Investigation of phenolic compounds and antioxidative potential of Chinese Brassica vegetables
Final Report Abstract
Several flavonoids and hydroxycinnamic acid derivatives were identified in Chinese cabbages. The main flavonoids are kaempferol derivatives, glycosilated and esterified with different hydroxycinnamic acids. Hydroxyferulic acid as a moiety of flavonoids such as kaempferol derivatives was characterized for the first time by NMR. The main hydroxycinnamic acids were derivatives of malic acid, which were identified for the first time in cabbages. Hydroxybenzoic acids were not detected as moieties of free phenolic compounds. However, different hydroxybenzoic- and hydroxycinnamic acids and aldehydes were identified bound to the cell wall. These compounds represent only minor amounts (0.3- 1%) of the total content of phenolic compounds in plants. Furthermore, no tendency was observable for differences in contents of free phenolic compounds distributed in outer and inner leaves. However, the contents in leaf blade were much higher than in leaf stem, particularly for the flavonoid derivatives. Flavonoids were not detected in leaf stem of plants cultivated under greenhouse conditions, but under field conditions in small amounts. However, significant concentrations were detected for anthocyanins in leaf stem, but lower contents in leaf blade of purple cabbage. Additionally, the contents of bound phenolic compounds of cell wall were also higher in leaf blade (approximately 140 μg/g cell wall) than in leaf stem (approximately 70 μg/g cell wall). Post-harvest treatments such as fermentation resulted in qualitative and quantitative polyphenol changes. The antioxidative potential of non fermented cabbage is comparable with data in literature, however, a distinct increase was observable for all fermented varieties after the fermentation procedure and might be explained by the qualitative changes of the polyphenol structures. The content of bound phenolic compounds did not change significantly during fermentation and seemed not to be involved in changes of antioxidative capacity during fermentation. Changes like an increase of polyphenols for all plant samples during storage treatments were observed and might be explained by an enhanced polyphenol biosynthesis due to higher plant stress during storage. Finally, naturally antioxidants such as polyphenols of Chinese cabbage have the ability to protect lipid compounds in processed foods by inhibiting the formation of lipid oxidation products.
