# Old-Fashioned, but Still a Superfood--Red Beets as a Rich Source of Bioactive Compounds
## 3 Flavonoids and Polyphenolic Acids
Such large differences were not observed in the case of flavonoids, their concentrations in most cases were rather similar. Attention to possible interactions between phenolic compounds in beetroot juice has been also drawn [25]. The content of the main active compounds in beetroot juice was determined by HPLC analysis before and after the addition of these compounds to the study juice, and the antioxidant activities of the obtained solutions were examined. Each of the studied additives had a great impact on the scavenging of the DPPH radicals in the juice. The addition of caffe acid, rhamnetin, and vitamin B5 resulted in a decrease in this ability, which was shown in Figure 4. Antioxidant interactions can be synergistic, additive, or antagonistic, depending on the specific substances and their concentration ratios. In the case of examined additives, only betanin caused an increase in the ability to neutralize the DPPH radical, in comparison to beetroot juice without any additives. There is a synergistic effect here, which is particularly visible in the initial stage of the kinetic curve. In contrast, an antagonistic effect is observed for the remaining additives. The kind of interaction depends on the specific antioxidants interacting in the system and the condition behind the evaluation, as noted above, however, the knowledge about the interaction between polyphenic compounds and vitamins as well as non-antioxidant compounds is still poor. A common theme in the scientific literature is that interactionsbetween antioxidant molecules do occur, but a mechanism that allows a prediction of synergistic and antagonistic interactions is not clear [55]. [PERSON] et al. studied the presence of bioactive compounds in industrial beetroot pomace extract obtained from different cultivars grown in Serbia [56]. The concentrations of total phenolics (45.68 mg gallic acid equivalents per gram), flavonoids (25.89 mg rutin equivalents per gram), and betalains (betanin, 4.09 mg g\({}^{-1}\); vulgaxanthin I, 7.32 mg g\({}^{-1}\)) were determined. Some studies reported that beets from organic cultivation are characterized by a higher content of bioactive compounds [57; 58; 59], while others reported the opposite relationship [60; 61]. According to [PERSON] and [PERSON], these inconsistent results may be due to the differences in the form and amount of fertilizer used, weather conditions, soil type, cultivation, and genetic factors [62]. It is generally accepted that plants from organic farming contain more polyphenic compounds than those from conventional cultivation. This, in turn, translates into the antioxidant properties of such foods, and their usefulness in preventing oxidative stress by neutralizing free radicals. Different reaction mechanisms between the polyphenic compounds and radicals have been proposed. They can be divided into two groups: H-atom abstraction and radical adduct formation. Moreover, H-abstraction processes can be divided into four subgroups of mechanisms: hydrogen atom transfer (HAT), proton coupled electron transfer (PCET), sequential proton loss electron transfer (SPLET), and single electron transfer followed by proton transfer (SET-PT). It should be mentioned that all of these mechanisms have the same result [63]. [PERSON] et al. mentioned that the mechanisms of action of many important food and beverage ingredients, such as chlorogenic acid, are not fully clarified [64]. Based on the electron-spin resonance spectroscopy [ESR] and density-functional theory [DFT], authors have concluded that in acidic and neutral media, chlorogenic acid rather takes the HAT or RAF pathway. On the other hand, [PERSON] et al. proved that SPLET is the dominant mechanism for the radical activity towards DPPH radicals in polar solvents [63]. Based on our experience in working with beetroot juice, we know that its pH, depending on the method of preparation and the variety of beetroot from which it was obtained, is in the range of 4-5.