**4. Color imparting bioactive compounds**

#### **4.1 Anthocyanins**

Root vegetables possess natural-colored pigments that give them their characteristic colors (see **Table 3**). Anthocyanins are water-soluble pigments generated from glycosylation of the anthocyanidin class of flavonoids. Till now, 670 such compounds have been isolated and identified from colored plants and flowers [52]. These pigments cover a wide spectrum of colors, from red to purple, depending on their structure [63]. Red radish contains anthocyanins that have similar properties as synthetic Red No. 40 and is stable to be used commercially [64]. The extraction and post-extraction concentration procedure affects the recovery of these natural


#### **Table 3.**

*Bioactive color pigments of root vegetables.*

colorants. Hydroethanolic and acidified water extraction system with membrane pertraction concentration is suitable for high anthocyanin recovery and led to 62.58 mg/100 ml anthocyanin recovery from red radish [65]. Hydrogen-rich water is reported to influence anthocyanin accumulation in the Yanghua cultivar of radish [66]. A study has indicated radish growth under UV-A light with hydrogen-rich water, and calcium chloride treatment promotes the accumulation of anthocyanins (reaching a relative anthocyanin content value of 42.06, which was at 9.72 in control plants) [67]. Purple yam is a rich source of anthocyanins with antimicrobial activity [21]. HPLC-DAD analysis has quantified 31 mg/100 g DW anthocyanin content in fresh yams. Cyanidin and peonidin glycosides acylated with hydroxycinnamic acids are leading anthocyanins of purple yam; however, processing of yams by blanching leads

#### *Bioactive Components of Root Vegetables DOI: http://dx.doi.org/10.5772/intechopen.105961*

to a total anthocyanin loss of 60% [20]. Purple radish has also identified the presence of acylated cyaniding sophoroside glycosides and diglycosides [52]. Within the biological system, anthocyanins are generated under the influence of light in the presence of chalcone synthase enzymes. Light-responsive genes are expressed in turnip at different levels in anthocyanin biosynthesis [68]. Among other sweet potato varieties, purple-fleshed sweet potatoes have indicated high amounts of peonidin anthocyanins (1039 mg//100 g DW), which is almost three times higher than cyanidin-based anthocyanins [53]. Purple Carrots are found to be a good source of anthocyanin, with content value reaching up to 33,876 mg/kg DM; however, air or freeze-drying of fresh carrots leads to anthocyanins loss [56]. A study on black carrots also has identified a considerable amount of anthocyanins [69].

#### **4.2 Carotenoids**

Carotenoids represent a class of natural products with lipid-soluble vibrant color pigments. These pigments are responsible for the yellow to red spectrum of colors in plants and other organisms [70]. Unlike flavonoid-based anthocyanins, carotenoids are tetraterpenoid molecules containing eight isoprenoid units that generate a C40 carbon skeleton [71]. In green tissues, carotenoids are located in chloroplasts, while in colored vegetables, accumulation occurs in chromoplasts imparting different colors to the plant [72]. Among root vegetables, carrot is the most prominent source of carotenoids. Carotene content varies depending upon the carrot cultivars. It is reported to be in the range of 58.15–102.02 mg/kg FW) with β-carotene constituting the major portion of total carotenoid content [73]. Studies have indicated the immense impact of light on carotenoids [74]. In carrots, the biosynthetic pathway of carotenoids is influenced by light, as it can induce repression of gene expression of β-carotene and α-carotene biosynthesis [75]. Parsnip, on the other hand, despite being a carrotlike root vegetable, only has minor amounts of carotenoids [76]. β-Carotene is also dominant in sweet potato varieties with orange flesh. The carbon chain of the majority of carotenoids contains trans double bonds. In sweet potatoes, 127 μg/g of total β-carotene contains 123 μg/g of all-E-β-carotene [58]. Turnip leaves are also reported to possess carotenoids with content values reaching up to 250 μg/g [77]; however, a study on white radish leaves has shown higher carotenoid content (486.95 μg/g DW) as compared to turnip leaves. Moreover, despite being a white-fleshed variety, a minute amount of carotenoids is also identified in radish roots [59].

## **4.3 Betalains**

Betalain pigments are betalamic acid derivatives divided into two subclasses: betacyanin (red-violet pigments) and betaxanthin (yellow pigments), depending upon cyclo DOPA and amine condensation on betalamic acid, respectively [61]. Black radish peel extracts appears yellowish and is reported to contain 22.5 mg/100 g DW of betaxanthin while only 7.7 mg/100 g DW of betacyanin pigments [78]. Beetroot is the richest vegetable source of betalains (17.24 mg/g DW), most prominently betacyanins responsible for its bright red color. A study has indicated approximately three times higher betacyanin content than betaxanthins in peels and other root sections of beetroot [61]. Beetroot peels are also betalain rich. An optimized study on betalain extraction from beetroot peels has identified 1.5% citric acid, 50% ethanol, and 52.52°C temperature and 49.9 min extraction time to be best suited for betalain recovery with the content value of 1.20 mg/g DW [79].
