**2. Phenolic compounds**

Phenolic compounds constitute a major class of biologically active plant metabolites (see **Table 1**, bioactive phenolic compounds) involving phenols with one or more hydroxyl groups. These phenolic compounds are divided into two main subclasses, phenolic acids, and polyphenolic compounds. Phenolic acids involve hydroxycinnamic acid and hydroxybenzoic acid derivatives [24]. Polyphenolic compounds constitute a diverse subclass, further divided into different groups such as flavonoids, stilbenes, coumarins, and lignans, depending upon the number of phenolic rings and structural diversity [25]. Root vegetables serve as a major source of phenolic compounds, and the phenolic content of some root vegetables is reported to be higher than non-root vegetables [26]. Among root vegetables, radish has a high phenolic content of 1315.83 μg gallic acid equivalent/g of extract, even higher than potatoes and kohlrabi [26]. The phenolic compounds tend to vary in different parts of the plants [27]. Carrots peels are reported to have more phenolic acids than vascular tissues. 5′-Caffeoylquinic acid is a major compound in different carrot varieties with the

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


#### **Table 1.**

*Major Bioactive phenolic compounds and bioactivties of some common root vegetables.*

highest content value in peels (15.04 mg/100 g FW), followed by cis-5′-chlorogenic isomer (3.51 mg/100 g FW) of Caffeoylquinic acid [9]. Another study has reported higher phenolic content in rutabaga sprouts (reaching up to 125.7 mg GAE/g DW) as compared to seeds (6.9 mg GAE/g DW) and roots (5.1 mg GA/g DW) [28]. Phenolic accumulation also differs depending upon the varieties, as a study has indicated higher phenolic content in black radish (13.7 mg GAE/g DW) compared to white and red varieties [29]. Higher phenolic content of plants correlates with antioxidant activity, such as parsnip, a carrot-related root vegetable rich with phenolic glycosides that positively correlate with antioxidant activity [15]. Beetroot with high phenolic content of 56.65 mg GAE/100 ml of juice, encapsulated as soybean proteins reaching up to 150 mg GAE/100 ml [11]. Turnip is reported to contain flavonoids and hydroxycinnamic derivatives. HPLC analysis of turnip tops has identified flavonoid glycosides and total phenolic content reaching up to mg 191.39 mg/100 g fresh weight [30]. Because turnip is usually consumed in processed form, phenolic compounds are negatively impacted upon applying heating procedures for processing. Franciso et al. [31] have studied the impact of different heat treatments on the turnip. Flavonoid content decreased to 4.58 μmol/g dry weight upon conventional boil initially at 13.85 μmol/g DW in raw samples. Likewise, high-pressure cooking also deteriorated flavonoids (5.00 μmol/g DW). A post-processed vegetable, Taro has high flavonoid content such as luteolin (44.5%), apigenin (52.7%), and chrysoeriol glycosides (2.63%) in dry land growth conditions [32]. On the other hand, sweet potato contains phenolic acid derivatives, including isomers of caffeoylquinic acid and dicaffeoylquinic acid, whose content is also reduced by applying processing techniques. Boiling is the least

favored technique for preserving phenolic compounds in sweet potatoes [18]. Cassava roots are widely consumed in African countries. Milled flour of dried cassava is used in many delicacies; however, drying impacts the phenolic composition of cassava. A study has indicated that roasting reduces the phenolic content (51.35 mg/g in roasted flour) as compared to sun drying (64.82 mg/g in sun-dried flour) [22]. However, other than processing techniques, extraction methods and parameters greatly impact the recovery of phenolic compounds from a vegetable source. A study has compared different techniques for black carrot extraction, reporting optimized microwaveassisted extraction (power = 348.06, time = 9.86 min, solvent: solid ratio = 19.3 g/ mL, solvent = 19.8% ethanol) to be best suited for phenolic recovery with the content value of 264.9 mg GAE/100 ml while conventional extraction to be least suited with approximately 2.5 times less phenolic yield than former technique [33].

Vegetables stored at room temperature lose sensory and composition quality due to the action of polyphenol oxidase and peroxidase enzymes. A study has recommended "refrigerate storing" to prolong the shelf life of vegetables [34]. Peels of vegetables are often discarded during food processing; however, incorporation of peels in food can be beneficial, as a study carried out on rutabaga has indicated higher phenolic content in peels (18.14 mg/GAE/g) as compared to a pulp (11.57 mg GAE/g) using ultrasoundassisted extraction [8].
