D. Physical Factors
Physical factors are other quality attributes that are neither grading factors nor chemical composition. Tests for physical factors provide additional information about the processing characteristics of corn for various uses, as well as its storability and potential for breakage in handling. The storability, the ability to withstand handling, and the processing performance of corn are influenced by corn’s morphology. Corn kernels are morphologically 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, and 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, 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.
1. Stress Cracks
Stress cracks are internal fissures 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 even if stress cracks are present.
The cause of stress cracks is pressure buildup due to large moisture 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 do not build up as much in the soft, floury endosperm as in the hard, horneous endosperm; therefore, corn with a higher percentage of horneous endosperm is more susceptible to stress cracking than softer kernels with a lower percent of horneous 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 operations).
- Alkaline Cooking: Non-uniform water absorption leading to overcooking or undercooking, which affects the process balance.
Growing conditions will affect crop maturity, timeliness of harvest, and the need for artificial drying which will influence the degree of stress cracking found from region to region. For example, late maturity or late harvest caused by weather-related factors such as raindelayed planting or cool temperatures may increase the need for artificial drying, thus potentially increasing the occurrence of stress cracks.
Stress crack measurements include stress cracks (the percent of kernels with at least one crack) and stress crack index (SCI) which is the weighted average of single, double and multiple stress cracks. Stress cracks measure 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, stress cracks are 50% and the SCI is 50. However, if all the cracks are multiple stress cracks, indicating a higher potential for handling issues, stress cracks remain at 50% but the SCI becomes 250. Lower values for stress cracks and the SCI are always better. In years with high levels of stress cracks, the SCI is valuable because high SCI numbers (perhaps 300 to 500) indicate the sample had a very high percent 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 9% in 2013 which was significantly higher than the 4% found in 2012 and 3% found in 2011.
- Stress cracks ranged from 0 to 86% with a standard deviation of 10% (5% and 3% in 2012 and 2011, respectively).
- Stress cracks distribution showed 80.0% (90.8% in 2012 and 96.0% in 2011) of samples with less than 10% stress cracks.
- In 2013, there were 11.5% with stress cracks above 20% which is higher than the 3% in 2012 or 2% in 2011.
- Stress crack averages for the Gulf, Pacific Northwest, and Southern Rail ECAs were 9%, 10% and 5%, respectively.
- SCI had a U.S. Aggregate average of 22.8 which is significantly higher than the 9.3 found in 2012 and 4.6 found in 2011.
- The SCI had a range of 0 to 324 with a standard deviation of 35.1 in 2013, much higher than the 14.1 in 2012 or 6.0 in 2011.
- Of the samples, 85.7% had SCI of less than 40, which is lower than the 95.0% of the samples in 2012 that had SCI of less than 40. However, 9.2% had SCI greater than 80 which is more than the 2.0% in 2012 or 1.0% in 2011.
- SCI averages for the Gulf, Pacific Northwest, and Southern Rail ECAs were 23.5, 27.4 and 11.7, respectively.
- Stress crack index and stress crack percentages were both lower for the Southern Rail ECA.
2. 100-Kernel Weight
100-kernel (100-k) weight indicates larger kernel size as 100-k weights increase. Kernel size affects drying rates. As kernel size increases, the volume-to-surface area ratio becomes higher, and as the ratio gets higher, drying becomes slower. In addition, large uniform-sized kernels often enable higher flaking grit yields in dry milling. Kernel weights tend to be higher for specialty varieties of corn that have high amounts of horneous (hard) endosperm.
- 100-k weights of U.S. Aggregate samples averaged 33.41 g in 2013, compared to 34.53 g in 2012, but similar to 33.11 g in 2011.
- 100-k weights ranged from 18.07 to 45.09 g. The 100-k weight standard deviation of 2.88 g in 2013 was similar to standard deviations of 2.76 g and 2.64 g for the two previous years.
- The 100-k weights were distributed so that 39.0% of the aggregate samples had 100-k weights of 35 g or greater in 2013, compared to 48.3% in 2012 and 26.2% in 2011.
- 100-k weights for the Pacific Northwest ECA were lowest with 30.33 g compared to the Gulf and Southern Rail ECAs averaged 34.10 and 34.23 g, respectively.
3. Kernel Volume
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 losses for processors and higher yields of fiber.
- Kernel volume averaged 0.27 cm3 for U.S. Aggregate corn in 2013 which was the same as the 0.27 cm3 found in 2012 and higher than 0.26 cm3 found in 2011.
- Kernel volumes ranged from 0.15 to 0.36 cm3. Kernel volume standard deviations remained nearly constant at 0.02 cm3 over all three years.
- The kernel volumes were distributed so that 14.9% of the aggregate samples had kernel volumes of 0.3 cm3 or greater in 2013, compared to 10.9% in 2012 and 4.0% in 2011.
