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Corn Export Cargo Quality Report 2011/2012

Contents

  1. Export Cargo Quality Highlights
  2. Survey Overview
  3. Grade Factors and Moisture
  4. Chemical Composition
  5. Physical Factors
  6. Mycotoxins
  7. Corn Export System
  8. U.S. Export Corn Flow
  9. Impact of the Corn Market Channel on Quality
  10. U.S. Government Inspection and Grading
  11. Survey Design and Sampling
  12. Statistical Considerations
  13. Corn Grading Factors
  14. Chemical Composition
  15. Physical Factors
  16. Grade Requirements and Conversions
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Physical Factors

There are tests for other quality attributes that are not grading factors, or chemical factors. 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, 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

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 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 do not build up as much in the soft, floury endosperm as in the horneous endosperm; therefore, corn with higher percentages of horneous endosperm is more susceptible to stress cracking than softer grain with lower percentages of horneous endosperm. A kernel may have one, two, or multiple cracks. High-temperature drying is the most common cause of stress cracks, but handling impacts can also increase stress cracks. The impact of high levels of stress cracks on various uses includes:

  1. 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. It also lowers germination.
  2. Wet Milling – Lower starch yield because the starch and protein are more difficult to separate. Stress cracks may also alter steeping requirements.
  3. Dry Milling – Lower yield of large flaking grits (the prime product of many dry milling operations).
  4. Alkaline Cooking – Non-uniform water absorption leading to overcooking or undercooking, which affects the process balance.

Growing conditions greatly affect the need for artificial drying and influence the degree of stress cracking found from region to region. Then, as corn moves through the market channel, some stress-cracked kernels break, increasing the proportion of broken corn. Concurrently, impacts of kernels on other kernels or metal during handling may cause cracks in new kernels. As a result of stress-cracked corn becoming broken corn and kernels with no previous stress cracks developing stress cracks during handling, the overall percentage of kernels with stress cracks may or may not remain constant through the merchandising channel. Whether or not the stress cracks levels remain constant depends on the severity of the impacts.

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 percentage of multiple stress cracks. Multiple stress cracks are generally more detrimental to quality changes than single stress cracks.

Highlights

  • Stress cracks of exported U.S Aggregate corn was higher than int he Harvest samples (10% versus 3%), but still at a very low level.
  • Stress cracks ranged from 0 to 33% with a standard deviation of 5.0%.
  • Distribution of percentage of stress cracks showed 93.4% of the samples with less than 20% stress cracks at export. While this was lower than the 98.1% at harvest, it indicates the corn should still handle very well with relatively low amounts of breakage. The percent of stress cracks for the Gulf, Pacific Northwest and Southern Rail ECAs was low with 12%, 5%, and 4%, respectively. The Gulf ECA’s average stress cracks were significantly higher than the other two ECAs.
  • Stress crack percentages for contracts loaded as U.S. No. 2 o/b were 9.0%, slightly lower than the 11.0% found for contracts loaded as U.S. No. 3 o/b. Not surprisingly, contracts loaded as U.S. No. 2 o/b had BCFM (2.7%) which was slightly lower than the 3.4% BCFM found for contracts loaded as U.S. No. 3 o/b. Thus, contracts with higher BCFM also had slightly higher stress crack percentages.
  • U.S. Aggregate SCI average of 30.8 at export was low, minimizing breakage during loading and discharge.
  • In the Export Report, 68.6% of the samples had SCI of less than 40, indicating relatively few kernels had double or multiple stress cracks.
  • The U.S. Aggregate SCI for contracts loaded as U.S. No. 2 o/b were 28.8, slightly lower than the 34.9 found for contracts loaded as U.S. No. 3 o/b.
  • The relatively low levels of stress cracks observed for 2011 corn should indicate reduced rates of breakage when corn is loaded and discharged, improved wet milling starch recovery, improved dry milling yields of flaking grits, and good alkaline processability.

