## 3 Conclusions  
Waste lemon peel obtained at about 50% of weight at lemon juice processing companies is an ideally suited raw material to produce, beyond valued pectin [9; 15] and lemon oil, the water-soluble and highly stable yellow #15 natural colorant. The dye, a water-soluble limocitrol glycoside, is a flavonoid suitable as an industrial alternative to both synthetic and natural yellow colorants such as curcumin, crocin, norbixin, or riboflavin (vitamin B2) [20].  
It is also relevant here to learn that between 1980 and 2016, the lemon yearly share of the citrus market increased from 5% to 13%, going from 5.2 to 17.5 million tonnes [19], whereas entrepreneurial efforts (undertaken, for example, in the Netherlands [21]) to have waste citrus peel recognized from a regulatory viewpoint as a raw material and no longer as waste have been successful.  
Extraction may be conducted via simple solid-liquid extraction in aqueous ethanol or via hydrodynamic cavitation of waste lemon peel in water, in both cases at affordable cost essentially dictated by the cost of labor and electricity.  
A comparison of various extraction procedures in terms of yield, purity, and cost will be carried out including also other extraction routes such as supercritical carbon dioxide that, in the case of citrus flavonoids, in the presence of 85% ethanol as modifier, affords significantly higher yield in the amount of flavonoids extracted, and significantly shorter extraction times [22].  
Coupled with the ease and greenness of the extraction processes suggested, the low cost and abundance of waste lemon peel, alongside the enhanced stability of the natural colorant and the health benefits of citrus flavonoids (cardioprotective, chemopre, and neuroprotective effects) [23], support industrialization of this new bioeconomy production.  
In the circular economy approach applied to citrus, resulting in what [PERSON], [PERSON], and [PERSON] in 2010 called \"the citrus biorefinery\" [24], production of the yellow biocolarant would take place concomitantly to production of all valued bioproducts obtainable from waste lemon peel, namely pectin, essential oil, cellulose, and other flavonoids.  
It is therefore important to develop new scalable green extraction technologies capable of affording all the aforementioned bioproducts, if possible in one pot and with high energy efficiency.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Year** & **Amount (Million Tonnes)** & **Share of Citrus Production** \\ \hline
1980 & 5.2 & 8\% \\ \hline
2016 & 17.5 & 13\% \\ \hline \hline \end{tabular}
\end{table}
Table 1: Amounts of lemons harvested globally in 1980 and 2016 (adapted from Ref. [19], with kind permission).  
Once said bio-based production will be industrialized, most likely directly at large citrus processing plants, this polymethoxylated flavonoid derived from lemon will be widely used in different beverage and food products including sports drinks, dairy and bakery products, fats, ice creams, dried fruits, and cereals, as well as in cosmetic and personal care products still using hazardous synthetic yellow dyes.  
Investigation, B.F. and R.C.; Methodology, F.M.; Supervision, M.H.; Writing--original draft, M.P.; Writing--review & editing, R.C., F.M., and M.T.H. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors declare no conflict of interest.