and CO2 exchanges and the respiration levels of the biotic portion of grains. Higher initial moisture content of grains tends to facilitate increased CO2 concentration due to higher respiration rates (Cardoso et a. 2008; Kumar and Kalita 2017).
Fig. 4.6. Interaction of grain moisture, storage temperature and equilibrium relative humidity at which different organisms can grow in storage.
Source: Bradford et al. (2018).
Three factors – moisture content of seeds, storage temperature, and equilibrium relative humidity (RH) – are the most important determinants of grains quality, as shown in Figure 4.6 (Bradford et al. 2018), including legumes (Sangeetha and Mohan 2020). Storage life of grains increases exponentially as the equilibrium RH humidity and temperature decrease (Bradford et al. 2018). Besides microbial and insects‐induced quality deterioration, the specific quality changes attributed to storage are associated with flavor (mustiness, sour/bitter), discoloration (browning, darkening), and “hard‐to‐cook” (HTC) defects (reduced imbibition, longer cooking time). It is well documented that under adverse storage conditions, storage defects such as “bin burn,” “hard‐shell” and HTC phenomena occur, resulting in a significant loss of bean quality and economic value (Paredes‐Lopez et al. 1989; Siqueira et al. 2018; Chu et al. 2020).
The improved utilization of dry beans can be maximized through a detailed understanding of the impact and control of postharvest handling, storage, and packaging. The overall final bean quality is directly associated with the control of critical physical, chemical, and biochemical processes during production and postharvest handling and storage (Uebersax and Siddiq 2012).
Bean storage facilities
Dry beans are commonly stored in a wide variety of structures constructed as wood cribs, concrete silos, or steel bins. These vary based on the availability of existing infrastructure and general market conditions. Concrete silos have been noted for their capacity, structural strength, and general mass. These facilities resist rapid temperature change, which impacts the internal temperature of both the beans and the interspatial air. Storage facilities, particularly in northern latitudes, are exposed to the heat of the southern sun, and such surface heating will cause differential temperature changes within the stored beans and result in moisture migration and localized regions of high moisture spoilage. Concrete has been demonstrated to resist this temperature gradient better than other materials (Roberston and Frazier 1978; Maraveas 2020).
Economical flat‐storage, in which beans are piled on a reinforced concrete floor, has gained popularity due to the availability of free‐span pole building construction, rapid and flexible filling with adequate control of seed coat checking and splitting, and a high capacity to cost benefit ratio. It is noted that this technique requires beans of stable moisture content, and full knowledge of the angle of repose (which is the naturally self‐aligning angle that the bean pile assumes). Exceeding this angle will result in lateral pressure and cause cascading avalanches that can readily damage or destroy the side walls of the facility (Uebersax and Siddiq 2012).
Steel bins are arguably the most common and are available in different sizes (height and diameter). These are used in both elevator and on‐farm systems. Steel bins are relatively simple and can be easily installed on site. They are positioned on a poured support footing, and side walls are made by bolting curved sections of corrugated steel into place to form a circular bin structure. Successive tiers are added to the designated height, and a steel roof cap is bolted in place. These bins are equipped with mechanical aeration systems and belt conveyors that deliver beans into and out of bins. All of these facilities are equipped with bean ladders (generally, a spiral chute or alternating plates in a zig‐zag formation) that allow the beans to descend into the silo or bin with minimum damage (Roberston and Frazier 1978).
Dry beans may be stored in pallet‐sized tote boxes constructed of either wood, cardboard, or polypropylene sacks each containing up to one ton of product. These systems provide direct and flexible handling (using a forklift truck) of small lots without excessive bean movement, thus reducing seed coat checking. It is common for beans with highly differentiated quality standards, such as dark and light kidneys, cranberry, and the limited quantities of specialty beans such as yellow eyes, soldier bean and heritage varieties to be handled on‐farm and in local elevator facilities using individual tote storage and handling. Totes may be carefully off‐loaded to bagging systems or shipped directly to the end users (Roberston and Frazier 1978; Uebersax and Siddiq 2012).
Bean drying and aeration
Dry beans must be aerated and monitored for temperature to reduce the accumulation of localized moisture during storage. High moisture beans (> 18%) will result in mold growth, which will lead to storage shelf‐life reduction and quality loss. Moisture migration and localized accumulation usually begins soon after harvest (typically initiated in the fall and continuing into the early stages of the winter season). As is typical in all stored grains, convection currents form within the grain mass as cooler weather causes air at bin walls to cool and settle, and warmer air to rise within the grain mass (Roberston and Frazier 1978). Condensation occurs when this warm air comes in contact with the cold grain and spoilage results. Convection air currents move within stored grain when declines in external temperatures chill the internal surfaces of the bin. Air near the bin walls is cooled and settles, causing the warmer air in the center of the bin to rise. When the warmer moist air from the center of the bin comes in contact with the cold grain at the surface, it is chilled, and moisture condenses on the upper bean layers. This moisture is absorbed by the beans in the surface layer, causing an increase in moisture content and resulting in mold growth, crusting, and spoilage. It is important that the bean mass is uniform and does not enable air current channelling. The scalping operation (pre‐cleaning) is essential to remove plant material, splits and fines that will likely cause disruption of airflow and inefficient and ineffective bean aeration. To provide uniform cooling and drying, duct systems located at the base of the bins are used (Uebersax and Siddiq 2012).
Aeration is the practice of moving large volumes of air at low flow rates sufficient to cool all beans within a bin. With the proper flow rate, relative humidity and temperature, stored bean quality can be stabilized. Aeration prevents moisture migration and also reduces mold growth since mold activity decreases rapidly at temperatures below 70°F. Most field and storage molds become inactive at 50°F. Aeration can also reduce, but not eliminate, musty odors and off‐flavors. It has been demonstrated that an airflow rate of 0.1−0.2 cubic feet per minute is desirable for on‐farm storage facilities. Specifically, any bean storage of greater than 1,000 bushels should be equipped with an integral aeration system (Maddex 1978).
Beans that are not sufficiently field‐dried at harvest are unsuitable for long‐term storage without artificial drying. This may be achieved by passing large volumes of warm air (generally between 105°F and 145°F) through the bean mass. Artificial drying of beans requires strict monitoring of the drying conditions. Excessively high drying temperatures will damage the external appearance (seed coat fracture and discoloration) and alter the inherent starch and protein functional properties. Beans may be dried in small batch lots on ventilated wagons or more commonly in designated drying bins. These batch systems establish moving air through static beans and will adequately remove moisture without seed coat damage. Rapid drying conditions can also produce case hardening in which seed surfaces are differentially dried relative to internal seed tissue, thereby resulting in excessive stress and increased seed coat damage (McWatters et al. 1988). Drying too slowly can create conditions favorable for mold growth,
