## 4 Conclusions  
Spent coffee grounds (SCG) are recognized as the most common form of coffee waste throughout the globe and are generated in notable amounts after coffee beverage preparation in commercial coffee establishments. Therefore, this investigation addresses the tremendous problem of coffee waste accumulation if it is inappropriately discharged into the environment. Our research demonstrates that multivariate statistical methods, including response surface methodology, can be applied effectively for modelling and optimizing the process of extracting antioxidant bioactive compounds from spent coffee grounds. This mathematical and statistical approach allows researchers to select the most crucial factors influencing the extraction process. The approach can also monitor the significance of the effect of each extraction process variable and their interactions with the chromatographic and spectrophotometric techniques used to evaluate responses. Additionally, the implementation of surface response methodology can determine the SCG extracts with the maximum recovery of antioxidant bioactive compounds whilst minimizing the required experiments and financial support. Our findings provide not only deeper insights into the influence of extraction parameters on the physiochemical and antioxidant properties of spent coffee grounds extracts, but also confront the urgent problem of finding an efficient and environmentally safe method for utilizing spent coffee grounds. The extraction yield of antioxidant bioactive compounds from spent coffee grounds depends significantly on the concentration of the organic solvent and the ratio of the solvent volume to the SCG solid matrix. Based on the experimental data, the ultrasound-assisted extraction technique, with 65% hydroethanonic solution (\(v/v\)) and a solvent-matrix ratio of 51 mL/g SCG for 30 min at 60 \({}^{\circ}\)C, proved to be an effective alternative technique for the extraction of bioactive compounds with high antioxidant activity. The SCG ethanolic extract produced with the most favourable variant of the extraction process was characterised by a high concentration of total phenolics (approx. 38 mg GAE/g SCG d.m. ), caffeine (approx. 5 mg/g SCG d.m.) and chlorogenic acids (approx. 8 mg/g SCG d.m.). It also had a high antioxidant activity according to both antioxidant assays (approx. 73 mg Trolox/g SCG d.m. and 81 g Fe(II)/g SCG d.m.). In conclusion, the obtained results clearly demonstrate that spent coffee grounds sourced from commercial coffee establishments should be considered as a substantial coffee by-product abundant in antioxidant bioactive compounds with the potential for innovative applications in the pharmaceutical and agro-industrial food processing areas. In addition, we recommend further research to investigate the application of spent coffee grounds in different forms as an innovative ingredient of functional food products or biodegradable packaging materials utilized in the agri-food processing chain. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Used RSM Response** & **Approximated Value** & **Real Value** & **Deviation (\%)** \\ \hline ABTS (mg Trolox/g SCG d.m.) & 74.49 & 73.04 & \(-1.9\%\) \\ FRAP (\(\mu\)mol Fe(II)/g SCG d.m.) & 78.00 & 80.76 & 3.5\% \\ TPC (mg GAE/g SCG d.m.) & 36.13 & 37.88 & 4.8\% \\ Caffeine (mg/g SCG d.m.) & 4.86 & 4.82 & \(-0.8\%\) \\
3-CQA (mg/g SCG d.m.) & 7.69 & 7.86 & 2.2\% \\ BI (Abs\({}_{420}\)) & 0.21 & 0.22 & 4.8\% \\ \multicolumn{4}{c}{Optimal extraction conditions} \\ \multicolumn{4}{c}{Organic solvent concentration: 64.9\%} \\ \multicolumn{4}{c}{SMR: 50.6 mL/g SCG d.m.} \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Verification of the developed mathematical models of the used RSM responses: comparison of the approximated values predicted by the statistical approach based on fitted equations and the real values of the validation analyses. **Author Contributions:** Conceptualization, R.B. and A.G.; methodology, R.B., A.G. and M.W.-W.; software, R.B.