A review of influence of environment and process parameters on glucosinolate-myrosinase system from Brassica

Valéria Dal Prá , Natália Silva Jardim, Carolina Bolssoni Dolwitsch, Marcio Antônio Mazutti, Carine Viana, Denise Bohrer, Paulo Cicero Nascimento, Leandro Machado de Carvalho, Marcelo Barreto da Silva, Camilo Amaro de Carvalho, Marcelo Barcellos da Rosa Laboratory of Chemical Analysis, Department of Chemistry, Federal University of Santa Maria, Av. Roraima, 1000, Santa Maria, RS, 97105-900, Brazil. Department of Chemical Engineering Federal University of Santa Maria, Av. Roraima, 1000, Santa Maria, 97105-900, Brazil. Federal University of Espírito Santo. Department of Biological and Agricultural Sciences. Br 101, Km 60. São Mateus, ES, 29932-540, Brazil. Departament of General Biology, Federal University of Viçosa, Av. Peter Henry Rolfs, s/n, Viçosa, 36570-000, Brazil.


* Corresponding Author
Email; marcelobdarosa@gmail.com (M.B.Rosa); Tel.: +55-55-3220-8870 Capsella bursa-pastoris (shepherd's-purse), Cardamine bonariensis (wild watercress), Coronopus didymus (Lesser swine-cress), Lepidium spp.(peppercress) and Raphanus raphanistrum (wild radish) (Souza and Lorenzi, 2005).The family Brassicaceae has been very studied due to the pharmacologic properties of its main metabolites, the glucosinolates (GLS).These metabolites, as well as, their hydrolysis products (isothiocyanate and nitriles) are powerful antioxidants and anti-carcinogenic agents (Paulino, 2008).The GLS are one of the greatest metabolic groups containing sulfur, which are found mainly in comestible vegetables.Until the year of 2004, approximately 120 GLS were identified from species of Brassicaceae and other families (Falk et al., 2004).The chemical structure of GLS consists of ester of (Z) hydroxylamine sulphate that had an atom of sulfur bonded to a β-D -glucopyranose that is a side chain derived from an amino acid.The chemical side chain is highly variable and can contain groups aliphatic (alkyl, alkenyl, hydroxy alkenyl, ω-methyl alkyl, ω-sulphinyl and ω-sulphinyl alkyl), aromatic (benzyl, substituted benzyl) or heterocyclic (indolic groups), depending of the amino acid precursor (Holst and Williamson, 2004;Padilla et al., 2007).The presence of sulphate in the molecule confers properties strongly acid.In this way, the GLS are not volatile and occur as salt (Holst and Williamson, 2004).The hydrolysis of GLS occurs due to action of an endogenous thioglucosidase myrosinase when the plant tissues are disrupted during processing, chewing, and digestion, since this enzyme is located within the vacuoles of the plant matrix.
In Table 1 are presented the chemical structures of the main compounds 76 detected in Brassica family.Table 1 The extent of hydrolysis of glucosinolates and the nature and composition of the breakdown products formed are known to be influenced by various characteristics of the hydrolysis medium.Intrinsic factors such as coexisting myrosinase and its cofactors ascorbic acid, epithiospecifier protein (ESP), or ferrous ions (Bones and Rossiter, 1996) and extrinsic factors such as pH and temperature (Ludikhuyze et al., 2000) can affect the hydrolysis of glucosinolates, because these factors present influence on myrosinase activity and stability, leading to increase or decrease the efficiency of hydrolysis of glucosinolates.By this reason, processing, extraction and quantification methods are likely to influence the extent of glucosinolate hydrolysis and the ratio of the derivatives produced (Verkerk et al., 1997).Although the related variables influence the myrosinase activity, the literature is focused mainly on the effects on glucosinolase content.There is a well documented review in the literature (Cartea and Velasco, 2008) that reports the influence of environmental conditions, processing and storage on the glucosinolates content of Brassica sp. and their effects on human nutrition and health.Based on these aspects, the present work reports the influence of climate, genotype and seasonal variation in the glucosinolate-myrosinase system, effect of heat treatment and high hydrostatic pressure on the stability of myrosinase as well as in the GLS content, and additionally, this work reports the isolation and quantification of these compounds, focusing in the methods of isolation and quantification of glucosinolates from biological matrices.

