# Characterization of Coffee Silver Skin as Potential Food-Safe Ingredient
## 3 Results
### Polyphenols, Melanoidin, and Radical-Scavenging Activity  
The extraction of the phenolic fraction was preliminarily carried out by using deionized water and two solutions containing 60% and 70% (\(v\):\(v\)) of methanol, respectively, in order to explore the extractive capability of solvent mixtures characterized by different polarities, widely used in the literature for vegetables [48]. Among the mixtures, the 70% methanol solution allowed the highest yield of the soluble fraction as evaluated by the content in total phenolic compounds (TPCs) and, for this reason, it was chosen for further investigations on the radical-scavenging activity as well as on the phenolic pattern. Moreover, as phenolic compounds can also occur in the bound form by either ester or ether bonds with sugars, alcohols, and other biomolecules from cell walls, the insoluble residue recovered from the first step was submitted to further extraction under acidified conditions. [PERSON] and [PERSON] [49] have recently reviewed all the factors interacting with the availability and extraction of soluble and bound phenols, remarking on the dynamic balance of this system that can change according to the conditions of the medium. The results on the content of phenolic compounds and antiradical activity of the sample, along with the content in melanoidins, caffeine, and chlorogenic and caffeic acids, are reported in Table 3.  
The TPC of the soluble fraction was significantly higher with respect to the bound fraction. Due to the differences in the solvent mixtures used, procedures adopted, and molecules used as reference compounds for the assays, the comparison of TPC results with others found in literature was very difficult; however, it can be generally affirmed that larger amounts were found with respect to Gemechu [8], whilst lower values were observed when the work of other authors was considered [36; 49; 50; 51; 52]. Besides the factors previously cited, such a discrepancy can also depend on the geographical provenience of CSS and the roasting process; however, a reduced content of these molecules can positively affect bitterness, astringency, and the cut grass smell sensations that are developed with cooking processes. As far as the radical-scavenging activity is concerned, the soluble and bound fractions differed significantly in the TEAC values, with the free fraction showing the highest value. This may be primarily related to the TPC values but maybe also a consequence of the different compositions in the patterns of phenolic compounds of the two extracts, with the soluble one being richer in molecules with higher antiradical activity with respect to the bound fraction. In general, the antiradical activity was comparable to results reported in the work by [PERSON] et al. [53]. To gain a more in-depth understanding of the phenolic composition of CSS and to quantify caffe content, the extracts were submitted to HPLC analysis (Figure 1) and in Table 3 the contents of the most representative phenolic acids--chlorogenic and caffeic acids--are reported. Indeed, the major phenolic compounds in CSS are chlorogenic acids, which are esters of cinnamic acids with quinic acid; several isomers have been detected in CSS. However, 5-O-caffeoylquinic acid (5-CQA) was found to be the main one [54]. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Phenolic** & **TPC** & **TEAC** & **Chlorogenic Acid** & **Caffeic Acid** & **Caffeine** \\
**Fraction** & **(mg GAE 100 g\({}^{-1}\))** & **(mmol Trolox eq 100 g\({}^{-1}\))** & **(mg g\({}^{-1}\))** & **(mg g\({}^{-1}\))** & **(mg g\({}^{-1}\))** \\ \hline Soluble & 578 \(\pm\) 64 & 1.57 \(\pm\) 0.04 & 0.39 \(\pm\) 0.02 & 0.31 \(\pm\) 0.01 & 17.45 \(\pm\) 0.69 \\ Bound & 467 \(\pm\) 29 & 1.11 \(\pm\) 0.07 & 0.18 \(\pm\) 0.01 & 0.21 \(\pm\) 0.01 & n.d. \\ \hline Sign. & ** & ** & ** & ** & - \\ \hline \end{tabular} Legend: n.d.: not detectable; asterisks indicate significance at *: \(p\) < 0.05 and **: \(p\) < 0.0.  
\end{table}