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Phenolic compounds are a large group of phytochemicals widespread in the plant kingdom. Depending on their structure they can be classified into simple phenols, phenolic acids, hydroxycinnamic acid derivatives and flavonoids. Phenolic compounds have received considerable attention for being potentially protective factors against cancer and heart diseases, in part because of their potent antioxidative properties and their ubiquity in a wide range of commonly consumed foods of plant origin. The Brassicaceae family includes a wide range of horticultural crops, some of them with economic significance and extensively used in the diet throughout the world. The phenolic composition of Brassica vegetables has been recently investigated and, nowadays, the profile of different Brassica species is well established. Here, we review the significance of phenolic compounds as a source of beneficial compounds for human health and the influence of environmental conditions and processing mechanisms on the phenolic composition of Brassica vegetables.
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Molecules 2011, 16, 251-280; doi:10.3390/molecules16010251
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Review
Phenolic Compounds in Brassica Vegetables
María Elena Cartea *, Marta Francisco, Pilar Soengas and Pablo Velasco
Misión Biológica de Galicia, Consejo Superior de Investigaciones Científicas (CSIC), Apartado 28,
36080 Pontevedra, Spain; E-Mails: mfrancisco@mbg.cesga.es (M.F.); psoengas@mbg.cesga.es (P.S.);
pvelasco@mbg.cesga.es (P.V.)
* Author to whom correspondence should be addressed; E-Mail: ecartea@mbg.cesga.es;
Tel.: +34986854800; Fax: +34986841362.
Received: 19 October 2010; in revised form: 6 December 2010 / Accepted: 28 December 2010 /
Published: 30 December 2010
Abstract: Phenolic compounds are a large group of phytochemicals widespread in the
plant kingdom. Depending on their structure they can be classified into simple phenols,
phenolic acids, hydroxycinnamic acid derivatives and flavonoids. Phenolic compounds
have received considerable attention for being potentially protective factors against cancer
and heart diseases, in part because of their potent antioxidative properties and their
ubiquity in a wide range of commonly consumed foods of plant origin. The Brassicaceae
family includes a wide range of horticultural crops, some of them with economic
significance and extensively used in the diet throughout the world. The phenolic
composition of Brassica vegetables has been recently investigated and, nowadays, the
profile of different Brassica species is well established. Here, we review the significance of
phenolic compounds as a source of beneficial compounds for human health and the
influence of environmental conditions and processing mechanisms on the phenolic
composition of Brassica vegetables.
Keywords: anthocyanins; antioxidant activity; biological activity; brassica; flavonoids;
health; hydroxycinnamic acids; polyphenols
1. Introduction
Plant-based foods contain significant amounts of bioactive compounds, which provide desirable
health benefits beyond basic nutrition. Epidemiological evidence suggests that consumption of a diet
OPEN ACCESS
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rich in vegetables and fruits has positive implications for human health. In the last decades, special
attention has been paid towards edible plants, especially those that are rich in secondary metabolites
(frequently called phytochemicals) and nowadays, there is an increasing interest in the antioxidant
activity of such phytochemicals present in diet. Recent reports suggest that cruciferous vegetables act
as a good source of natural antioxidants due to the high levels of carotenoids, tocopherols and ascorbic
acid, and strong epidemiological evidence shows that these compounds may help to protect the human
body against damage by reactive oxygen species. In addition to carotenoids, tocopherols, and ascorbic
acid, most of the antioxidative effect related to plant food intake is mainly due to the presence of
phenolic compounds, which have been associated with flavour and colour characteristics of fruits and
vegetables. In this aspect, the popularity and consumption of vegetable Brassica species is increasing
because of their nutritional value. Brassica crops have been related to the reduction of the risk of
chronic diseases including cardiovascular diseases and cancer. Brassica foods are very nutritive,
providing nutrients and health-promoting phytochemicals such as vitamins, carotenoids, fiber, soluble
sugars, minerals, glucosinolates and phenolic compounds [1,2].
The family Brassicaceae (=Cruciferae) consists of 350 genera and about 3,500 species, and
includes several genera like Camelina, Crambe, Sinapis, Thlaspi and Brassica. The genus Brassica is
the most important one within the tribe Brassiceae, which includes some crops and species of great
worldwide economic importance such as Brassica oleracea L., Brassica napus L. and Brassica rapa
L. The same species can be utilized for several uses according to different forms or types. The genus is
categorized into oilseed, forage, condiment, and vegetable crops by using their buds, inflorescences,
leaves, roots, seeds, and stems. Brassicaceae vegetables represent an important part of the human diet
worldwide, are consumed by people all over the world and are considered important food crops in
China, Japan, India, and European countries. The main vegetable species is B. oleracea, which
includes vegetable and forage forms, such as kale, cabbage, broccoli, Brussels sprouts, cauliflower and
others; B. rapa includes vegetable forms, such as turnip, Chinese cabbage and pak choi, along with
forage and oilseed types; B. napus crops are mainly used like oilseed (rapeseed), although forage and
vegetable types like leaf rape and nabicol are also included; finally, the mustard group which is formed
by three species, B. carinata, B. nigra and B. juncea, is mainly used as a condiment although leaves of
B. juncea are also consumed as vegetables and they are widely used for both fresh and processed
markets in Asian countries (Table 1).
2. Phenolic Compounds
The beneficial effects of Brassica vegetables on health improvement have been partly attributed to
their complex mixture of phytochemicals possessing antioxidant activity. In recent years, considerable
attention has been directed towards the identification of natural antioxidants, namely those plant-
derived that may be used for human consumption regarding health promotion and disease prevention.
Among phytochemicals possessing antioxidant capacity, phenolic compounds are one of the most
important groups [2]. “Phenolic compounds” is a generic term that refers to a large number of
compounds (more than 8,000) widely dispersed throughout the plant kingdom and characterized by
having at least one aromatic ring with one or more hydroxyl groups attached. Phenolics are produced
in plants as secondary metabolites via the shikimic acid pathway. Phenylalanine ammonialyase (PAL)
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is the key enzyme catalyzing the biosynthesis of phenolics from the aromatic amino acid
phenylalanine.
Table 1. Main vegetable Brassica species, crops, and plant parts used for consumption.
Species Group Common name Organ
Brassica oleracea acephala Kale, collards Leaves
capitata capitata Cabbage Terminal leaf buds (heads)
capitata sabauda Savoy cabbage Terminal leaf buds (heads)
costata Tronchuda cabbage Loose heads
gemmifera Brussels sprouts Vegetative buds
botrytis botrytis Cauliflower Inflorescences
botrytis italica Broccoli Inflorescences
gongylodes Kohlrabi Stem
albogabra Chinese kale Leaves
Brassica rapa chinensis Pak choi, bok choy Leaves
dichotoma Brown sarson, toria Seeds
narinosa Chinese flat cabbage,
wutacai
Leaves
nipposinica Mibuna, mizuna Leaves
oleifera Turnip rape, rapeseed Seeds
pekinensis Chinese cabbage,
pe-tsai
Leaves
perviridis Komatsuna, Tendergreen Leaves
parachinensis Choy sum Leaves
rapa Turnip, turnip greens,
turnip tops
Roots, leaves and shoots
ruvo Broccoleto Shoots
trilochularis Yellow sarson Seeds
Brassica napus pabularia Leaf rape, nabicol Leaves
napobrassica Swede, rutabaga Roots
Brassica juncea rugosa Mustard greens Leaves
capitata Head mustard Heads
crispifolia Cut leaf mustard Leaves
Phenolics range from simple, low molecular-weight, single aromatic-ringed compounds to large
and complex tannins and derived polyphenols [3,4]. They can be classified based on the number and
arrangement of their carbon atoms in flavonoids (flavonols, flavones, flavan-3-ols, anthocyanidins,
flavanones, isoflavones and others) and non-flavonoids (phenolic acids, hydroxycinnamates, stilbenes
and others) [3] and they are commonly found conjugated to sugars and organic acids. The most
widespread and diverse group of polyphenols in Brassica species are the flavonoids (mainly flavonols
but also anthocyanins) and the hydroxycinnamic acids.