100-Kernel Weight, Kernel Volume and Kernel True Density

100-kernel (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 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.

Highlights

  • 100-k weight averaged 35.14 g for U.S. Aggregate Export corn sampleswith a range of 28.24 to 39.30 g. The Export Report samples had greater uniformity than the Harvest Report corn as indicated by a tighter range and lower standard deviation.
  • The 100-k weights were lowest for the Pacific Northwest ECA.
  • The 100-k weights were distributed such that 55.7% of the aggregate samples had 100-k weights of 35 g or greater.
  • Kernel volume averaged 0.27 cm3 for U.S. Aggregate export corn samples and ranged from 0.22 to 0.30 cm3. The range and standard deviations were less in Export than with Harvest Report samples, therefore, showing more uniformity.
  • The kernel volumes were smallest (0.26 cm3) for the Pacific Northwest ECA than for the other ECA’s.
  • About 84.1% of the U.S. Aggregate samples had kernel volumes equal to or greater than 0.26 cm3.
  • Kernel true density averaged 1.291 g/cm3 for U.S. Aggregate Export Report corn samples and ranged from 1.244 to 1.327 g/cm3, slightly higher than for Harvest Report samples. This apparent increase in true density is likely due in part to lower moisture at export (14.3% as compared to the 15.6% aggregate average for harvest samples) and that true density tests were performed on only whole, fully intact kernels.
  • For the export samples, 88.2% had kernel true density equal to or above 1.275 g/cm3.
  • Among ECAs, Pacific Northwest had a lower average true density than the other two ECAs with 1.276 g/cm3 for export samples; similarly, it also had lowest average true density for the harvest samples.

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, any breaks in the kernel pericarp affect 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. Secondly, intact whole kernels are less susceptible to mold invasion during storage and to breakage during handling. Some companies pay extra premiums for contracted corn delivered above a specified level of whole kernels.

Highlights

  • Whole kernels averaged 87.5% for U.S. Aggregate corn at export level.
  • Whole kernel averages for Gulf, Pacific Northwest, and Southern Rail were significantly different with 87.5%, 88.9%, and 85.2%, respectively.
  • At the export level, 25.6% of the samples had whole kernels greater than 90%. Another 55.4% of the export samples were distributed in the 85 to 89.9% range.
  • The whole kernel percentages for contracts loaded as U.S. No. 2 o/b were 87.6%, essentially the same as the 87.8% found for contracts loaded as U.S. No. 3 o/b.
  • Whole kernels in Export Report samples were still relatively high and should help corn maintain quality during storage and enable relatively low breakage during handling.

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.

Highlights

  • Horneous endosperm averaged 84% for U.S. Aggregate corn at both export and harvest levels. Its range of 71 to 94% was essentially unchanged from the harvest level.
  • Horneous endosperm percentages varied little between the Gulf and Southern Rail ECAs but were significantly higher in the Pacific Northwest ECA.
  • Horneous endosperm percentages were similar between contracts loaded as U.S. No. 2 o/b and those loaded as U.S. No. 3 o/b.
  • U.S. Aggregate corn in Export Cargo Report samples had 97.9% of the samples with greater than 80% horneous endosperm, whereas in the Harvest Report, only 78.9% of the samples had greater than 80% horneous endosperm.

Physical Factors Summary

Highlights

  • The low levels of stress cracks (10%) in the Export Report samples indicate good 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 processability.
  • Kernel true densities (1.291 g/cm3) were higher for Export Report samples than Harvest Report samples. The slightly lower moistures at export may account for part of this increase as well as the fact that true densities were performed on completely whole, intact kernels. The lowest true densities were found in the Pacific Northwest ECA.
  • The relatively high whole kernels (87.5%) in combination with the low stress cracks (10%) at export indicate the corn should have reduced breakage during loading and discharge of the cargo.
  • Approximately 60% of Export Cargo Report samples had horneous endosperm less than 85%, indicating availability of corn with desirable softness for the wet millers and feeders.