Antioxidants of 15 onions with white, yellow, and red colors and their relationship with pungency, anthocyanin, and quercetin
Introduction
Onions have many health benefits and are one of the major sources of various biologically active phytomolecules, including phenolic acids, flavonoids, cepaenes, thiosulfinates, and anthocyanins (Fossen and Andersen, 2003, Slimestad et al., 2007). The quercetin compounds are major flavonoids in onions and are related to skin colors and disease resistance in plants (Trammell & Peterson, 1976). Quercetin has various human health benefits and inhibits cancer cell growth (Chu, Chang, & Hsu, 2000). It also functions as an antioxidant, chelating agent (Torel, Cillard, & Cillard, 1986), and free radical scavenger (Murota & Terao, 2003). The onion ranked highest in quercetin compounds among 29 vegetables and 9 fruits (Galdón, Rodríguez, & Romero, 2008) and has been suggested as a healthy food to prevent coronary heart disease (Hertog, Hollman, & Katan, 1992).
The red onion color is caused by anthocyanins consisting of mainly cyanidin or peonidin glucosides that are acylated with malonic acid or nonacylated (Mazza & Miniati, 1993). There are at least 25 different anthocyanins from red onions. The content of anthocyanin is approximately 10% of the total flavonoid content or 39−240 mg/kg fresh weight (Slimestad et al., 2007). Anthocyanins are also reported to be a source of antioxidant activity (Geetha, Ponmozhi, Saravanakumar, & Suganyadevi, 2011). However, white onions contain less flavonoid content than colored onions (Herrmann, 1976).
The pungent flavor of onions is caused by numerous volatile sulfur compounds produced when alliinase reacts on the flavor precursor compounds (S-alk(en)yl-cysteine-sulfoxides) after the tissues are mechanically damaged (Lancaster & Boland, 1990). One of the sulfur volatiles, thiopropal S-oxide, is a lachrymatory factor uniquely found in onions (Thomas & Parkin, 1994). Pyruvic acid is also produced as a byproduct and has been used to measure the pungency of onions (Randle & Bussard, 1993). The sulfur compounds in onions and garlic have been reported to reduce or prevent many chronic diseases, such as cancer, coronary heart disease, obesity, and hypercholesterolemia (Lanzotti, 2006).
Reactive oxygen species (ROS) are produced in cells by cellular metabolism and other exogenous environmental agents. The overproduction of ROS can damage cellular biomolecules, such as nucleic acids, proteins, lipids, and enzymes, and can result in several diseases (Prior, Wu, & Schaich, 2005). Antioxidant scavengers of these free radicals are associated with a reduced risk of cancer and cardiovascular diseases. Onions have high levels of antioxidant properties (Stajner & Varga, 2003). Therefore, onions are a food associated with a reduced risk of various human diseases (Sanderson, Mclauchlin, & Williamson, 1999). The total antioxidant capacity is highest in onion peel ethanol extract and plasma quercetin, and the isorhamnetin levels in rats were markedly increased by feeding the onion peel powder and extract (Park, Kim, & Kim, 2007).
There are various ways to measure antioxidant activity (AOA) in plant extracts. The Folin–Ciocalteu (F–C) assay has been widely used due to its convenience of measurement (Prior et al., 2005). Although originally developed to measure phenolic content in protein (Folin & Ciocalteu, 1927), this assay can also be used to estimate AOA because the chemistry is based on the reduction of the F–C reagent (Prior et al., 2005). This reagent reacts with phenolic and many non-phenolic compounds, such as ascorbic acid, reducing sugars, and amino acids (Singleton et al., 1999, Waterhouse, 2002). Therefore, the chemical nature of a sample may cause significant interference with the total AOA levels. The 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay is also widely used due to its simple and convenient procedure (Prior et al., 2005). The color absorption of the reactant steadily declines over time, and approximately 2 h is recommended to obtain reliable results (Yoo, Lee, & Patil, 2011). Because there are several types of reactions between various reduction compounds in samples and assay reagents, every method measures different reduction compounds. Thus, it is common that two different assays result in quite different responses or patterns (Prior et al., 2005).
We have been developing low pungency, high quercetin containing red onion cultivars in our breeding program to provide more health benefits, including antioxidants. However, the relationship between antioxidant activities vs. onion pungency and color has not yet been characterized. In this study, we investigated the antioxidant activity of 15 short-day onions with white, yellow, and red bulb colors using the F–C and DPPH assays to elucidate the relationships between pungency, anthocyanin, and quercetin contents and the AOA levels.
Section snippets
Chemicals
All chemicals and solvents used in this study were purchased from Sigma (St. Louis, MO., USA) or Fisher Scientific (Pittsburg, PA, USA).
Plant materials
Fifteen short-day onion genotypes with different colors (2 white, 10 yellow, 3 red) shown in Fig. 1 were used. The onions were planted in mid-October and harvested in late-April from the field of the Texas A&M University Research and Extension Center at Weslaco (26°15′N, 97°98′W) in the Rio Grande Valley of South Texas, USA. The onions were managed according to
Onion pungency and total soluble solids
The onion pungency measured by pyruvic acid content is presented in Fig. 1A. The mean pyruvic acid content ranged between 4.1 (‘Legend’) to 6.3 (‘D17’) μmol/mL, which are mild and moderately hot, respectively according to Lee et al. (2009a). There was no distinctive relationship between bulb color and pungency in the tested genotypes. However all 3 red genotypes had more than 5 μmol/mL pyruvic acid and were regarded as moderately hot (http://vfic.tamu.edu/onion_information/; Wall & Corgan, 1992
Conclusion
In this study, we have investigated the relationship of AOA vs. pungency, TSS, anthocyanin, or quercetin and extraction solvent using various genotypes. Contrary to our general understanding, we could not identify any single parameter that was directly related with AOA levels in onion. Each parameter responded differently with the AOA assays and reflected the complex nature of the reducing compounds in the onion juices and reaction mechanisms of each assay. We concluded that eating red onions
Acknowledgments
The present study is based upon work supported by a USDA-NIFA Grant (No. 2010-34402-20875) titled “Designing Foods for Health” through the Vegetable & Fruit Improvement Center, Texas A&M University, College Station, Texas, USA.
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