Solar-Powered Sensor-Controlled Microclimate Storage and its Effect on Physiological Weight Loss in Dadin-Kowa Red Onion (Allium cepa L.) under Tropical Nigerian Conditions

Postharvest losses of onion (Allium cepa L.) in sub-Saharan Africa are estimated at 30–40% of total yield, driven largely by inadequate storage infrastructure and the absence of microclimate control. This study evaluated a solar-powered, sensor-controlled microclimate storage system for Dadin-Kowa Red onion under tropical Nigerian conditions, using a 3 × 3 factorial design (three temperatures: 25, 30, 35°C; three relative humidity levels: 60, 65, 70%) monitored weekly over an 8-week storage period, with weight loss (WL, %) as the primary response variable. The system maintained temperature within ±0.5°C and relative humidity within ±3% of set-points for over 90% of the trial. Temperature was the dominant driver of weight loss (mean WL across 8 weeks: 21.9% at 25°C, 26.9% at 30°C, 46.7% at 35°C; all pairwise comparisons p < 0.001), with the 35°C treatments separating from the lower-temperature treatments within the first week of storage. Relative humidity had a smaller effect that was confined almost entirely to 25°C, where 65% RH produced the lowest weight loss (20.98% mean), and became statistically undetectable at 35°C. The two lower temperatures, 25°C and 30°C, were statistically indistinguishable for approximately the first three weeks of storage, diverging only from around Day 28 onward. These findings refine the temperature–humidity storage recommendation for this cultivar and indicate that, for storage cycles under three weeks, humidity management at 25°C offers the only detectable additional benefit over temperature control alone.