Physical factors are other quality attributes that are neither grade factors nor chemical composition. Physical factors include stress cracks, kernel weight, kernel volume and true density, percent whole kernels, and percent horneous (hard) endosperm. Tests for these physical factors provide additional information about the processing characteristics of corn for various uses, as well as corn’s storability and potential for breakage in handling. These quality attributes are influenced by the physical composition of the corn kernel, which is in turn affected by genetics and growing and handling conditions. 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, and consists of soft (also referred to as floury or opaque) endosperm and of horneous (also called hard or vitreous) endosperm, as shown above. The endosperm contains primarily starch and protein, the germ contains oil and some proteins, and the pericarp and tip cap are mostly fiber.
Summary: Physical Factors
Average U.S. Aggregate stress cracks (4%) and stress crack index (SCI) (8.4) were slightly higher than 2015, but lower than 2014 and 5YA, indicating corn’s susceptibility to breakage should be similar to last year, but below 2014 and 5YA.
Among the ECAs, the Southern Rail ECA had the lowest SCI average in 2016, 2015, 2014, and 5YA. The Southern Rail also had the lowest stress cracks averages in 2016 and 5YA.
Average U.S. Aggregate 100-k weight (35.20 g) in 2016 was higher than 2015, 2014, and 5YA.
Average U.S. Aggregate kernel volume (0.28 cm3) in 2016 was higher than 2015, 2014, and 5YA. There was also a higher percentage of large kernels in 2016, compared to the previous two years.
The Pacific Northwest ECA had the lowest kernel volume average and lowest 100-k weight average of the ECAs in 2016, 2015, 2014, and 5YA.
U.S. Aggregate kernel true density averaged 1.258 g/cm3 in 2016, which was higher than 2015, similar to 2014, but lower than 5YA. Over the past six years, true densities have tended to be higher in years with higher protein.
True density kernel distributions above 1.275 g/cm3 in 2016 indicate slightly softer corn in 2016 and 2015 than in 2014. Of the ECAs, the Pacific Northwest had the lowest true density and lowest test weights in 2016, 2015, 2014, and 5YA.
U.S. Aggregate whole kernels averaged 95.2% in 2016, higher than 2015, 2014, and 5YA.
There was a higher percentage of whole kernels in 2016 and 2015 than in 2014. The relatively high percentages of whole kernels and low stress crack percentages indicate that the 2016 corn crop should handle well with minimal breakage.
Average U.S. Aggregate horneous (hard) endosperm (79%) was same as 2015, and lower than 2014 and 5YA. The distributions of horneous endosperm percentages indicate a higher percentage of corn samples with soft endosperm in 2016 and 2015 than in 2014.
Horneous endosperm and true density appear to change in the same direction, with higher values in a drought year, such as 2012, and lower values in higher-yielding years, such as 2016 and 2015.