## 3 Results and Discussion  
### The Selection of Extraction Method  
The research goal was to optimize the green extraction of polyphenols from Grasevina grape pomace. Ultrasound was chosen as the extraction method and we initially sought to determine the most effective ultrasound extraction method, considering its potential for industrial application to obtain ready-to-use extracts that could be easily transferred to the industry. UAE was chosen as the technique considering its both non-thermal and eco-friendly characteristics coupled with benchmarked applicative characteristics in industry. The effects associated with ultrasound energy include increased convective forces, acoustic cavitation, and the generation of highly reactive radicals, enhancing solubility and accelerating the dissolution of target molecules from biomass.  
The research began by exploring the possibilities of UAE and its potential for continuous industrial-scale application. Water was selected as a solvent in the optimization of ultrasound reactors due to its versatility as a universal solvent, making it capable of dissolving a wide range of compounds, polyphenols too. Additionally, its non-toxic nature, environmental friendliness, and cost-effectiveness align with the practical considerations for the intended ultrasound-assisted process. These experiments took several different forms: (i) mixing (conventional extraction), (ii) batch extractions (bUAE), and (iii) continuous extraction using an ultrasonic probe (cUAE) (Figure 1). The choice of extraction conditions was selected by balancing between achieving optimal extraction efficiency, preserving sensitive compounds, and ensuring cost-effective and energy-efficient processes.  
The results presented in Figure 1 represent the concentrations of total polyphenols (TPs) obtained at the end of the process, at 15 min for extractions without heating to compare with UAE conditions, in the ultrasonic bath, and batch extraction using a 34 mm diameter probe, and at 15 min for continuous extraction using a 34 mm diameter probe. The polyphenol contents were extracted in the following order, from highest to lowest: 34 mm probe, batch > 34 mm probe, continuous > extractions without heating (Figure 1). Furthermore, the measured final energy input for the continuous process was 191 180 Ws, while for a batch process, it was 91 915 Ws. Therefore, an ultrasonic batch reactor was selected as the extraction method.  
Figure 1: TP content (mgGAE g\({}^{-1}\) dw) of prepared extract with different extraction methods with water. Contents of polyphenols were expressed as the means (n = 3) \(\pm\) S.D. Presented values followed by different lower-case letters (a,b) are significantly different (\(p\) < 0.05) as measured by [PERSON]’s HSD test. cUAE—continuous UAE. bUAE—batch UAE.  
In the industry, the shift towards unconventional methods, notably ultrasound technology, is increasingly observed due to their efficiency and potential for energy savings, primarily attributed to the cavitation phenomenon. Continuous systems are preferred due to their ability to streamline processes; nonetheless, a critical evaluation of their merits and demerits is crucial. Following the selection of the method, the choice of probe diameter becomes imperative, as it significantly affects the distribution and intensity of ultrasonic waves, thereby influencing the efficiency and effectiveness of the extraction process [18; 23]. A larger probe diameter leads to a broader cavitation zone, promoting more extensive and uniform extraction throughout the sample. It enhances penetration depth, ensuring interaction with a larger volume of the extraction matrix, and influences power distribution, impacting the breakdown of cell walls and release of target compounds. Optimizing the probe diameter is crucial for achieving efficient and scalable ultrasonic-assisted extraction processes, and results are presented in Figure 2. A 50 mm probe was selected as optimal, and extraction conditions were optimized for it.