Basic nutritional investigationQuantitative evaluation of the antioxidant properties of garlic and shallot preparations
Introduction
Many plant extracts used in treatment of human health problems have shown antioxidant and free radical scavenging capacities against external and endogenous agents [1], [2], [3], [4], [5]. Among these, extensive studies carried out on garlic extracts (Allium sativum L.) have reported the presence of two main classes of antioxidant components, namely flavonoids [6], [7] and sulfur-containing compounds, diallyl sulfide and trisulfide, and allyl-cysteine [8]. These are likely to play a role in the widely demonstrated biological effects of garlic, which include inhibition of benzo[A]pyrene-induced forestomach neoplasia in Sprague-Dawley rats by garlic oil and inhibition by aged garlic extracts of lipid peroxidation induced by Cu2SO4 in low-density lipoprotein or by tert-butyl hydroperoxide in erythrocytes [9], [10], [11]. However, the detailed mechanism of these effects remains unclear. In comparison, there are no reports of the potential antioxidant properties of the other onion, shallot (Allium ascalonicum L.), which is a major component of many Asian diets and is widely believed to be beneficial to health. This bulb is darker than garlic and has a stronger odor that correlates with its sulfide content [12]. Analysis of shallot extracts has confirmed the presence of flavone and polyphenolic derivatives such as quercetin, quercetin 4′-glucoside, quercetin 7,4′-diglucoside, quercetin 3,4′-diglucoside, and quercetin mono-d-glucose, suggesting that it also may have antioxidant properties [13], [14], [15].
The aim of this study was to detect any antioxidant and free radical-scavenging properties of aqueous and organic solvent extracts of the red shallot and to compare them with extracts of garlic and with a range of well-known antioxidants. The antioxidant potentials were measured by the ABTS·+ bleaching assay [16], which is commonly used for this purpose. We also tested the capacity of the extracts to protect the biologically more relevant proteins and lipids from oxidation by radiation-generated hydroxyl radicals and for the ability to decrease preformed lipid and protein peroxides. The results showed that shallot extracts have antioxidant properties similar to or slightly higher than those of fresh garlic preparations. Parallel measurements on aged garlic preparations from commercial sources showed significantly lower antioxidant activities. The relative antioxidant potentials of the fresh extracts were related to their content of phenolic and sulfur compounds. It seems likely that the frequently reported health benefits of shallot and garlic may be explained at least in part by their ability to enhance or maintain the total antioxidant capacity of individuals who include these bulbs in their diet.
Section snippets
Materials and reagents
Glassware was cleaned by heating overnight in concentrated HNO3, washing in distilled water, and drying. Solutions were made up in distilled water that was purified by passage though a four-stage Milli Q system (Millipore-Waters, Sydney, Australia) equipped with a 0.2-μm-pore final filter. Bovine serum albumin (BSA; Cohn fraction V, fat fee) and catalase (EC 1.11.1.6, 65,000 U/mg) were supplied by Boehringer-Mannheim (Mannheim, Germany), and 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)
Water content
The water contents of shallot and garlic bulbs were 83.79 ± 0.6% and 37.87 ± 0.6%, respectively. The pH of extract solutions was 6 to 7. The lyophilized solids expressed as fractions of the weight of the fresh bulbs subjected to pressing, water, and hexane extraction were 6.42 ± 0.95%, 8.68 ± 0.56%, and 2.78 ± 0.25% in shallot and 7.29 ± 0.85%, 17.6 ± 0.68%, and 6.84 ± 0.62% in garlic. Tests showed that more than 95% of antioxidant activities of the extracts were contained in the lyophilized
Discussion
Garlic and shallots are widely consumed components of the diet of many populations, as flavor enhancers and because of widespread belief in their preventive and even curative potential against a range of debilitating conditions and diseases. Although numerous scientific studies have been undertaken on garlic to test the basis and validity of these beliefs, little parallel work has been carried out with shallots. Current knowledge of the properties and constituents of shallots and analogy with
Acknowledgments
The authors are grateful to Somdej Srichairattanakul for assistance with the HPLC assays and to Sugunya Wongpornchai for help with the GC/MS analyses.
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This study was supported by the S.M. Pharmaceutical Company (Thailand) and by Macquarie University (Sydney, Australia).