## 1 Introduction
In this sense, it should be mandatory to build up the efficient and environmentally friendly utilization method of isolating bioactive antioxidant compounds from coffee silverskin. The great variety of methods can be applied for recovering bioactive antioxidant compounds from plant natural resources and their by-products, which include conventional extraction techniques such as liquid-liquid and solid-liquid extraction methods, as well as non-conventional extraction techniques [35; 36]. Among these extraction methods, solid-liquid extraction is still widely employed to recover different groups of bioactive antioxidant compounds. This extraction method is based on mixing fragments of solid plant matrix with various organic solvents, both polar and non-polar or their aqueous solutions, and holding the resulting mixture for a desired time, which is needed to start the process of dissolving particles of a solid plant matrix by the used solvent. However, the effectiveness and efficiency of the solid-liquid extraction method, and in consequence a commercial applicability of the extraction process in this system, may be affected by several operational parameters, including the type of used solvent and its concentration, the solid-to-liquid ratio, extraction time and temperature, pH, agitation speed, particle size of the used plant food matrix, and the number of extraction steps [37; 38]. For these reasons, finding the appropriate conditions for the best output for an extracting process of bioactive antioxidant compounds is the main purpose of optimization, which can be achieved by using the traditional approach. This approach involves monitoring the effect of one independent process variable on the response whilst other variables remain at constant level. It does not consider the interactions between variables, which leads to an incomplete understanding of the bioactive antioxidant compounds extracting process. The traditional form of optimizing the extraction process requires conducting of numerous experiments that result in their unprofitable time and financial management [39; 40]. Considering these limitations of traditional optimization approach, multivariate statistical optimization techniques such as response surface methodology (RSM) should be employed for modeling and optimizing the extraction process of bioactive antioxidant compounds [41]. The exploitation of RSM, introduced by [PERSON] and [PERSON] [42], as a statistical and mathematical optimization tool needs to be carried out via several stages, which comprise: screening of independent process variables and their adequate ranges, selection of experimental design and performance of real experiments, generation of regression model equations, verification of model adequacy, graphical representation of the obtained model, and determination of optimal process conditions. By a designation of mathematical models based on the quantitative data of the used analysis responses, RSM allows the significance of independent process variables to be evaluated while minimizing the number of assays performed during extraction process optimization. Moreover, this technique can identify the interrelationship between applied variables and determine the optimum conditions for the operational extraction variables [41; 43; 44; 45]. Taking all of the above into consideration, the main goal of the present research was to optimize the extraction method of bioactive compounds obtained from coffee silverskin by the use of response surface methodology. Initially, the influence of different solvents, such as methanol, ethanol and their aqueous solutions, and distilled water, as well as other crucial extraction factors (solvent concentration, solid-to-liquid ratio, extraction duration and equipment), on the extraction technique of antioxidant compounds was estimated to produce extracts rich in phenolic compounds with high antioxidant activity.