Stress cracks are internal fissures in the horneous (hard) endosperm of a corn kernel. The pericarp (or outer covering) of a stress-cracked kernel is typically not damaged, so the kernel may appear unaffected at first glance, even if stress cracks are present.
The cause of stress cracks is pressure buildup due to moisture 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 hard, horneous endosperm; therefore, corn with a higher percentage of horneous endosperm is more susceptible to stress cracking than softer grain. A kernel may vary in severity of stress cracking and can have one, two, or multiple stress cracks. The most common cause of stress cracks is high-temperature drying that rapidly removes moisture. The impact of high levels of stress cracks on various uses includes:
General: Increased susceptibility to breakage during handling. This may lead to processors needing to remove more broken corn during cleaning operations, and a possible reduction in grade and/or value.
Wet Milling: Lower starch yields due to the increased difficulty in separating starch and protein. Stress cracks may also alter steeping requirements.
Dry Milling: Lower yield of large flaking grits (the prime product of many dry milling operations).
Alkaline Cooking: Non-uniform water absorption leading to overcooking or undercooking, which affects the process balance.
Growing conditions will affect crop maturity, timeliness of harvest, and the need for artificial drying, which will influence the degree of stress cracking found from region to region. For example, late maturity or late harvest caused by weather-related factors, such as rain-delayed planting or cool temperatures, may increase the need for artificial drying, thus potentially increasing the occurrence of stress cracks.
Stress crack measurements include “stress cracks” (the percentage 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” measures only the number of kernels with stress cracks, whereas SCI shows the severity of stress cracking. For example, if half the kernels have only single stress cracks, “stress cracks” is 50% and the SCI is 50 (50 X 1). However, if half the kernels have multiple stress cracks (more than two cracks), indicating a higher potential for handling breakage, “stress cracks” remains at 50%, but the SCI becomes 250 (50 X 5). Lower values for “stress cracks” and the SCI are always more desirable. In years with high levels of stress cracks, the SCI provides valuable information 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.
U.S. Aggregate stress cracks in 2016 averaged 4%, above 2015 (3%), but below 2014 (8%) and 5YA (5%).
U.S. Aggregate stress cracks standard deviation (6%) in 2016 was higher than 2015 (5%), lower than 2014 (9%), but same as 5YA (6%).
Stress cracks ranged from 0 to 84% in 2016, whereas the ranges were from 0 to 75% in 2015, and 0 to 100% in 2014.
There was a high percentage of samples with less than 10% stress cracks in 2016 (91.7%), similar to 2015 (93%), and more than 2014 (79%). Also in 2016, 4.8% of the samples had stress cracks above 20%, which is similar to 2015 (3%), but lower than 2014 (9%).
Stress crack distributions indicate that 2016 corn should have low susceptibility to breakage, similar to that found in 2015.
Stress crack averages in 2016 for Gulf, Pacific Northwest, and Southern Rail ECAs were 4%, 5%, and 3%, respectively. Among all ECAs, the Southern Rail either had the lowest stress cracks or tied for lowest stress cracks in 2016, 2015, 2014, and 5YA.
U.S. Aggregate SCI in 2016 averaged 8.8, above 2015 (6.6), but below 2014 (20.2) and 5YA (12.7).
U.S. Aggregate SCI was more variable in 2016 (standard deviation of 16.6) than in 2015 (11.7), but less than in 2014 (27.7) and 5YA (18.9).
The 2016 SCI had a range of 0 to 268, wider than 2015 (0 to 180) and narrower than 2014 (0 to 410).
Of the 2016 samples, 94.4% had SCI of less than 40, which is about the same as 2015 (96%) but higher than 2014 (89%) samples. Only 3.4% of the 2016 samples had SCI higher than 80, compared to 2% of the 2015 samples and 7% of the 2014 samples.
SCI averages for the Gulf, Pacific Northwest, and Southern Rail ECAs were 8.9, 10.3, and 5.8, respectively.
The Southern Rail ECA had the lowest SCI in 2016, 2015, 2014, and 5YA. The lower SCI found for the Southern Rail ECA is likely related to greater field drying potential typically found in the states that constitute the Southern Rail ECA.
The high percentage of the 2016 crop having near 75% Good or Excellent crop growing conditions came with good maturation and grain filling, early harvest conditions, and favorable field drying weather. This led to less artificial drying needed than in wetter years and the relatively low stress cracks and SCI found in 2016.