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Polyphenolic antioxidant compounds were studied in three onion varieties (red -cv. Karmen, yellow -cv. Všetana and white -cv. Ala) regarding their total content and qualitative composition of flavonoid complex. The effects of temperature and storage period during onion storage were also studied. The total content of polyphenols was estimated spectrophotometrically with Folin-Ciocalteau's phenolic reagent and individual flavonoid components by HPLC method using Waters TM chromatograph on Watrex 250 × 4 mm Sepharon SGX C18 7μm column with gradient elution. The highest amount of polyphenolic compounds was found in red variety Karmen (108,300 mg/kg DM), the lowest amount in white variety Ala (26,445 mg/kg DM) while the yellow variety Všetana had average content of total polyphenolic compounds 65,210 mg/kg DM. An increase in total polyphenols during storage was found in red and yellow varieties, esp. during storage at a laboratory temperature (22°C). Among the flavonoid and polyphenolic compounds as major constituents spiraeoside (quercetin-4´-O-β-D-glucoside), rutin and quercetin and three other not identified compounds were found. Significant varietal differences were found. The compound with the highest content was spiraeoside (32,234 mg/kg DM in red cv. Karmen, 23,283 mg/kg DM in yellow cv. Všetana and 265 mg/kg DM in white cv. Ala). Rutin ranged from 15 mg/kg DM in cv. Ala to 157 mg/kg DM in cv. Karmen and similarly quercetin from 1 mg/kg DM in cv. Ala to 163 mg/kg DM in cv. Karmen. During storage an increase in flavonoids could be observed, esp. at a laboratory temperature compared to storage at a lower temperature.

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Lachman J., Hejtmánková A., Sýkora J., Karban J., Orsák M., Rygerová B. (2010): Contents of major phenolic and flavonoid antioxidants in selected Czech honey. Czech J. Food Sci., 28: 412-426.The chemical constitution of antioxidants contained in honey is derived from its origin. Forty honey samples (harvest 2006), which came from various locations of the Czech Republic and varied in their origins, were evaluated spectrophotometrically for their total polyphenol content, total flavonoids and 3',4'-dihydroxyflavones and flavonols, and major antioxidants were identified by HPLC-DAD and GC-MS. The kind of honey, location, and date of the honey harvest were shown to have a significant effect on the contents of phenolic antioxidants (average content 11.02 mg gallic acid equivalents/100 g), total flavonoids (0.66 mg quercetin equivalents/100 g), and 3',4'-dihydroxyflavones and flavonols (4.32 µg quercetin equivalents/100 g). In the Czech honey, ferulic acid (0.11 mg/100 g) and chrysin (0.06 mg/100 g) and other minority phenolics and flavonoids were identified and quantified as honey phenolic antioxidants contained. The results obtained support and extend complete knowledge on the contents of bioactive phenolics in the Czech honey, which could serve as a good source of natural antioxidants effective in reducing the risk of the occurrence of heart disease, cancer, cataracts, different inflammatory processes and immuno-system decline.

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