Influence of Pretreatment and Agro-ecological Origin on the Solar Drying Kinetics and Moisture Diffusivity of Vernonia amygdalina Vegetable.

Vernonia amygdalina (bitter leaf), is an indigenous African leafy vegetable that is rich in bioactive compounds. The high moisture content (> 80% wet basis) of the leaves however limits its shelf life. While drying is essential in preserving the quality of the vegetable to a nutritionally safe level, the combined influence of pretreatment type and agro-ecological origin on the drying kinetics and mass transfer behaviour of V. amygdalina remains insufficiently characterised.

Fresh Vernonia leaves harvested from urban (Bodija, UB) and rural (Ekere, RR) sites in Ibadan, Nigeria were subjected to squeeze washing (SQW), blanching (BLC), and water steeping (SPP) before drying.  The experiment was carried out in a fabricated hybrid solar dryer (1.22 x 0.91 x 1.52 m, fitted with a 1500  w electric heater, a 30 w, 12 V, 7 Amp solar panel powering three 12V axial fans). Experimental inlet temperature was 32 – 48 ^OC with outlet temperature of 31.9 – 42.3 ^OC and a relative humidity of 21 – 61 %.

The moisture ratio data were fitted to five thin-layer mathematical models (Newton, Page, Henderson-Pabis, Wang-Singh, and Lewis) evaluated by R² and residual sum of squares (RSS). The effective moisture diffusivity (D_eff) was computed from the best-fit model parameters using a measured leaf half-thickness of 0.13 mm.

The Initial moisture content ranged from 83.70 ± 0.17 – 86.23 ± 0.12 % (wet basis), with rural samples recording higher values. The Page model provided the best fit across all treatment combinations (R² = 0.9806 – 0.9897; RSS = 0.018 – 0.034). Effective moisture diffusivity ranged from 3.76 × 10⁻¹² to 5.53 × 10⁻¹² m²/s with UBSQW recording the highest (5.53 × 10⁻¹² m²/s) and RRSPP the lowest (3.76 × 10⁻¹² m²/s). Blanched samples: RRBLC and UBBLC achieved a respective final moisture contents of 8.00 ± 0.00 % and 10.67 ± 1.15 % falling within the recommended safe storage range (8.00 – 10.00 %), The steeped samples on the other hand showed the slowest moisture loss.

The study shows that pretreatment and agro-ecological origin significantly (p ≤ 0.05) influence the solar drying behaviour and moisture diffusivity of V. amygdalina.  Future work focusing on drying optimisation and bioactive compound retention profiling across pretreatment regimes is recommended.