2.4.1. Factor Selection and Organization of Experiments

The organization of the experiments was carried out using the experimental design obtained by the central composite experimental design (CCD). Three independent variables were selected (temperature, initial pH and concentration of sugars extracted from dates). Table 1 shows the domain of study with coded levels and real values of studied variables.


**Table 1.** Coded levels and real values of studied variables.

In the central composite design, the −1 and +1 correspond to the lower and the higher level, respectively. The value 0 represents the central value of the rangeand α has the value of 1.68 (α = 4√ *Nf* , where *N<sup>f</sup>* is a number of experiments).

The CCD matrix employed for three independent variables is given in Table 2. Each column represents the different variables (factors) and each line represents the different experiments (20).

**Table 2.** The central composite experimental design (CCD) matrix for different variables (coded levels).


**Table 2.** *Cont.*


The CCD matrix is composed of a complete factorial design, 2<sup>3</sup> ; two axial points on the axis of each design variable at a distance of α = 1.682 from the design center and 5 points at the domain center. The actual experimental values corresponding to the coded levels used for the creation of the experiment matrix are presented below (Table 3).


**Table 3.** Actual values for the three independent variables.
