Differences in Thermal Stability of Glucosinolates in Five Brassica Vegetables
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Czech J. Food Sci. Vol. 27, 2009, Special Issue Differences in Thermal Stability of Glucosinolates in Five Brassica Vegetables M. DEKKER*, K. HENNIG and R. VERKERK Wageningen University, Product Design and Quality Management Group, 6700 EV Wageningen, The Netherlands, *E-mail: [email protected] Abstract: The thermal stability of individual glucosinolates within five different Brassica vegetables was studied at 100°C for different incubation times up to 120 minutes. Three vegetables that were used in this study were Brassica oleracea (red cabbage, broccoli and Brussels sprouts) and two were Brassica rapa (pak choi and Chinese cabbage). To rule out the influence of enzymatic breakdown, myrosinase was inactivated prior to the thermal treatments. The stability of three glucosinolates that occurred in all five vegetables (gluconapin, glucobrassicin and 4-methoxyglucobrassicin) varied considerably between the different vegetables. The degradation could be modeled by first order kinetics. The rate constants obtained varied between four to twenty fold between the five vegetables. Brussels sprouts showed the highest degradation rates for all three glucosinolates. The two indole glucosinolates were most stable in red cabbage, while gluconapin was most stable in broccoli. These results indicate the possibilities for plant breeding to select for cultivars in which glucosinolates are more stable during processing. Keywords: food matrix; phytochemicals; processing; degradation; broccoli; red cabbage; Brussels sprouts; pak choi; Chinese cabbage INTRODUCtiON kerk et al. 1997, 2001; Verkerk & Dekker 2004; Rungapamestry et al. 2006, 2007; Volden et al. Glucosinolates are a group of plant secondary 2008), while the effects of industrial processes as metabolites. In the human diet they are mainly freesing, fermenting and canning are less studied found in Brassicaceae vegetables. Over 120 differ- (Tolonen et al. 2002; Dekker & Verkerk 2003; ent glucosinolates have been identified to this date. Oerlemans et al. 2006). During thermal process- Glucosinolates and their breakdown products are ing of Brassica vegetables, glucosinolate levels can of particular interest in food research because of be reduced because of several mechanisms: enzy- their proposed anticarcinogenic properties. There matic breakdown, thermal breakdown and leaching are clear indications that they block tumour initia- into the heating medium. Thermal degradation of tion by modulating the activities of Phase I and glucosinolates in red cabbage has been studied by Phase II biotransformation enzymes and suppress Oerlemans et al. (2006). Differences in stability tumours by apoptosis (Mithen et al. 2000; Ver- were observed between the different types of glu- kerk et al. 2009). Many steps in the food produc- cosinolates (aliphatic, indoles, aromatic). In this tion chain, such as cultivation, storage, processing study we investigate the differences in thermal and preparation of vegetables, may have an impact stability of three glucosinolates that occur in five on levels and thus intake of phytochemicals (De Vos different Brassica vegetables upon incubation at & Blijleven 1988; Dekker et al. 2000; Dekker 100°C. From the experimental data the kinetic & Verkerk 2003). Domestic treatments of the degradation rate constants are estimated. The Brassica vegetables such as chopping, cooking, results indicate the possibilities for plant breeding steaming and microwaving have been shown to to select for cultivars in which glucosinolates are affect the glucosinolate content considerably (Ver- more stable during processing. S85 Vol. 27, 2009, Special Issue Czech J. Food Sci. MatERiaL AND METHOds In Figure 1 the concentration of the glucosi- nolates is given for all five vegetables during the Five different Brassica vegetables were purchased incubation at 100°C. Large differences in stability from a local supermarket: Brassica oleracea (red between the glucosinolate are observed as was cabbage, broccoli and Brussels sprouts) and Brassi- reported by Oerlemans et al. (2006). ca rapa (pak choi and Chinese cabbage). Edible Surprisingly also large differences are observed parts were taken and vegetables were microwave in the stability of the same glucosinolate in differ- treated (300 g, 5 