Читать книгу Dry Beans and Pulses Production, Processing, and Nutrition - Группа авторов - Страница 79

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, resulting in deterioration of quality.