Influence of climate and genotype to seasonal variation in the glucosinolate-myrosinase system
Cultivar, location, and growing conditions play important roles in the production of bioactive compounds in Brassica sp.(Rosa et al., 1997).The concentration and composition of GLS, phenolics, and vitamin C in Brassica sp. is genotype dependent (Sarikamis et al., 2009).Moreover, climatic factors such as temperature, irradiation, and water supply also have an important influence on the phytochemical content in Brassica sp.(Martinez-Villaluenga et al., 2009).According to Schmidt et al., (2010), GLS breakdown product levels are due to the combination of GLS content in the plant and myrosinase activity (Ludikhuyze et al., 2000).The activity of this enzyme depends on the genetic variation (Rask et al., 2000), on some intrinsic (metal ions, ascorbic acid, pH) and on some extrinsic factors (temperature) (Bones and Rossiter, 1996;Ludikhuyze et al., 2000).Therefore, cultivar selection should be tailored to specific environmental factors at each location to achieve optimization in phytochemical content of Brassica sp.In addition, selected white cabbages with an optimized bioactive compound content could be used as raw material for sauerkraut production, enhancing the human dietary intake in health promoting compounds.
Brassicas are economically important crops that show high intra-specific variation in morphological and chemical traits (Hanson et al., 2009).Previous studies have shown that concentrations and profiles of GLS show considerable variation within species and that they vary with environmental conditions and developmental stage (Potter et al., 2000;Rosa and Rodrigues, 2001;Castro et al., 2004;Poelman et al., 2008;Hanson et al., 2009).
Among the cultivated Brassicaceae, broccoli attracted attention after the discovery that it contains high levels of the isothiocyanate sulforaphane [1-isothiocyanate-(4R)-(methylsulfinyl) butane], and of other glucosinolate derivatives thought to have anticarcinogenic properties (Beecher, 1994;Zhang et al., 1992;Cover et al., 1998).The ability of sulforaphane or indole-3-carbinol to protect against tumorigenicity is dose and time dependent.Therefore, selection of cultivars accumulating high levels of isothiocyanates may be important.Based on the perceived beneficial effects, broccoli has received widespread attention as a medicinally significant food, its consumption being recommended throughout the year.To meet this requirement, and because it is a very perishable vegetable, producers tend to grow suitable cultivars under mild climatic conditions in spring and summer, principally for the fresh market, although some cultivars are more suited to freezing.
Variation in GLS has been of interest to ecologists and nutritional chemists alike.Ecological studies have investigated the effects of variation in GLS concentrations and profiles of Brassica oleracea on above ground plant-insect interactions.Both generalists and specialists herbivores can be influenced by GLS (Gols et al., 2007;Poelman et al., 2008).
Members of the Brassicaceae vary considerably in GLS quantity and composition (Rask et al., 2000;Fahey et al., 2002) and insect herbivores specialized on Brassicaceae differ in their performance on different members of this plant family (Fox et al., 1996;Ohsaki and Sato, 1999;Sznajder and Harvey, 2003).Differences in GLS quantity and composition between crucifer species may thus contribute to differences in herbivore performance.Plutella xylostella L. (Lepidoptera, Plutellidae) is a specialist herbivore that is known to feed on a number of species in the Brassicaceae.Adult females and larval stages use GLS as oviposition and feeding stimulants (Pivnick et al., 1994).However, an increase in GLS concentration does not always positively correlate with larval performance (Li et al., 2000).
Some GLS may have anticarcinogenic effects (Lund, 2003;Moreno et al., 2006) and nutritional studies have mainly focused in the aerial parts of the plants.Root GLS levels, although less studied in the context of human health, are important for resistance against soil pests and may be used for biofumigation (Smetanska et al., 2007).In aerial parts, GLS concentrations depend on a variety of factors including temperature, time of day, water content, and nutrient supply (Rosa et al., 1994;Rosa, 1997;Pereira et al., 2002;Gols et al., 2007).Cole (1980) observed, based on measurements of volatile myrosinase hydrolysis products, a substantial decline in levels of aliphatic glucosinolates over the first few days of development in seedlings of turnips, Chinese cabbage, fodder rape (B.campestris L), cauliflower (B.oleracea var botrytis) and radish (Raphanus sativus L).A thorough understanding of glucosinolate metabolism in plants requires intensive studies of their distribution between plant organs and changes during the various development stages.Major differences in the relative amount of individual GLS have been observed between the different parts of developing rapeseed plants McGregor (1988), indicating that them have defined distribution patterns.Furthermore, large differences between seed, leaf and root glucosinolate profiles of several brassicas have been described by Sang et al., (1984).Rosa et al., (1994) reported that under mild conditions, GLS in the leaves of young cabbage plants showed significant variation throughout a single day.Although the data suggested a rapid metabolism of GLS, the causes of this variation are not fully understood.Even under controlled greenhouse conditions, GLS concentrations in leaves of Brassica species fluctuate when sown over a time period of several months, which was ascribed to abiotic seasonal changes (Gols et al., 2007).Soil characteristics like pH influenced GLS concentrations in leaves of kale (Petersen et al., 2002;Velasco et al., 2007;Gerendas et al., 2008;Pongrac et al., 2008).Variation on the amount and pattern of GLS has been attributed to genetic and environmental factors, including plant age, temperature, water stress, and soil type (Fenwick et al., 1983;Rosa et al., 1997;Farnham et al., 2004).Distribution of the GLS varies depending on plant part, with both quantitative and qualitative differences among roots, leaves, stems, and seeds.Van Dam et al., (2009), in his review article reports that the roots usually have a higher concentration of glucosinolates compared with other parts of the plant.Agronomic factors, such as soil type, moisture, and mineral nutrient availability, are known to exert a significant effect on GLS content.Soil fertility has significant effects on levels of specific GLS in the growing plants (Rosa et al., 1997).Total and indolic GLS concentrations have been correlated with climatic factors in several crops of B. oleracea (Charron et al., 2005).Winter seasons seem to induce lower GLS levels due to short days and cool temperatures accompanied by frost (Rosa et al., 1997).
In addition, the profile and concentration of GLS are affected by developmental stage of the plant (Petersen et al., 2002) and by biotic interactions.Above ground plant-animal interactions can cause up regulation of specific GLS, depending on the tissues attacked and the identity of the herbivore (Textor and Gershenzon, 2009).In B. oleracea herbivory by the generalist Myzus persicae up regulates indolic GLS( Kim and Jander, 2007), whereas damage resulting from the specialist Pieris rapae also increases aliphatic GLS (Agrawal and Kurashige, 2003).There is especially little information on root GLS.

