## 2 Red-Colored Foods and Bioactive Compounds
### Biological Activities of Flavonoids
#### 2.1.1 Flavonols, Flavones, Flavanols, and Flavanones  
A flavonol is a flavonoid metabolite that is hydroxylated at the C-3 position of the heterocyclic ring [50]. Flavonols are widely present in vegetables, fruits, and grains. They are generally present as an aglycon-based glycoside such as kaempfer, quercetin, myricetin, isonhamentin, and rhametin [51]. Common flavonoids and aglycones have a minimum of 279 and 347 different glycoside combinations, respectively [52]. Among them, quercetin 3-O-glycosides such as isoiquericitin, quercetin, and ruin are categorized as representatives of the flavonol family, one of the subclasses of flavonoid compounds, and are the most abundant flavonoids [53]. In particular, they are frequently found in cranberry, strawberry, apple, kale, and red pepper [51]. Compared with flavonoids, flavone-containing plants are limited. The main flavones in edible plants are glycosides of apigenin, luteolin, and diosmetin [51]. Flavones are structurally very similar to flavonoids, and there are few hydroxyl groups at the C-3 position. Flavones have three functional groups: hydroxy, carbonyl, and conjugated double bonds. Thus, flavones show a characteristic reaction of the three functional groups [54]. Flavanols have no double bond between C-2 and C-3 and no carbonyl at C-4 in the heterocyclic ring. Therefore, there are two chiral centers (C-2 and C-3) that result in four isomeric structures for each flavan-3-ol molecule. Common examples of flavanols are catechin (_trans_) and epicatechin (_cis_), which exist in two isomeric forms. (+)-catechin and (--)-epicatechin are often found in food plants [55]. Skins of grapes, apples, and blueberries are also rich in flavanols [56; 57]. Unlike other flavonoids, flavanols are not glycosylated \begin{table}
\begin{tabular}{p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt}} \hline \hline
\begin{tabular}{c} Diastolic \\ blood \\ pressure \(\downarrow\) \\ \end{tabular} \\ \hline \end{tabular}
\end{table} 
Table 5: Biological activities of other red-colored food-derived compounds. in food [58]. Catechin and epicatechin can easily condense into oligomeric procyanidins, which produce anthocyanins under aqueous acidic conditions with heating [59; 60]. Flavanones are widely distributed in approximately 42 larger plants. In particular, they are found in the Compositae, Leguminosae, and Rutaceae. Heterocyclic flavanones also contain a ketone group, but there is no unsaturated carbon-carbon bond [61]. Flavanones, called dihydroflavones, lack the double bond between C-2 and C-3 in the heterocyclic ring of the flavonoid skeleton, which is present in flavones and flavonoids, and flavanone is found in high concentrations in citrus fruits. They are also found in tomatoes and aromatic plants, but their main sources are citrus fruits, especially grapefruit. Flavanones naturally exist in the form of aglycon and glycoside, while the major aglycones are naringenin in grapefruit and hesperetin in oranges [62]. In many studies, the therapeutic effects of red beetroot have been shown under a variety of conditions, including complications associated with metabolic syndrome [63]. One of the main mechanisms by which flavonoids can reduce cardiovascular risk is their vasodilatory and antihypertensive effects [64]. The data suggest improvements in endothelial function, an effect that would also be expected to reduce the risk of hypertension and the development of atherosclerosis. One study examined the blood pressure-lowering effects of 0, 100, or 200 mg red raspberry extracts/(kg\(\cdot\)d) in normal and spontaneously hypertensive rats. After 5 weeks, the red raspberry extracts demonstrated a dose-dependent antihypertensive effect in spontaneously hypertensive rats, effects that coincided with an increase in nitrogen oxide (NO) activation, a decrease in vasoconstratrictive endothelin-1, dose-specific antioxidative actions, and improved vascular endothelial dysfunction [65].