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Glucosinolates in Brassica foods: Bioavailability in food and significance for
human health
Article in Phytochemistry Reviews · July 2007
DOI: 10.1007/s11101-007-9072-2 · Source: OAI
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Phytochem Rev (2008) 7:213–229
DOI 10.1007/s11101-007-9072-2
Glucosinolates in Brassica foods: bioavailability in food
and significance for human health
Marı́a Elena Cartea Æ Pablo Velasco
Received: 11 June 2007 / Accepted: 21 September 2007 / Published online: 20 October 2007
Springer Science+Business Media B.V. 2007
Abstract Glucosinolates are sulphur compounds
that are prevalent in Brassica genus. This includes
crops cultivated as vegetables, spices and sources of
oil. Since 1970s glucosinolates and their breakdown
products, have been widely studied by their beneficial
and prejudicial biological effects on human and
animal nutrition. They have also been found to be
partly responsible for the characteristic flavor of
Brassica vegetables. In recent years, considerable
attention has been paid to cancer prevention by
means of natural products. The cancer-protective
properties of Brassica intake are mediated through
glucosinolates. Isothyocianate and indole products
formed from glucosinolates may regulate cancer
cell development by regulating target enzymes,
controlling apoptosis and blocking the cell cycle.
Nevertheless, variation in content of both glucosinolates and their bioactive hydrolysis products
depends on both genetics and the environment,
including crop management practices, harvest and
storage, processing and meal preparation. Here, we
review the significance of glucosinolates as source of
bioactive isothiocyanates for human nutrition and
M. E. Cartea (&) P. Velasco
Department of Plant Genetics, Misión Biológica de
Galicia, Spanish Council for Scientific Research (CSIC),
Apartado 28, 36080 Pontevedra, Spain
e-mail: ecartea@mbg.cesga.es
P. Velasco
e-mail: pvelasco@mbg.cesga.es
health and the influence of environmental conditions
and processing mechanisms on the content of
glucosinolate concentration in Brassica vegetables.
Currently, this area is only partially understood.
Further research is needed to understand the mechanisms by which the environment and processing
affect glucosinolates content of Brassica vegetables.
This will allow us to know the genetic control of
these variables, what will result in the development
of high quality Brassica products with a healthpromoting activity.
Keywords Brassica Bioactive compounds Chemoprevention Isothiocyanates Sulforaphane Indole-3-carbinol Seasonal variation Thermal degradation
Introduction
Brassicaceae family includes vegetables as broccoli,
Brussels sprouts, cabbage, collards, kale, turnip
greens or leaf rape. They are commonly grown and
consumed worldwide. Among these, broccoli intake
and cancer prevention research have been widely
studied, having multiple references related to different types of cancers in scientific literature. This fact is
due to the presence of a type of bioactive components: glucosinolates. Glucosinolates are the major
class of secondary metabolites found in Brassica
crops. The molecule comprises a b-thioglucoside
123
214
N-hydroxysulphate, containing a side chain and a
b-D-glucopyranose moiety. Glucosinolates can be
grouped into three chemical classes: aliphatic, indole
and aromatic, according to whether their amino acid
precursor is methionine, tryptophan or an aromatic
amino acid (tyrosine or phenylalanine), respectively
(Giamoustaris and Mithen 1996). The most important
glucosinolates found in Brassica vegetables are
methionine-derived glucosinolates (Mithen et al.
2003). However, several glucosinolates belonging to
the three chemical classes have been identified in the
edible parts of Brassica crops.
The first observations on the unique properties of
glucosinolates and isothiocyanates were recorded at
the beginning of the 17th century as a result of the
study on the chemical origin of the sharp taste of
mustard seeds (Fahey et al. 2001). Glucosinolates
known as sinigrin (2-propenyl) and sinalbin
(4-hydroxybenzyl) were isolated early in the 1830s
from black (Brassica nigra) and white (Sinapis alba)
mustard seeds, respectively. In 1956, Ettlinger and
Lundeen (1956) proposed the correct structure of
glucosinolates and they described the first chemical
glucosinolate synthesis (Fahey et al. 2001).
Although approximately 120 classes of glucosinolates have been identified in plants, each plant
species contains up to four different glucosinolates in
significant amounts (Fahey et al. 2001). Glucosinolates are not bioactive until they have been
enzymatically hydrolysed to various bioactive breakdown products by the endogenous plant enzyme
myrosinase (thioglucoside glucohydrolase, E.C.
3.2.1.147). These breakdown products include isothiocyanates, nitriles, thiocyanates, epithionitriles,
oxazolidine-2-thiones, and epithioalkanes (Grubb
and Abel 2006) depending on the substrate, pH
conditions, availability of ferrous ions and the level
and activity of specific protein factors such as the
epithiospecifier protein (ESP). At physiological pH,
isothiocyanates are the major products, whereas
nitriles are formed at more acid pH (Halkier and
Du 1997).
Comparative studies of glucosinolate profiles
indicate significant differences among cruciferous
crops (VanEtten et al. 1976; Carlson et al. 1987;
Kushad et al. 1999; Ciska et al. 2000). Apart from
glucosinolate profile, large differences in the levels of
both aliphatic and indole glucosinolates have been
observed in Brassica plants, presumably due to the
123
Phytochem Rev (2008) 7:213–229
use of different varieties, analytical methods and
growing conditions. Moreover, the chemical structure
and glucosinolate concentrations in cruciferous plants
vary considerably, depending on the stage of development, tissue type and environmental conditions
(Velasco et al. 2007).
Several published studies have described the
glucosinolate composition of different vegetable
Brassica species and these have been reviewed
extensively (see Table 1). Fenwick et al (1983a)
provide a comprehensive review on the glucosinolate
content of crop plants, and more recently, Rosa et al.
(1997) and Rosa (1999) reported the glucosinolate
content of different Brassica vegetables. Each type of
cruciferous vegetables shows a characteristic glucosinolates profile, differing substantially, even though
they are all part of the same species (Fenwick et al.
1983a; Kushad et al. 1999). In B. oleracea, there is a
considerable variation in glucosinolate structure and
in the total amount of glucosinolates. All the different
B. oleracea types contain glucobrassicin (3-indolylmethyl) and glucoiberin (3-methylsulfinilpropyl)
and most contain substantial amounts of sinigrin. For
example, sinigrin, glucobrassicin and glucoiberin
have been identified as the major glucosinolates in
kales and cabbages. Sinigrin makes the major
contribution of glucosinolates to kales, while glucobrassicin or glucoiberin do so to cabbage leaves
(Cartea et al. 2007b). In broccoli, common glucosinolates are glucoraphanin (4-methylsulfinylbutyl),
sinigrin, progoitrin (2-hydroxy-3-butenyl), gluconapin (3-butenyl) and the indole glucosinolates
glucobrassicin and neoglucobrassicin (1-methoxy3-indolylmethyl) (Kushad et al. 1999). The predominant glucosinolate is glucoraphanin (making up more
than 50% of the total glucosinolates), being sinigrin
levels comparatively low. In Brussels sprouts, collard
and cauliflower, the predominant glucosinolates are
sinigrin, progoitrin and glucobrassicin (VanEtten
et al. 1976; Carlson et al. 1987; Kushad et al.