Effect of thermal treatment on the glucosinolate-myrosinase system of Brassica
Glucosinolate hydrolysis products, and in particular isothiocyanates and indoles, have received a special interest in food research because of their anticarcinogenic properties (Verhoeven et al., 1997;Wallig et al., 1998).Moreover, glucosinolates and their hydrolysis products are associated with important taste, aroma and flavour attributes in Brassica vegetables (Van Doorn et al., 1998;Drewnowski and Gomez-Carneros, 2000;Coogan et al., 2001).Like other vegetables, most Brassica vegetables are heat processed before consumption.This leads to myrosinase inactivation and hence stops the hydrolysis of GLS into beneficial breakdown products.GLS can be hydrolyzed by myrosinases existing in the human gut, but the production level of isothiocyanates is three times greater when GLS are hydrolyzed by plant myrosinase (Conaway et al., 2001).Controlling myrosinase activity during processing is, therefore, of particular interest.Earlier investigations showed different thermal stability of myrosinase based on Brassica source (Table 2).This table shows that broccoli myrosinase has the lowest thermal stability compared to other myrosinase sources, whereas it is the highest in the case of rapeseed myrosinase.However, Matusheski et al., (2004) found that heating fresh broccoli florets to 60 ºC prior to homogenization simultaneously increased sulphoraphane formation, which indicates a much higher thermal stability of broccoli myrosinase than what has been reported by Ludikhuyze et al., (1999).However, some of the differences could be due to the limitation of heat transfer to whole vegetable.Earlier work has indicated that the glucosinolate-myrosinase system is modified during the processing of Brassica vegetables due to partial or total inactivation of myrosinase, thermal breakdown of GLS and their hydrolysis products, loss of enzymatic cofactors, leaching of GLS and their derivatives into the cooking medium, or volatilization of the derivatives (Dekker et al., 2000).The extent of these losses probably depends on the duration and type of heat treatment, the degree of material disintegration, and the vegetable matrix itself (Rosa and Heaney, 1993).High hydrostatic pressure (HHP) is a non-thermal technology used in the food industry to inactivate microorganisms and spoilage enzymes without affecting the quality of fresh products (San Martin et al., 2002).Moreover, the application of pressure processing has been found to retard thermal inactivation (Hendrickx et al., 1998).HHP stability has been investigated for broccoli and mustard seed myrosinases (Ludikhuyze et al., 1999;Van Eylen et al., 2008).However, a very wide difference in pressure stability has been reported.Green cabbage is widely consumed as either cooked or processed products; however, no published data on the thermal and pressure inactivation of myrosinase from green cabbage are yet available in the literature.
Vegetables are primarily consumed in the cooked form and are processed by various techniques.Blanching is a short heat treatment that is typically applied to vegetables prior to further processing with the aim of enhancing both safety and quality attributes.Blanching imparts benefits, such as destruction of surface microflora of vegetables and enhancing the colour and texture and also the keeping quality of vegetable products.The quality of blanched product depends significantly on the time and temperature of blanching and also on the size of vegetable to be blanched.Under-blanching speeds up the activity of enzymes and is worse than no blanching.Over-blanching causes loss of texture, colour, phytochemicals and minerals.Industrial blanching processes involve temperatures ranging from 70 to 95 °C and times usually no higher than 10 min (Morales-Blancas et al., 2002) whereas for domestic purposes vegetables are generally blanched for 10-12 min in boiling water (98-100 °C).A considerable amount of research has been done to understand the effects of blanching on texture, colour, phytochemical content and antioxidante activity of different vegetables.Volden et al., (2008) showed the effects of blanching of red cabbage on the levels of glucosinolates, polyphenols and anthocyanins, as well as for the antioxidant potential by the ferric reducing ability power (FRAP) and oxygen radical absorbance capacity (ORAC) assays.Data on the effect of blanching on physicochemical properties of cabbage is scarce (Amin and Lee, 2005; Volden et al., 2008).

