Soybean downgrading due to immature (green and semi-green) color at harvest, caused by frost conditions, poses a significant loss to producers and processors. After harvest, drying and storage are important for preserving the quality of the harvested produce. This study investigated the impact of drying on color change in harvested immature soybeans and the effect of the soybean moisture content, storage environment (temperature, light, and aeration), and storage period on color change and oil quality of immature soybeans. Soybeans were harvested at three different maturity stages: R6 (green) and R7 (semi-green) in pods and R8 (fully matured) in seed. The soybeans in pods were dried, shelled, and conditioned to moisture contents of 12% and 17% (wet basis) prior to storage in 12 storage chamber (box) environments. The chambers were built to have four environments of “light” and “no light” with and without aeration and were stored at temperatures of either 4 °C or 23.5 °C for 24 weeks. Samples were taken every 2 weeks for 2 months and then bimonthly in storage. Soybean color change during drying and their chlorophyll, color, peroxide value (PV), and free fatty acid (FFA) status in storage were determined. Visual observation showed that R6 (green) soybean color faded after 48 h drying, which was supported with a colorimeter reading as the “a” value increased from −8.89 to −3.83 and −8.89 to −1.71 with 37 °C and 27 °C drying temperatures, respectively. The ANOVA analysis showed that light had the greatest contribution (~81%) to the color change compared to the other three storage environment factors of temperature (~9.1%), aeration (~8%), and moisture content (~1.5%) with <10% separate effects. During storage, the R6 green and R7 semi-green soybean color continued to fade with color a-values that exceeded the initial values of the R8 matured (control) by 353% and 350%, respectively, by the end of the storage period. Low amounts of peroxide and free fatty acids (FFA) were recorded throughout the storage period. Only the FFA of 17% M.C. soybeans stored at 23.5 °C exceeded acceptable limits at the end of the storage period. Exposing immature (green and semi-green) soybeans to light resulted in the fading of the green color. Seed producers in regions prone to frost can extend harvest time by allowing immature soybeans to field-dry.

Soybean (Glycine max L.) is one of the world’s most important sources of plant-based protein, with a protein content exceeding 35–40% (dry basis), along with other essential nutritional benefits. Ideally, soybeans are field-dried to approximately 13% moisture content (wet basis, wb); however, adverse weather conditions can necessitate harvesting at elevated moisture levels sometimes exceeding 20% (wb). In such cases, mechanized drying systems, particularly in northern U.S. regions, become essential for safe storage and quality preservation. This study investigated the effects of drying temperature, airflow rate, and initial moisture content on drying kinetics and kernel integrity using mathematical modeling. Drying behavior was modeled using fractional calculus and compared to the empirical Page model, while kernel cracking and breakage were analyzed using logistic regression. Both fractional and Page models exhibited strong agreement with experimental data (R² = 0.903–0.993). The fractional model achieved superior predictive accuracy, improving RMSE and MAE by 83.7% and 81.2%, respectively, compared to the Page model. Cracking and breakage were more strongly influenced by drying temperature than by initial moisture content, with the greatest quality degradation occurring at high temperatures. Optimal drying conditions were identified as temperatures below 27 °C and initial moisture contents between 19 and 20% (wb), which best preserved kernel quality. Logistic models more accurately predicted breakage than cracking, confirming their effectiveness in assessing mechanical damage during drying. The results affirm the suitability of fractional order models for accurately capturing drying kinetics, while logistic models offer robust performance for evaluating physical quality degradation. These modeling approaches provide a framework for efficient and quality-preserving soybean drying strategies in regions reliant on off-field drying systems.

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.