Growers take extra care by flushing and cleaning their harvesting equipment when switching classes between fields (Bingen and Siyengo 2002). Buyers specify limits to mixed classes of beans based on quality although not injurious to health. Cross‐contamination with soybeans presents a significant allergen risk and is cause for load rejection at the elevator. Growers are diligent to maintain a barrier between fields and quite often use dedicated combines if they have both crops on their farm operation. Extra care is also taken to make adjustments within their respective harvests processes to minimize damage to the actual seed coat of the bean. The lower the percentage of cracked or checked seed coat the greater the quality because the bean maintains its integrity when canned. The higher the percentage of seed coats that have been fractured, the poorer the beans’ function when canned or cooked (J. Cramer 2020, personal communication).
Growers are paid for their beans on a cleaned, dry weight. Upon delivery at the elevator/processing facility, the load will be sampled and graded. Grading includes a visual inspection for safety, testing for moisture, screening for foreign material, and hand sorted to determine what percentage of the load is suitable for consumption. The elevator operator/processor also uses this grading process to determine where the load will be received (Bingen and Siyengo 2002). The lot is managed to assess need for additional mechanical drying, making beans suitable for long term storage.
POSTHARVEST OPERATIONS
Selected postharvest operation from harvesting to conveying and cleaning/sorting/grading and warehousing of dry beans are shown in Figure 4.2, which include:
1 Direct harvest combine for cutting upright beans. The cutter heads may be 40 feet across, and the full hopper of beans is directly transferred to a field truck for transport to elevator.
2 Dumping/unloading of truck at receiving station of elevator. The total weight and sample taken for quality grading that is used for payment to the grower.
3 Conveying beans: Beans are conveyed on rubber conveying belts (to minimize damage) for in‐plant bean cleaning.
4 Beans conveyed over air aspiration and “gravity table” for cleaning. Cleaning employs air aspiration (for removal of pods and light debris), size screening (removal of over and under sized beans and debris), and “gravity table” (removal of stones and mud balls).
5 Electric eye sorting machine. Beans are distributed in channels and individually passed over optical sensors. Out of specification beans are rejected with a blast of air (for removal of off‐colored beans).
6 Packaging in polyethylene (PE) bags. Beans in 100‐pound (45.5‐kg) PE bags are stacked on pallets for truck or rail car shipment.
7 Packaging in the “super sack” PE totes. Beans in 2,200‐pound (1,000‐kg) PE totes are warehoused for rail or truck shipment.A variety of high‐efficiency equipment is used to perform above‐mentioned postharvest operations; further details are discussed in the following sections.
Fig. 4.2. Harvesting through packaging and warehousing of dry beans. (For color detail, please see color plate section.)
Source: Original images by authors.
Conveying and transfers
Following harvest, beans are delivered to the elevator where samples are taken from each load to check for quality, color, and foreign material. All edible dry beans have a thin seed coat layer that is easily damaged in handling or drying. The bean moisture content is among the most important considerations following harvest (Rodiño et al. 2011; Bradford et al. 2018). Unless specific handling equipment is available, most farm operators should not consider moving beans to market if the moisture content is below 15% or above 18% (Maddex 1978). Most beans are harvested with combines, which results in direct mechanical damage to the beans in part due to impact loading. Mechanical damage can result from bean impact velocity, moisture content, temperature, and the size of bean.
It is essential to minimize seed coat checking and splitting during all stages of physical handling of dry beans (Aguilera and Rivera 1990). Bean seed coat damage is cumulative during each stage of handling from harvest to final distribution and preparation. Damage is most readily associated with mechanical dropping and shattering during conveyor transfers. Augers and poorly designed bucket lifts are particularly detrimental and result in high checked seed coats due to shearing action and fractional abuse to the seed. Conveyor belts that result in stationary aggregated beans during conveyance are superior to system designs in which “individual beans are dynamic” resulting in bean‐to‐bean and bean‐to‐conveyor abrasion.
Bean dropping has a profound influence on seed coat damage (Shahbazi et al. 2011). Emptying of combine bins into wagons or trucks and truck unloading during elevator receipt can result in severe bean impact. The filling of bins and silos or dumping into free‐standing piles are areas where significant seed coat damage may occur. It is recommended that minimum drop distances be maintained. It is recommended to establish a continuous flow from truck to bean pile rather having beans striking floors or grates when pit dumping. The length of drop into the bin is critical to control seed coat damage. Bean ladders are used to reduce the “free fall of beans” when they are filled into the storage facility. Construction of bean ladders within bins and silos enables beans to descend by sliding on a spiral ramp to levels ranging from the floor to any subsequent elevation within the storage structure.
Conveyor belts are used to move beans in and out of storage to reduce damage that may occur with auger conveyors. Although belt conveyors are more expensive than general grain handling augers, the reduction in damage typically makes the investment worthwhile. Belt conveyors for under‐bin‐flow unloading or for replacement of augers in dryers are custom‐made items. Belt conveyors are usually larger in size than auger conveyors thus require additional space (Uebersax and Siddiq 2012).
Receiving, cleaning, and separation
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