Isolation and quantification of glucosinolates from Brassica
Due to their physicochemical properties, the separation and the isolation of GLS is an extremely difficult task.The presence of the sulfate group and of the thioglucose moiety cause the octanol-water partition coefficient of GLS (log Po/w) to fall in the low value domain (−4.30 for glucoiberin to −1.38 for neoglucobrassicin), thus suggesting they are very hydrophilic and always water-soluble entities (Holst and Williamson, 2004).The characterization and quantification of GLS, either in pure state or within mixtures, is possible through the desulpho-glucosinolates technique (Wathelet et al., 1995;Kiddle et al., 2001).
Moreover, many ion pair chromatography based methods have been developed after the initial work published by Helboe et al., (1980) to purify GLS (Prestera et al., 1996;Toribio et al., 2007).They exploit the well-known property of alkyl-ammoniums (tetramethyl ammonium, tetraoctyl ammonium, tetradecylammonium, etc.) to form ion pairs with sulfate groups (Prestera et al., 1996).This is the strategy used by plants to transport GLS.Indeed, a GLS anion is sometimes associated with an aromatic choline ester cation, such as sinapine, in sinalbin from mustard or glucoraphanin from broccoli (Butzenlechner, 1996).Recently, Fahey et al., (2002) successfully resolved glucoraphanin and glucoiberin from crude plant homogenates using high speed counter-current chromatography (HSCCC) in the elution mode and a highly-salted and highly polar biphasic solvent system: 1-propanol/acetonitrile/saturated aqueous ammonium sulfate/water (1:0.5:1.2:1).This protocol was optimized and scaled up by Fisher et al., (2005) where 15 g of enriched glucoraphanin extract were injected in a MIDI-dynamic extraction centrifuge apparatus equipped with a 928mL column.
There are different methods of extraction and characterization of glucosinolates in Brassica family.As can be seen in Table 3, extraction with methanol is widely reported, but other methods such as decoction, headspace solid-phase microextraction are also employed.
Among the methods for detection and quantification, there is a wide variation, although liquid and gas chromatography coupled or not the mass detector predominated, since they allowed confirmation of the compounds analyzed.Regarding the isolated glucosinolates, each species has its particularity in relation to them, which can also vary within the same species due to climate change, seasonal variation, aspects that were discussed earlier.
Compounds that exhibit prominent, ie appear in a large number of species are sinigrin and sinalbina addition of sulfur compounds, allyl, benzyl isothiocyanates and dimethyl sulfide.Herzallah and Holley (2012), developed a method to reversed phase-high performance liquid chromatography method was to quantify sinigrin, sinalbin, allyl isothiocyanate and benzyl isothiocyanate present in aqueous and freeze-dried yellow and Oriental (brown) mustard extract samples using two pre-treatment methods (autoclaving, boiling) to prevent degradation by myrosinase.Toribio et al., (2007) purified the glucosinolates sinalbin and glucoraphanin by strong ion-exchange displacement centrifugal partition chromatography (SIXCPC).The optimized conditions involved the biphasic solvent system ethyl acetate/n-butanol/water, the lipophilic anion-exchanger.Amounts as high as 2.4 g of sinalbin and 2.6 g of glucoraphanin were obtained in starting from 12 and 25 g of mustard and broccoli seed aqueous extracts