2.1. Flavonoids
Flavonoids are polyphenolic compounds comprising fifteen carbons with two aromatic rings
connected by a three-carbon bridge, hence C6-C3-C6 (Figure 1). They are the most numerous of the
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phenolics and are found throughout the plant kingdom [3,4]. They are present in high concentrations in
the epidermis of leaves and fruits and have important and varied roles as secondary metabolites, being
involved in processes like UV protection, pigmentation, stimulation of nitrogen-fixing nodules and
disease resistance [3,4]. Flavonols are the most widespread of the flavonoids. Quercetin, kaempferol
and isorhamnetin, the main flavonols in Brassica crops, are most commonly found as O-glycosides.
Conjugation occurs most frequently at the 3 position of the C-ring, but substitutions can also occur at
the 5, 7, 4´, 3´ and 5´ positions [3,5,6]. The number of sugar conjugates is elevated but in Brassica
vegetables they appeared mainly conjugated to glucose. They are also commonly found acylated by
different hydroxycinnamic acids.
Figure 1. Flavonoid aglycones found in vegetable Brassica crops.
Within the colored flavonoids, anthocyanins are the most important group of plant pigments, also
considered as multifunctional components of food due to their antioxidant activity and other beneficial
biological properties [7-9]. Anthocyanins are the sugar-conjugated forms of anthocyanidins, which are
compounds widely dispersed throughout the plant kingdom. They are particularly evident in fruit and
flower tissues where they are responsible for red, blue and purple colors. In addition, they are also
found in leaves, stems, seeds and root tissue. Nevertheless, in certain fruits and vegetables,
anthocyanins exist in smaller amounts and only some of them exist in such an amount that they can
determine the proper color. They are involved in the protection of plants against excessive light and
also have an important role in attracting pollinating insects [3,10]. The chemical structure of the
anthocyanin determines the stability, color intensity and potential biological activity. The most
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common anthocyanins are pelargonidin, cyanidin, delphinidin, peonidin, petunidin and malvidin, being
cyanidin the most common in Brassica crops [9,11,12].
2.2. Hydroxycinnamic acids
Hydroxycinnamic acids are a kind of non-flavonoid phenolics characterized by the C6-C3 structure
(Figure 2). These compounds are abundant in plants and are used in both structural and chemical plant
defense strategies. They can occur freely or as components of plant polymers (cell wall). Derivatives of
cinnamic acid are present in numerous vegetables and fruits. In Brassica vegetables the most common
are p-coumaric, sinapic and ferulic acids, often found in conjugation with sugar or other
hydroxycinnamic acids [13-16].
Figure 2. Hydroxycinnamic acids found in vegetable Brassica crops.
2.3. Biological activities
In the past two decades, there has been an increasing interest in the bioavailability and biological
effects of phenolics and flavonoids in food plants. Phenolic compounds have multiple additional roles
in plants, including attracting insects for seed dispersion and pollination. They are also part of the
natural defense system against insects, fungi, viruses and bacteria and they can act as plant hormone
controllers. Moreover, in recent years, phenolic compounds have been intensively investigated because
of their potential health-promoting effects [2,17-19]. They have been reported to possess many useful
properties for human health, including anti-inflammatory, enzyme inhibition, antimicrobial,
antiallergic, vascular and cytotoxic antitumor activity, but the most important action of phenolics is
their antioxidant activity [1,19-24]. Furthermore, phenolic compounds possess other properties such as
hydrogen peroxide production in the presence of certain metals, the ability to scavenge electrophiles
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and inhibit nitrosation reactions and chelate metals and, therefore, they act by blocking the initiation of
several human diseases [4,25-27].
The antioxidant activity of phenolic compounds is related with its chemical structure that confers
them redox properties. They can play an important role in adsorbing and neutralizing reactive oxygen
species (ROS), quenching singlet and triplet oxygen, or decomposing peroxides. Reactive oxygen
species, derived from oxidation processes, are an important part of the defense mechanisms against
infection, but excessive generation of free oxygen radicals may damage the tissue. When there is an
imbalance between ROS and antioxidant defense mechanisms, the ROS lead to the oxidative
modification in cellular membranes or intracellular molecules and result in the peroxidation of
membrane lipids, leading to the accumulation of lipid peroxides. This oxidative stress has been linked
to cancer, aging, atherosclerosis, inflammation and neurodegenerative diseases such as Parkinson’s
(PA) and Alzheimer’s disease (AD) [28,29]. Therefore, antioxidants, such as phenolic compounds, are
considered as possible protective agents, reducing the oxidative damage from ROS in the human body
and retarding the progress of many chronic diseases as well as the oxidation of low-density
lipoproteins (LDL), which is thought to play an important role in atherosclerosis.
The antioxidant ability of flavonoids and phenolic acids is related to the number and position of
hydroxyl groups in the molecule; an increase in the number of hydroxyl groups leads to a higher
antioxidant activity. Compounds with three hydroxyl groups on the phenyl ring of phenolic acids or
the B ring of flavonoids have a high antioxidant activity. The loss of one hydroxyl group decreases
activity slightly, whereas the loss of two hydroxyl groups significantly decreases the activity.
Moreover, glycosylation results in a lower antioxidant activity for some flavonoids such as quercetin,
the addition of a sugar moiety decreases the activity of the aglycon and the addition of a second moiety
further decreases the activity, probably due to steric hindrance by addition of sugar moieties [24]. The
antioxidant capacity of Brassica species has been related to its phenolic profile and content, especially
flavonoids, since phenolic compounds have demonstrated a higher antioxidant activity than vitamins
and carotenoids [1,20,22,30]. Many studies have shown the antioxidant power of particular flavonoids
and flavonoid-rich extracts. Flavonoids can also inhibit, and sometimes induce, a large variety of
mammalian enzyme systems; some of these enzymes are involved in important pathways that regulate
cell division and proliferation, platelet aggregation, detoxification and inflammatory and immune
response. Over the last 15 years, numerous publications have demonstrated that besides the in vitro
antioxidant capacity, certain phenolic compounds such as anthocyanins, catechins, proanthocyanidins
and other non-colored flavonoids may regulate different signaling pathways involved in cell survival,
growth and differentiation [1,18,26]. The effects of flavonoids on various stages of the cancer process,
on the immune system and on homeostasis in cell systems and animals have been also described
[6,27]. However, nowadays, it is widely accepted that if flavonoids have any preventive or curative
activity through their ingestion, this effect must involve, not only their antioxidant potential, but also
the modulation of multiple cellular pathways that are crucial in the pathogenesis of those diseases
[19,25].
Flavonoids are involved in a vast array of biological functions. Quercetin, a major representative of
the flavonol subclass and which is found at high concentration in broccoli, has received considerable
attention. This flavonoid has displayed the ability to prevent the oxidation of LDL by scavenging free
radicals and chelating transition metal ions. These properties are conferred due to the dihydroxylated
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B-ring, unsaturation at the C-ring and a 4-oxo function at the C-ring. As a result, quercetin may aid in
the prevention of certain diseases, such as cancer, atherosclerosis and chronic inflammation by
retarding oxidative degradation and inducing enzymes that detoxify carcinogens and also blocking the
formation of cancer by deactivating at least 30 types of agents that may cause cancer [26,27,31]. Some
flavonoids, derived from quercetin or 3-methoxyquercetin have also been described in literature as
acetylcholinesterase (AChE) inhibitors [32]. In addition, kaempferol also revealed a strong antioxidant
potential [31]. Some kaempferol derivatives found in high quantities in Brassica species are acylated
with caffeic acid. The presence of an O-dihydroxy structure in the caffeoylmoiety confers great
stability to their radical scavenging capacity [33]. Higher intakes of kaempferol resulted in a lower risk
of coronary heart disease. It has been demonstrated recently that quercetin and kaempferol
synergistically suppress cell proliferation in human gut cancer lines [26]. Ayaz et al. [34] found that
phenolic fractions extracted from kale leaves (B. oleracea), rich in quercetin and kaempferol
derivatives, effectively inhibited the growth of the Gram-positive bacteria Staphylococcus aureus,
Enterobacter faecalis, Bacillus subtilis and the Gram-negative bacterium Moraxella catarrhalis, which
is known to be a major respiratory pathogen in humans. Furthermore, isorhamnetin isolated from
mustard leaf showed a strong activity in reducing serum levels of glucose in Diabetes mellitus through
an antioxidant activity test [35]. Isorhamnetin revealed distinct vasodilator effects in animal models as
well, suggesting vascular protective effects in human cardiovascular diseases [17,19,36]. Besides,
quercetin, kaempferol and isorhamnetin were shown to have an anti-inflammatory effect on activated
macrophages [36].
