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Upon delivery to the local elevator, the bean truck loads are weighed and then immediately dumped into a handling pit in the floor of the receiving area. This pit provides for aggregation of the beans for conveyance to subsequent cleaning operations. Again, caution is required in the flow rates and dropping distances of beans to minimize shattering of the dry seed coats (Bingen and Siyengo 2002).

The initial major unit operation is the direct application of high‐velocity air to pass through the beans and lift lighter‐than‐bean material, such as stems, leaves, pods, checked seed coats, and other forms of trash and plant material (Rodiño et al. 2011). A wide array of debris is readily removed from the dry beans during this forced air aspiration. If this material were contained in the beans during long‐term storage, it would have an adverse effect on airflow and moisture content, and the potential for excessive mold growth and subsequent off‐flavor development (Uebersax and Siddiq 2012). Thus, it is important to remove this lighter‐than‐bean material prior to long‐term storage of beans.

The next unit operation involves the removal of materials that are heavier than the beans. This is a complex operation designed primarily for removing stones and mud balls, which are a result of the harvest. The use of a gravity table provides a large surface area with vibrating mechanical force to remove these heavier materials. This shaking action and the slight incline of this gravity table impart greater energy to the denser stones or mud balls. Because of the density differences, the heavier materials proceed to move up the incline and are segregated over the edge of the table.

In the gravity separator, a large volume of dry beans is spread in a very thin layer and continuously moves over the gravity table. As this table rapidly shakes, it sorts the stones and mud balls and allows the beans to pass forward. The stones and mud balls are separated from the dry beans and are collected in a waste disposal bin. Stones and mud balls are characteristic of the geophysical structures within the bean‐growing region and may vary in size, shape, and density.

Bean size separation is achieved by screening over a series of sieves. This screening provides increased uniformity of size and shape within a specified lot of beans and separates materials, which are both larger and smaller than the desired bean. Thus, beans are collected as either “overs” or “throughs” with respect to the individual sieve size. The overs are materials that do not pass through the screen and the throughs are materials that do pass directly through the sieve. With the proper size configuration of a series of screens, the vast majority of both oversized and undersized material will be readily eliminated. Large seeds, small seeds, splits, stones, and additional mud balls can be removed through various screening operations (Uebersax and Siddiq 2012).

A final cleaning stage incorporates color sorting by equipment that possesses a series of photoelectric cells commonly referred to as electric eyes. Each individual bean is viewed for appearance. Each bean is channelled past a photoelectric cell positioned within subdivided individual lanes; discolored beans are rejected by a blast of air. This highly sophisticated cleaning system will result in improved uniformity of beans possessing “good” color with limited damage (Uebersax and Siddiq 2012). It is noted that beans of contrasting classes and other grains are removed based on color differences.

Bingen and Siyengo (2002) reported that more sophisticated electro‐mechanical cleaning and sorting equipment has set the conditions for elevators to respond to canning and processing standards, but it has also strengthened the role for individual expertise in grading. Instead of supplanting individual judgment, the investments in more sophisticated technologies require continued reliance on human expertise (the human eye) for grading beans.

Figure 4.3 shows the appearance of beans and rejected debris samples taken at various stages of dry bean cleaning operations. As with all agricultural commodities, appropriate and specific bean quality standards must be established to enable adequate commercial commerce. It is important that thorough sampling and systematic inspection of each bean lot is conducted to assure compliance with designated quality specifications.

Fig. 4.3. Appearance of samples taken at various stages of dry bean cleaning/sorting operations. (For color detail, please see color plate section.)

Source: Original images by authors.