Cleaning operations consist of eliminating impurities from the grain mass. Sometimes it has to be done more than once through the postharvest system. It may be accompanied by a sorting of the beans according to quality. Cleaning is necessary before artificial drying, storage, marketing or further processing of the beans. After threshing, soybeans are contaminated by earth, plant and insect waste, small pebbles, etc. The beans are susceptible to physical damage during threshing and harvesting, resulting in seed breakage. The broken kernels and all kind of impurities hinder drying operations and make them longer and more costly. In addition, these impurities lower the quality of the product and are also the target for potential infestation during storage.

In the United States, generally, grain is not cleaned when it comes from off the farm. It is placed in bins according to quality so that it can be blended with grains of different quality when loaded out. In Argentina, since there is a premium for No. 1 soybeans, most soybeans are cleaned to less than 1 percent foreign material. In Brazil, soybeans that exceed Brazilian export quality (foreign material 1 percent) are cleaned. In Canada, very little cleaning is done at first point of receipt (Krischik, 1995).

The simplest cleaning method consists of tossing the beans into the air and letting the wind carry off the lightest impurities. This cleaning method does not eliminate the heavier impurities. Cleaner-separator machines (Figure 15) are used when large quantities of beans have to be cleaned. They are motor-driven and consist mainly of a reception hopper, a fan and set of vibrating sieves. Cleaning is done by repeated suction of the lightest impurities, followed by siftings of the beans.

Aspiration cleaning of soybean samples containing 0.5 to 4.0 percent foreign material and 3 to 22 percent splits was reported by Hurburgh et al. (1996). Air velocities of 19 m/s (3 500 ft/min) and 10 m/s (1 970 ft/min) were used to clean the samples. The high airflow rate removed 1.1 percent of whole soybeans compared with 0.4 percent at low airflow rate. At either airflow rate, the aspirator removed less saleable material and more non-grain material than previously reported for screen cleaning.

 Figure 15. Grain cleaning equipment, left: overall view; right: close-up.