D. Physical Factors
1. 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 that measures to the nearest 0.1 mg. The averaged 100-kernel weight is reported in grams.
The kernel volume for each 100-kernel replicate is calculated using a helium pycnometer and is expressed in cubic centimeters (cm3) per kernel. Kernel volumes usually range from 0.14 cm3 to 0.36 cm3 per kernel for small and large kernels, respectively.
True density of each 100-kernel sample is calculated by dividing the mass (or weight) of the 100 externally sound kernels by the volume (displacement) of the same 100 kernels. The two replicate results are averaged. True density is reported in grams per cubic centimeter (g/cm3). True densities typically range from 1.20 g/cm3 to 1.30 g/cm3 at “as is” moisture contents of about 12% to 15%.
2. Stress Crack Analysis
Stress cracks are 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 horneous or 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) one crack; (3) two cracks; and (4) more than two cracks. Stress cracks, expressed as a percent, are all kernels containing one, two or more than two cracks divided by 100 kernels. Lower levels of stress cracks are always better since higher levels of stress cracks lead to more breakage in handling. If stress cracks are present, singles are better than doubles or multiples. Some end users will specify by contract the acceptable level of cracks based on the intended use.
Stress crack index is a weighted average of the stress cracks. This measurement indicates the severity of stress cracking. Stress crack index is calculated as: [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 stress crack index can range from 0 to 500, with a high number indicating numerous multiple stress cracks in a sample, which is undesirable for most uses.
3. Whole Kernels
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 then 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 97.0% equates to a cracked & broken rating of 3.0%.
4. Horneous (Hard) Endosperm
The horneous (or hard) endosperm test is performed by visually rating 20 externally sound kernels, placed germ facing up, on a backlit viewing board. 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% to 100%, though most individual kernels fall in the 70% to 90% range.