Slimestad Rune, Fossen Torgils, Vågen Ingunn Molund
PlantChem, Saerheim Research Centre, N-4353 Klepp station, Norway.
J Agric Food Chem. 2007 Dec 12;55(25):10067-80. doi: 10.1021/jf0712503. Epub 2007 Nov 13.
Onion bulbs (Allium cepa L.) are among the richest sources of dietary flavonoids and contribute to a large extent to the overall intake of flavonoids. This review includes a compilation of the existing qualitative and quantitative information about flavonoids reported to occur in onion bulbs, including NMR spectroscopic evidence used for structural characterization. In addition, a summary is given to index onion cultivars according to their content of flavonoids measured as quercetin. Only compounds belonging to the flavonols, the anthocyanins, and the dihydroflavonols have been reported to occur in onion bulbs. Yellow onions contain 270-1187 mg of flavonols per kilogram of fresh weight (FW), whereas red onions contain 415-1917 mg of flavonols per kilogram of FW. Flavonols are the predominant pigments of onions. At least 25 different flavonols have been characterized, and quercetin derivatives are the most important ones in all onion cultivars. Their glycosyl moieties are almost exclusively glucose, which is mainly attached to the 4', 3, and/or 7-positions of the aglycones. Quercetin 4'-glucoside and quercetin 3,4'-diglucoside are in most cases reported as the main flavonols in recent literature. Analogous derivatives of kaempferol and isorhamnetin have been identified as minor pigments. Recent reports indicate that the outer dry layers of onion bulbs contain oligomeric structures of quercetin in addition to condensation products of quercetin and protocatechuic acid. The anthocyanins of red onions are mainly cyanidin glucosides acylated with malonic acid or nonacylated. Some of these pigments facilitate unique structural features like 4'-glycosylation and unusual substitution patterns of sugar moieties. Altogether at least 25 different anthocyanins have been reported from red onions, including two novel 5-carboxypyranocyanidin-derivatives. The quantitative content of anthocyanins in some red onion cultivars has been reported to be approximately 10% of the total flavonoid content or 39-240 mg kg (-1) FW. The dihydroflavonol taxifolin and its 3-, 7-, and 4'-glucosides have been identified in onions. Although the structural diversity of dihydroflavonols characterized from onions is restricted compared with the wide structural assortment of flavonols and anthocyanins identified, they may occur at high concentrations in some cultivars. From bulbs of the cultivar "Tropea", 5.9 mg of taxifolin 7-glucoside and 98.1 mg of taxifolin have been isolated per kilogram of FW.
洋葱鳞茎（葱属植物）是膳食类黄酮最丰富的来源之一，在很大程度上构成了类黄酮的总体摄入量。本综述汇编了已报道的洋葱鳞茎中类黄酮的定性和定量信息，包括用于结构表征的核磁共振光谱证据。此外，还总结了根据洋葱品种中以槲皮素衡量的类黄酮含量进行的索引。据报道，洋葱鳞茎中仅含有属于黄酮醇、花青素和二氢黄酮醇的化合物。黄洋葱每千克鲜重（FW）含有270 - 1187毫克黄酮醇，而红洋葱每千克FW含有415 - 1917毫克黄酮醇。黄酮醇是洋葱的主要色素。至少已鉴定出25种不同的黄酮醇，槲皮素衍生物在所有洋葱品种中最为重要。它们的糖基部分几乎全是葡萄糖，主要连接在苷元的4'、3和/或7位。在最近的文献中，槲皮素4'-葡萄糖苷和槲皮素3,4'-二葡萄糖苷在大多数情况下被报道为主要的黄酮醇。山奈酚和异鼠李素的类似衍生物已被鉴定为次要色素。最近的报道表明，洋葱鳞茎的外层干皮除了含有槲皮素与原儿茶酸的缩合产物外，还含有槲皮素的低聚结构。红洋葱的花青素主要是被丙二酸酰化或未被酰化的矢车菊素葡萄糖苷。其中一些色素具有独特的结构特征，如4'-糖基化和糖部分不寻常的取代模式。红洋葱总共至少已报道了25种不同的花青素，包括两种新的5-羧基吡喃矢车菊素衍生物。据报道，一些红洋葱品种中花青素的定量含量约为类黄酮总含量的10%或39 - 240毫克/千克FW。已在洋葱中鉴定出二氢黄酮醇紫杉叶素及其3-、7-和4'-葡萄糖苷。尽管与已鉴定出的黄酮醇和花青素广泛的结构种类相比，从洋葱中鉴定出的二氢黄酮醇的结构多样性有限，但它们在某些品种中可能以高浓度存在。从“特罗佩亚”品种的鳞茎中，每千克FW已分离出5.9毫克紫杉叶素7-葡萄糖苷和98.1毫克紫杉叶素。
