Influences of drying temperature and storage conditions for preserving the quality of maize postharvest on laboratory and field scales

Corn is widely produced and consumed due to its nutritional value and use in food and biofuel industries. Postharvest losses of grains can reach 25–30% across harvest, transport, drying, storage, and handling stages. Drying is necessary to reach safe storage moisture but may compromise physicochemical quality if not properly managed. Key parameters include drying air and grain temperatures, initial/final moisture, airflow, and ambient conditions. During storage, temperature and moisture drive biological activity and deterioration; excessive drying can also cause losses. Aeration cools grain masses to slow reactions; hermetic technologies that reduce O2 and increase CO2 can suppress respiration, microbes, and insects. The study aims to understand drying kinetics at high temperatures and their effects on corn quality in association with different storage technologies and conditions, evaluating how drying temperatures and storage systems can preserve maize quality at laboratory and field scales.

Prior studies highlight that improper drying can damage grain physical and chemical quality, increasing losses during storage. Temperature and moisture of stored grain determine respiration and microbial activity, with higher levels accelerating deterioration. Aeration (natural or forced) reduces grain temperature and slows biochemical reactions; artificial cooling has been studied for beans, soybeans, and rice, with fewer studies for corn. Hermetic storage that lowers O2 and raises CO2 can inhibit biological activity of microbes and pests. Physical properties like bulk density, porosity, and thousand kernel weight are critical for postharvest handling and are influenced by farming practices, hybrid type, and postharvest operations. Moisture migration during storage can prompt rewetting/drying cycles that damage kernels. The literature underscores the need to balance drying rate and capacity, and to match storage technology to regional conditions to minimize losses.

Grain material: Hard-type transgenic hybrid corn kernels (Herculex 30S31H) were hand-harvested on the cob at ~27% moisture, threshed, and cleaned. Field-scale drying: Continuous-flow commercial dryer (KW Dryer; capacity 100 t h−1; airflow 220 m³ h−1) operated at three drying-air temperatures: 80, 100, and 120 °C. Storage systems and grain lots: Dried lots were stored for six months in four systems: (1) hermetic (100-L PET containers), (2) bags (permeable nylon, 1000 kg), (3) aerated vertical silo (20 t), and (4) non-aerated vertical silo (20 t). Three grain quality lots were used: whole/clean (no defects), normal (2–4% impurities), and broken (5–7% broken kernels). Sampling scheme: For each storage system and grain type, samples were taken at 0, 3, and 6 months for physical quality and at 0 and 6 months for physicochemical quality. Spatial sampling included upper, middle, and lower positions in hermetic containers and upper/lower in silos. Laboratory wetting/drying cycles: From each drying treatment (80/100/120 °C), 150 kernels were subjected to controlled wetting in a BOD chamber (10 °C; 90% RH) for 0, 20, 40, 60, 80, 100, and 120 min to simulate intensive wetting during storage, followed by drying at the same temperature and duration as the wetting phase. Measurements: kernel water content, amounts of water absorbed/desorbed, volumetric contraction/expansion, and electrical conductivity to assess cellular tissue deterioration. Controlled-environment storage quality: Additional samples dried at 80, 100, 120 °C were stored for six months at two environments: refrigerated 10 °C/40% RH and ambient 23 °C/60% RH, monitoring physicochemical quality at start and end. Analyses: Moisture (oven method 105 ± 1 °C for 24 h). Physical parameters: length, width, thickness, volume, projected area, sphericity, circularity, porosity, apparent bulk density, and thousand kernel weight. Physicochemical parameters: electrical conductivity (EC), crude protein, acidity (0.1 N NaOH index), and ash. Statistics: ANOVA; Tukey tests at 1% and 5%; linear regression. Multivariate analyses: Principal component analysis (PCA) with biplots (first two components), k-means clustering, and Pearson correlations. Analyses performed in R using ggfortify.

