Phenolic acids, flavonoids and total antioxidant capacity of selected leafy vegetables

Abstract

Phenolic compounds and total antioxidant capacity of eight leafy vegetables, namely Komatsuna, Mizuna, Pok choi, Mitsuba, Salad spinach, Lettuce, Red amaranth and Green amaranth were determined. The phenolic compounds were characterized as hydroxybenzoic acids, hydroxycinnamic acids and flavonoids. Salicylic acid was, by far, the most common hydroxybenzoic acid, ranging from 4.40 to 117.36 μg/g fresh frozen weight (ffw). Vanilic acid, gallic acid, caffeic acid, chlorogenic acid, p-coumaric acid, ferulic acid and m-coumaric acid were commonly found in all of these vegetables. Isoquercetin and rutin, the most common flavonoids, ranged from 3.70 to 19.26 and 1.60 to 7.89 μg/g ffw, respectively, and hyperoside was highest (38.72 μg/g ffw) in Mizuna. Total antioxidant capacity values varied widely between ABTS⁺ and DPPH assay methods, with values reported as equivalents to trolox, quercetin and ascorbic acid. Among these vegetables, total antioxidant capacity was found in the following order: Pok choi > Komatsuna > Mizuna > Mitsuba > Red amaranth > Lettuce > Green amaranth > Salad spinach.

Introduction

In recent decades, polyphenols have attracted considerable interest in food technology research. Their antioxidant properties, availability in diets and probable roles in countering deadly diseases such as cancer, neuro-degenerative and cardiovascular diseases have been highlighted in various studies (Manach, Scalbert, Morand, Rémésy, & Jiménez, 2004). Currently, natural antioxidants, particularly in fruits and vegetables, have gained the attention of both researchers and consumers. Vitamins, phenolic substances and carotenoids are the three major groups of natural antioxidants that are available in fruits and vegetables and that are involved in defences against several diseases (Thaipong, Boonprakob, Crosby, Cisneros-Zevallos, & Byrne, 2006). Phenolic substances, which exist in plant tissues as secondary metabolites, can be categorized into simple phenols, phenolic acids (both hydroxybenzoic acids and hydroxycinnamic acids), flavonoids, lignins and tannins. These substances are responsible for colour, bitterness, acerbic taste, flavour, odour and antioxidant properties (Kartika, Li, Wall, Nakamoto, & Iwaoka, 2007). Antioxidants are compounds that can delay the oxidation of other molecules by inhibiting the initiation or propagation of oxidizing chain reactions caused by free radicals and thereby may reduce oxidative damage to the human body (Namiki, 1990).

Phenolic acids as well as flavonoids are a large group of phytochemicals that are widely available in the plant kingdom and are produced from phenylalanine and tyrosine via the shikimic acid pathway (Herrmann, 1995). Phenolic compounds are involved in attracting insects for seed dispersion and pollination, natural defences against harmful insects, fungi, viruses, bacteria and even controlling plant hormones. Flavonoids are important for a wide array of biological functions. Quercetin, a major representative of the flavonoid, prevents the oxidation of low-density lipoprotein by scavenging free radicals (Cartea, Francisco, Soengas, & Velasco, 2011). Ellagic acid is a naturally occurring polyphenolic metabolite which is produced within vacuoles as water soluble ellagitannins. Studies of ellagic acid have reported its antimutagenic and anticarcinogenic responses, which are beneficial for human health (Maas, Galletta, & Stoner, 1991).

Vegetables and fruits, in particular, are very important for human health because they occur naturally as whole foods and are rich in a large variety of nutrients. Leafy vegetables are not only good sources of minerals but also contain vitamins, antioxidants and pigments. A large number of vegetables, such as kale, spinach, brussel sprout, broccoli, onion, eggplant and cucumber, are rich sources of antioxidants (Gazzani et al., 1998, Velioglu et al., 1998, Vinson et al., 1998).

In Japan, commercially available leafy vegetables include: Komatsuna (Japanese mustard spinach, Brassica rapa var komatsuna, family: Brassicaceae), Mizuna (Japanese mustard, Brassica rapa, var japonica, family: Brassicaceae), Pok choi (Chinese celery cabbage, Brassica rapa var. chinenesis, family: Brassicaceae), Mitsuba (Japanese parsley, Cryptotaenia japonica, family: Apiaceae), Horenso (salad spinach, Spinacea oleracea, family: Amaranthaceae), Lettuce (condiment, Lactuca sativa, family: Asteraceae), Red amaranth (Amaranthus tricolor, family: Amaranthaceae) and Green amaranth (Amaranthus tricolor, family: Amaranthaceae). Mustard group vegetables are important food crops throughout the world (Cartea et al., 2011). Recent reports have suggested that cruciferous vegetables are good sources of natural antioxidants. They contain high levels of carotenoids, tocopherols and ascorbic acid, and there is strong epidemiological evidence that these compounds help protect the human body against damage from reactive oxygen species (Cartea et al., 2011). However, the phenolic compounds of these traditional vegetables have not been adequately studied. The present study selected eight leafy vegetables (mentioned above) and investigated their phenolic profiles and antioxidant capacities. Therefore, the objectives of this study were (1) to characterise the phenolic compounds by identifying and quantifying their phenolic acids and flavonoids, and (2) to evaluate their antioxidant capacities using ABTS⁺ and DPPH assays, with values reported as equivalents to trolox, quercetin and ascorbic acid. The findings of this study will improve our understanding of the antioxidant functions of these leafy vegetables for food nutritionists and consumers.