Chemical composition, nutritional value and antioxidant properties of Allium caepa L. Var. tropeana (red onion) seeds

Abstract

Chemical analysis of Allium caepa L. var Tropeana (red onion) seeds showed high amounts of oil (20.4%), fibre (22.4%), crude protein (24.8%), calcium (175.0 mg/100 g), potassium (1010 mg/100 g), low amounts of sodium (11.2 mg/100 g) and six cysteine derivatives, of which the S-propylmercapto-cysteine has never been reported in onion before. The antioxidant capacity of seed extracts containing cysteine derivatives (SECCD), before and after boiling the seeds, and of cooking water extracts containing cysteine derivatives (CWECCD), was also evaluated, by the ferric reducing antioxidant power (FRAP) and 1,1-diphenyl-2-picrilhydrazyl (DPPH) assays. The extracts showed discrete antioxidant capacity which increased after boiling, although cooking methods caused significant losses of the cysteine derivatives in water.

Introduction

Onion is one of the most important vegetable crops, with a world production of about 55 million tonnes (FAO, 2004). Its consumption is attributed to several factors, mainly heavy promotion that links flavour and health and the popularity of onion-rich ethnic foods. Onion bulbs are the main edible part, with a distinctive strong flavour and pungent odour. Onion seeds are also eaten, especially in some indian dishes, they do not affect the breath as strongly as bulbs do, nevertheless, their commercial availability is currently limited. Perhaps, if consumers were much more acquainted with onion seed nutritional and functional properties, there would be a boost in the trade market for this product. At the moment, no data about onion seed chemical value are available in literature. Onion represents a source of cysteine derivatives, which makes it a good antioxidant additive for food (Ostrowska et al., 2004), increasing its potential usability as a functional food and in ethnomedicine (Tram Ngoc et al., 2005). Oxidation is one of the major causes of chemical spoilage, resulting in rancidity and/or deterioration of the nutritional quality, colour, flavour, texture and safety of foods (Antolovich, Prenzler, Patsalides, McDonald, & Robards, 2002). Dietary antioxidants are important components because they protect against free radicals, such as reactive oxygen species in the human body. Free radicals are known to be the major contributors to degenerative diseases of aging and are recognised as major factors causing cancer, cardiovascular disorders and diabetes. At present, there is an increasing interest both in industry and scientific research in spices and aromatic herbs because of their strong antioxidant and antimicrobial properties. These properties are due to many substances, including some vitamins, flavonoids, terpenoids, carotenoids, phytoestrogens, minerals, etc. and render spices and some herbs or their antioxidant components as preservative agents in food (Calucci, Pinzono, Zandomeneghi, & Capocchi, 2003). The spices, in view of the many promising health beneficial physiological effects, have assumed the status of “Nutraceuticals” and are considered as a natural and necessary components of our daily nutrition. Moreover, since modern consumers are increasingly asking for natural products, free of synthetic additives, the application of natural antioxidants will probably continue in the future and it will be necessary to study their changes and interactions in more details. The aim of this work is the study of the nutritional value, including cysteine derivative quality, of the seeds of a low pungency or so-called “sweet” onion variety, the Tropea red onion, to make available appropriate nutritional labelling so that consumers can be better informed towards this product. Since these seeds are generally consumed cooked, the antioxidant activity of seed extracts containing cysteine derivatives (SECCD) before and after boiling the seeds, and of cooking water extracts containing cysteine derivatives (CWECCD), was also evaluated, by using DPPH and FRAP tests, in order to establish how a traditional cooking method could affect it. Several in vitro analytical methods are available to characterise the antioxidant propensity of bioactive compounds in plant foods and supplements. The use of more than one method is recommended to give a comprehensive prediction of antioxidant efficacy. A factor that provides a distinct challenge in the assay of antioxidant capacity is that within biological systems, there are at least four general sources of antioxidants: enzymes (superoxide dismutase, glutathione peroxidase and catalase); large molecules (albumin, ceruloplasmin, ferritin and other proteins); small molecules [ascorbic acid, glutathione, uric acid, tocopherol, carotenoids, (poly)phenols] and some hormones (estrogen, angiotensin, melatonin, etc.). Furthermore, there are multiple free radical and oxidant sources [e.g. ${\text{O}}_2^{}$, HO, NO, ONOO⁻, HOCl, RO(O), LO(O)] and both oxidants and antioxidants have different chemical and physical characteristics. Therefore, there is no simple universal method by which antioxidant capacity can be measured accurately and quantitatively. The mechanism of antioxidant action in vitro may involve direct inhibition of the generation of reactive oxygen species, or the scavenging of free radicals. We used FRAP assay because it is the only one that directly estimates the capacity of antioxidants or reductants in a sample and is based on the ability of the analyte to reduce Fe³⁺/Fe²⁺ couple. The reaction detects compounds with redox potentials of <0.7 V (the redox potential of Fe³⁺-TPTZ), so the FRAP test is a reasonable screen for the ability to maintain redox status in cells or tissues. Reducing power appears to be related to the degree of hydroxylation and extent of conjugation in polyphenols (Ou, Huang, Hampsch-Woodill, Flanagan, & Deemer, 2002). However, the FRAP test cannot detect compounds which act by radical quenching (H transfer), particularly thiols and proteins (Cao, Sofic, & Prior, 1997). In addition, since reduced metals are active propagators of radical chains via hydroperoxide reduction to RO, it would be interesting to evaluate whether high FRAP values correlate with the tendency of polyphenols to become pro-oxidants under some conditions. This has been shown for some flavones and flavanones (Delgado-Andrade, Rufián-Henares, & Morales, 2005), which also have high FRAP values. The DPPH test determines radical scavenging activities of compounds by measuring the inactivation potential of radicals in an aqueous mean. It is simple and rapid but it has generally a relatively small linear reaction range of only 2-3-fold and small molecules that have better access to the radical site have higher apparent antioxidant activity with this test. Furthermore, the DPPH radical is decolourised by reducing agents as well as H transfer, which also contributes to inaccurate interpretations of antioxidant capacity (Beretta et al., 2005, Noruma et al., 1997). Interpretation of data is further complicated when the tested compounds, carotenoids in particular, have UV spectra which overlap with those of the DPPH complex at 515 nm (Benzie & Strain, 1996).