## 3 Results and Discussion
### Grapefruit Polyphenol-Rich Extract Composition
#### 3.2.2 Phenolic Compounds' Identification and Quantification  
The qualitative and quantitative analyses of the phenolic compounds in the GF peels' polyphenol-rich extracts are crucial for upcycling food ingredients. Understanding the specific BCs present can offer insights into the extract's bioactivities and elucidate the underlying biological mechanisms involved. The LC/MS chromatograms of each polyphenol-rich extract are presented in Figure 3.  
Compared to the commercial standards, sixteen phenolic compounds were identified and quantified in the GF-CE and fourteen in the GF-PE (Table 3). The identification of these compounds led to their distribution into three main structurally related classes, i.e., hydroxycinnamic acids (four compounds), hydroxybenzoic acids (five compounds), and flavonoids (seven compounds). Figure 3: LC/MS chromatograms for (**a**) GF-CE and (**b**) GF-PE. The presence of such BCs is mainly associated with the antioxidant and antimicrobial activities of GF peels and their derived extracts [65; 66; 67]. The most abundant compounds found in GF peels' polyphenol-rich extracts were naringenin, narinutin, and hesperidin, which agrees with the literature available for GF [62; 67; 68; 69]. The values for naringenin, narinutin, and hesperidin ranged from 7.72 to 48, from 15 to 28.7, and from 0.28 to 0.67 mg/g DE, respectively. An effective extraction technique is essential to achieve a greater yield of BCs, significantly impacting the composition, yield, and bioactivities of the extract. In the food industry, the preference lies in utilizing non-toxic and easily manageable solvents for the extraction of by-products [70]. Water stands out as the most safe and cost-effective environmentally friendly solvent option [71]; it demonstrates effectiveness in extracting polar molecules, whereas for less polar compounds, organic solvents or combination solvent systems are utilized. However, most of the polyphenols are poorly soluble in water. Flavonoid glycosides are usually more water-soluble than aglycones, which have a better solubility in organic solvents [72]. Therefore, to enhance the extraction, mixtures of water-alcohol are used [73]. Typically, insoluble phenolic compounds, such as flavonoids, are in the cell wall, while soluble phenolic compounds are found inside the cell vacuoles [74]. At high temperatures, the integrity of the cell wall is compromised, and this may promote the delivery of cell wall polysaccharides into the solvent and allow the phenolic compounds to diffuse [75; 76]; so, their extractability can also increase. Comparing our results (Table 3) to the reported data, it was observed that the concentration of phenolic acids, mainly hydroxycinnamic acids, was higher in the GF-PE obtained after the extraction of the EO by hydrodistillation during 3 h. This process may weaken the cell wall integrity, resulting in an increase in the contents of low-molecular-weight insoluble phenolics and the flavonoid aglycons when extracted by ethanol (80%) [57]. However, certain flavonoid compounds may undergo degradation when exposed to elevated temperatures. For instance, rutin initiates degradation at 75 \({}^{\circ}\)C, with its concentration declining progressively as the temperature rises until it becomes non-detectable at 90 \({}^{\circ}\)C [77]. On the other hand, gallic acid exhibits instability at higher temperatures. The degradation rates increase as the temperature increases from 60 \({}^{\circ}\)C to