Other phenolic compounds as sinapoyl esters and proanthocyanidins (condensed tannins) are
considered undesirable compounds in human nutrition [37,38]. Their presence in oilseed rape meal has
antinutritive effects in both monogastric and ruminant livestock feeds [38]. They have been shown to
cause a dark colour and a bitter taste in rapeseed meal and derived protein products and, therefore, they
are one of the principal factors currently limiting the use of canola seeds (Brassica napus L.) and other
oilseed crops of the genus Brassica including B. rapa, B. juncea or B. carinata [37,38]. Indeed,
proanthocyanidins are able to form soluble and insoluble complexes with proteins, polysaccharides
and other macromolecules from the diet, thus reducing their bioavailability [38]. Both compounds are
abundant in developing seeds of Brassica oilseed crops but this paper will focus on Brassica
vegetables.
2.4. Bioavailability
The extent of the absorption of dietary phenolics is an important unsolved problem in judging their
potential health effects. The health effects of polyphenols depend on the amount consumed and on
their bioavailability. In the last decades, numerous studies have been carried out to determine the
bioavailability of different phenolic compounds in diet by using animal models and human assays
[39-41]. These studies have stated that, in general, most of the phenolic compounds have a low
bioavailability since they are detected in very small amounts both in plasma and tissues.
It is well known that the structural properties of polyphenols affect the rate and extent of their
absorption in the small intestine and colon of humans, as well as the formation and occurrence of
metabolites in plasma [17,40]. Moreover, the absorption, physiological functions and bioavailability of
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cell wall-bound phenolic compounds differ from those of free phenolic compounds. Phenolic
compounds are present as free, as well as cell wall-bound compounds in plants. Bioavailability of
flavonoid glycosides takes place in the colon by means of intestinal microflora [41,42].
Flavonoids are thought to be poorly absorbed because the naturally occurring glycosides sugar
moieties elevate the hydrophilicity of molecules, and no enzyme is known to split the glycosidic bond.
Aglycones, sugar-free flavonoids can efficiently pass through the gut wall, but flavonoids are rarely
found as aglycones in plants. It has been suggested that the colon has intestinal microflora that can
hydrolyze the glycosidic bond, thus creating the aglycone, but the process also degrades the
compound. The aglycone is then absorbed in the large intestine easily because of its lipophilicity, and
then metabolized in the liver [42,43]. Bioavailability also differs greatly from one polyphenol to
another, so that the most abundant polyphenols in our diet are not necessarily those leading to the
highest concentrations of active metabolites in target tissues [44]. To date, many studies analyzing the
bioavailability of flavonoids have shown contradictory results, suggesting that the absorption of
flavonoids depends on the variety and position of the sugar groups attached. Therefore, research is
needed on the bioavailability of phenolics to allow us to correlate phenolic intake with one or several
accurate measures of bioavailability (such as concentrations of key bioactive metabolites in plasma
and tissues) and with potential health effects in epidemiological studies [41,45].
Most of the information on subsequent metabolism is derived from animal studies but little data in
humans is available. The limited source of knowledge on the absorption and metabolism of flavonoids
has been generated by studying isolated flavonoids and individual foods. More studies are needed on
the investigation of the absorption and metabolism of various flavonoids in individual and
combinations of foods.
3. Phenolic Compounds in Brassica Vegetables
As mentioned above, the nutritional interest of Brassica crops is partly related to their phenolic
compound contents. These crops are generally rich in polyphenols, but the phenolic compound
composition can be quite different among species and even among crops from the same species. The
polyphenol composition of different Brassica species has been described, revealing distinct qualitative
and quantitative profiles. For example, Podsedek [1] did an extensive review on phenolic profiles in
different Brassica species. Several authors have studied the antioxidant capacity of different
cruciferous vegetables and, in general, they found that red cabbage had the highest antioxidant activity,
followed by green cabbage, mustard cabbage, Chinese cabbage and Chinese white cabbage. In a study
focused on B. oleracea, Podsedek et al. [23] found a comparable antioxidant activity between red
cabbage and Brussels sprouts, which were from 5 to 2.2-fold higher than that for white and savoy
cabbages. Authors suggest that the highest antioxidant activity of red cabbage compared to the other
green and white cultivars may be due to the presence of different antioxidant components such as
phenolic compounds.
Methods for identification and quantification of the main phenolic compounds have been generally
based on HPLC coupled to DAD or MS detectors [13,46-49]. Polyphenol composition of members of
the Brassicaceae family has been investigated. Nowadays, it is widely known that Brassica vegetables
contain flavonoids, and especially flavonols. A large number of flavonoid glycosides have been found;
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among them, glycosides of kaempferol and quercetin, their derivatives in combination with
hydroxycinnamic acids as well as sinapic acid derivatives have been found to be the most important
phenolic compounds in Brassica species [18,50,51]. Most studies deal with B. oleracea crops like
broccoli [52], cabbage [22,51] and kale [53]. In the last years, phenolic composition of other vegetable
species like B. rapa, B. napus and B. juncea has also been studied [54-56]. In the following sections
we will review the phenolic composition of the four vegetable species of the genus Brassica
summarized in Table 2 and of other cruciferous crops from the family Brassicaceae in Table 3.
3.1. Phenolic composition in Brassica oleracea crops
Several studies have reported the presence of phenolic compounds in different B. oleracea crops
[13,48,49,56,57]. Phenolic compounds content depends on the analysis method used as well as on
numerous environmental factors. For this reason, phenolic content comparisons among crops evaluated
under different conditions and with different analytical methods are not too accurate; therefore, this
review mainly deals with the phenolic profile of Brassica crops, i.e. their qualitative differences rather
than differences on concentrations.
Phenolics are distributed differently depending on the crop and on the plant part evaluated. External
and internal leaves of different B. oleracea crops like tronchuda cabbage [58] and savoy cabbage [59]
were found to be different in terms of total phenolic content. Quercetin, kaempferol and phenolic acids
derivatives from the external and internal leaves, seeds and sprouts leaves of tronchuda cabbage have
been reported by several authors [48,58,60,61] and the different composition seems to be determinant
for the antioxidant activity displayed by each. Leaves contained higher amounts of phenolic
compounds than stems and inflorescences. In another study, Sousa et al. [62] presented a similar
qualitative phenolic composition in kales and tronchuda cabbage inflorescences, exhibiting several
complex kaempferol derivatives and 3-p-coumaroylquinic acid (Table 2).
Among crops included into B. oleracea species, broccoli has been the most exhaustively studied
with regard to polyphenol composition. Numerous recent studies have shown that this crop contains a
high antioxidant potential linked to a high level of phenolic compounds [45,49,63] and it is a good
source of flavonol and hydroxycinnamoyl derivatives. Llorach et al. [49] found 22 compounds that
were identified as several derivatives of kaempferol and ferulic and sinapic acids. The two main
flavonol glycosides present in broccoli florets are quercetin and kaempferol 3-O-sophoroside-7-O-
glucoside and the other minor glucosides were isoquercitrin, kaempferol 3-O-glucoside and
kaempferol 3-O-diglucoside [13,52] (Table 2). In the same way, more than 20 compounds were found
in other B. oleracea crops such as kale (B. oleracea var. acephala), curly kale (B. oleracea var.
sabellica), white cabbage (B. oleracea var. capitata), black cabbage (B. oleracea var. acephala DC.
subvar. viridis), cauliflower (B. oleracea L. var. botrytis) and tronchuda cabbage (B. oleracea var.
costata), where the main phenolics were kaempferol and quercetin 3-O-sophoroside-7-O-glucoside
and its combinations with different hydroxycinnamic acids, mainly kaempferol and quercetin 3-O-
(caffeoyl/sinapoyl)-sophoroside-7-O-glucoside [14,30,48,53,56,57,64] (Table 2). Heimler et al. [65]
compared the main phenolics in several B. oleracea crops and reported that broccoli and kale varieties
exhibit the highest content of both total phenolics and flavonoids.
