Physical Factors
100-Kernel Weight, Kernel Volume and Kernel True Density
The 100-kernel weight is determined from the average weight of two 100-kernel replicates using an analytical balance with a minimum of four decimal places. The averaged 100-kernel weight is reported in grams.
The kernel volume is determined using a helium pycnometer to determine the volume (displacement) of the two replicates, and is expressed in cm3/100. Kernel volumes usually range from 0.18-0.30 cm3 per kernel for small and large kernels, respectively.
True Density is calculated as the mass (or weight) of the two replicates of 100 externally sound kernels by the volume (displacement) of the same 100-kernels. The replicate results are averaged. True Density is reported in grams per cubic centimeter (g/cm3). True densities typically range from 1.20 to 1.35 g/ cm3 at “as is” moistures of about 12 to 15%.
Stress Crack Analysis
Stress Crack Percent is evaluated by using a backlit viewing board to accentuate the cracks. A sample of 100 intact kernels with no external damage is examined kernel by kernel. The light passes through the hard endosperm so the severity of the stress crack damage in each kernel can be evaluated. Kernels are sorted into four categories: (1) no cracks; (2) 1 crack; (3) 2 cracks; and (4) more than 2 cracks. Stress Crack Percent is expressed as all kernels containing one, two or more than two cracks divided by 100 kernels. Lower Stress Crack Percent is always better since high stress crack percentages lead to more breakage in handling. If stress cracks are present, singles are better than doubles or multiples. Some corn end-users will specify the acceptable level of cracks based on the intended use.
Stress Crack Index (SCI) is a weighted average of the stress cracks. This measurement indicates the severity of stress cracking. SCI is calculated as
SCI = [SSC x 1] + [DSC x 3] + [MSC x 5]
Where
SSC is the percentage of kernels with only one crack,
DSC is the percentage of kernels with exactly two cracks, and
MSC is the percentage of kernels with more than two cracks.
The SCI can range from 0 to 500, with a high number indicating numerous multiple stress cracks in a sample, which is undesirable for most uses.
Whole Kernels/Cracked & Broken
In the Whole Kernels Test, 50 grams of cleaned (BCFM-free) corn are inspected kernel by kernel. Cracked, broken, or chipped grain, along with any kernels showing significant pericarp damage are removed, the whole kernels are weighed, and the result is reported as a percentage of the original 50 gram sample. Some companies perform the same test, but report the “Cracked & Broken” percentage. A Whole Kernels score of 97% equates to a Cracked & Broken rating of 3%.
% Horneous Endosperm
The horneous (or hard) endosperm test is performed by visually rating 20 externally sound kernels, placed germ facing up, on a light table. Each kernel is rated for the estimated portion of the kernel’s total endosperm that is horneous endosperm. Soft endosperm is opaque and will block light, while horneous endosperm is translucent. The rating is made from standard guidelines based on the degree to which the soft endosperm at the crown of the kernel extends down toward the germ. The average of horneous endosperm ratings for the 20 externally sound kernels is reported. Ratings of horneous endosperm are made on a scale of 70-100%, though most individual kernels fall in the 70-95% range.
Mycotoxin Testing
Detection of mycotoxins in corn is complex. The fungi producing the mycotoxins often do not grow uniformly in a field or across a geographic area. As a result, the detection of any mycotoxin in corn, if present, is highly dependent upon the concentration and distribution of the mycotoxin in a lot of corn, whether a truck load, a storage bin or a rail car.
GIPSA’s protocol requires a minimum of a 4,540 gram (10 pound) sample from large lots such as barges/sublots to grind for aflatoxin testing. The large sample size is used so the quantitative testing reflects the average mycotoxin concentration of the entire lot of corn in parts per billion (ppb). The objective of the GIPSA sampling process is to minimize under-estimating or over-estimating the true mycotoxin concentration since accurate results are imperative for corn exports. However, the objective of the Harvest Report assessment of aflatoxins was only to report the frequency of occurrences of the mycotoxin in the current crop, but not specific levels of the mycotoxin in corn exports. It was not feasible to collect 4,540 grams per sample for the Harvest Report aflatoxins testing, so a smaller sample size was used. Using a smaller sample size for testing for aflatoxins increases the potential for over- or under-estimating the specific level of aflatoxins in the sample if the aflatoxins levels are reported. However, only the number of instances above the specified threshold is being reported.
For this study, a 200 gram laboratory sample was subdivided from the 2 kg survey sample of shelled kernels for the aflatoxin analysis. The sample was ground in a mill to pass a 20 mesh screen. From this well-mixed comminuted material, a 40-gram test portion was removed for the testing. EnviroLogix AQ 109 BG test kits were used for the analysis, and the manufacturer – Envirologix – specifies extracting aflatoxins from 20 to 50 gram test portions. The aflatoxins were extracted with 50% ethanol (2:1). The extracts were tested using the Envirologix QuickToxTM lateral flow strips, and the aflatoxins were quantified by the QuickScanTM system. GIPSA has issued a Certificate of Conformance for the EnviroLogix QuickToxTM kit for QuickScanTM for quantitative aflatoxin determination in corn.
For the DON testing, the Romer AgraQuant test method, as approved by the USDA/GIPSA, was used. An approximately 1350-gram portion was ground by a Romer Mill to a particle size which would pass through a number 20 wire mesh sieve and divided down to a 50-gram sample using a riffle divider. The sample was then processed as the USDA/GIPSA DON (Vomitoxin) Handbook requires. The DON was extracted with 250 ml of distilled water, and the extracts were tested using the Romer AgraQuant micro well test kits. The DON results were read using the StatFax Reader.