1999). Each of these vegetables also contains smaller
amounts of other glucosinolates (Table 1).
In contrast to this glucosinolate variation within
B. oleracea species, there is a relatively little
variation in glucosinolate structure of B. rapa crops.
Chinese cabbage accumulates gluconapin and glucobrassicanapin (4-pentenyl) and their hydroxylated
forms, progoitrin and gluconapoleiferin (2-hydroxy4-pentenyl), respectively. In turnip roots, the
Phytochem Rev (2008) 7:213–229
215
Table 1 Principal glucosinolates identified in leaves of Brassica vegetable crops
Crop
Aliphatic glucosinolates
Indole glucosinolates
Aromatic
GIB PRO SIN GAL GRA GNA GBN GIV GER GNL GBS NGBS 4HGBS 4MGBS GST
Brassica oleracea
White cabbagea,b,c,d
+
+
+
+
+
+
+
+
+
–
+
+
+
+
+
a,b,c,d
Savoy cabbage
+
+
+
–
+
+
+
+
–
–
+
+
–
+
+
Red cabbagea,c,d
+
+
+
–
+
+
–
+
–
–
+
+
–
–
–
Kalea,c,d,e
+
+
+
–
+
+
–
+
–
–
+
+
+
+
+
Collarde
+
+
+
–
–
–
–
+
+
–
+
–
–
–
–
Tronchuda cabbagec,f +
+
+
+
+
+
+
+
–
–
+
+
+
+
+
Broccolie,g
+
+
+
+
+
+
+
–
+
+
+
+
+
+
+
Brussels sprouts
d,e,g
+
+
+
–
+
+
–
+
–
–
+
+
–
–
–
Cauliflowerd,g
+
+
+
–
+
–
–
+
–
–
+
+
–
–
–
Kohlrabid
+
+
+
+
+
+
–
+
–
–
+
+
+
+
–
+
+
–
–
–
+
+
–
+
+
+
+
+
+
+
Turnip greens
+
+
–
+
+
+
+
+
–
+
+
+
+
–
+
Turnip topsi
Chinese cabbageb
+
+
+
+
–
–
–
–
–
–
+
+
+
+
+
–
–
–
–
+
+
+
+
+
+
–
–
+
+
+
–
+
–
–
+
–
+
–
–
+
+
+
+
+
+
–
+
–
+
–
+
+
+
–
+
+
+
–
+
+
Brassica rapa
Turnipb
h
Brassica napus
Swedeb
Leaf rape
j
Major glucosinolates found in each crop are shown in bold
GIB: glucoiberin (3-methylsulfinylpropyl); PRO: progoitrin (2-hydroxy-3-butenyl); SIN: sinigrin (2-propenyl); GAL: glucoalysiin
(5-methylsulphinylpentyl); GRA: glucoraphanin (4-methylsulphinylbutyl); GNA: gluconapin (3-butenyl); GBN: glucobrassicanapin
(4-pentenyl); GIV: Glucoiberverin (3-methylthiopropyl); GER: glucoerucin (4-methylthiobutyl); GNL: gluconapoleiferin
(2-hydroxy-4-pentenyl); GBS: glucobrassicin (3-indolylmethyl); NGBS: neoglucobrassicin (1-methoxy-3-indolylmethyl); 4HGBS:
4-hydroxyglucobrassicin (4-hydroxy-3-indolylmethyl); 4MGBS: 4-methoxyglucobrassicin (4-methoxy-3-indolylmethyl), GST:
Gluconasturtiin (2-phenylethyl)
Sources
a
VanEtten et al. (1980)
b
Sones et al. (1984)
c
Cartea et al. (2007b)
d
Ciska et al. (2000)
e
Carlson et al. (1987)
f
Rosa et al. (1996)
g
Kushad et al. (1999)
h
Padilla et al. (2007)
i
Rosa (1997)
j
Cartea et al. (2007a)
predominant glucosinolates found by Sones et al
(1984) were progoitrin and gluconasturtiin (2-phenylethyl), while gluconapin and glucobrassicanapin
have been identified as the most abundant glucosinolates in the edible parts of turnip greens (Kim et al.
2003; Padilla et al. 2007) and turnip tops (Rosa
1997). Vegetable crops of B. napus include leaf rape
and swedes. Cartea et al. (2007a) found that glucobrassicanapin followed by progoitrin and gluconapin
proved to be the most abundant glucosinolates in a
variety of leaf rape called ‘nabicol’. In swedes,
glucobrassicin, progoitrin and gluconasturtiin have
123
216
been found as the major glucosinolates (Carlson et al.
1987).
As components of feed and food, some abovementioned glucosinolates as source of bioactive compounds
(i.e., isothiocyanates, thiocyanates, nitriles and epithionitriles) have been recognized for long for their
distinctive benefits to human nutrition and plant
defence. This feature has lead to consider Brassica
foods as possible functional foods. The term ‘functional
foods’ describes foods that, if they are normal dietary
constituents, can provide sufficient amounts of bioactive
components that are valuable for health improvement.
In order to acquire the full benefit of functional foods, it
is necessary to know the natural variation in content of
bioactive food components (Dekker and Verkerk 2003).
Such variation might be regulated genetically or it might
result from changes in the growing environment or from
differences in post-harvest handling, processing, storage
or food preparation.
Here, we review the significance of glucosinolates
as bioactive compounds for human nutrition and
health and the influence of environmental conditions,
processing, and storage on glucosinolate concentration in Brassica vegetables.
Human nutrition and anticarcinogenic activity
Different epidemiological studies have indicated that
diet and cancers are closely interlinked. Over the past
30 years, various studies showed that fruits and
vegetables contain natural phytochemicals such as
glucosinolates that have anticarcinogenic properties
(Block et al. 1992; Talalay and Zhang 1996; Hecht
2000; Talalay and Fahey 2001; Anilakumar et al.
2006). Consequently, cruciferous vegetables have
been very interesting in the last years for their
potential use in cancer chemoprevention (Rosa et al.
1997; Farnham et al. 2004; Smith et al. 2005).