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Ferreres et al. [66] and Taveira et al. [67] characterized the phenolic compounds and evaluated the
antioxidant potential of shoots of B. oleracea var. costata grown in vitro, finding several compounds
distinct from those described previously from material grown in the field. Authors detected a high
number of chlorogenic acids, flavonoids (prevailing hydroxycinnamic acid esters of kaempferol and
quercetin glycosides) and hydroxycinnamic acyl glycosides (with predominance of synapoyl
gentiobiosides).The results obtained by these authors are relevant since they indicate that the in vitro
production of shoots can become important as a dietary source of compounds with a health
protective potential.
Anthocyanins have also been identified on Brassica vegetables [2,9,68]. For example, the red
pigmentation of red cabbage and purple cauliflower is caused by anthocyanins. The major
anthocyanins identified in cruciferous crops like red cabbage [68] or broccoli sprouts [9] are cyanidin
3-O-(sinapoyl)(feruloyl)diglucoside-5-O-glucoside and cyanidin 3-O-(sinapoyl)(sinapoyl)diglucoside-
5-O-glucoside, with quantitative differences among species and crops within the species. For example,
differences among cauliflower and red cabbage were found by Lo Scalzo et al. [12] in their
anthocyanin profiles. Cyanidin-3,5-diglucoside was absent in cauliflower, while it was well
represented in red cabbage. Anthocyanins in the genus Brassica present unusually complex structures
with one or more cinnamic acids. The p-coumaryl and feruloyl esterified forms of cyanidin-3-
sophoroside-5-glucoside were predominant in cauliflower, while the sinapoyl ester was mostly present
in red cabbage [12].
In broccoli sprouts [9] qualitative and quantitative differences among varieties in the anthocyanin
composition were found. These authors identified cyanidin-3-O-diglucoside-5-O-glucoside acylated
and double acylated with p-coumaric, sinapic, caffeic, ferulic or sinapic acids with at least three
predominant anthocyanins isomers of cyanidin 3-O-(acyl) diglucoside-5-O-glucoside, cyanidin 3-O-
(acyl1)(acyl2) diglucoside-5-O-glucoside, and cyanidin 3-O-(acyl1)(acyl2) diglucoside-5-O-(malonyl)
glucoside.
Significant levels of hydroxycinnamic acids have also been reported in B. oleracea species, like
kale, cabbage, broccoli, and cauliflower. In these crops, hydroxycinnamoyl gentiobiosides and
hydroxycinnamoylquinic acids were found to be the most abundant [13,69]. The predominant
hydroxycinnamic acids conjugates have been identified as 3-caffeoyl quinic acid, 3-p-coumaroyl
quinic acid, 1,2-disinapoylgentiobiose, 1-sinapoyl-2-feruloylgentiobiose, 1,2,2’-trisinapoylgentiobiose
and 1,2´-disinapoyl-2-feruloylgentiobiose. More recently, Ayaz et al. [34] identified gallic,
protocatechuic, p-hydroxybenzoic, vanillic, syringic, salicylic, p-coumaric, caffeic, ferulic and sinapic
acids as the most abundant in kales. Significant levels of chlorogenic acids have previously been
reported in leafy Brassica species, like kale, cabbage and Brussels sprouts. Inflorescences from kales,
tronchuda cabbage and turnip tops exhibited the same six organic acids (aconitic, citric, pyruvic, malic,
shikimic and fumaric acids), but kales presented a considerably higher amount [62].
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Table 2. Variation of phenolic compounds found in main vegetable Brassica crops.
Brassica oleracea Brassica rapa Brassica
napus Brassica
juncea
Phenolic compound Tronchuda
cabbage1 Cauliflower2 Kale3 Broccoli4White
cabbage5Turnip
greens/tops6Pak
choi7 Leaf
rape8 Leaf
mustard9
Quercetin (Q) derivatives
Q-3-O-sophorotrioside-7-O-sophoroside x X
Q-3-O-sophorotrioside-7-glucoside x X x x
Q-3-O-sophoroside-7-O-glucoside x x x X x x
Q-3,7-di-O-glucoside x X x x x x
Q-3-O-sophoroside x X x x x
Q-7-O-glucoside x
Q-3-O-glucoside x X x x
Q-3-O-(caffeoyl)-sophorotrioside-7-O-glucoside X x
Q- 3-O-(sinapoyl)-sophorotrioside-7-O-glucoside X x
Q-3-O-(feruloyl)-sophorotrioside-7-O-glucoside X x
Q-3-O-(p-coumaroyl)-sophorotrioside-7-O-
glucoside X
Q-3-O-(caffeoyl)-sophoroside-7-O-glucoside x X x x x x
Q-3-O-(methoxycaffeoyl)-sophoroside-7-O-
glucoside x x x
Q-3-O-(sinapoyl)-sophoroside-7-O-glucoside x x x x x
Q-3-O-(feruloyl)-sophoroside-7-O-glucoside x
Q-3-O-(p-coumaroyl)-sophoroside-7-O-glucoside X
Q-3-O-(feruloyl)-sophoroside x X x x x
Kaempferol (K) derivatives
K-3-O-tetraglucoside-7-O-sophoroside x
K-3-O-sophorotrioside-7-O-sophoroside x x x X x x
K-3-O-sohorotrioside-7-O-glucoside x x X x x
K-3-O-sophoroside-7-O-diglucoside x x X x
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Table 2. Cont.
Brassica oleracea Brassica rapa Brassica
napus Brassica
juncea
Phenolic compound Tronchuda
cabbage1 Cauliflower2Kale3 Broccoli4White
cabbage5Turnip
greens/tops6Pak
choi7 Leaf
rape8 Leaf
mustard9
K-3-O-sophoroside-7-O-glucoside x x x X x x x x
K-3,7-di-O-glucoside x X x x x x
K-3-O-sophoroside x X x x x
K-7-O-glucoside x x x x x x
K-3-O-glucoside x X x
K-3-O-(caffeoyl)-sophorotrioside-7-O-
sophoroside X
K-3-O-(methoxycaffeoyl)-sophorotrioside-7-O-
sophoroside X
K-O-(sinapoyl)-sophorotrioside-7-O-sophoroside X
K-O-(feruloyl)-sophorotrioside-7-O-sophoroside X
K-3-O-(p-coumaroyl)-sophorotrioside-7-O-
sophoroside X
K-3-O-(caffeoyl)-sophorotrioside-7-O-glucoside X x
K-3-O-(methoxycaffeoyl)-sophorotrioside-7-O-
glucoside X
K-O-(sinapoyl)-sophorotrioside-7-O-glucoside X
K-O-(feruloyl)-sophorotrioside-7-O-glucoside X x
K-3-O-(caffeoyl)sophoroside-7-O-glucoside x x x X x x x x x
K-3-O-(methoxycaffeoyl)sophoroside-7-O-
glucoside x x x x x x x
K-3-O-(sinapoyl)-sophoroside-7-O-glucoside x x x x x x x x
K-3-O-(feruloyl)-sophoroside-7-O-glucoside x x x x x x x
K-3-O-(p-coumaroyl)-sophoroside-7-O-
glucoside x x x x x
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Table 2. Cont.
