## 3 Carotenoids and Betalains: Natural Source and Cellular Localization  
Carotenoids are lipophilic isoprenoid molecules synthesized by almost all photosynthetic plants and are found in larger quantities in some organs and tissues. These pigments in plants are synthesized and stored in chromoplasts (Figure 1), organelles that arise from chloroplasts and/or other non-green plastids, such as amyloplast, leucoplast, etc. [18]. Special membranes develop in chromoplasts, called internal membranes of chromoplasts, whose chemical composition has been extensively studied and has been found to consist mainly of lipids (e.g., galactolipids) and special proteins (e.g., carotenoid enzymes) [46].  
In detail, carotenoids are stored in carotenoid-lipoprotein substructures and/or plast  
Figure 1: Schematic representation of the main sources and chromoplast structures containing carotenoids.  
Especially some vegetables such as carrot root, red papaya, tomato and watermelon are characterized by hyperaccumulation of \(\beta\)-carotene and lycopene as crystalline structures associated with the formation of crystalline chromoplasts [47].  
Tomatoes and carrots are used in various culinary preparations and produce a huge amount of waste, such as peel and pomace. Lycopene is the main carotenoid present in tomatoes (especially in the outer pericarp), with a range between 2.62 and 629 mg/100 g, while other carotenoids are between 0.23 and 2.83 mg/100 g [18; 49; 19]. Orange carrots contain a significant amount of carotenoids (mainly \(\beta\)-carotene and, to some extent, \(\alpha\)-carotene). Their distribution depends on the localization, which varies in the xylem (nucleus) and in the phloem (flesh) of the root (where it is most abundant) [18]. Other sources of carotenoids are pumpkin [\(\beta\)-carotene (78 \(\upmu\)g/g), \(\alpha\)-carotene (48 \(\upmu\)g/g)] [50], mango [\(\beta\)-carotene (5-32 \(\upmu\)g/g)] [51], watermelon [lycopene (60-70 \(\upmu\)g/g)] [52] and sweet potato [\(\beta\)-carotene (310 \(\upmu\)g/g)] [53]. As reported by [PERSON] et al. [12], several green approaches (e.g., microwave-, pressurized liquid-, ultrasound-, supercritical fluid- and pulsed electric field assisted extraction) may be applied for the recovery of carotenoids from natural sources and agricultural/food waste. These methods substantially differ in the mechanism applied for destructuring plant cells, as well as for operating conditions (e.g., temperature, pressure and solvent) [18].  
Unlike carotenoids, belatains are synthesized in the cytoplasm at the level of the endoplasmic reticulum, where their key biosynthetic enzymes are present. Their biosynthesis occurs mainly in plant tissues at the level of epidermal and subepidermal structures of plants. Being secondary metabolites, betalains are stored as glycolates in the vacuole (Figure 2) [40], whose membrane is made up of approximately 62% proteins, phospholipids and sterols [54].  
Betalains are found in the edible parts of a few plants but also in the leaves, flowers, stems and bracts [43]. Red beet (390-445 mg/100 g) [34] is the only vegetable currently and commercially used for the extraction of betalains. Such pigments from _Amaranthaceae_ plants used in traditional Chinese medicine, such as _Amaranthus_ sp. (0.95-6.02 mg/100 g) [47] and _Celosia_ sp., were tested for their feasibility to color food, although they are used only locally. The most promising family among betalain-bearing plants is the _Cactaceae_[55], and the main genera producing edible fruits are _Opuntia_ (41-89 mg/100 g) [34], _Hyloreus_ (82.79 mg/100 g) [56] and _Stenocreus_ (479 mg/100 g) [57].  
Among the different green methods applied for the recovery of betalains from natural sources and agricultural/food waste, pulsed electric field-, microwave- and ultrasound-assisted extraction are the most promising [12].  
Figure 2: Schematic representation of the main sources and vacuole structure containing betalains.