*2.2. Experimental Design*

Without needing many experimental runs, and keeping a confidence interval [31], D-optimal experimental design ensures an optimal selection of spray drying conditions for maximizing the content of antioxidants. Then, for this purpose, two independent continuous variables (inlet temperature (T) and maltodextrin concentration (C)), and one categorical independent variable (type of MX) were tested. The minimum and maximum levels of these variables are described in Table 1. Content of resveratrol was set as unique response variable. The D-optimal experimental design consisted of 25 experiments, necessary to achieve a quadratic model in the quantitative factors. From these 25 experiments, five runs were repeated, i.e., runs 8, 10, 11, 12 and 14 were repeated with runs 21, 22, 23, 24 and 25, respectively. All the experiments were executed randomly. Experimental design and RSP were carried out employing Matlab 2013a (Natick, MA, USA).


**Table 1.** Testing levels of variables in D-optimal experimental design.