Brassica oleracea Brassica rapa Brassica
napus Brassica
juncea
Phenolic compound Tronchuda
cabbage1 Cauliflower2Kale3 Broccoli4White
cabbage5Turnip
greens/tops6Pak
choi7 Leaf
rape8 Leaf
mustard9
K-3-O-(methoxycaffeoyl)-sophoroside x X x x x
K-3-O-(sinapoyl)-sophoroside x x x x x
K-3-O-(feruloyl)-sophorotrioside x
K-3-O-(feruloyl)-sophoroside x x x x x
K-3-O-(p-coumaroyl)-sophoroside x x x
Isorhamnetin (I) derivatives
I-3-sophorotrioside-7-sophoroside
I-3,7-di-O-glucoside x x x
I-3-glucoside x x x
Hydroxycinnamic acids
3-caffeoyl quinic acid x x x x x x x
5-caffeoyl quinic acid x
3-p-coumaroyl quinic acid x x x x x x
4-p-coumaroyl quinic acid x x
4-caffeoyl quinic acid x x
Sinapylglucoside x x x x
Ferulic acid x x x x x
4-feruloyl quinic acid x x x
Sinapic acid x x x x x x x
1,2-disinapoylgentiobiose x x x X x x x x x
1-sinapoyl-2-feruloylgentiobiose x x x X x x x x x
1,2,2’-trisinapoylgentiobiose x x x X x x x x x
1,2’-disinapoyl-2-feruloylgentiobiose x x x X x x x x x
1Tronchuda: [48,62]; 2Broccoli: [13,45,49,52,63]; 3Cauliflower: [64]; 4White cabbage: [56,57,59]; 5Kale: [14,56]; 6Turnip greens: [55,72]; 7Pak choi: [47];
8Leaf rape: [56]; 9Leaf mustard: [15,70]
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Table 3. Phenolic compounds in different cruciferous crops.
Crop Compound Reference
Moricandia
arvensis 3,4’-di-O-β-D-glucopyranoside-7-O-α-L-rhamnopyranoside
β-D-glucopyranosyl 4-O- β-D-glucopyranosylcaffeate
methyl 3-O- β-D-glucopyranosyl-5-
hydroxycinnamate
β-D-glucopyranosyl 4-O-β-D-glucopyranosylbenzoate
β-D-glucopyranosyl 4-hydroxybenzoate
methyl 4-O-β-D-glucopyranosylcaffeate
1-O-caffeoyl-β-D-glucopyranoside
2-phenylethyl-β-D-glucopyranoside
[80]
Bunias
orientalis Kaempferol monosinapoyl di-O-glycoside
Kaempferol monomalonyl/monosinapoyl di-O-glycoside
Kaempferol di-O-glucoside and tri-O-glucoside
demethylated sinapic acid
[82]
Diplotaxis
erucoides/Eruca
sativa
Kaempferol di-O-glycoside
Isorhamnetin mono-O-,di-O-, and tri-O-glycosides
Quercetin monosinapoyl tri-O-glycoside
Quercetin tri-O-glycoside
Quercetin tetra-O-glycoside
Quercetin monosinapoyl di-O-glycoside
Quercetin di-O-, tri-O-, and tetra-O-glycosides
[82]
Diplotaxis
tenuifolia Quercetin-3,3‘,4‘-triglucoside
Quercetin-3,4‘-di-glucoside-3‘-(6-methoxycaffeoyl-glucoside)
Quercetin-3,4‘-di-glucoside-3‘-(6-caffeoyl-glucoside)
Quercetin-3,4‘-di-glucoside-3‘-(6-sinapoyl-glucoside)
Quercetin-3,4‘-di-glucoside-3‘-(6-feruloyl-glucoside)
Quercetin-3,4‘-di-glucoside-3‘-(6-p-coumaroyl-glucoside)
Quercetin-3-(2-methoxycaffeoyl-glucoside)-3‘-(6-sinapoyl-
glucoside)-4‘-glucoside
Quercetin-3-(2-caffeoyl-glucoside)-3‘-(6-sinapoyl-glucoside)-4‘-
glucoside
Quercetin-3-(2-sinapoyl-glucoside)-3‘-(6-sinapoyl-glucoside)-4‘-
glucoside Quercetin-3-(2-feruloyl-glucoside)-3‘-(6-sinapoyl-
glucoside)-4‘-glucoside
Quercetin-3-(2-feruloyl-glucoside)-3‘-(6-feruloyl-glucoside)-4‘-
glucoside
Kaempferol-3,4‘-di-glucoside
Isorhamnetin-3,4‘-di-glucoside
[83]
Eruca vesicaria Quercetin-3-glucoside
Kaempferol-3-glucoside
Kaempferol-3,4‘-di-glucoside
Kaempferol-3-(2-sinapoyl-glucoside)-4‘-glucoside
Isorhamnetin-3-glucoside
Isorhamnetin-3,4‘-di-glucoside
[83]
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Table 3. Cont.
Crop Compound Reference
Nasturtium
officinale Quercetin 3-O-triglucoside-7-O-rhamnoside
Quercetin 3-O-gentiobioside-7-O-rhamnoside
Quercetin 3-O-sophoroside-7-O-rhamnoside
Quercetin 3-O-sophoroside-7-O-(caffeoyl)-rhamnoside
Quercetin 3-O-triglucoside-7-O-(caffeoyl)-rhamnoside
Quercetin 3-O-triglucoside-7-O-(sinapoyl)-rhamnoside
Quercetin 3-O-triglucoside-7-O-(feruloyl)-rhamnoside
Kaempferol 3-O-triglucoside
Kaempferol 3-O-sophoroside
Kaempferol 3-O-gentiobioside
Kaempferol 3-O-gentiobioside-7-O-rhamnoside
Kaempferol 3-O-triglucoside-7-O-rhamnoside
Kaempferol 3-O-triglucoside-7-O-(caffeoyl)-rhamnoside
Kaempferol 3-O-triglucoside-7-O-(sinapoyl)-rhamnoside
Kaempferol 3-O-triglucoside-7-O-(sinapoyl)-rhamnoside
Kaempferol 3-O-sophoroside-7-O-rhamnoside
Kaempferol 3-O-sophoroside-7-O-(caffeoyl)-rhamnoside
[50]
Hydroxycinnamic acids in leaves and stems of tronchuda cabbage varieties were also evaluated by
Ferreres et al. [58]. In this work, authors found that hydroxycinnamoyl gentiobiosides of ferulic and
sinapic acid were predominant in the inner leaves of tronchuda cabbage, while hydroxycinnamoyl-
quinic acids were more abundant in leaves and stems.
3.2. Phenolic composition in Brassica rapa crops
In contrast to B. oleracea vegetables, and despite being much appreciated and highly consumed,
phenolic composition of B. rapa crops has been less investigated. However, this subject has received
an increasing interest in the last years. At present, it is known that crops from this species contain a
high amount of phenolic compounds and as result, they are an appreciable source of polyphenols,
especially flavonoids. The main polyphenols identified in B. rapa vegetables are acylated mono-, di-,
tri- and tetraglucosides of quercetin, kaempferol and isorhamnetin as well as esters of
hydroxycinnamic acids with malic acid, glycosides, and quinic acid [47,70]. In fact, the main
difference between B. oleracea and B. rapa species is the presence of isorhamnetin derivatives in the
B. rapa group, which are always absent in B. oleracea [71].
Ferreres et al. [72] and Francisco et al. [55] identified more than 20 acylated and nonacylated
flavonol glycosides and ferulic and sinapic acids in Brassica rapa var. rapa by HPL-DAD-ESI-MSn
(Table 2). The content of phenolic compounds was studied in turnip edible parts including leaves,
stems, flower buds and roots [54,55]. The authors found that kaempferol 3-O-sophoroside-7-O-
glucoside, kaempferol 3-O-(feruloyl/caffeoyl)-sophoroside-7-O-glucoside, kaempferol 3,7-di-O-
glucoside, isorhamnetin 3,7-O-diglucoside and sinapic acid were the major phenolics quantified by
HPLC–DAD analysis. In a comparative study carried out by Sousa et al. [62] in Brassica
inflorescences, it has been shown that B. rapa exhibited several phenolic acids and flavonoids distinct
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from those found in the B. oleracea species, mainly isorhamnetin derivatives. In another B. rapa
variety, namely var. sylvestris, Romani et al. [71] also found some common compounds described in
Brassica rapa var. rapa such as derivatives of kaempferol 3,7-di-O-glucoside, kaempferol-3-O-
glucoside, isorhamnetin 3,7-O-diglucoside and hydroxycinnamoyl gentiobiosides, being kaempferol-3-
O-glucoside and quercetin-3-O-(sinapoyl)-sophotrioside-7-O-glucoside the most abundant compounds.
