The activation energy that is required for the onset of the drying process, namely water activation in the seed (Vo?a et al., 2007), could be determined using the Arrhenius equation. It is the energy barrier that must be overcome in order to activate moisture diffusion (Perea-Flores et al., 2012). The activation energies for the WS and CS of Jatropha were determined from the slopes of the plots of ln(k) versus T?1 for all the tested model.
Fig. 2a and b show the plot of ln(k) versus T?1 for WS and CS of Jatropha, respectively at the five drying temperatures when Avhad and Marchetti model (Avhad and Marchetti, 2016) was used. The estimated value for the activation energies and pre-exponential factors for the four studied models are also presented in Table 1. The activation energy value for the whole seeds and crushed seeds of Jatropha varied from 23.67 to 36.06 and 32.
88 to 45.75 KJ mol?1, respectively for all mathematical models used. The computed activation energies of WS and CS of Jatropha were in agreement with those reported for other agricultural products such as Sorghum (Resende et al., 2014), grape seeds (Roberts et al., 2008), sliced, and crushed Hass avocado seeds (Avhad and Marchetti, 2016) and castor oil seeds (Perea-Flores et al.
, 2012). As it can be observed from Table 1, the activation energy of the CS was greater than that of the WS and this was unexpected as the rate of water evaporation in the crushed seed was faster than that of the WS. In the study of activation energy of water release rate from corn kernel, Vo?a, et al. (Vo?a et al.
, 2007) found that the drying rate constant k significantly increased with the increasing of drying air temperature, and described activation energy as the energy that needs to be supplied to kernels for initiating the moisture release. The authors concluded that if the activation energy is higher, moisture release from kernels would be slower. In general, high values of activation energy are related to the nature of materials where water is bounded more strongly to the structure and consequently more difficult to be removed it (Bezerra et al., 2015). The present result was in contrary to the finding of Avhad and Marchetti (Avhad and Marchetti, 2016) in which the activation energy of the crushed Hass avocado seeds (24-32 KJ mol?1) was found to be less than that of the sliced (34-36 KJ mol?1) and non-pretreated (43-129 KJ mol?1) seeds.