CONCLUSIONS
From this review it was seen that the vegetables of the Brassica family are rich in glucosinolates, which have a valuable anticarcinogenic action.Glucosinate-myrosinase system may be influenced by factors such as climate, soil, genotype, with seasonal variation.Therefore, cultivar selection should be tailored to specific environmental factors at each location to achieve optimization in phytochemical content of Brassica sp.In addition, selected white cabbages with an optimized bioactive compound content could be used as raw material for sauerkraut production, enhancing the human dietary intake in health promoting compounds.
The study of the effects of processing on the concentrations of GLS and the parameters related to their hydrolysis in Brassica vegetables has a pivotal role in complementing research on the epidemiology of the consumption of Brassica vegetables and chemoprevention.An understanding of the physical and biochemical changes occurring before the ingestion of processed Brassica vegetables may help to interpret the metabolic fate of GLS in experimental studies in animals and humans and inform the subsequent formulation of dietary strategies to optimize the uptake of isothiocyanates in vivo.Hydrolysis of vegetables at high temperatures leads to destruction of myrosinase, an enzyme responsible for converting glucosinate in active substances, as well as treatment at high pressures, which is used in the food industry leads to destruction of this enzyme.Therefore, when one wishes to obtain constant characteristics in a plant of the Brassica family all these aspects should be taking in account.

Table . 1
: Chemical structure of some glucosinolates.

Table . 2
: Thermal stability of myrosinase from different Brassica sources.

Table . 3
: List of glucosinolates identified in Brassica family, extraction methods and characterization.