Results have consistently shown that the chemoprotective agents derived from this class of vegetables of
the Cruciferae family have an influence on carcinogenesis during the initiation and promotion phases of
cancer development. Similarly, reports from epidemiological studies and clinical trials support this
notion.
Isothiocyanates and indoles are two major groups
of autolytic breakdown products of glucosinolates.
Both of them exhibit protective activities against
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Phytochem Rev (2008) 7:213–229
many types of cancer. In vitro and in vivo studies
have reported that these compounds affect many
stages of cancer development, including the induction
of detoxification enzymes (Phase II enzymes) and the
inhibition of activation enzymes (Phase I enzymes)
(Zhang and Talalay 1994; Hecht 2000; Fahey et al.
2002; Anilakumar et al. 2006).
Phase II enzymes such as quinone reductase,
glutathione-S-tranferase, UDP-glucuronyl transferase
and NADPH reductase are able to conjugate with
activated carcinogens and turn them into inactive
water soluble compounds. They can be excreted
through the urine, what results in the neutralisation of
potential carcinogens from mammalian cells. Inductions of Phase II cellular enzymes are largely
mediated by the antioxidant responsive element
(ARE), which is regulated by the transcriptional
factor, Nrf2. The most powerful inducers of the Phase
II enzymes are the isothiocyanates sulforaphane,
iberin and erucin, which are the hydrolysis products
of glucoraphanin, glucoiberin, and glucoerucin
(4-metylthiobutyl), respectively (Nilsson et al. 2006).
Another mechanism for glucosinolate breakdown
products requires the inhibition of the enzymes
involved in cancer induction. Phase I enzymes are
responsible for in vivo and in vitro metabolic activation of most carcinogens in human and animal
cells. Under certain conditions, products of Phase I
enzymes (cytochrome P450 enzymes) serve as substrates for Phase II enzymes, which turn them into
electrophilic carcinogens that can be eliminated by
the kidney (Talalay and Zhang 1996). Cytochrome
P450 enzymes are a battery of Phase I enzymes. It has
been shown that they activate metabolically chemical
carcinogens such as nitrosamines and aflatoxins.
In addition to the modulation of Phase I and Phase II
enzymes, apoptosis and cell cycle perturbations appear
to be yet other potential chemopreventive mechanisms
due to isothiocyanates, especially with respect to the
effects on initiated tumor cells (Mithen et al. 2000).
Glucosinolates and their breakdown products may
regulate cancer cell development by blocking the cell
cycle and promoting apoptosis (Mithen et al. 2003;
Lund 2003). Apoptosis or programmed cell death is
the genetically encoded destruction of an active cell,
what reduces tumor invasion and metastasis. Moreover, it has been shown that glucosinolate breakdown
products prevent and/or suppress estrogen-dependent
cancers, such as cervical and breast, in both animal and
Phytochem Rev (2008) 7:213–229
human cells by blocking the estrogen receptor function
(Keum et al. 2004).
Aliphatic glucosinolates
A great deal of research on functional foods like
anticarcinogens has focused on broccoli and on a
single bioactive component within broccoli, which is
called sulforaphane (Zhang et al. 1992; Fahey et al.
1994; Talalay and Zhang 1996; Fahey et al. 2002),
the isothiocyanates derived from its cognate glucosinolates glucoraphanin (Fig. 1). Initial interest in
sulforaphane was due to its powerful ability to induce
Phase II enzymes coupled with the inhibition of
cytochrome P450 (Phase I enzyme) (Zhang and
Talalay 1994; Talalay and Zhang 1996; Fahey et al.
1997). Moreover, sulforaphane serves as an indirect
antioxidant and it induces cell cycle arrest and
apoptosis. Rose et al (2005) studied an actively
proliferating HT29 cell line established from human
colon cancer and they identified sulforaphane as
an inductor of cell cycle arrest followed by apoptosis.
In a similar way, several studies have also stated the
ability of sulforaphane to induce apoptosis in a range
of cell lines including prostate, lymphocyte and
217
mammary. Moreover, sulforaphane shows potential
for treating Helicobacter pylori, which caused stomach cancer (Fahey et al. 2002). These results are
motivating efforts to increase sulforaphane content of
broccoli and to promote the health benefits of this
vegetable. Currently, the evidence of health benefits
from sulforaphane is strong enough to demand the
development of broccoli sprouts with a uniform and
high sulforaphane concentration.
Although the most characterized isothiocyanate
compound is sulforaphane, other isothiocyanates may
also contribute to the anti-carcinogenic properties of
crucifers. Glucosinolate hydrolysis products from
glucoiberin, sinigrin and progoitrin have also been
identified as suppressing agents, protecting human
and animal cells against carcinogenesis. These glucosinolates may well exert comparable levels of
biological activity to sulforaphane by either inducing
Phase II detoxification enzymes or inhibiting Phase I
enzymes (Fahey et al. 1997; Nilsson et al. 2006).
Indole and aromatic glucosinolates
Indole-3-carbinol, the degradation product of the
indole glucosinolate glucobrassicin, and benzyl and
Fig. 1 The most studied glucosinolates and their hydrolysis products implicated in human nutrition in each chemical class: (A)
aliphatic, (B) aromatic, (C) indole
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218
phenethyl isothiocyanates, the degradation products
of two aromatic glucosinolates, glucotropaeolin and
gluconasturtiin, respectively, have been investigated
for their potential as cancer chemoprotective agents
(Fig.1). These compounds may be responsible for the
selective induction of cancer cells to apoptosis,
supporting the potential preventive and/or therapeutic
benefit of the glucosinolate hydrolysis products
against different type of cancers (Zhang and Talalay
1994; Fahey et al. 1997).
Isothiocyanates formed from indole glucosinolates
are unstable and they separate spontaneously into
indole-3-carbinol. This compound may condense
under the acid conditions of the stomach to form
toxic compounds. Despite this toxicity, it has proved
to be successful against breast cancer (Telang et al.
1997) and respiratory papilloma (Rosen et al. 1998).
It has been shown that indole-3-carbinol elevates
quinone reductase, glutathione transferase (Phase II
detoxication enzymes) and CYP1A (a Phase I
enzyme) contents and, in some cases, does so
synergistically. However, the association between
broccoli consumption, cancer risk and glutathione
S-transferase genotype suggests that isothiocyanates
may be more important than indole compounds in
modulating risks. For more detailed action of this
dual nature, see reviews by Fenwick et al. (1983a)
and Rosa et al. (1997).