- Kernel volumes for the Gulf, Pacific Northwest and Southern Rail ECAs averaged 0.27, 0.24, and 0.27 cm3, respectively.
- The Pacific Northwest ECA had lower kernel volumes and lower 100-k weights than the other two ECAs.
4. Kernel True Density
Kernel true density is calculated as the weight of a 100-k 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.
- Kernel true density averaged 1.258 g/cm3 for U.S. Aggregate corn in 2013 which is significantly lower than the 1.276 g/cm3 found in 2012 and the 1.267 g/cm3 found in 2011.
- In 2013, true densities ranged from 1.157 to 1.326 g/ cm3. True density standard deviation was 0.021 in 2013, compared to 0.017 and 0.019 g/cm3 in 2012 and 2011, respectively.
- Kernel true densities were distributed so that only 34.6% of the samples were at or above 1.275 g/cm3 in 2013 compared to 52.3% of the samples in 2012 and 39.0% in 2011. This would indicate kernels in 2013 tend to have softer endosperm than in the two previous years.
- In 2013, kernel true densities for the Gulf, Pacific Northwest and Southern Rail ECAs averaged 1.261, 1.241, and 1.267 g/cm3, respectively. Similarly, the lowest test weight (56.5 lb/bu) was also found in the Pacific Northwest ECA.
- Similarly, test weight was significantly lower (57.9 lb/bu) in 2013 than in 2012 (58.8 lb/bu) and also somewhat lower than the 58.1 lb/bu found in 2011. The adjacent figure illustrates the positive relationship between kernel true density and test weight for the 2013 samples. The correlation coefficient was 0.84.
5. Whole Kernels
Though the name suggests some inverse relationship between whole kernels and BCFM, the whole kernels test conveys different information than the broken corn portion of the BCFM test. Broken corn is defined solely by the size of the material. Whole kernels, as the name implies, is the percent 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, above contracted premiums, for contracted 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 configuration 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 than 25%) will usually lead to more damage to grain than harvesting at lower moisture levels (less than 18%).
- Percent of whole kernels averaged 92.4% for U.S. Aggregate corn which was significantly lower than the 94.4% in 2012 and 93.8% in 2011.
- In 2013, whole kernels ranged from 73.6 to 99.6%. The standard deviation of 3.7% was similar to 3.4% and 3.9% in the previous two years.
- Of the U.S. Aggregate samples, 76.9% of the samples tested had 90% or higher whole kernels, compared to 89.5% in 2012 and 88.2% in 2011.
- Percent of whole kernel averages for Gulf, Pacific Northwest, and Southern Rail were 92.4%, 92.5%, and 92.5%, respectively. Thus, the percentages of whole kernels were essentially constant across all ECAs.
- Percent of whole kernels while lower than the previous two years was still relatively high when farm corn was delivered to local elevators.
6. Horneous Endosperm
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. Medium-high to medium hardness is desired for alkaline cooking. Moderate to soft hardness is used for wet milling and livestock feeding.
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.
- Horneous endosperm averaged 82% for U.S. Aggregate corn in 2013 and was down from 85% in 2012, and 84% in 2011 but was still relatively high for commodity corn.
- Of the U.S. Aggregate samples, 67.6% were equal to or greater than 80% horneous endosperm in 2013, which was considerably below the 87% found in 2012 and 78% found in 2011.
- Horneous endosperm averages for Gulf, Pacific Northwest, and Southern Rail were 83%, 80%, and 83%, respectively.
- Horneous endosperm ranged from 71 to 96% in 2013 compared to 74 to 97% in 2012. Thus, the lower percentages of horneous endosperm found in 2013 are consistent with the lower levels of true density found in 2013. The adjacent figure shows the weak but positive relationship (a correlation coefficient of 0.73) between horneous endosperm and true density for the 2013 samples.
SUMMARY: PHYSICAL FACTORS
- While average stress cracks and SCI were higher in 2013 than in 2012 and 2011, stress cracks were still sufficiently low.
- Average 100-k weights were lower in 2013 than in 2012 but similar to those in 2011. Average kernel volumes were the same as those in 2012. Kernel weights, volumes and true densities were much lower for the Pacific Northwest ECA than for the Gulf and Southern Rail ECAs.
- Kernel true densities were significantly lower in 2013 than in 2012 and also somewhat lower than those in 2011. Similar to true density, U.S. Aggregate average test weight and horneous endosperm percentages were lower in 2013 than in 2012. The horneous endosperm average of 82% indicates corn will be moderately lower in hardness than the previous two years but this average is still relatively hard for commodity corn.
- The average percentage of whole kernels was lower than that found for the previous two years. This still means that 92.4% of the kernels had pericarps fully intact which should store well and in combination with relatively low stress cracks should enable low breakage in handling.
- The moderately lower true density combined with relatively high starch content should indicate good availability of corn for wet milling.