Effect of Field Drying and Storage Conditions on the Color and Quality of Desiccated Immature (Green and Semi-Green) Soybeans

Early frost during the R6 and R7 maturity stages of soybean (Glycine max L.) usually causes immature (green or semi-green) crops to be harvested. These immature soybean seeds have a shrunken appearance, green tone, and high chlorophyll content in the oil, leading to heavy discounts for farmers at the elevator. Previous lab-scale storage studies have shown that seed color can change under light and warm temperatures; however, light cannot be added to a commercial storage bin. Therefore, this study examined the effect of field drying and storage conditions on immature soybean color and oil quality. Soybean planted in two plots were desiccated at the R6 and R7 maturity stages and then allowed to field dry. The field-dried desiccated soybeans were conditioned to moisture contents (MCs) of 12 and 17% and stored in airtight plastic bags at respective temperatures of 4 °C and 22.5 °C for 24 weeks. Seed color, mold, and oil quality were analyzed at intervals of 0, 4, 8, 16, and 24 weeks. The desiccated R6 seeds’ color “a” value significantly changed during field drying from (−9.75 to +0.19) and (−8.96 to +1.95) for Plot 1 and Plot 2, respectively. This means that the color changed from green to a golden yellow or light greenish-brown color after field drying. The chlorophyll content of the desiccated soybeans after field drying at the two maturity stages for both plots was less than 3 mg kg−1 of oil and was relatively stable throughout storage. During storage, at 17% moisture content and 22.5 °C, mold counts increased significantly for R6, R7, and R8 (frozen) control soybeans between weeks 0 and 4 to 4.36 CFU g−1, 5.93 CFU g−1 and 6.22 CFU g−1, respectively. Peroxide and free fatty acid values were within acceptable limits across all storage temperatures and moisture contents. This study suggests that favorable weather conditions for field drying after an early frost have the potential to improve the color of harvested and stored soybeans, similar to mature soybeans.