Phenethyl isothiocyanate is the hydrolysis product
of the glucosinolate gluconasturtiin. It is found at
high levels in some minor crops such as watercress
and radishes. This compound may inhibit Phase I
enzymes which are related to the activation of
carcinogens and it has been shown that this compound inhibits induction of lung and esophageal
cancer in both rat and mouse tumor models. The
effect of phenethyl isothiocyanate metabolites on
human leukemia cells in vitro has been proved as
well as its role to inhibit tobacco smoke-induced lung
tumors in mice. Kuang and Chen (2004) have shown
the effects of indole-3-carbinol, phenethyl isothiocyanate and benzyl isothiocyanate on the induction of
apoptosis in human cell lung cancer. The results
indicated that all compounds tested are able to inhibit
the growth of A549 cells by inducing apoptosis at low
concentrations and necrosis at high concentrations.
Indole-3-carbinol showed an antiproliferative effect
in A549 cells, but not through induction of apoptosis.
A recent study by Rose et al. (2005) showed that
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Phytochem Rev (2008) 7:213–229
extracts of broccoli and watercress inhibit the invasive potential of the human breast cancer cell line
in vitro. This study also suggested that their phytochemical constituents, isothiocyanates, are a new
kind of invasion inhibitors.
Anti-nutritional effects
The degradation products of some glucosinolates
have been traditionally considered as damaging or
prejudicial because of their goitrogenic and growth
retardation activities. Their presence in the seeds of
oilseed cruciferous crops reduces in a significant way
the quality of the seed meal left after oil extraction.
For these reasons, in the past 40-years glucosinolates
have assumed major agricultural significance because
of the increasing importance of rapeseed cultivars of
B. napus, B. rapa, and B. juncea as oil crops in
temperate and subtropical areas of the world. The
most remarkable of the prejudicial degradation
products is oxazolidine-2-thione, derived from progoitrin (Rosa et al. 1997). This glucosinolate is
accumulated in the seeds of oilseed rape and it
causes goiter and other harmful effects on animal
nutrition, such as depressed growth, poor egg
production and liver damage. However, there is no
evidence of any goitrogenic effect on humans
because of Brassica consumption (Mithen 2001)
and an international study indicated no relationship
between glucosinolate intake and the incidence of
thyroid cancer.
Goitrin is an inhibitor of thyroid peroxidase and it
prevents iodide from oxidation into iodine for
subsequent iodination of tyrosine residues in the
biosynthesis of the thyroxines T3 and T4. Thiocyanate anions act as a competitive inhibitor of iodide,
and thereby they prevent iodide uptake by the
thyroid. In addition to the alterations in size, structure
and function of the thyroid, it has been shown that
goitrin can undergo nitrosation reactions which might
have implications for human health where nitrate
levels in water are high (Luthy et al. 1984). The
precise cause of this is not fully understood, but it
may be related either to the presence of intact
glucosinolates or the production of nitriles in the
digestive tract.
Plant breeders have drastically reduced glucosinolate levels of the seed to allow the protein-rich seed
Phytochem Rev (2008) 7:213–229
cake (the residue left after crushing to obtain oil) to
be sold as an animal feed supplement. Efforts to
avoid the goitrogenicity of mustard/rapeseed oil cake
led to the successful development of the oilseed crop
‘Canola’, which was developed in the 1970s by a
plant-breeding program designed to develop cultivars
of oilseed rape with low levels of glucosinolates and
erucic acid. Canola seed contains about 40% of oil
and this oil must contain less than 2% of erucic acid.
The seed meal, which is used for feed animals after
the oil extraction, must have less than 30 lmol of
glucosinolates per gram of meal.
In conclusion, the potential negative effects of
glucosinolates require a further examination, as this
topic has been scarcely researched in recent years.
For more detailed treatment of the ´antinutritional’
nature of these compounds, see reviews by Fenwick
et al. (1983a), Rosa et al. (1997), Griffiths et al.
(1998) and Anilakumar et al. (2006).
Sensorial and organoleptic effects
Apart from the medicinal value of these sulfur
compounds, glucosinolates are responsible for the
characteristic flavour and odour of Brassica vegetables. Glucosinolate-derived isothiocyanates produce a
pungent and bitter flavour and sulfurous aroma when
the plant tissue is injured, playing a significant
organoleptic role in Brassica products (Chong et al.
1982; Fenwick et al. 1983a; Rosa et al. 1997).
However, the direct relation between glucosinolate
content and sensory properties is complex.
Literature describes a bitter effect mainly because
of isothiocyanates formed from sinigrin, gluconapin
and progoitrin, but also from glucobrassicin and
neoglucobrassicin, but in different intensities. In
contrast, alkyl glucosinolates as glucoerucin, glucoiberverin (3-methylthiopropyl), glucoiberin and
glucoraphanin do not contribute to the attribute
‘bitter’. In Brussels sprouts, isothiocyanates derived
from sinigrin and progoitrin have been related to
bitterness (Fenwick et al. 1983b; Van Doorn et al.
1998) while in cooked cauliflower, neoglucobrassicin
and sinigrin glucosinolates were found responsible
for the bitter taste and, thereby, they were considered
responsible for consumers’ acceptance (Engel et al.
2002; Schonhof et al. 2004). In cabbage, isothiocyanates derived from progoitrin and gluconasturtiin are
219
pungent and very bitter compounds (Fenwick et al.
1983b; Rosa et al. 1997). In turnip greens, gluconapin and glucobrassicanapin have been described as
flavour compounds (Rosa 1997; Padilla et al. 2007).
A sensory profile of turnip greens, affected by variety
and maturity, has been reported by Jones and Sanders
(2002), who concluded that both sources had a
significant effect on their sensory characteristics and
preferences.
Sensorial analysis comparing bitterness with variation in glucosinolate concentration suggests that
these compounds and their breakdown products are
not the only determinants of the characteristic flavour
of Brassica vegetables (Hansen et al. 1997; Baik
et al. 2003; Padilla et al. 2007). Bitter taste is
probably a synergistic property of various phytochemicals, not just of hydrolysis products derived
from some glucosinolates. Indole hydrolysis products, phenolic content or flavonoids could have some
influence on organoleptic properties and on the bitter
flavour, as it has already been reported (Drewnowski
et al. 2001).
Influence of environmental conditions
on glucosinolate content
Variation on the amount and pattern of glucosinolates
has been attributed to genetic and environmental
factors, including:
Genetics
Glucosinolate diversity varies widely in families and
species, suggesting that diversification has accompanied speciation (Rosa et al. 1997). For this reason,
glucosinolates have been used as chemical markers in
chemotaxonomy. Glucosinolate levels of vegetative
tissues and seeds vary regardless of each other.
Therefore, they are probably controlled by different
genetic and physiological mechanisms. Biosynthesis
and genetics of glucosinolates have been the target of
several comprehensive reviews (Giamoustaris and
Mithen 1996; Rosa et al. 1997; Fahey et al. 2001;
Halkier and Gershenzon 2006; Grubb and Abel,
2006). These features are not the focus of this review.
