Impact of Traditional Processing Methods on Total and Soluble Oxalate Levels in Five Hibiscus Species from Kwilu Province, DR Congo

Abstract

Aims: Hibiscus species are widely recognised for their rich nutritional profile, which includes essential micronutrients such as iron, calcium, magnesium, and vitamins A and C. They also contain a range of compounds classified as anti-nutritional factors, including oxalates, cyanogenic glycosides, tannins and others, which can interfere with the bioavailability and absorption of key nutrients.   Study Design:  In this experimental study we investigated the oxalate content across five Hibiscus species subjected to traditional processing methods. Treatments included sun drying, soaking, and the application of artisanal potash. Quantitative analyses were conducted to evaluate the impact of each practice on total and soluble oxalate levels, with species and provenance as variables. Place and Duration of Study: Hibiscus samples were collected from Kikwit and Idiofa cities across Kwilu Province, Democratic Republic of Congo (DRC). Laboratory analyses were subsequently performed at the Department of Chemistry, Higher Institute of Education (ISP), Kikwit, Kwilu Province, DRC. Methodology: Hibiscus species were collected from Kikwit and Idiofa (Kwilu State, DRC), and their total and soluble oxalate contents were determined before and after traditional treatments using volumetric titration with a standard potassium permanganate solution.    Results: Analysis revealed that traditional processing practices exert variable effects on the oxalic acid content of Hibiscus leaves, with oxalate levels differing by species and provenance. Sun drying significantly influenced oxalate concentrations in sorrel (H. sabdariffa), resulting in a reduction of total oxalate by 1-31% and soluble oxalate by 51–91.4%. Notably, H. sabdariffa var. sabdariffa exhibited an unexpected increase of approximately 50.5% in total oxalate following sun drying. The application of indigenous potash (Kompos) also impacted oxalate levels: total oxalate decreased by 3-29%, while soluble oxalate increased across all samples, ranging from 56-87%. Furthermore, soaking dried sorrel leaves in water led to a marked reduction in oxalate content, with total oxalate decreasing by 3.5-32.4% and soluble oxalate by 35.5-55.9%, compared to untreated controls. Conclusion: Five Hibiscus species analyzed in this study exhibited significant oxalate levels, shaped by their geographic origin. Traditional processing, specifically sun drying and soaking, reduced soluble oxalate concentrations by 51-92%. While treatment with local potash decreased total oxalate, it simultaneously elevated soluble oxalate, likely through the transformation of insoluble forms mediated by bicarbonate and hydroxide ions.