## 3 Results and Discussion
Looking holistically at the obtained response surface (Figure 4), i.e., in the full ranges of variability of the two variability factors (i.e., ethanol concentration and extraction time), the main and strongest factor was the alcohol concentration in linear terms. It can be assumed that the higher the concentration of ethanol, the better the extraction efficiency of chlorogenic acid. If we want to analyze the individual areas of volatility a bit more carefully, it is worth focusing on the ranges that interest us. The graph in Figure 4 is similar in its distribution of curves in the range from 40% to approximately 60% ethanol content. There are clearly dominant lines running parallel to the OY axis, i.e., the axis on which the extraction time is shown. This means that in this range, the concentration of ethanol has the main impact and the greatest impact on the content of chlorogenic acid; the higher it is, the higher the content of acid in the extract. In the second range of ethanol concentration, from about 60% to 95%, an increasing influence of the extraction time can be clearly seen, which is manifested by the shape of the croissant curves. Moreover, the square component (Q) of the ethanol concentration, as well as the interaction of extraction time and ethanol concentration, influenced this characteristic shape of the response surface. A practical conclusion resulting from the shape of the curves in Figure 4 is the indication of a point or a narrow area for which the extraction efficiency can be optimized with a minimum of time and raw material expenditure. For example, at about the twentieth day of extraction with 68% ethanol, the process can be stopped if its efficiency is at a satisfactory level. It is about 346 ug/cm\({}^{3}\) here, i.e., slightly lower than after 30 days of extraction and similar to the level with the use of 95% ethanol for the same time. On the basis of the obtained response areas, it is easier to decide on the shortest possible extraction time and the lowest concentration of ethanol to use in order to obtain a product with a satisfactory content of the active substance in the extract, which in this case, is chlorogenic acid. Suggestions of one of the authors indicated that the efficiency of the extraction of biologically active substances with ethanol can be increased if it is warm when added to the herbal raw material [26]. In order to empirically verify this statement, the first variant of the experiment, in which warm ethanol was used at the beginning (heated to 50 \({}^{\circ}\)C), was compared with the second variant, in which cold ethanol was used in summer conditions (25 \({}^{\circ}\)C). For this reason, the first variant (discussed above) was implemented \"hot\" and the second was implemented \"cold\". The shape of the response surface of this second variant is shown in Figure 6.  
The Pareto analysis (Figure 5b) showed that all variability factors had significant impacts on the shapes of the curves shown in Figure 6. As in the previous variant, the linear aspect of the ethanol concentration had the greatest impact. The second factor of variability in terms of the strength of the impact on the RSM model was the interaction between the two main variables (LxL), and the third was the extraction time (L). The other two quadratic aspects of concentration and time were less significant, but the shapes of the curves show some similarity to the RSM model referring to the first variant. When comparing the absolute values of the contents of chlorogenic acid in extracts obtained by flooding dried elderberry flowers with warm or cold ethanol, a better effect (i.e., a higher concentration) could be found in the case of using warm ethanol. The differences were statistically significant.