Evaluation of naturally ventilated onion storage structure for a specific period

Naturally ventilated onion bulb storage structure was designed and constructed in NCAM, Kwara State, Nigeria to minimize post-harvest losses by controlling internal temperature and relative humidity which could help extend the shelf life of onion. Storage was made with saw mail wood of high standard for high durability with dimension 1.68m  1.2m. Structure was constructed as raised platform 0.91m above the ground level with down ventilation. Onions were stored for 90 days and they were in four different compartments C1, C2, C3 and C4 at 1.13 m, 1.35m, 1.57m and 1.79m to the ground level respectively. Internal and external temperature, relative humidity within and outside the storage were monitored and the physiological weight loss were recorded every two weeks interval. Data obtained were subjected to descriptive (mean and standard deviation) and inferential (ANOVA) statistics. Internal and external temperature of the structure ranged from 12.8°C to 29.5°C and 15.5 °C to 37.7°C, 18.3°C to 36.6°C and 24.6 °C to 42.8°C in the morning and afternoon respectively. Internal storage temperature parameters in the morning were within the optimum temperature range for bulb onion storage while in the afternoon is slightly above optimum temperature range for bulb onion storage. The Pearson Correlation, r, between the internal and external temperature in the morning and afternoon is 0.974 and 0.322 respectively. This indicates that there is large significance difference between the internal and external temperature in the morning while medium significance variation occurred in the afternoon. Relative humidity of both internal and external surrounding of the storage structure ranged from 38% - 75% and 40% - 83% in the morning, 34% - 69% and 35% - 75% in the afternoon. The internal relative humidity range in the morning is slightly above the recommended optimum value for bulb onion storage while in the afternoon is within the recommended optimum value. The Pearson Correlation, r, between the internal and external relative humidity in the morning and afternoon is 0.966 and 0.767 respectively, which show that there is large significance difference between them. The onions stored in compartment C1 had a lowest physiological loss in weight of 7.31% followed by compartment C4 with a 9.09% while compartment C2 had 11.77% of highest loss in weight and compartment C4 recorded a 9.84% weight loss. This study concluded that the design needs ventilation and roofing material that would aid significantly in the control of both temperature and relative humidity within the storage, hence the structure has been able to reduce internal temperature to minimal level near to suit onion storage.