## 3 Results and Discussion
### Kinetic Modeling of Substrate Consumption  
The sugar consumption rate was mainly a function of cell growth rate, product formation and rate of substrate uptake for \(Z\)[36]. The experimental values were fitted to Equation (18). The model parameters of substrate consumption for each medium are shown in Table 1. The values of \(Z\) and \(\gamma\) ranged from 0.0012 gS/gX.h to 0.0284 gS/gX.h and from 5.3409 gS/gX to 5.7230 gS/gX. Their minimum and maximum values were determined by the utilization of Media A and B for \(Z\) and Media B and E for \(\gamma\), respectively (Table 1 and Figure 1C,F,I,L,O). The \(\gamma\) values coincided with the results in Table 2.  
Typically, when energy and carbon sources were high, microorganisms tended to distribute excess energy and carbon through the formation of storage compounds or extracellular products. Some products such as acetic acid, lactic acid and even ethanol that occur during fermentation negatively affect cell growth [36]. Therefore, a maintenance coefficient (\(Z\)) was used to describe the specific substrate uptake rate for cellular maintenance, in which it is varied for each medium tested. It was reported that the \(Z\) may vary according to the specific growth rate [48]. Later on, it was stated that \(Z\) represents the energy expenditure needed to repair damaged cellular components, to transfer certain nutrients and products in and out of cells, to move and to adjust the osmolarity of the cells in the inner volume [30]. It was also mentioned that the values of \(Z\) may vary from 0.02 to 4 gS/gX.h based on the environmental conditions surrounding the cell and growth rate [36; 49]. As seen from results in Table 1, the \(Z\) values of all fermentations were very low, indicating that the yeast mainly utilizes the sugars in the carob extract for ethanol and biomass production. Besides, this obviously indicated that the MLP model was capable of estimating the results of substrate consumption with a good amount of accuracy for each medium tested. On the other hand, the values of \(Z\) in Table 3 were close to zero except for that of [PERSON], [PERSON], [PERSON] and [PERSON] [24], [PERSON], [PERSON], [PERSON] and [PERSON] [26] and [PERSON] and [PERSON] [29] (as a substrate banana waste), which assisted our results. The high values of \(Z\) in Table 3 indicate that an important part of the carbon source used was employed for maintenance in strains [25]. Subjective comparisons of the sugar consumption curve with MLP model were given by plotting both the actual data and the estimated values from the models (Figure 3). The results indicate that the MLP model estimated slightly lower values in the log phase of the fermentation in Medium A and in the stationary phase of the fermentation in Medium B. Besides, the data at the end of the log phase of the fermentation in Medium B were slightly overestimated with the MLP model. On the other hand, the experimental sugar consumption in all phases of the fermentation in Medium C was successfully estimated with the MLP model. However, the data in all phases of the fermentation in Medium D were predicted to be slightly higher with the MLP model. For Medium E, the experimental data in the stationary phase were only slightly high predicted by the model. Finally, the comparison and validation of the MLP model were performed based on the values of RMSE, MAE, \(R^{2}\), slope, BF, and AF (Table 1). For different nutritional compositions, the values of RMSE and MAE varied between 0.38 and 2.19 g/L and 0.20 and 1.34 g/L, respectively. Their lowest and highest values were determined by the usage of Media C and D, respectively. On the other hand, satisfactory results were yielded with \(R^{2}\), which were above 0.992. Similar to \(R^{2}\), the slope values were equal to 1 for Media A, B, and C while it was 0.99 for Media D and E (Table 1). Additionally, BF values except for that of Medium D (0.86) were all between 0.96 and 1.04.