Bitter leaf is a widely consumed African vegetable. The deep green colour of the leaves serves as a visual quality indicator which the international diaspora market demands even when the leaf is in its dehydrated form.
This study investigates and models how pretreatment and raw material source affects the colour of solar-dried bitter leaf. Fresh bitter leaf samples were harvested from a rural (Ekere) and an urban community (Bodija) in Oyo State, Nigeria. The samples were pretreated by squeeze-washing, blanching, and steeping prior to solar drying. Using colourimetry measurements in the CIE L*a*b* colour space, this study monitored the colour profiles of the leaves before and during solar drying. The data was fitted into
four models (First-Order, Zero-Order, Fractional Conversion, and Arrhenius equations) using nonlinear regression analysis. The coefficient of determination (R²) and sum of squared errors (SSEs) were used as fitness indicators. The results showed that after pretreatment, control samples from both sources exhibited higher initial L* values (31.45, 25.30), while blanched samples had lower initial L* readings (28.48, 27.68). Blanched samples also had higher initial a* values (17.33, 18.64). The study found that solar drying caused lightness fluctuations in all samples from Ekere, a decrease in redness in a blanched Bodija sample, and an increase in yellowness in control and squeeze-washed samples obtained from both sources. During solar drying, the squeeze-washed rural sample showed the highest average ΔE value (70.13), and the most significant (p<0.05) colour change. Zero-Order kinetics provided the best overall fit (average R² = 0.7949) followed by Fractional Conversion (R² = 0.5619), with samples sourced from Bodija exhibiting slightly superior model fits. Based on this study's findings, bitter leaf processors should be sourced from urban locations; they should undergo minimal pretreatment, and blanching and squeeze-washing should be avoided to preserve the colour characteristics demanded by the export market.