Kenneth J. Hellevang, Agricultural & Biosystems Engineering
Department, NDSU, Fargo, ND
Pinto beans at 16% moisture content exposed to ultraviolet light darkened dramatically within weeks, which is deterioration in color quality. The Hunter-L value (whiteness) decreased from 49.2 to 45.1 during two months of storage. The color darkened more quickly at the beginning of the storage period, with 44% of the darkening occurring during the first 2-weeks, and 66% occurring during the first month. The Hunter-a values increased from 4.5 to 5.7 indicating an increase in the redness of the beans, which is also deterioration in color quality. Beans at 16% moisture content exposed to light representing a light spectrum similar to sunlight darkened dramatically within weeks similar to those exposed to ultraviolet light.
This indicates a need to minimize exposure to light in storage and possibly even before storage. Approximately 70% of bean handlers use flat storage (buildings), which exposes a large surface area to potential damage due to exposure to light.
Pinto beans were stored in covered 5 gallon pails for about 8 months. The beans, at 14%, 16% and 18% moisture contents, were stored at 40°F and 80°F. The higher temperature was selected to represent summer storage temperatures and the lower temperature was selected to be cold enough to minimize mold growth and still be above freezing.
The beans stored at 40°F and 14% moisture content decreased in Hunter colorimeter lightness (L-value) from 49.2 to 47.2, which was only a minor decrease in color quality. In contrast the beans stored at 80°F and 18% moisture content decreased in L-value from 49.2 to 36.0, which was a drastic deterioration in color quality. At 40°F there was little difference in color quality at the various moisture contents after the storage period; 14% - 47.2, 16% - 47.3, and 18% - 47.1. At 80°F, the higher moisture content beans deteriorated more in color lightness than the drier beans; 14% - 41.4, 16% - 38.5, and 18% - 36.0.
There was a dramatic difference in the amount of increase in bean redness for the various treatments. There was little difference in the Hunter colorimeter redness value (a-value) of beans stored at 40°F and 14% moisture content; increasing from 4.5 to 4.7. In contrast the beans stored at 80°F and 18% moisture content increased in a-value from 4.5 to 8.6, which was a considerable deterioration in color quality. At 40°F there was little difference in color quality at the various moisture contents after the storage period; 14% - 4.7, 16% - 4.7, and 18% - 4.8. At 80°F, the higher moisture content beans deteriorated in color more than the drier beans; 14% - 7.0, 16% - 8.1, and 18% - 8.6.
The pinto beans were checked for the hard-to-cook characteristic after four months of storage. Shorter cooking times are desired. There was a dramatic difference in cooking times for the various treatments. The beans that were stored at 80°F had cooking times that were two to four times longer than the beans stored at 40°F. The beans that had decreased in color quality (decreased lightness and increased redness) had much longer cooking times. At 80°F, the shortest cooking time was for the beans stored at 14% moisture content, the beans which experienced the least loss in color quality. For the beans stored at 40°F, the shortest cooking was for the 18% moisture beans.
Currently pinto beans at 14%, 16% and 18% moisture are being stored at 20°F, 40°F, 60°F, and 80°F. After five months of storage, the color change occurring appears to be similar to that which occurred during the first year of the study. The beans stored at 80°F show the most change and those stored at 20°F and 40°F show very little change. The beans will be stored for 10 months while continuing to monitor bean quality.
The results show that it is extremely important to store pinto beans at cool temperatures to maintain color and cooking quality. Beans should be cooled as rapidly as possible using aeration.
Natural Air Drying
Beans should be harvested at moisture contents exceeding that which is safe for storage to minimize damage such as cracking, minimize losses such as beans left in the field during harvest and to maintain bean color quality. Harvesting at higher moisture contents also provides more time and flexibility for harvest. Harvesting at higher moisture contents minimizes the moisture shrink (weight loss as the beans lose moisture). For example, the shrink that occurs as beans dry from 15% to 12% is 3.4%. That is a loss of about $7.50 per acre for beans yielding 1,460 pounds per acre that are worth $15.00 per cwt. This is a loss of about $2.6 million annually on 355,000 acres of pinto beans in North Dakota and Minnesota. Drying is required to obtain the benefits of harvesting at moisture contents above those safe for storage.
Pinto beans were successfully dried using a natural air drying system from about 20% down to about 13.5% moisture content using two airflow rates during September 2000. There was no measurable change in color during the drying period. Dirt in the beans caused considerable dusting on the dried beans, so cleaning prior to drying is preferred. An attempt to increase the moisture content of the beans by operating the fans just during the higher humidity times of the day during October was not successful.
Resistance to Airflow Through Dry Edible Beans
Pinto bean color, a primary quality parameter, can be maintained by lowering storage temperature using aeration. To properly design the aeration system (select a fan), the resistance to airflow must be known. Also, existing information suggests that beans should be harvested at moisture contents exceeding that safe for storage to minimize damage during harvest, to maintain color quality, and to maximize the weight of beans harvested. To properly design a drying system, the resistance to airflow through the beans must be known.
The resistance to airflow through pinto and navy beans was measured over a wide range of airflow rates using a constructed laboratory facility. These two types of beans account for 81% of the dry beans grown in North Dakota and Minnesota. The resistance to airflow of the two types of beans were almost the same and were approximately equal to that of soybeans.
Travis Lee, Agricultural & Biosystems Engineering student, completed much of the research work. Colorimeter and pin cooking tests were conducted in the laboratory of Sam Chang, Professor, Cereal Science Department, North Dakota State University.