## 3 Results and Discussion
### Optimization of the Extraction Process Parameters
The maximum predicted value of caffeine content was 5.4 mg/g SCG d.m., which should be achieved with 54% hydroalcoholic solvent solution and 44.3 mL/g SCG d.m. The model introduced by [PERSON] et al. [19] showed the maximum predicted content of caffeine in SCG reached 6.14 mg/g SCG d.m. These results also accord with those of [PERSON] et al. [46], who evaluated the quality of SCG from  
Figure 2: The surface response plots and their matching contour plots showing the ABTS radical scavenging assay (**a**), FRAP assay (**b**), the total polyphenols content (TPC) assay (**c**), caffeine content (**d**), and chlorogenic acid 3\(-\)CQA content (**e**), the spectrophotometric determination of (BI) browning index (**f**). beans of different geographical origins (1.94-3.91 mg caffeine/g SCG d.m.). The caffeine content in SCG presented in this study is comparable with the caffeine content in guarana seeds as described by [PERSON] et al. [47]. Generally, the increase in ethanol concentration in the aqueous solvent solutions above a critical threshold results in significant decreases in the extraction yield of caffeine due to the hydrophilic nature of caffeine [48]. With regard to the chlorogenic acid content, the effects of all linear (A, B) and quadratic terms (A\({}^{2}\), B\({}^{2}\)) were observed to be statistically significant. The optimization revealed that the maximum yield of chlorogenic acids (7.8 mg 3-CQA/g SCG d.m.) could be obtained with an ethanol solvent concentration of 64% and 46 mL/g SCG d.m. These results are in agreement with data collected by [PERSON] et al. [49], who proposed an environmentally safe procedure for isolation of bioactive compounds from spent coffee grounds. The antioxidant activity of coffee and its by-products, especially spent coffee grounds, is known to be substantially affected by the roasting process, as well as coffee beverage preparation techniques [50]. The different phenolic compounds, including chlorogenic acids, originally identified in coffee beans are partially degraded during the roasting process [51]. Despite this, the antioxidant activity of coffee and its by-products can be maintained by the formation of new antioxidant constituents called melanoidins [52]. Melanoidins are known as water-soluble, nitrogen-containing macromolecular browning compounds, which are responsible for the brownish color and unique aroma properties of roasted coffee beans. It has also been reported in the literature that diverse biological activities, namely, antioxidant, antimicrobial, anticarcinogenic, anti-inflammatory, antihypertensive, and antiglycative activities, have been attributed to these compounds [53]. For these reasons, it is important to estimate the content of the final Maillard reaction chemical products. Therefore, the browning index is recognized to be a straightforward indicator of the content of browning compounds generated via the caramelisation and Maillard reactions [54]. Our findings demonstrated that for the browning index levels, both linear (A, B) terms and their interactions (A \(\times\) B) were statistically important. The optimization results showed maximum BI levels of 0.29 that could be achieved with an 88% organic solvent concentration and 11 mL/g SCG d.m. Additionally, the current study found that increasing both the SMR and the concentration of the organic solvent promotes a greater level of absorbance, resulting in greater amounts of melanoidins in the SCG extracts. The previously cited scientific investigation observed that the brown color can be directly associated with the concentration of Maillard reaction products. Notwithstanding this fact, in our experiment, we have not calculated the melanoidin content but have just concentrated on the brown color of SCG extracts as an indirect estimator of the amount of the final Maillard reaction products, which contain chromophore groups with a typical maximum absorption at 420 nm in their macromolecular structure.