Abstract
Cowpea (Vigna unguiculata) is a widely consumed legume in West Africa. Cowpea seeds have a high nutritional quality, due to its high content of proteins, starch, minerals and vitamins (folates). However, cowpea also contains anti-nutritional factors such as alpha-galactosides. The aim of this thesis was to understand the mechanisms that allow to reduce the alpha-galactosides content and preserve the folates during germination, soaking and cooking. Two coupled phenomena have been explored and interpreted to optimize the soaking/cooking conditions:i)The diffusion mechanisms of water, alpha-galactosides and folates within the seed and transfer in the soaking/cooking water, in relation to the structure of the seed;ii)The biochemical reactions (mainly enzymatic) of these molecules in the seed and soaking/cooking water.The contents of raffinose, stachyose, verbascose and 5 major folate forms were measured in the seed and in the soaking water. A model representing the diffusion and reaction phenomena of these two kinds of molecules has been developed. The diffusivities of water, alpha-galactosides and folates have been identified. The enzymatic reactions were modeled by a Mickaelis-Menten law for alpha-galactosides and simplified by a first order kinetics for folates.During the soaking process, the water absorption mainly occurs through the micropyle. During cooking, the water absorption takes mainly place through the testa. The results showed a low diffusion of alpha-galactosides and folates during soaking at 30 °C (about D = 2 × 10-13 m2.s-1) and higher at 60 °C and 95 °C (from D = 2 to 6 × 10-11 m2.s-1). This increase of apparent diffusivity (× 100) of alpha-galactosides and folates between 30 °C and 60 °C was explained by a cyto-hisological study, which revealed a middle lamella degradation of parenchymenteous cells of cotyledons. Alpha-galactosides and folates are degraded and significantly interconverted during soaking at 30 °C. An optimal enzymatic activity was identified at 35 °C and at pH = 5.8. Concerning the folates, the soaking allows an interconversion of the storage forms (formylated) in active form (methylated) for the organism. Germination also generates these interconversions, but also allows a high folate production after 48 hours. Therefore, the transfer and reaction model highlights the importance of the soaking and/or germination process in order to reduce the alpha-galactosides content while preserving/increasing the folates and their bioavailability. Thus, the germination and or soaking process allows to limit the cooking process, allowing then to gelatinize the starch and make the seed consumable.