It is often said that grain quality can never improve after it’s been harvested. No matter how well an elevator dries their grain, monitors its bin temperature, manages moisture and controls pests, not much can be done to bring the quality up a notch once it’s taken from the plant. For the most part, this notion holds true with the exception of one measure: grain cleaning. Taking this step right after harvesting can get the whole process off to a smooth start. While grain cleaning doesn’t physically add anything to the grain, it certainly adds value by removing foreign materials, making it a more marketable product.
Clean grain is achieved through a variety of products such as screens, vibrators, scalpers and aspirators. Grain passes over screens while different sized holes allow unwanted pieces to fall through, leaving only the desired size or shape grain behind. Also, grain is screened to remove oversized and foreign materials. Elevator operators determine how much foreign matter, or dockage, to remove based on specifications handed down from their customers or the Federal Grain Inspection Service. These specifications depend on the type of grain and its intended destination.
According to experts in the cleaning field, the dockage is the most important player in the grain cleaning game. Knowing the desired specifications is a good starting point, but developing the best cleaning approach to achieve those specs depend on what you need to remove from the incoming product.
Dockage refers to several types of foreign materials such as straw, chaffs, hulls, seeds, dust, sand, pests and damaged or immature kernels. The FGIS puts strict standards on how much of this material can be present within the crop. The website has published the official U.S. standards for all grains, such as corn, barley, wheat, soybeans and sorghum, among others.
The FGIS defines dockage differently for each grain. According to the FGIS website, corn dockage is “all matter that passes readily through a 6/64 round-hole sieve and all matter other than corn that remains on top of the 12/64 round-hole sieve.”
The FGIS provides detailed information about the sieve size to which it refers. A 6/64 round-hole sieve is a 0.032-inch thick metal sieve with round perforations 0.0937 (6/64) inch in diameter. The holes must be placed so that each center is 5/32 inch apart. The perforations of each row must be staggered in relation to the adjacent row.
A 12/64 round-hole is the same thickness screen, but with larger perforations that are 0.1875 (12/64) inch in diameter. These holes must be spaced apart a quarter inch from the center of one hole to the center of the next. The perforations of each row must be staggered, as well.
Reasons for cleaning
The simplest explanation as to why grain should be cleaned is to remove dockage. But clean grain operations do not exist simply for the sake of cleanliness. Cleaning grain aids in providing the end user with a more consistent product that can be further processed for milling, malting, human or animal food consumption or seed processing.
By cleaning grain before storage, the handling characteristics of the product improve by helping prevent spoilage and making it easier to dry. Steve Schmidt, senior applications engineer at ROTEX Global, LLC., explains why cleaning grain before storage is an advisable practice: “Removing the cobs and fines that tend to accumulate in the center of the storage bin allows for uniform airflow and monitoring during storage. As mold grows faster on broken kernels, efficient cleaning of the grain will remove the broken kernels and reduce the chance of mold in storage,” Schmidt says.
For exporters, meeting specifications is imperative to ensure shipments won’t be rejected. Countries conduct diagnostics once the product reaches their port and if it doesn’t meet a certain grade requirement, significant penalties will be assessed. Grain exporters should aim for only 1% dockage in order to avoid the possibility of rejection.