Drying, Storing, and Handling Dry Edible Beans
August 21, 2008
Research has shown pinto beans should be stored at temperatures of 40 F or cooler to maintain color and cooking quality. The Hunter-L values, a measure of the lightness, for beans stored at specific temperatures and moisture contents are shown in Table 1. Lower numbers indicate a darker color. After 10 months of storage, pinto bean cooking times of 16% and 18% moisture beans stored at 20 F were only 1.2 times longer than before storage and at 40 F were only 1.7 times longer as shown in Table 2.
Table 2: Median pin cooking times for beans stored for 10 months at specified moisture contents and temperatures. Median cooking time before storage was 18.4 minutes. Shorter cooking times are preferred.
If the beans cannot be kept cool, the moisture content must be low enough to permit storage without deterioration at typical summer temperatures. The recommended moisture content for edible beans to minimize the growth of mold is about 13% at 70 F.
Pinto beans darken rapidly when exposed to light, so they should be stored in a dark environment. It is important to follow good storage management practices such as measuring the temperature and moisture content of the beans at least monthly. Whenever there is more than a 10-degree differential between the average outdoor temperature and the bean temperature during the fall, the beans should be cooled with aeration. This should continue until the beans are cooled at least to 40 F for short term storage and about 25 F for long-term storage.
To minimize the potential for mechanical damage, beans should be handled at moisture contents of about 16% or greater and at warm temperatures. Research shows that the potential for mechanical damage of pinto and navy beans increases at bean moisture contents of about 15% or lower, Table 3. Research also shows that the potential for mechanical damage of pinto and navy beans increases at lower bean temperatures, Table 4.
Belt conveyors are preferred due to their gentleness in conveying. The speed of auger rotation should be reduced and augers operated full to minimize damage. Elevator legs need to be adapted for handling beans, including reducing the discharge velocity and utilizing a method of gently slowing the beans at the bottom of spouts. The amount of damage as beans are dropped into a storage bin, either onto concrete or onto other beans, is large enough to justify using a bean ladder. The amount of damage to pinto beans and navy beans at 12%-13% moisture content increases linearly with drop height. Research showed the amount of damage, cracking, to navy beans when dropped onto concrete was 2% from 5 ft., 6% from 10 ft., and 12% from 15 ft. The amount of damage to navy beans when dropped onto other beans was 2% from 5 ft., 3% from 10 ft., and 8% from 15 ft., slightly less but still excessive. The amount of damage to pinto beans when dropped onto concrete was 3% from 5 ft., 9% from 10 ft., and 15% from 15 ft. The amount of damage to pinto beans when dropped onto other beans was 1% from 5 ft., 7% from 10 ft., and 10% from 15 ft. Beans at 16% moisture content had less damage than beans at 13%, but it was still excessive. This validates the need to use a bean ladder when placing beans into storage or other places where the beans might be dropped.
Natural air drying will work well for drying edible beans during mid-September to mid- October in North Dakota. Based on average climatic conditions the beans are expected to dry to about 12 to 14% moisture if the fans are operated continuously.Shutting fans off during the warmest and driest part of the day will permit drying the beans to about 15% to 16%, but will lengthen the drying time. Fans should run during the night with higher humidity to permit drying the beans nearer to the desired final bean moisture content. Adding supplemental heat reduces the final moisture content of the beans and will likely result in beans dried to moisture content lower than desired. Shut fans off during foggy or rainy weather, but do not leave the fan off for more than a couple days to minimize the potential for bean spoilage. Recommended minimum air-flow rates for various moisture contents and the corresponding estimated drying times are shown in Table 5.
The static pressure associated with moving air through pinto and navy beans is equivalent to that of soybeans. Design the drying system using the data for soybeans.
Edible beans require special care when drying with a high temperature column dryer. The relative humidity of the drying air should not be lower than about 30%. Normally the drying should occur with the air heated less than about 20 F above the outdoor air temperature, to keep the relative humidity above 30%. The beans need to be monitored continuously to assure that the beans are not being damaged. The drying process needs to be slow to minimize damaging the beans and will be slow in comparison to drying cereal grain.
Dr. Kenneth Hellevang is with the NDSU Extension Service, Agricultural & Biosystems Engineering Department