## 2 Technical and Economic Aspects  
Lemon yellow #15 was isolated from the ethanol extract of dried _Citrus limon_ zest obtaining 4 mg of pure dye starting from 132.9 g dried zest obtained after lyophilization of 305.6 g raw peel [8]. The lyophilized zest was extracted with EtOH in a sonicator for one hour, after which the extract underwent column chromatography according to the procedure recently described [8]. Eventually, yellow #15 was isolated via chromatography on Sephadex LH-20 column and HPLC. The dye easily dissolves in water from which it can be isolated via simple freeze-drying as a yellow powder (Figure 1). Following careful molecular structure and configuration assignment based on NMR spectroscopy, optical rotation measurements, and comparison of the experimental CD with calculated electronic circular dichroism (ECD) spectral data, the colorant was shown to be the limocitrol glycoside esterified with 3-hydroxymethylglutaric acid (Figure 2), limocitrol 3-O-6\"-[3-hydroxyl-3-methylglutary])-\(\beta\)-p-glucopyranoside. Figure 1: Lemon-derived yellow #15 in aqueous solution (**a**), and in powder form (**b**) (pictures by Dr. [PERSON] and Prof. [PERSON], The Medical University of South Carolina). Figure 2: Chemical structure of yellow #15: limocitrol 3-O-6”-(3-hydroxy-3-methylglutary)-\(\beta\)-p-glucopyranoside. This explains why yellow #15 is considerably more soluble in water than curcumin, and more soluble than limocitrol-3-O-\(\beta\)-p-glucopyranoside, the glycoside of the lemon flavonoid limocitrol first isolated in the form of yellow crystals in 1964 [11]. Indeed, after one-month exposure to sunshine of equimolar solutions of Yellow 15, crocin, and curcumin, significant color degradation of crocin and curcumin was observed, whereas color of the limocitrol-based dye showed little or no color degradation [8]. The proposed scale-up procedure to extract yellow #15 from the zest of _Citrus limon_ includes three extraction steps of lemon zest with 95% water and 5% ethanol, followed by another three extraction steps with 50%:50% water:ethanol mixture (Figure 3) [8]. Following addition of the extracts to absolute (200 Proof) EtOH, a lemon EtOH fraction precipitated out of solution (Figure 3b) concentrated in highly stable and water-soluble food dye yellow #15. The colorant can be used as such to impart the yellow color to foods and aqueous beverages. Displayed in Scheme 1, the hydroalcoholic extraction involves a first extraction of waste lemon peel with EtOH (5% _v/v_, \(\times\)3) followed by a subsequent extraction with EtOH (50% _v/v_, \(\times\)3), in each case at room temperature. Valued ethanol can be efficiently recovered by evaporation under reduced pressure and reused in subsequent extraction runs. The extraction of the dye with aqueous ethanol from waste lemon peel at room temperature offers numerous advantages. First, ethanol is a common by-product of most citrus companies producing orange, lemon, or grapefruit juice. For example, EtOH 91% (_v/v_) is commonly obtained following fermentation of the sugar-rich pulp residues of the centrifuge originating both from citrus essential oil and juice residues and also from the depulped and other residues of citrus squeezing [12]. Second, aqueous bioethanol is an eminently green extraction solvent [13] meeting several of the principles of Green Extraction, including the first and second principles (use of renewable solvent) due to the agricultural origin of the solvent [14]. Figure 3: Ground lemon zest extracted three times (1,2,3) with 95:5 H\({}_{2}\)O:EtOH followed by three new extraction rounds (4,5,6) with 50:50 H\({}_{2}\)O:EtOH (**a**). The solid precipitate upon addition of the extracts to pure EtOH (**b**). (Pictures by [PERSON] and Professor [PERSON], The Medical University of South Carolina). Third, the high solubility of this lemon flavonol glycoside in water makes its isolation a straightforward process using water, the cleanest possible solvent.