Water uptake and texture formation in tiny cereal grains during cooking: The case of fonio (Digitaria exilis)

Abstract

Understanding heat and mass transfer in cereal grains is essential for optimizing cooking, yet the underlying mechanisms remain poorly understood. This study used fonio (Digitaria exilis) as a case study to investigate water transfer, starch conversion, and texture development during soaking, steaming, and absorption cooking. Gravimetry, texture analysis, optical microscopy, and X-ray microtomography were combined to characterize hydration and structural changes. Fonio grains (equivalent diameter 0.83 ± 0.05 mm; specific surface area = 7598 ± 428 1/m; porosity 7.7 ± 2.7 %) exhibited a dense microstructure. Water uptake followed a biphasic pattern, with rapid initial water uptake within 1 min (0.11 to ∼ 0.55 kg/kg db), followed by slower diffusion-controlled hydration (∼0.70 kg/kg db after 24 h), without measurable swelling. Filling-ratio analysis suggested that part of the retained water was located outside the μCT-resolved pore volume. Repeated soaking–steaming cycles increased hydration to ∼1.50 kg/kg db and starch conversion to ∼82 %, while steaming caused little additional water uptake. Absorption cooking increased stickiness up to tenfold through solute leaching, whereas hardness depended mainly on hydration and starch conversion. Decoupling hydration from heating improves texture control while reducing steaming time in fonio processing.