Thereby, they will not be discussed in the next
sections.
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Phytochem Rev (2008) 7:213–229
Plant development
There is a great variation in the profile and the
amount of glucosinolates of Brassica species (Cartea
et al. 2007a, b; Padilla et al. 2007). Besides, there is a
great variation inside the plant depending on the plant
development. In a variety of kale (B. oleracea
acephala group), Velasco et al. (2007) detected an
increasing concentration of aliphatic glucosinolates in
leaves, from the early stage until the prebolting stage.
Indole glucosinolates were increasing from the early
stage to five months after sowing, and then they
started to decrease (Fig. 2). This result agrees with
Fieldsend and Milford (1994) who found that total
glucosinolate and most of individual glucosinolate
content in oilseed rape increased from vegetative to
reproductive stages and maturity. Aliphatic and
indole glucosinolates showed unlike variation
throughout the plant development. The highest contents of all aliphatic glucosinolates, including sinigrin
and glucoiberin, were noticed in flower buds, whilst
the highest levels of indole glucosinolates, as glucobrassicin, and the aromatic glucosinolate levels were
observed in leaves harvested at the optimum
70
Aliphatic
Indolic
Aromatic
60
Flower buds
Concentration
50
40
30
20
Leaves
10
Flower buds
Leaves
0
1
2
3
Plant stages
4
5
Fig. 2 Mean concentration (lmol g–1 dw) for the three classes
of glucosinolates through plant growth (form Velasco et al.
2007). Plant stages: (1): the initial plant stage before
transplanting at the five or six leaves stage, 30 days after
sowing (DAS); (2) the vegetative phase at the first optimum
consumption stage, 90 DAS; (3) the vegetative phase at the
second optimum consumption stage, 180 DAS; (4) the
vegetative-reproductive phase transition, at the last consumption stage 300 DAS; (5) the reproductive phase before bolting,
390 DAS
123
consumption stage, 180 days after sowing (Booth
et al. 1991; Velasco et al. 2007). Glucosinolate
differences between plants and large changes in the
types of glucosinolates that occur in vegetative and
floral tissues during ontogeny, makes a comprehensive characterization of plant material necessary
when sampling for analysis and consumption.
Plant part
Glucosinolate content depends on the plant part, and
its levels in stems and petioles proved to be lower
than those of roots and heads (Rosa et al. 1997). In
general, seeds had the highest glucosinolate concentration, followed by leaves, roots and stems in
B. napus (Velasco et al. submitted). In Arabidopsis,
Brown et al. (2003) found that dormant and germinating seeds had the highest glucosinolate
concentration (2.5–3.3% by dry weight) followed
by inflorescences, siliques, leaves and roots.
Environmental factors
The synthesis and degradation of glucosinolates can
happen in a wide range of climate conditions. Bible
and Chong (1975) showed that climate can influence
amounts of glucosinolates. They concluded that cold
unit accumulation is the most reliable index of root
radish glucosinolate content, regardless of soil and
climate. Thiocyanate content was negatively correlated with mean daily air temperature on organic soil
and positively correlated with rainfall. Velasco et al.
(2007) found that temperature could have a positive
effect on the glucosinolate concentration throughout
the vegetative cycle of plants, as low temperatures
caused a reduction on glucosinolate content. This was
also established by Ciska et al. (2000), who found
that a high average temperature increased in a
significant way the glucosinolate content of different
Brassica crops. Temperature was not the only
environmental factor to take into account, since a
lower average rainfall also increased glucosinolate
content. In watercress plants, Engelen-Eigles et al.
(2006) stated that the major environmental influence
on gluconasturtiin content is daylength. In this sense,
plants grown under long days contained a 30–40%
higher gluconasturtiin concentration than plants
Phytochem Rev (2008) 7:213–229
grown under short days. But with the same daylength,
plants grown under temperatures between 10 and
15C had a 50% higher gluconasturtiin content, but a
lower fresh weight than plants grown at 20–25C.
These data suggested that gluconasturtiin content
can be increased by growing plants under lowtemperature and long-day regimes.
Besides, crop season has an effect on glucosinolate
concentration. Winter or autumn seasons seem to lead
to lower glucosinolate levels, due to short days,
wetter conditions, cool temperatures and less radiation (Rosa and Heaney 1996; Rosa et al. 1997).
Charron et al. (2005) found that total and indole
glucosinolate concentrations in different groups of
B. oleracea were significantly affected by the season,
and they were generally higher in spring than in fall
season. The effect of season was mainly explained by
the average temperature, daylength and photosynthetic photon flux. Cartea et al. (2007b) confirmed
this result since they found that local varieties of
cabbage planted during the fall season had a 40% less
of total glucosinolate than the same varieties planted
during the spring season. This difference was mainly
due to indole glucosinolates with a 55% less in fall
season.
Cultural practices: soil type and nutrients
It has been said that different cultural practices affect
glucosinolate content, specially plant irrigation and
density. Closer spacing affects the morphology of the
plant, i.e., by reduction of head size, increasing
concentrations of glucosinolates (Rosa et al. 1997).
In other plants like rapeseed, a higher density does
not lead to an increase in glucosinolates concentration in leaves or seeds. Following Ju et al. (1980), the
soil composition has an influence on glucosinolate
content, with plants in organic soils having the
highest total glucosinolate concentration.
The addition of different nutrients to the soil has a
different effect on glucosinolate content. For example, Brassica crops require more sulphur than most
other crops due to its role in the synthesis of
glucosinolates, as well as sulphur aminoacids and
proteins (Rosa et al. 1997). Sulphur application
increases the glucosinolate content in oilseed rape
leaves and flowers (Booth et al. 1991). Nitrogen is
also a constituent of the glucosinolate molecule, but
221
different studies have shown that a high dose of
nitrogen application tended to give lower glucosinolate levels. Kim et al. (2002) found that
glucosinolate content from the edible parts of
B. rapa is strongly affected by nitrogen and sulphur
applications. This was also stated by Rosen et al.
(2005) who showed that total glucosinolates and
glucobrassicin content were maximized in cabbage
cultivars grown at low nitrogen and high sulphur
application rates.
The environmental effect in the hydroxylation step
that links gluconapin and progoitrin in the aliphatic
pathway was reported by Zhao et al. (1994). They
showed that sulphur deficiency reduces the aliphatic
glucosinolate concentration and increasing nitrogen
results in higher proportions of progoitrin, what
suggests that the hydroxylation step is favoured.