- Drying kinetics: Higher drying temperatures (80 → 100 → 120 °C) accelerated moisture removal to below 12% storage target. - General quality trends: Higher drying temperature increased deterioration risk; wetting during storage reduced quality. - Physical properties under storage (Tables 1–2): - Aerated and hermetic systems better preserved kernel physical dimensions; largest changes occurred in non-aerated silos and bag storage. - Porosity increased markedly over storage time across grain conditions, e.g., from ~44–46% at time zero to ~59–67% by six months, regardless of storage system. - Apparent bulk density decreased with time; worst in non-aerated silos, while hermetic storage best preserved thousand kernel weight over time. - Germination and vigor (Table 3): - Germination decreased over time in all storage forms; broken kernels showed the poorest performance. - Hermetic storage maintained higher germination percentages over time. For normal corn, hermetic storage was ~97.5% at 3 months and ~95.0% at 6 months; for whole corn, hermetic remained ~98.5% at 3–6 months. - Electrical conductivity (EC) increased over time in all storage forms, indicating membrane deterioration; aerated storage often showed the highest EC increases, hermetic the lowest. Example: cracked corn in aerated storage rose from 678 to 961 μS cm−1 g−1 (0→6 months), whereas whole corn in hermetic rose from 114 to 138 μS cm−1 g−1. - Physicochemical changes (Table 4): - Moisture content increased over time across storage forms; larger increases observed in aerated storage and in broken grain lots. - Crude protein generally decreased with storage time for whole and normal lots across systems; values were relatively better preserved in aerated and hermetic systems but still declined over six months. - Acidity index generally decreased over six months across treatments, particularly in normal (mixed) lots; ash content also tended to decrease with time, with final ash values similar across storage forms. - Effects of drying temperature and rewetting cycles (Figures 2–4; Table 5): - Rewetting during storage, followed by redrying, caused increased EC and volumetric shrinkage/expansion cycles, reflecting cumulative physical damage to cellular structures. Greater drying temperatures and longer times intensified these effects. - Drying at 120 °C yielded the lowest initial acidity index (1.94 mL 0.1 N NaOH) compared to 80 °C (2.46) and 100 °C (2.33), likely because slower low-temperature drying prolongs warm, moist conditions conducive to fermentation and higher acidity. - Increasing drying temperature reduced crude protein content (e.g., at time zero: 100 °C ~9.00% vs 120 °C ~8.00%). Storage time further reduced crude protein across temperatures. - Ash content patterns varied with temperature and time; generally, ash values changed with increased drying temperature and over storage, with significant differences between temperatures and periods. - Storage temperature effect (controlled environments): Storage at 10 °C preserved physical and physicochemical quality over six months, whereas storage at 23 °C resulted in reductions. - Multivariate analyses (PCA, Pearson): Treatments grouped by grain type and storage time; broken kernels associated with higher EC (lower vigor). Germination, thousand kernel weight, and volume were strongly positively correlated. Physicochemical PCA showed time-driven grouping: zero-time associated with higher acidity; six months associated with higher protein and ash in the multivariate space, with acidity negatively correlated to protein and ash. - Practical equivalency: Healthy, whole corn dried at 80 °C and stored in silos with natural aeration produced satisfactory quality, comparable to more tightly controlled drying and storage under airtight, low-temperature conditions.

The study demonstrates that optimizing the combination of drying temperature and storage system is critical for preserving maize quality from harvest to processing. Faster moisture reduction at higher drying temperatures reduces time at high moisture but can exacerbate physical damage and biochemical deterioration. Storage system choice modulates these effects: hermetic and aerated systems effectively maintained chemical quality (lower EC increases, better protein retention) relative to non-aerated silos and bags. Moisture migration and rewetting cycles during storage were shown to be particularly damaging, increasing EC and inducing volumetric changes, underlining the importance of environmental control and uniform preprocessing. Broken kernels consistently underperformed across metrics, emphasizing the need to minimize mechanical damage prior to storage. Cooling storage at 10 °C mitigated six-month deterioration in both physical and chemical attributes versus ambient 23 °C, supporting refrigeration as a preservation strategy. Overall, to address the core question—how drying temperature, storage technology, and storage conditions interact to preserve maize quality—the findings recommend moderate drying temperature (e.g., 80 °C) paired with effective aeration or hermetic containment and preferably cool storage. This balance limits deterioration while achieving safe moisture, aligning postharvest operations with quality preservation.

- Increasing drying-air temperature accelerates moisture reduction to safe storage levels but elevates deterioration risk; rewetting during storage reduces grain quality. - Over six months, ambient (23 °C) storage reduced physical and physicochemical quality, whereas refrigerated storage (10 °C) preserved quality. - Hermetic and aerated storage systems better maintained chemical quality (lower EC, better germination) than non-aerated silos and bags, particularly for intact (whole) grain lots. - Grain quality heterogeneity matters: lots with broken kernels degraded more rapidly regardless of storage system. - A practical, field-scale strategy—drying healthy, whole kernels at 80 °C and storing with natural aeration—achieved quality outcomes comparable to airtight, low-temperature storage with tighter controls. Future work could extend storage duration, include additional hybrids, and explore dynamic aeration and real-time CO₂/O₂ monitoring to further refine control strategies.