Effect of cooking on the thermal behavior of the cowpea bean oil (Vigna unguiculata L. Walp)

Abstract

Cowpea Bean belongs to the Vigna unguiculata species and arouses interest because it has great climate adaptation and nutritional qualities. It is frequently found in the African continent and in Brazilian North and Northeast regions. It is a legume that needs to be cooked for its usual consumption. The main purpose of this study was the investigation of the lipid profile and thermal behavior of the oil from raw and cooked cowpea beans. The fatty acid composition of this oil indicates that there is a predominance of polyunsaturated fatty acids with ~37 % linoleic acid and 24 % α-linolenic acid, against ~25 % of saturated fatty acids (mostly palmitic). Details concerning the thermal behavior of these oils were evaluated by thermogravimetry and differential scanning calorimetry (DSC), under nitrogen and synthetic air atmospheres. The kinetic parameters were evaluated from several heating rates with sample mass of 5 and 20 mg in open crucibles under synthetic air and nitrogen atmospheres. The obtained data were evaluated with the iso-conversional kinetic method, where the values of activation energy (E a/kJ mol−1) were evaluated in function of the conversion degree (a). The results indicate that the kinetic behavior of the cooked oil under nitrogen and synthetic air atmospheres are different, which was attributed to the several sample masses used. In addition, this oil also was evaluated by DSC from 25 to −60 °C, where it was verified a phase transition behavior.