Influence of storage and processing
on glucosinolate content
The use of Brassica vegetables to improve human
health and the interpretation of epidemiological data
require an understanding of glucosinolate chemistry
and metabolism across the whole food chain, from
production and processing to the consumer. Glucosinolate and related isothiocyanate contents of
Brassica vegetables are affected by methods of
storage and food processing, e.g., cutting, chewing,
cooking, fermenting or freezing (Song and Thornalley 2007). Nevertheless, given the importance of
glucosinolates in terms of their anticarcinogenic
activity, few thermal degradation studies have been
carried out with regard to food processing. Most of
the research has been done in crude extracts and
information about the plant myrosinase stability
throughout processing is limited. When vegetables
are sliced and washed as part of processing,
conditions for myrosinase activity are probably
optimal. This enzyme is temperature-sensitive and
consequently, throughout conventional thermal processing, it will be inactivated and it cannot transform
glucosinolates into beneficial products. Thereby, it is
important to maintain the levels of beneficial glucosinolates after harvest and to provide the correct
way of cooking to ensure optimal health benefits.
The effect of storage, processing and cooking
methods on glucosinolate content has been studied in
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222
several Brassica vegetables as broccoli, Brussels
sprouts, cauliflower and cabbage. However, results
are often confusing (Verkerk et al. 2001; Vallejo
et al. 2002; Verkerk and Dekker 2004). Recently, the
influence of the environmental factors, post-harvest
and processing on the content of glucosinolates and
other bioactive components of broccoli have been
reviewed by Jeffery et al. (2003), Jones et al. (2006)
and Song and Thornalley (2007).
Storage
The effect of storage on glucosinolate preservation is
unclear and it depends on glucosinolates, being
indole glucosinolates more sensitive to storage conditions than the aliphatic or aromatic glucosinolates
(Verkerk et al. 2001; Jeffery et al. 2003). Indole
glucosinolate levels were evaluated in stored chopped
broccoli, showing a significant 2–4-fold increase after
48 h (Verkerk et al. 2001). This suggests that not
only conditions in the field, but also conditions during
harvest and handling may alter indole glucosinolate
synthesis.
When stored at ambient temperature (12–22C),
there was not a significant decrease in glucosinolate
content of Brassica vegetables such as broccoli,
Brussels sprouts, cauliflower and cabbages (Song and
Thornalley 2007). When these vegetables were stored
in a domestic refrigerator (4–8C), the total glucosinolate contents decreased 11–27% for 7 days and
minor changes were observed during the first 3 days
of storage. Regarding individual glucosinolates,
losses of glucoiberin, glucoraphanin and glucoalyssin
(5-methylsulphinylpentyl) were higher than those of
sinigrin, gluconapin and progoitrin. The loss of
glucoiberin in broccoli was 40–50%, whereas with
regard to gluconapin the loss was 5–10% in all
vegetables studied.
A great deal of attention has been paid to
glucoraphanin and its decomposition product, the
isothiocyanate sulforaphane, because of its anticarcinogenic properties. Although investigations on the
effects of storage and cooking on the glucosinolate
content of broccoli have been performed (Goodrich
et al. 1989; Vallejo et al. 2003), most recent studies
(Jones et al. 2006; Winkler et al. 2007) have examined the effects of storage or cooking on sulforaphane
formation in dietary broccoli. Rodrigues and Rosa
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Phytochem Rev (2008) 7:213–229
(1999) reported a decrease in the glucoraphanin
content of broccoli stored at 4C for 5 days. Rangkadilok et al. (2002) found no significant losses in the
glucosinolate content of broccoli stored at the same
temperature for 7 days, while the decline of glucoraphanin content happened at 20C in 7 days. In
contrast, Hansen et al. (1997) stated that glucoraphanin and glucoiberin contents increased when broccoli
was stored at 10C for 7 days. Winkler et al. (2007)
concluded that there were no significant differences
between storage temperatures, storage times and
marketing temperatures as for glucoraphanin content.
Relative humidity only appears to be a critical
factor for glucosinolate retention when postharvest
temperatures rise above approximately 4C. A high
relative humidity of 98–100% is recommended to
maintain postharvest quality in broccoli. Rangkadilok
et al. (2002) found that broccoli heads stored in
plastic bags with a high relative humidity ([90%)
showed no significant loss at 20C, whereas heads
stored at low relative humidity and at the same
temperature showed a 50% decrease of glucoraphanin
content during the first 3 days of storage. Similarly,
Rodrigues and Rosa (1999) found that glucoraphanin
content decreased by [80% in broccoli heads left at
low relative humidity and at 20C for 5 days.
However, when broccoli was stored at 4C, there
was no difference in glucoraphanin content after
7 days either in open boxes at ambient humidity
(approximately 60% RH) or in plastic bags (approximately 100% RH) (Rangkadilok et al. 2002).
The effect of controlled atmosphere storage and
the modified atmosphere packaging on glucosinolate
content is confusing. Jones et al (2006) summarized
several studies about both features. Authors concluded that both controlled atmosphere storage and
modified atmosphere packaging appear to be useful
tools in maintaining glucosinolate content after
harvest. Either the atmospheres reached or relative
humidity achieved may have prevented membrane
degradation and subsequent mixing of glucosinolates
with myrosinase. However, more work is necessary
to clearly elucidate the atmospheres that may best
maintain glucosinolate content. On the other hand,
little is either known about the effect of other
postharvest factors, such as ethylene or cytokinin
application on glucosinolate content.
To sum up, the most important postharvest conditions necessary for maintaining broccoli quality are
Phytochem Rev (2008) 7:213–229
low temperature (less than 4C) and a high relative
humidity (Rodrigues and Rosa 1999; Jones et al.
2006). These conditions maintain cellular integrity
and the process seems to maintain glucosinolate
content by preventing the mixing of glucosinolates
with myrosinase.
Domestic treatments: cooking, steaming
and microwaving
All the factors and operations through the postharvest
chain will activate complex reaction mechanisms,
and physical and physiological processes change
glucosinolates levels and, subsequently, their breakdown products (Jeffery et al. 2003). Prior to
consumption, Brassica vegetables are subject to
different ways of processing. Although some Brassica vegetables as broccoli, cauliflower or cabbage
can be eaten raw in salads or pickled form, most of
them are cooked before consumption. It has been
generally shown that conventional cooking methods
such as boiling, steaming, pressure cooking and
microwaving reduce the intake of glucosinolates by
approximately 30 to 60%, depending on the method,
intensity and type of compound (Rodrigues and Rosa
1999; Verkerk et al. 2001; Rangkadilok et al. 2002;
Verkerk and Dekker 2004). Therefore, increased
bioavailability of dietary isothiocyanates may be
achieved by avoiding boiling of vegetables. Cooking
at high temperatures denatures myrosinase into
vegetable material, resulting in a lower conversion
of glucosinolates to isothiocyanates. Glucosinolate
levels can be reduced because of enzymatic breakdown, thermal breakdown and leaching into the
cooking water. Leaching of glucosinolates may not
represent a dietary loss when culinary practices use
this cooking water for soups (Rosa and Heaney
1993).
