Farm Storage Recommendations for Dry Edible Beans
April 15, 2003
Ag. & Biosystems Engineering
North Dakota State University
During the 2001 harvest, it was observed that pinto beans harvested near the end of September were darker than beans harvested near the beginning of September. Darker beans may have a $1 to $6 per hundred weight lower value than lighter colored pinto beans. Eleven farmers from different areas in North Dakota collected bean samples while harvesting during September 2002. There was a lot of variability, but the lightness trended lower with later harvest dates. The measured reduction in lightness values from 57 to 55 is visually observable darkening. A difference in lightness for the different varieties was noted, but the sample sizes were not large enough to validate a difference. The lightness values observed at the various harvest dates are shown in Figure 1. A darkening of the pinto beans was noted when the length of the growing period exceeded 120 days. Beans harvested after a 110 days growing period had a 57 L-value and those harvested after a 130 days growing period had a 54.5 L-value. The data is shown in Figure 2. Some increase in the "redness" of the beans was noted with later harvest dates and longer growing periods. The harvest moisture content did not affect the lightness of the pinto beans.
The colorimeter used to measure the lightness and redness of the pinto beans is shown in the pictures below.
Bean breakage caused by harvesting and handling can be very costly. Research was conducted to determine the breakage susceptibility of pinto and navy beans related to bean moisture content and temperature. The amount of navy bean breakage was constant at moisture contents of 19% to 16%, then increased dramatically at moisture contents less than 16%; 4% at 16 - 19% moisture, 15% at 15% moisture, 22% at 14% moisture, 37% at 13% moisture, and 49% at 12% moisture.
The amount of pinto bean breakage was minimal and increased slowly as moisture contents decreased from 19% to 16%, then increased more rapidly at moisture contents less than 16%. The amount of pinto bean breakage was 2% at 19% moisture, 3% at 18% moisture, 5% at 17% moisture, 6% at 16% moisture, 9% at 15% moisture, 16% at 14% moisture, 20% at 13% moisture, and 26% at 12% moisture.
With both navy and pinto beans at 16% moisture content, there was an increase in breakage with a decrease in temperature. There was more breakage even at 60ºF than there was at 75ºF. The rate of increase in breakage was greater below about 30ºF for pinto beans, but the rate of increase in breakage was uniform for navy beans as temperatures decreased. The amount of pinto bean breakage was 6% at 75ºF, 11% at 59ºF, 16% at 45ºF, 21% at 31ºF, 29% at 18ºF, and 41% at 9ºF. The amount of navy bean breakage was 5% at 76ºF, 13% at 59ºF, 15% at 46ºF, 18% at 29ºF, 26% at 21ºF, and 32% at 8ºF. By harvesting and handling the beans at optimal moisture contents and temperatures, bean breakage can be greatly reduced.
The apparatus used to determine the breakage susceptibility of pinto and navy beans at differing moisture contents and temperatures is shown in the pictures at above, left.
Research was conducted to determine if the amount of damage as beans are dropped into a storage bin, either onto concrete or onto other beans, was 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. 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.
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. There was a difference in the amount of damage occurring between when beans are dropped onto concrete or onto other beans, but the amount of damage was high for both conditions. 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.
Harvesting beans at moisture contents exceeding 16% to 18% minimizes bean damage such as cracking, provides more harvest flexibility, reduces harvest losses, and maintains bean color quality. Research was conducted to determine a natural-air drying system that would dry pinto beans to 15%-16% moisture. An airflow rate of about 2 cfm/bu. was used for the drying. The beans were cleaned prior to drying, because previous work showed that dirt in beans resulted in a dust on the beans after drying. The initial bean moisture content was about 25%. The drying fans were controlled by a humidistat set to shut the fans off if the relative humidity was below 60% to prevent over drying. The rain sensor shut the fans off if it rained more than 0.5 inch.
The beans at the bottom of the bins dried down to about 15% first, and then the drying moved to the next stage while the moisture content of the beans at the bottom of the bin fluctuated within a percentage point for the remainder of the drying period. After about 132 hours the bottom of the bin dried down to the desired moisture content. It took about 520 hours before the middle of the bin dried down to 15%, and about 830 hours to complete the drying of the beans at the top of the bin. This is the actual hours of fan operation, which is different than calendar days. Since a humidistat and rain sensor was used, the fans didnt run all the time, only during the desired relative humidity range of 60% to 100% and when it was not raining.
The color quality, lightness and redness, of the beans was maintained during drying.
I want to acknowledge the work of students Nick Johnson and Jeff Duchscher.