In a recent study, the phenolic profiles of fifteen B. rapa crops, including B. rapa var. pekinensis, B.
rapa var. chinensis, B. rapa var. oleifera, B. rapa var. ruvo and B. rapa L. var. rapa were reported by
Lin and Harnley [15]. Those groups included vegetable crops which are among the major Brassica
vegetables consumed in China and some Asian countries. The major phenolic compounds identified
were kaempferol 3-O-sophoroside-7-O-glucoside derivatives, isorhamnetin 3-O-glucoside-7-O-
glucoside, hydroxycinnamoyl gentiobioses, hydroxycinnamoylmalic acids and hydroxycinnamoyl-
quinic acids. The phenolic compounds identified on these Asian Brassica vegetables were similar to
those of the abovementioned European B. rapa.
Turnip roots seem to be a less interesting edible part compared to leaves or inflorescences due to
the very low amounts of phenolic compounds and the small antioxidant capacity found in this organ.
In a study performed by Fernandes et al. [54], only ferulic and sinapic acids and their derivatives were
detected in significant amounts in this organ.
3.3. Phenolic composition in Brassica napus crops
Most efforts in this species have been focused on crops like rapeseed (Brassica napus var. oleifera).
It is known that seeds of winter rapeseed varieties contain high amounts of phenolic compounds. In
fact, canola seeds are much richer in phenolic compounds compared to other oilseeds. The most
significant phenolic compounds are sinapic acid derivatives [73,74], although other minor phenolics in
rapeseed include p-hydroxybenzoic, vanillic, gentisic, protocatechuic, syringic, p-coumaric, ferulic,
caffeic and chlorogenic acids. In leaves of oilseed rape, four hydroxycinnamic acids (caffeic, p-
coumaric, ferulic and sinapic acid) were identified in the water-soluble phenolic fraction of the leaves.
Although B. napus crops are mainly used as oilseed, this species also include forage and vegetable
types likes rutabaga or swede and leaf rape or nabicol (Table 2). Contrary to other Brassica vegetables,
few studies have been done to evaluate the phenolic profile of vegetable B. napus. Velasco et al. [56]
identified phenolics present in nabicol leaves, which is a vegetable Brassica crop widely grown in the
Northwest of Spain. They identified 17 flavonoids, mostly derivatives from kaempferol and eight
hydroxycinnamic acids, being sinapic acid the most abundant (Table 2). Li et al. [75] generated
transgenic B. napus plants over expressing the Arabidopsis PAP1 (AtPAP1) gene responsible for the
production of the anthocyanin pigment 1 and identified and quantified the leaf phenolics in transgenic
plants and non-transgenic controls. They identified derivatives of quercetin, kaempferol, sinapic acid,
cyanidin and pelargonidin and also found that all of them, except for the kaempferol derivatives were
dramatically increased in the leaves of transgenic plants as compared to the non-transgenic controls.
3.4. Phenolic composition in Brassica juncea crops
Brassica juncea is mainly used as a condiment because of its seeds, along with the other mustards
of the genus Brassica, Brassica carinata and Brassica nigra. Nevertheless, B. juncea leaves are also
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consumed as vegetables in Asian countries (Table 1). The phenolic composition of this species has
been less studied with regard to B. oleracea and B. rapa. The main flavonols in this species are
quercetin and kaempferol. The phenolic compound variation of two Chinese leaf mustard cultivars
grown under field conditions was studied by HPLC–ESI–MSn analysis [70,76]. Authors identified the
free polyphenol content in the outer and inner leaves as well as in their leaf blades and leaf stalks, and
concluded that hydroxycinnamic acids and flavonoids were higher in the leaf blade than in the stalk,
whereas small amounts of flavonoids were detected in the stalks. The main flavonoids are kaempferol
derivatives (mono-, di-, triglucosides). Isorhamnetin and hydroxycinnamoyl gentiobioses were also
detected, but no quercetin derivatives. The main hydroxycinnamic acids were malate derivatives of
sinapic, ferulic, hydroxyferulic and caffeic acids. Ferulic acid content was significantly higher in the
leaf blade than in the stalk.
Fang et al. [77] determined the contents of the total free phenolic acids, the total phenolic acids, the
total phenolics and the antioxidant activities in leaf mustard as well as the effects of pickling methods
on these compounds and they identified several hydroxycinnamic acids as caffeic, p-coumaric, ferulic
and sinapic along with benzoic acid derivatives as gallic, protocatechuic, p-hydroxybenzoic, and
vanillic acids.
3.5. Phenolic composition in other cruciferous crops
Besides vegetable Brassica species belonging to the U triangle [78], Brassicaceae is a large family
of plants that include important vegetable crops. Several species within the family Brassicaceae have
been surveyed for their flavonoid profiles. In these studies, flavonol glycosides were the only
flavonoids present in leaves and flowers of the genera Brassica and Sinapis. In a chemosystematic
survey on wild Brassica relatives, Aguinagalde et al. [79] identified 21 different flavonoid glycosides,
all based on the flavonol skeleton. Braham et al. [80] identified phenolic compounds in Moricandia
arvensis (Table 3) and evaluated their antioxidant capacity, finding that some of the compounds
detected possess a high scavenging activity.
Flavonols were also reported in Diplotaxis and Eruca leaves by Weckerle et al. [81]. More recently,
Bennet et al. [82] studied the phenolic compound composition in several cruciferous species such as
Diplotaxis erucoides L., Diplotaxis tenuifolia L., Eruca sativa L., and Bunias orientalis L., all of them
known as rocket crops. These crops had significant levels of polyglycosylated flavonoids, with/without
hydroxycinnamoyl acylation and the most abundant flavonoids were kaempferol, quercetin and
isorhamnetin. This composition was found in all tissues, except roots. Hydroxycinnamate derivatives
of either the disaccharide gentiobiose or the quinic acid were detected at very low levels, in leaf and
floral tissues of Bunias, Diplotaxis and Eruca species (Table 3). Martinez-Sanchez et al. [83] also
studied the flavonoid profile of Eruca vesicaria and D. tenuifolia (Table 3). They found important
differences between flavonoid profiles of these two species. E. vesicaria contained kaempferol
derivatives as principal compounds, whereas D. tenuifolia instead accumulated quercetin derivatives.
Some diacyl derivatives found in these species have not been identified in other Brassicaceae. Like
other Brassicaceae, the flavonoids found in watercress (Nasturtium officinale) were quercetin and
kaempferol derivatives glycosylated and acylated, but unlike them, they showed a characteristic
glycosylation pattern with rhamnose at the 7 position [50] (Table 3).
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In another study, Onyilagha et al. [84] presented a comprehensive review of flavonoid distribution
in different tissues of crops from the family Brassicaceae. Authors also studied the leaf flavonoids in
several cruciferous species, including Sinapis alba, Thlaspi arvense, Camelina sativa, Crambe spp.
and several other genera of the family Brassicaceae. These authors reported the accumulation of
derivatives of flavonols, quercetin, kaempferol and isorhamnetin in S. alba; quercetin, in C. sativa;
quercetin and kaempferol in C. hispanica var. glabrata and derivatives of the flavones, apigenin and
luteolin, in C. abyssinica, C. hispanica var. hispanica and T. arvense leaves. Various kaempferol,
quercetin, and isorhamnetin glycosides were identified in the leaves of flowering Diplotaxis species. In
leaves of B. orientalis, various hydroxycinnamate derivatives, including sinapoylglucose were found;
no data on flavonoids were reported. Quercetin triglucosides were found in E. sativa by qualitative
analyses by liquid chromatography/mass spectrometry (LC/MS) and NMR.
4. Variation on Phenolic Content in Brassica Vegetables
Biosynthesis and concentration of phenolic compounds in plants depends on genetic and
environmental factors. Several studies have demonstrated that there is a substantial and significant
variation for the antioxidant phytochemicals into Brassica species, both within and among species, and
even among crops of the same species; thus, the potential health benefits provided by cruciferous crops
will depend firstly on the genotype. The phenolic compound composition may differ between
cultivars, as well as among parts within the individual plant as shown in several crops like turnip
greens and turnip tops [85], pak choi [47] and tronchuda cabbage [54,58]. These compounds are also
susceptible to ontogenic variation but the published studies that have addressed this topic have been
inconsistent.
