Analysis of chickpea soaking by simultaneous water transfer and water–starch reaction Original Research Article

The soaking process of five spring and five winter chickpea genotypes were investigated in water between 20°C and 100°C. Samples did not differ in initial water content (IWC), water absorption capacity (WAC), swelling capacity (SC), and seed coat thickness in terms of the growing season. While WAC decreased with increasing temperature, SC was not affected by temperature. The process was considered to be a simultaneous unsteady-state water diffusion and first order irreversible water–starch reaction phenomenon. The seed coat effectively controlled the water absorption up to 60% (d.b.) water content in all samples. The spring and winter samples showed no significant difference in the diffusivity (Deff) and true reaction rate constant (k) within the given temperature range except for Deff below 30°C. Spring samples had greater Deff values than winter samples below 30°C possibly due to more permeable seed coat structures of the former. The magnitude of Deff and k were between 10−10 and 10−9 m2 s−1 and 10−6 and 10−4 s−1, respectively, within the given temperature range. They increased with increasing temperature and the effect of temperature was evaluated through an Arrhenius type equation. Two distinct activation energies were observed below and above 55°C for both Deff and k. It was 48 and 18 kJ mol−1 for Deff and 23 and 41 kJ mol−1 for k below and above 55°C, respectively. The trend of the activation energies suggested that diffusion was more effective on the process than the reaction below 55°C and vice versa above 55°C, and significant textural changes start to take place in chickpea around 55°C. The internal effectiveness factor (η) increased from 0.61 to 0.74 with temperature from 20°C to 100°C. Its trend corroborated the above conclusion on the relative effects of the diffusion and reaction on the soaking process below and above 55°C. The birefringence of samples indicated that the gelatinization of chickpea starch starts around 55°C, which may explain why the activation energies of Deff and k changed around 55°C.