## 4 Discussion  
### Solvent Selection  
Figure 1 presents the result of the extraction of resveratrol by different solvents. Among the extractions with different concentrations of ethanol, the 80% ethanol solvent stood out with the highest content of resveratrol extracted (66.86 mg/kg dry weight (D.W.)). Of the methanol solvents, 99.9% methanol extracted the highest concentration of resveratrol (84.06 mg/kg D.W.), exceeding 80% ethanol solvent. Series 10-99.9% acetone solvents had the lowest concentration of resveratrol after extraction. By adding 1% HCl to the 99.9% methanol solvent positively improved the extraction yield, reaching 92.06 mg/kg D.W. By adding 19% water to this solvent, the extraction yield dropped. Among the solvents with ethanol-diethyl ether in different ratio, the ethanol-diethyl ether 4:1 ratio stood out, obtaining the highest concentration of resveratrol (147.14 mg/kg D.W.), exceeding all solvents. In the work of [PERSON] et al. (2008) a broad spectrum of protic and aprotic solvents were examined for extracting trans-resveratrol. Aprotic solvents gave optimum yields at solvent dielectric constants (\(\epsilon/\varepsilon_{0}\)) between 36 and 42. That indicates that any solvent mixture giving this volumetrically averaged (\(\epsilon/\varepsilon_{0}\)) range would give an optimum analyte yield. This optimal polarity range matches up with solutions of 70:30 and 80:20 ethanol:water (\(v/v\)) for protic systems. Changes in the protic solvent polarity from this optimum quickly reduced trans-resveratrol yields [38]. This could explain the higher yields of trans-resveratrol for 80:20 ethanol:water among the ethanolic solvents. [PERSON] et al. (2015) compared methanol and acetone as extraction solvents for resveratrol from vine shoots. Their findings are in agreement with the results of this study, as they conclude that the extraction yield is decreasing when acetone is used. The highest difference between methanol and acetone resulted when extractions were performed on cut vine shoots at laboratory temperature, in which case the extracts obtained with acetone solvents had a very low content of trans-resveratrol in comparison with the extracts obtained with methanol [39]. A few studies are focused on establishing the optimal solvents for trans-resveratrol extraction, and the majority tend to search for the optimal solvents for extracting all polyphenols or stilbenes from grape cane [40] and grape pomace [41]. Therefore, all of them recommend ethanol and methanol as extracting solvents in different concentrations (60-80%). [PERSON] et al. (2001) established in their study that the optimal solvent for extracting trans-resveratrol from grape berry skins is ethanol 80% and the lowest concentration was obtained using acetone 99.9% [42]. The result that shows resveratrol is more soluble in solutions with a higher content of alcohol were confirmed by [PERSON] et al. (2016), who separated the vine stems extract in two fractions, containing either ethanol or water-soluble compounds. The aqueous fraction was essentially free of resveratrol and the ethanol fraction had 284.6 mg/kg trans-resveratrol [2]. The most frequent solvents used for the extraction of resveratrol from pruning wastes were ethanol 80% [1; 42; 43] and methanol 80% [39]. Although methanol offers high extraction yields due to the fact that it is a polar organic solvent, it is not a selective solvent for polyphenols [37]. Resveratrol has also been extracted from the bark of _Pinus sibirica_[44] and from the bark of spruce _Pica mariana_[45] by using diethyl ether with great results. Furthermore, despite the fact that it is a fairly nonpolar solvent, diethyl ether is more recommended for extraction due to the selectivity for polyphenols, including trans-resveratrol. In addition, due to the selectivity for polyphenols, this solvent eases the further steps of the purification methodology [46]. In the attempt to partially purify the dry extracts of vine shoots obtained by maceration with 80% ethanol solvent, [PERSON] et al.