On the other hand, secondary metabolites present in Brassica crops are very susceptible to changes
in environmental conditions. Phenolic contents are affected by biotic stresses (insect attack and
pathogen infection) and abiotic stresses (light, temperature, nutrient supplies, water availability,
growing conditions and UV radiation) besides storage conditions, post-harvest treatments and the
estimation methods [86,87]. All these factors, besides the biosynthesis of phenolic antioxidant
compounds, affect the final concentration of polyphenols in plant tissues. As it was previously
explained, phenolics are produced in plants as secondary metabolites via the shikimic acid pathway.
Phenylalanine ammonialyase (PAL), the key enzyme catalyzing the biosynthesis of phenolics from the
aromatic amino acid phenylalanine, was found to be responsive to biotic and abiotic stresses.
In the following sections some factors like fertilization, processing, cooking methods or storage
conditions are discussed in detail.
4.1. Influence of fertilization and cropping systems on phenolic content
A crucial question about Brassica vegetables that deserves full attention in scientific literature is the
edaphic conditions in which they are grown. Plant quality can be modified by growing plants in a high
mineral-containing medium, thus attaining high levels of nutritionally important minerals that can be
used to produce foods and food ingredients dietary supplements. Plant nutrients can be thus important
factors in determining the secondary metabolism synthesis within plants. Nitrogen is one of the most
important essential plant nutrients in controlling quality and yield of vegetables. Moreover, nitrogen
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modulates the biosynthesis of secondary metabolites (e.g., phenolic compounds, glucosinolate,
carotenoid, etc.). Many plant species, particularly Brassicaceae crops, incorporate sulfur into a wide
range of secondary compounds such as the sulfation of flavonol and desulfoglucosinolates, choline and
gallic acid glucoside. Sulfur is an essential plant macronutrient found in cysteine and methionine
amino acids, as well as in a variety of secondary metabolites. However, few studies have investigated
the impact of nitrogen and sulfur application on the total phenolic concentrations and
antioxidant activity.
Several studies reported that an increase in sulfur fertilization significantly promotes the total
phenolic contents and the antioxidant activity in leaf mustard [88], broccoli [89] and ‘friarello’, a local
B. rapa crop widely grown in Southern Italy [87]. Li et al. [88] also determined the effect of nitrogen
supply on leaf mustard and found that the total phenolic content was considerably decreased by
increasing nitrogen fertilization. These studies provide clear evidence that nitrogen and sulfur nutrition
can be used to manipulate total phenolic concentrations of Brassica crops with potential benefits to
human health and as a result, it can be concluded that sulphur fertilization may improve the nutritional
value of these crops.
Like other secondary metabolites, phenolic compounds may be affected by agronomic practices.
Several studies have compared the contents of certain phytochemicals between organic and
conventional fruits and vegetables [60,90,91]. Sousa et al. [60] studied the content of phenolic
compounds in tronchuda cabbage under organic and conventional agriculture and they conclude that
generally, leaves from organic culture have higher amounts of phenolics, probably due to the
interference of mineral fertilizers and pesticides with the biosynthetic pathway of phenolic compounds.
In another study, Young et al. [91] found that pak choi samples grown organically had higher levels of
total phenolics than conventional samples, but these differences among organic and conventional
growing were not found in collards. They concluded that the production method did not increase the
biosynthesis of phenolics but the organic system provided an increased opportunity for insect attack,
resulting in a higher level of total phenolic agents in pak choi. Thus, insect attack might be a biotic
stress factor contributing to higher levels of total phenolic agents in some vegetables from organic
production systems.
4.2. Influence of processing and cooking on phenolic composition
Phenolics in vegetables exist in both free and conjugated forms. Generally, only conjugated
flavonoids are present in fresh vegetables, but aglycones may be found as a result of food processing.
It is known that processing may affect the concentration and biological activities of different
compounds present in plants to a significant extent. This aspect seems to be very important taking into
account that only some vegetables are consumed in a raw state and most of them are processed before
consumption. The recent literature data show a consistent trend for the effects of thermal processing on
the total antioxidant activity in vegetable Brassica crops when comparing to other vegetable crops;
however, when compare to the total flavonoid or total phenolic content, the results did not show such
consistency. This suggests that the effect of thermal processing on phenolic, flavonoid or total
antioxidant activities is different in different products and deserve further research. In addition,
differences in processing methods may have different effects on the content of distinct phytochemicals.
Industrial processing such as blanching, canning, sterilizing and freezing, as well as cooking methods
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are expected to affect the yield, chemical composition and bioavailability of antioxidants [92].
Brassica vegetables containing phenolic compounds usually undergo domestic processing before
cooking. Some of them such as broccoli or cauliflower are cut; others such as kale and cabbage are
prepared in julienne, which may have similar effects to chopping. Operations such as cutting and
slicing may cause a rapid enzymatic depletion of several naturally occurring antioxidants as a result of
the cellular disruption, which allows contacts of substrates and enzymes. During vegetable cooking,
qualitative changes, antioxidant breakdown and their leaching into surrounding water may influence
the antioxidant activity of the vegetables [1].
Some antioxidant compounds like ascorbic acid and carotenoids are very sensitive to heat and
storage and are lost during different vegetable processing steps [93]. However, flavonoids and some
phenolic compounds are quite stable at high temperature and over long periods of storage [94]. Several
studies have shown that blanching has a significant effect on the contents of ascorbic acid and total
phenolics, and on the antioxidant activity of green leafy vegetables. Blanching of vegetables does not
necessarily cause the loss of antioxidant properties. In some vegetables, blanching might actually
increase the availability of the natural occurring antioxidant components besides improving the
palatability of vegetable crops.
The literature data have shown that the loss of dietary antioxidants is caused by the cooking
conditions, such as the type of cooking (conventional, steaming, microwaving, etc.), cooking time and
amount of water. A loss of antioxidant capacity after boiling has been observed for several vegetables
[93]. It is well known that the cooking process drastically reduces the vitamin C content of vegetables
and several other authors report a loss in the phenolic content of vegetables after cooking. The overall
loss of antioxidants (for oxidation, as in the case of vitamin C or for a simple diffusion in the cooking
water, as in the case of phenolics) results in the decrease of antioxidant capacity. In another study,
Vallejo et al. [94] compared the losses in phenolics compounds when broccoli was submitted to high-
pressure boiling, low-pressure boiling, steaming and microwaving. The authors found clear
disadvantages when microwave cooking was used, noticing losses of 97, 74 and 87% in flavonoids,
sinapic acid derivatives and caffeoylquinic acid derivatives, respectively. To this respect, Zhang and
Hamauzu [93] reported losses in the total phenolics of 62% in broccoli florets and of 43% in broccoli
stems. However, they did not find differences with traditional boiling, which suggests that, as no
soaking effect is produced, losses may be attributed to degradation. Similar results were found by
Francisco et al. [85] who reported a loss of 65–75% of flavonoids and 70–80% of hydroxycinnamic
acids under conventional and high-pressure cooking in turnip tops. Nevertheless, losses were reduced
to 20–30% by steaming cooking, showing that this is the ideal method to preserve secondary
metabolites in Brassica crops.
Lin and Chang [95] examined the antioxidant activity of broccoli under different cooking
treatments and found that a precooking and/or cooking treatment had no profound effect on the
antioxidant properties of broccoli. In another study, Sultana et al. [96] reported the effects of different
cooking methods (boiling, frying and microwave cooking) on the antioxidant activity of some selected
vegetables including cabbage, cauliflower, yellow turnip and white turnip and concluded that all the
cooking methods affected the antioxidant properties of these vegetables; however, microwave
treatment exhibited more deleterious effects when compared to those of other treatments. Most
phenolic compounds are water soluble and they are recovered in the water after cooking [97]. Authors
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found that the steam-cooking of broccoli results in an increase in the content of flavonoids and
phenolic acids as compared to fresh broccoli, whereas cooking in water has the opposite effect. The
increase in the content of polyphenols and carotenoids is related to their enhanced availability whereas
the observed losses of the compounds are mainly due to their leaching into the cooking-water.