Rosa and Heaney (1993) found a reduction of total
glucosinolate content when boiling Portuguese cabbage and they reported 40 to 80% leaching of
glucosinolates from broccoli heads, cabbage leaves
and Brussels sprouts into the cooking water. Ciska
and Kozlowska (2001) also observed a decrease of
glucosinolate content after 5 min of cooking (35%),
which gradually decreased to 87% of loss after
30 min in white cabbage. The individual and total
glucosinolates content was measured in Portuguese
223
cabbage and white cabbage before and after cooking,
and about 50% of total glucosinolates was lost during
the cooking process (Pereira et al. 2002).
Gliszczynska-Swiglo et al. (2006) proved that
water-cooking resulted in a decrease of the total
glucosinolate content, as well as in the main glucosinolates of broccoli if compared with fresh broccoli,
whereas steam-cooking had the opposite effect.
Thermal degradation of indole glucosinolates has
been examined in details (Slominski and Campbell
1989; Bones and Rositer 2006). It was observed that
heat treatment resulted in a substantial decomposition
of indole glucosinolates with thiocyanate and indoleacetonitriles as products, while autolysis (macerated
tissue) gave little indoleacetonitriles, but high levels
of thiocyanate and carbinols.
Steaming, microwaving and stir-fry cooking are
popular cooking methods that had little effect on the
total glucosinolate contents of Brassica vegetables.
Jones et al. (2006) reported that steaming for 2 min is
the most effective way to maintain glucosinolate
content. Vallejo et al. (2002) investigated the effect
of steaming on the quality of broccoli. Results
indicated that steaming induces the isothiocyanate
content, which was approximately 3-fold lower in
steamed than in fresh broccoli. This was presumably
due to the effects of myrosinase activity in broccoli,
which increased the conversion of glucosinolates into
corresponding isothiocyanates rather than to the loss
of glucosinolate content during steaming. Similar
observations were found by Goodrich et al. (1989)
who indicated that large glucosinolate losses occur in
blanched broccoli but not in blanched Brussels
sprouts.
Microwave cooking is an efficient alternative for
cooking vegetables due to the low amount of cooking
water required, and it has been used as a good method
to inactivate myrosinase with a negligible loss of
glucosinolates. The effect of microwave treatment on
myrosinase activity has been studied. Nevertheless,
most studies about microwave cooking and its
relation to glucosinolates are confusing. The effects
of microwave treatment on the glucosinolate content
measured in red cabbage showed higher levels of
these compounds than in untreated vegetables
(Verkerk and Dekker 2004). This feature is probably
due to the increased extractability of glucosinolates
by using microwave treatment. However, Rouzaud
et al. (2004) reported that microwaving of cabbage
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224
resulted in a loss of sinigrin (8%), and according to
Vallejo et al. (2002), microwaving of broccoli florets
resulted in a 74% loss of total glucosinolates and it
produced significant decreases of glucoiberin, progoitrin and gluconapin. Moreover, these losses were
not associated with the leaching of glucosinolates
into the cooking water.
Stir-fry cooking is gradually becoming one of the
major cooking methods used worldwide for Brassica
vegetables. Song and Thornalley (2007) cooked
Brassica vegetables by means of the stir-fry method
for 3–5 min. Glucosinolate content of several vegetables as broccoli, cabbage, cauliflower or Brussels
sprouts did not change significantly through this
cooking procedure, presumably because the temperature in the stir-fry process was lower than expected.
These authors stated that the stir-fry procedure
inhibited myrosinase activity rapidly without any
effect on glucosinolate content. This may explain
why glucosinolate contents were preserved.
Glucosinolate levels do not necessarily decline
rapidly after the chopping and cooking process and
even induction can occur. Mithen et al. (2000)
discuss two opposing mechanisms that take place
through vegetable processing: hydrolysis of glucosinolates by myrosinase and the induction of indole
glucosinolates by an unknown mechanism. In fact,
Verkerk et al. (2001) observed increased levels of
indole glucosinolates and some aliphatic after chopping. Matusheski et al (2004) also found that heating
fresh broccoli simultaneously increased sulforaphane
formation and decreased sulforaphane nitrile formation, which is the primary hydrolysis product when
the plant tissue is crushed at room temperature.
Recent evidence suggests that sulforaphane:sulforaphane nitrile ratio appeared to be genetically
determined (Mithen et al. 2003; Matusheski et al.
2004) and it is influenced itself by hydrolysis
conditions and the action of the epithiospecifier
protein. As sulforaphane had a far more potent effect
on Phase I and II enzymes than sulforaphane nitrile,
the health effects of predominant sulforaphane production could be significant.
Industrial processes
The effects of industrial processes as freezing,
fermenting and canning on glucosinolate variation
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Phytochem Rev (2008) 7:213–229
have been little studied (Dekker and Verkerk 2003).
Freezing broccoli is commonly used in food industry.
Commercial production of frozen vegetables
involves, however, steam treatments during blanching (Rodrigues and Rosa 1999) that inactivated
myrosinase and decreased glucosinolate breakdown
to isothiocyanates. If there was not a blanching step
before freezing, glucosinolates would be completely
broken down by myrosinase soon after thawing (Rosa
et al. 1997; Mithen et al. 2000).
Fermenting is another type of industrial process
used in some Brassica vegetables. The most common among fermented Brassica (e.g., cabbage)
products is sauerkraut, which is manufactured either
by natural or controlled fermentation. Apart from
myrosinase enzymes found in plants and in the
human intestinal flora, it has been reported that some
lactic acid bacteria strains degrade glucosinolates
and possess myrosinase-like activity. Ciska and
Pathak (2004) investigated the relationship between
the contents of degradation products in fermented
cabbage and the glucosinolates in raw cabbage.
Ascorbigen formed from one of the degradation
products of glucobrassicin was found to be the major
compound in fermented cabbage. Storage of fermented cabbage caused a reduction in the contents
of isothiocyanates. The lowest relative contents
(expressed as a percentage of the initial glucosinolate content) of degradation products were found for
the products of sinigrin degradation, whereas the
highest were found for the products of glucoraphanin
degradation.
