Natural Air Drying of Pintos
April 15, 2002
Kenneth Hellevang, Agriculture & Biosystems Engineering Dept., North Dakota State University
Dry edible beans should be harvested at higher moisture contents to minimize mechanical damage, minimize losses such as beans left in the field during harvest, and to minimize quality loss such as beans becoming darker. Harvesting at higher moisture contents also provides more time and flexibility for harvest and reduces lost value due to beans weighing less at lower moisture contents. The shrink that occurs as beans dry from 16% to 13% moisture is 3.4%; a loss of about $7.50 per acre for beans yielding 1,460 pounds per acre that are worth $15.00 per cwt. Rewetting beans dried below 16% will increase the value of the beans and reduce the potential for damage during handling.
Pinto beans were successfully dried using a natural air-drying system from about 20% down to about 13.5% moisture content during the fall of 2000 with no measurable change in bean color. Dirt in the beans caused considerable dusting on the dried beans, so it is advisable to clean the beans before drying. 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 2000 was not successful. A laboratory study during the summer of 2001 found that the beans dried to 12-13% rewet very slowly.
Pinto beans were harvested in September 2001 at about 22% and dried with a time clock controlling the fan so the fan operated from 6 pm to 10 am, which was drying the beans to about 16% moisture. When the fan operated during a 3-day rainy period, the beans rewet to about 23% moisture. This rapid rewetting was unexpected based on previous experience. The beans were rapidly dried to minimize deterioration of the beans. Current laboratory work is determining if the initial bean moisture content affects the rate of moisture absorption.
Pinto beans will be dried in 2002 using humidistats to control the drying fans to determine if that permits drying the beans to about 15 to 16% moisture content while maintaining bean quality. Also, it will be determined if the moisture content of over dried pinto beans can be increased to about 16% using a natural air drying fan controlled by a humidistat.