However, other studies reported very slight losses of total flavonoids and caffeoylquinic derivatives
in broccoli (11% and 8% respectively), while no loss of total sinapic and feruloyl derivatives occurred
[94]. During steaming, phenolic compounds can remain in the edible part of broccoli, probably owing
to the inactivation of oxidative enzymes [94]. Natella et al. [98] concluded that microwave and
pressure cooking are less detrimental than boiling to the phenolics content of several vegetables,
including cauliflower. As a conclusion, and because data in this sense are still ambiguous, moderate
blanching time, proper handling, and an appropriate method might be sought for the processing of
vegetable Brassica crops in order to preserve their antioxidant properties.
4.3. Influence of storage on phenolic content
Several studies indicate an increasing content of polyphenols for material stored under different
modified atmosphere packaging for few days at low temperature. This might be an indication of
further biosynthesis of polyphenols for plant protection in the first days after harvest [99], presumably
triggered as a reaction to stress in the plants. Furthermore, it has been reported that longer storage
times resulted in a decreased phenolic content, e.g. for flavonol glucosides in several vegetable crops
[16]. In Brassica crops, the influence of storage on free phenolic content was recently studied in
different cultivars of Chinese cabbage cultivated in Germany under field conditions [70]. These
authors found that storage at 20 ºC resulted in rapid yellowing and floppy leaves which resulted in an
undesirable appearance and sensory quality for consumers. The increasing levels of polyphenols
observed in different works in the plant from post-harvest treatments (storage) open up possibilities for
increased phenolic content in vegetables and foods.
5. Future Perspectives
Given the mounting data in support of the role of phenolics in the prevention of different chronic
diseases including several types of cancer, improving the phenolic load of plant-origin-foods would be
of a clear benefit to human health through dietary intervention. Plant geneticists have attempted to
improve phytochemical levels through traditional breeding programs or through bioengineering of
Phenylalanine ammonialyase (PAL) leading to secondary metabolite accumulation. To our knowledge,
breeding programs to increase or decrease the content of a particular phenolic compound related to
human health with horticultural Brassica crops have not been carried out. However, the modification
of the synthesis of phenolic compounds is being currently carried out in different crops and in the
model species Arabidopsis thaliana, to reduce or to increase the final concentration of a certain type of
phenolic compound.
To date, most of the structural and several regulatory genes of the synthesis pathways of phenolic
compounds have been cloned, characterized and used in gene transformation experiments to modify
their content. Respecting flavonoids pathway, early attempts to manipulate flavonoid biosynthesis
were made to generate novel flower colors, such as the use of a maize DFR gene to produce a new
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flower colors in Petunia [100]. The use of structural genes in the metabolic engineering of flavonoids
becomes more important when attempting to direct flavonoid synthesis towards branches that are
normally absent in the host plant. This approach was used by Jung et al. [101], who introduced the IFS
gene into Arabidopsis in order to convert naringenin, which is ubiquitous in higher plants, to the
isoflavone genistein. An example of modification of transcriptional factors can be found in
Schijlen et al. [102]. They introduced the transcriptional factors LC and C1 from maize into
Arabidopsis and tobacco and this resulted in an accumulation of anthocyanins in tissues where they are
not normally synthesized.
With regard to the modification of the hydroxycinnamates pathway, an example can be found in B.
napus. Seeds of oilseed rape accumulate high amounts of antinutritive sinapate esters [103]. Together
with the high fiber content, the sinapate esters contribute to the antinutritive characteristics of the B.
napus seed protein fraction as revealed by their bitter taste, astringency and low digestibility [104].
Therefore, reduction of the amount of sinapate esters in the seeds is a major goal in B. napus breeding.
So far, conventional plant breeding has not been able to produce oilseed rape lines with a low-sinapate
ester trait. Several studies reported on a large genetic variability of sinapate ester content and
composition in seeds [105]. Analysis of variance, however, showed a highly significant effect of the
environment as well. Thus, the transgenic approaches provided so far the best strategy to follow to
finally obtain B. napus cultivar with low content in sinapate esters.
Following this research line, and based on the absence of soluble sinapate esters in the A. thaliana
mutant sin1, which is impaired in ferulate-5-hydroxylase (F5H), the homologous gene from B. napus
was identified and the sequence information used for an antisense suppression strategy [106]. This
approach resulted in transgenic plants displaying a seed sinapine content reduced to 40% compared to
control lines. Transgenic lines of B. napus were homozygous for a single insertion of a dsRNAi
cassette designed to suppress the BnSGT1 gene in seeds. These lines showed a sinapate ester content
of only about 30% relative to control plants [103]. Bhinu et al. [107] based in knowledge gained from
Arabidopsis, developed transgenic B. napus lines which showed up to 90% reduction in sinapine.
Information gains from these and other experiments carried out in other species will greatly contribute
to a better undestanding of phenolic compounds synthesis and may be useful for Brassica crop
improvement.
On the other hand, and it was previously explained, the flavonoid content is quite high in some
Brassica species. For this reason, consumption of Brassica vegetables is highly recommended.
However, a question to resolve nowadays is that of the adequate intake of these beneficial vegetables
since as it has been noted by Jahangir et al. [2], the potentially toxic effects of excessive flavonoid
intake are still largely ignored. At high doses, flavonoids may act as mutagens and, therefore, their
unfavorable effects may well balance up their beneficial ones. Further research must be led about the
toxicological properties of flavonoids, thus, clarifying the balance of potentially adverse and beneficial
effects included in their mechanisms of action.
It can be concluded that vegetables belonging to the family Brassicaceae are rich food sources of
natural antioxidants and essential nutrients (vitamins, phenolics, minerals, fibre, etc.) and the
vegetables of this family possess a high potential to manage against oxidative stress and, thus, act as
strong anticancerous as well as antidegenerative foods. Therefore, for improving the quality and
production of these vegetables, breeding programs are necessary in order to enhance the antioxidant
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potential of our daily food supply. Therefore, the potential of these phytochemical compounds for the
maintenance of health and protection against heart disease and cancer is also raising interest among
scientists and food manufacturers as consumers move towards functional foods with specific health
effects. An interesting aspect for future research is to clarify the genotype × environmental interactions
on the flavonoid composition in plants. By combining the knowledge gained from the studies
concerning the effects of different flavonoid compounds on human health, it might be possible to
produce plants with even better health properties.
Understanding the bioavailability, transport and metabolism of polyphenols after consumption of
Brassica vegetables as food is a prerequisite for understanding the mechanisms of their protective
effects in humans. Research is also needed on the bioavailability and metabolism of polyphenols to
allow scientific backed statements and recommendations on dietary intake, effective dosage, daily
allowance and dietary guidelines for nutrition and health applications. Epidemiological and
intervention studies examining the effects of phenolics in humans and animal models with relevant and
reliable biomarkers of safety, ingestion, metabolism and functional activity might be improved to
optimize phenolic bioavailability and realize their chemopreventive and chemotherapeutic effects in
vivo. Furthermore, a better understanding of the dietary phenolic and gut microbiota relationship
should help in the prevention of diseases as well as in improvement of human health.
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Several vegetables were selected to study their flavonoid contents and antioxidant activities. The results showed that both green and purple leaves of sweet potatoes (185.01 and 426.82 mg kg(-1) respectively) and the outer leaves of onion (264.03 mg kg(-1)) possessed higher amounts of flavonoids, and more than 85% of free radical scavenging activities were evaluated by using 1,1-diphenyl-2-picrylhydrazyl (DPPH), superoxide and hydroxyl radicals. In addition, green leaves of sweet potatoes and the outer leaves of onion showed higher reducing power and higher antioxidant activity in a Linoleic acid system as compared to cabbage, spinach, potato and crown daisy. Blanching of green leaves of sweet potatoes for 30-60 s retained more flavonoids and higher free radical scavenging activities as compared to more than 1 min of blanching. The storage test showed that green leaves of sweet potatoes stored at 4-10 degrees C maintained better quality than those stored at room temperature. (C) 2000 Society of Chemical Industry.
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