Canning is another way for the consumers to find
Brassica vegetables. Dekker and Verkerk (2003)
showed a significant reduction of glucosinolates
content in canned cabbage as compared to fresh and
frozen cabbage. Most likely, the reason for this
reduction, although it is unclear, was either thermal
and/or enzymatic degradation. Canned vegetables
undergo an important heat treatment and, therefore,
the thermal degradation of glucosinolates is thought
to be the most important mechanism.
The thermal degradation of glucosinolates in red
cabbage has been studied in some details by first
inactivating myrosinase (Oerlemans et al. 2006). It
was found that aliphatic glucosinolates were less
sensitive to heat treatment compared to indole
glucosinolates (8% and 38%, respectively). These
authors identified two indole glucosinolates
Phytochem Rev (2008) 7:213–229
4-hydroxyglucobrassicin (4-hydroxy-3-indolylmethyl)
and 4-methoxyglucobrassicin (4-methoxy-3-indolylmethyl) to be the most thermolabile glucosinolates
throughout cooking. Several studies agree with this
remark: Rosa and Heaney (1993) identified that
neoglucobrassicin and 4-methoxyglucobrassicin along
with an aliphatic glucosinolate (glucoiberin) were more
thermolabile than other glucosinolates. In broccoli,
glucobrassicin was identified to be more thermolabile
than glucoiberin and glucoraphanin (Vallejo et al.
2002; Oerlemans et al. 2006). Brassica vegetables,
such as broccoli, Brussels sprouts, cauliflower and
cabbages were evaluated by Song and Thornalley
(2007). Authors found that regarding individual glucosinolates, losses of glucoiberin, glucoraphanin and
glucoalyssin by thermal degradation were higher than
those of sinigrin, gluconapin and progoitrin. The loss of
glucoiberin in broccoli was 40–50%, whereas the loss
of gluconapin was 5–10% in all vegetables studied.
Influence of the human digestive tract
on glucosinolate content
Raw cruciferous vegetables yield isothiocyanates and
nitriles, while residual glucosinolates in cooked
vegetables with thermally inactivated myrosinase
are degraded into isothiocyanates by myrosinases
also present in the microflora of the human digestive
tract (Shapiro et al. 2001). Glucosinolates are broken
down by plant myrosinase in the small intestine or by
bacterial myrosinase in the colon, and metabolites are
detectable in human urine 2–3 h after the intake of
Brassica vegetables.
The effect of cooking on isothiocyanate production
from glucosinolates during and after cabbage ingestion
was examined in human individuals. Isothiocyanate
content was larger after consumption of raw vegetables. However, isothiocyanates still arise, even though
to a lesser extent, when cooked vegetables are
consumed. This suggests that the colon microflora
catalyses glucosinolates. The formation of allyl isothiocyanates from sinigrin has been shown in the
digestive tract of rats associated with Bacteroides sp.,
a human colonic strain. In that study, sinigrin content
and the nature of the inoculum used were shown to
significantly affect the allyl isothiocyanate production.
Very little work has been carried out on the
degradative enzymes of the gut microflora. However,
225
more recently, several groups have identified bacterial strains associated with glucosinolate degradation
(see review by Bones and Rossiter 2006). Recent
evidence (Cheng et al. 2004) suggests that strains of
Bifidobacterium sp., B. longum, B. pseudocatenulatum and B. adolescentis were able to digest in vitro
both sinigrin and glucotropaeolin (benzylglucosinolate), causing a reduction in the medium pH. All these
findings allow us to conclude that these bifidobacteria
species could be involved in the digestive degradation of glucosinolates in the human intestinal tract,
affecting the final bioavailability of glucosinolates
present in Brassica foods.
Conclusions
With an increased interest in diet and health, it is
necessary to have information about glucosinolates
profiles and levels in plants. Glucosinolates from most
economic important Brassica vegetables as broccoli,
cauliflower or cabbage have been widely studied.
Conversely, few reports concerning the glucosinolate
concentration on other minor Brassica vegetables as
kale, turnip greens or leaf rape are found in scientific
literature. Our group has recently published the
glucosinolate profile and content of these crops. These
studies provide valuable information for developing
new cultivars with an appropriate glucosinolate profile, from which high quality added value products can
be produced. Investigations on degradation products of
glucosinolates (mainly isotiocyanates) and other nutritional phytochemicals caused by different processing
mechanisms are currently in progress in our group.
Genes necessary to alter glucosinolate profiles have
been found within Brassica genus and Arabidopis
thaliana. Aliphatic glucosinolate content is highly
heritable and they vary among Brassica crops and
varieties of the same crop (Kushad et al. 1999). On the
other hand, indole glucosinolates are common in
Brassica vegetables, although their levels are not only
subject to environmental fluctuations but to conditions
during harvest and processing. In fact, qualitative
differences observed among aliphatic composition may
be due to allelic variation in a few genes encoding key
regulatory enzymes at key points in the glucosinolate
pathway. For example, biosynthesis of gluconapin
requires a functional allele at the Gsl-alk locus that
turns glucoraphanin into its alkenyl homolog,
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226
gluconapin. Li and Quiros (2003) obtained transformed
Arabidopsis plants with a reduced concentration of
glucoraphanin, which was turned into gluconapin.
All these findings enhance the possibility of
studying genes involved in aliphatic glucosinolate
regulation and the feasibility of modifying aliphatic
glucosinolate profiles in specific plant genotypes.
Considering the chemical classes, previous studies
have shown that the synthesis of indole glucosinolates is regulated in a very different way to the
aliphatic glucosinolates (Brown et al. 2002; Kim
et al. 2003). Synthesis of aliphatic glucosinolates is
clearly regulated by the genotype. In contrast, the
effects of the genotype, the environment and environment · genotype on the content of indole
glucosinolates appeared reversed, with regulation
being primarily environmental. Moreover, it has been
shown that indole glucosinolates are more sensitive to
storage and processing conditions than the aliphatic
or aromatic glucosinolates (Verkerk et al. 2001).
Consequently, the development of enhanced Brassica hybrids with a high glucosinolate content is
possible for aliphatic glucosinolates such as glucoraphanin. Nevertheless, it may be necessary to
determine the mechanisms whereby environment
and processing cause upregulation of indole glucosinolates before developing hybrids with a specific
content of this last class of glucosinolates. The most
promising varieties for future breeding purposes
would be those with the highest total glucosinolate
content and, particularly, glucosinolates with beneficial effects related to human health. Isothiocyanates
and some indole and aromatic compounds derived
from these glucosinolates have, as it was previously
explained, a chemoprotective effect, related to a
reduction of the risk of certain cancers in humans.
Further research is needed to understand the
genetic and environmental factors and mechanisms
causing variability in the production of glucosinolates
and their breakdown/hydrolysis products, in order to
develop high quality Brassica foods with humanhealth promoting activity.
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