
- Stress cracks of U.S Aggregate corn averaged 3.0%.
- Stress cracks ranged from 0% to 40% with a standard deviation of 3.0%2.
- Stress cracks distribution showed 96.2% of samples with less than 10% stress cracks.
- The percent of stress cracks for all regions including the Gulf, Pacific Northwest and Southern Rail areas was extremely low averaging only 2.0% to 3.0%.
- Stress crack index (SCI) had a very low Aggregate average of 4.6 from a range of 0 to 129, which indicates a very low amount of stress-cracked kernels had multiple stress cracks; samples with high SCI were few and far between.
- Over 97% of the samples had an SCI of less than 40, indicating very few kernels had double or multiple stress cracks. This is the normal expectation at the first point of delivery.
- The low levels of stress cracks observed should indicate reduced rates of breakage when corn is handled, improved wet milling starch recovery, improved dry milling yields of flaking grits, and good alkaline process ability.
100-Kernel Weight, Kernel Volume and Kernel True Density
100-kernel weight (100-k weight) indicates larger kernel size as 100-k weights increase. Large kernels affect drying rates and large uniform-sized kernels often enable higher flaking grit yields in dry milling. Kernel weights tend to be higher for varieties with high amounts of horneous endosperm.
Kernel volume in cm3 is often indicative of growing conditions. If conditions are dry, kernels may be smaller than average. If drought hits later in the season, kernels may have lower fill. Small or round kernels are more difficult to degerm. Additionally, small kernels may lead to increased cleanout loss for processors and higher yields of fiber.
Kernel true density is calculated as the 100-kernel weight of a sample divided by the volume, or displacement, of those 100 kernels. True density is a relative indicator of kernel hardness, which is useful for alkaline processors and dry millers. True density, as a relative indicator of hardness, may be affected by the genetics of the corn hybrid and the growing environment. Corn with higher density is typically less susceptible to breakage in handling than lower density corn, but it is also more at risk for the development of stress cracks if high-temperature drying is employed. True densities above 1.30 g/cm3 would indicate very hard corn desirable for dry milling and alkaline processing. True densities near the 1.275 g/cm3 level and below tend to be softer, but will process well for wet milling and feed use.
- 100-k weight averaged 33.11 g for U.S. Aggregate corn with a range of 16.59 to 44.48 g/100 kernels. This shows a wide range of kernel sizes was found across all regions.
- The 100-k weights were distributed so that over 81% of the aggregate samples had 100-kernel weights of 30.0 g or greater.
- Kernel volume averaged 0.26 cm3 for U.S. Aggregate corn and ranged from 0.14 to 0.34 cm3.
- There was little difference in kernel volume among ECAs.
- Kernel true density averaged 1.267 g/cm3 for U.S. Aggregate corn. It ranged from 1.163 to 1.328 g/cm3.
- Between regions, Pacific Northwest had slightly lower average true density with 1.252 g/cm3.
- The Southern Rail region had the highest true densities averaging 1.273 g/cm3.
Though the name suggests some relationship between whole kernels and BCFM, the whole kernels test conveys different information than the broken corn portion of the BCFM test. Broken corn (BC) is defined solely by the size of the material. Whole kernels, as the name implies, is a measure of the quantity of fully intact kernels in the sample.
The exterior integrity of the corn kernel is very important for two key reasons. First, it affects water absorption for alkaline cooking operations. Kernel nicks or cracks allow water to enter the kernel faster than intact or whole kernels. Too much water uptake during cooking can result in expensive shutdown time and/or products that do not meet specifications. Some companies even pay extra premiums, over and above contracted premiums, for corn delivered above a specified level of whole kernels.
Second, an intact whole kernel is important for all corn that has to be stored or handled. Fully intact whole kernels are less susceptible to storage molds and breakage in handling. While hard endosperm texture lends itself to preservation of more whole kernels than soft corn, the primary factor in delivering whole kernels is handling during and after harvest. This begins with the combine confguration followed by the type, number and length of conveyance required to go from the farm to end user. All subsequent handling will generate additional breakage to some degree. Harvesting at higher moisture contents (e.g., greater that 25%) will usually lead to more damage to grain than harvesting at lower moisture levels (less than 18%).
- Whole kernel percentages averaged 93.8% for U.S. Aggregate corn with a range of 57.0% to 99.8%.3
- Whole kernel averages for Gulf, Pacific Northwest, and Southern Rail were 94.0%, 93.6%, and 93.2%, respectively.
- Over 88% of the U.S. Aggregate samples had whole kernels percentages of > 90%.
- Whole kernel percentages were relatively high and represent farm corn inbound to country elevators. The relatively high initial whole kernel percentages should reduce storage risk, and in combination with the low stress cracks enable reduced breakage in handling.
The % horneous endosperm test measures the percent of horneous or hard endosperm with a potential value from 70 to 100%. The greater the amount of horneous endosperm relative to soft endosperm, the harder the corn kernel is said to be. The degree of hardness is important depending on the type of processing; hard corn is needed to produce high yields of large flaking grits in dry milling, moderate to soft hardness for wet milling and livestock feeding, and medium to medium-high hardness is desired for alkaline cooking.
Hardness has been correlated to breakage susceptibility, feed utilization/efficiency and starch digestibility. As a test of overall hardness, there is no good or bad value for % horneous endosperm; there is only a preference by different end users for particular ranges. Many dry millers and alkaline cookers would like greater than 90% horneous endosperm, while wet millers and feeders would typically like values between 70% and 85%. However, there are certainly exceptions in user preference.
- Hard endosperm averaged 84% for U.S. Aggregate corn with a range of 71% to 95%.
- Hard endosperm percentages did not vary substantially across ECAs.
- U.S. Aggregate corn had 78.9% of the samples with greater than 80% hard endosperm.
Physical Factors Summary
- The low levels of stress cracks observed should indicate the potential for reduced rates of breakage when corn is handled, improved wet milling starch recovery, improved dry milling yields of flaking grits, and good alkaline process ability, but this potential may yet be affected by further drying and handling.
- Kernel true densities were in a medium range which should be good for wet milling and feeding, yet samples at the high levels (over 1.30 g/cm3) indicate availability of corn for dry milling and alkaline processing uses.
- The relatively high initial whole kernel percentages (93.8%) in combination with the low stress cracks percentage (3%) provides indication of good storable corn that should also have reduced breakage in handling.