Physical factors are other quality attributes that are neither grade factors nor chemical composition. Physical factors include stress cracks, kernel weight, kernel volume, 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, as well as 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 (9%) in 2017/2018 was slightly higher than 2016/2017 and 2015/2016, but slightly lower than 5YA.
- Of the 2017/2018 export samples, 16.0% had 15% or higher stress cracks, compared with only 3 to 5% in the previous two years. The higher harvest moisture in 2017 (16.6%) than in 2016 (16.1%) may have, in part, contributed to higher stress cracks found this year as compared with 2016/2017. The higher stress cracks indicate more breakage may occur during additional handling in 2017/2018 than in the previous two years.
- Average U.S. Aggregate stress crack index (SCI) (22.4) was slightly higher than 2016/2017 and 2015/2016, but slightly lower than 5YA.
- In 2017/2018, 48.7% of the samples had SCI of 20 or higher, compared with 20% in 2016/2017 and 21% in 2015/2016. This indicates more samples had double or multiple stress cracks in 2017/2018 than in the two previous years.
- Average U.S. Aggregate 100-kernel (100-k) weight (36.07 g) was higher than the past two years and 5YA, indicating larger kernels in 2017/2018 than in the previous two years.
- Average 100-k weight for the Gulf ECA (37.45 g) was higher than the Pacific Northwest ECA (31.12 g) but similar to the Southern Rail ECA (36.80 g).
- Average U.S. Aggregate kernel volume (0.28 cm3) was higher than 2016/2017, 2015/2016 and 5YA. Average kernel volume at export was lower than for 2017 harvest.
- Average kernel volume was lower for the Pacific Northwest ECA (0.25 cm3) than for the Gulf and Southern Rail ECAs (both 0.29 cm3) in 2017/2018. The Pacific Northwest ECA had either the same or the lowest average kernel volume for the previous three years and 5YA, indicating Pacific Northwest has usually had smaller kernel sizes than the Gulf and Southern Rail ECAs.
- Average U.S. Aggregate kernel true density (1.287 g/cm3) was slightly higher than 2016/2017, higher than 2015/2016 and similar to 5YA. For the 2017/2018 export samples, 83% had kernel true densities equal to or above 1.275 g/cm3, indicating a similar percentage of kernels with high true densities compared with 2016/2017 but higher than 2015/2016.
- The average percent of whole kernels at export (84.4%) was lower than 2016/2017, 2015/2016 and 5YA.
- The percentage of 2017/2018 export samples with whole kernels greater than or equal to 90% was 14.7%, compared with 39% in 2016/2017 and 50% in 2015/2016, indicating a much lower percentage of whole kernels in 2017/2018 than in the previous two years.
- Average U.S. Aggregate horneous endosperm (81%) was higher than 2016/2017, slightly higher than 2015/2016 and slightly lower than 5YA.
- Of the 2017/2018 export samples, 72.0% had at least 80% horneous endosperm, in contrast to 25% in 2016/2017 and 55% in 2015/2016. This indicates a higher percentage of the 2017/2018 samples contained high amounts of horneous endosperm than in the two previous years.