min, 900 W) to inactivate myrosi- ent vegetables. Brussels sprouts show the lowest nase. After this the samples were cooled, blended stability for all three glucosinolates. The two indole in liquid nitrogen to a fine powder and stored at glucosinolates (glucobrassicin and 4-methoxyglu- –20°C until thermal treatments (Oerlemans et cobrassicin) were most stable in red cabbage, while al. 2006). gluconapin was most stable in broccoli. The incubation at 100°C was done with samples The degradation could be modeled by first order of 5 g frozen blended vegetable in a heating block. kinetics. The estimated degradation rate constants After the incubation time the samples were cooled (kd) are given in Table 2. The order of stability on ice (Oerlemans et al. 2006). Glucosinolate for the glucosinolates was identical for all five analysis was done by HPLC according to Verkerk vegetables: gluconapin > glucobrassicin (indole) et al. (2001). > 4-methoxyglucobrassicin (indole), this was also Kinetic parameters were obtained by analysing reported for red cabbage by Oerlemans et al. the breakdown data with a first order degradation (2006). –kdt model: Ct/C0 = e using Microsoft Excel. Large differences between the vegetables are also observed for the degradation rate constants. Gluconapin is twenty fold more stable in broccoli RESULts AND DisCUssiON compared to Brussels sprouts. Glucobrassicin is six fold more stable in red cabbage compared The five different vegetables contain each be- to Brussels sprouts. 4-Methoxyglucobrassicin is tween seven and nine different glucosinolates. four fold more stable in red cabbage compared to Three glucosinolates occur in all five vegetables: Brussels sprouts. gluconapin, glucobrassicin and 4-methoxygluco- In this research the stability of the glucosinolates brassicin. These three glucosinolates have been was studied in an environment close to their natu- analysed for their thermal stability. In Table 1 the ral environment (the cellular structure has been glucosinolate content of the vegetables before the disrupted and enzymes have been denatured). Ap- thermal treatment is given. parently the environment (food matrix) in which Table 1. Initial content of the three glucosinolate in the five different Brassica vegetables Initial content (µmol/100 g FW) red cabbage broccoli Brussels sprouts pak choi Chinese cabbage Gluconapin 28.7 16.4 37.5 12.1 8.5 Glucobrassicin 13.0 28.3 78.0 2.6 3.5 4-Methoxyglucobrassicin 1.6 3.6 14.6 1.7 11.3 Table 2. First order degradation rates of the three glucosinolate in the five different Brassica vegetables at 100°C –2 –1 kd × 10 (min ) red cabbage broccoli Brussels sprouts pak choi Chinese cabbage Gluconapin 0.2 0.1 2.1 0.3 1.9 Glucobrassicin 0.8 1.5 4.9 2.5 2.7 4-Methoxyglucobrassicin 1.7 5.0 6.8 3.3 2.9 S86 100 80 (%) Czechȱ 100 J. Food Sci. Vol. 27, 2009, Special Issue 60 80 40 (%) ȱ (a)Gluconapin the observed differences in cellular environment 10060 20 that determine the degradation rates. This would indicate possibilities for plant breeding to select 4080 Gluconapin 0 (%) ȱ for cultivars in which glucosinolates are more 20600 20 40 60 80 100 120 stable during processing. Further research should Timeȱ(min) investigate these possibilities in more detail. 400 Gluconapin 0 20 40 60 80 100 120 20 Timeȱ(min) Reference s 100 0 Dekker M., Verkerk R. (2003): Dealing with Variability 80 0 20 40 60 80 100 120 (%) ȱ in Food Production Chains: A Tool to Enhance the (b) Timeȱ(min) 60100 Sensitivity of Epidemiological Studies on Phytochemi- cals. European Journal of Nutrition, 42: 67–72. 4080 Dekker M., Verkerk R., Jongen W.M.F. (2000): Predic- (%) ȱ Glucobrassicin tive modelling of health aspects in the food produc- 2010060 tion chain: A case study on glucosinolates in cabbage. 04080 Trends in Food Science & Technology, 11: 174–181. (%) ȱ Mithen R.F., Dekker M., Verkerk R., Rabot S., John- Glucobrassicin 0 20 40 60 80 100 120 2060 son I.T. (2000): The nutritional significance, biosyn- Timeȱ(min) thesis and bioavailability of glucosinolates in human 40 0 foods. Journal of the Science of Food and Agriculture, Glucobrassicin 0 20 40 60 80 100 120 20 80: 967–984. (c)) Oerlemans K., Barrett D.M., Bosch Suades C., 100 Timeȱ(min) 1ȬRedȱcabbage ) 0 Verkerk R., Dekker M. (2006): Thermal degradation (% 2ȬBroccoli ȱ 80 0 20 40 60 803ȬBrussels 100ȱsprouts 120 of glucosinolates in red cabbage. Food Chemistry, 95: 19–29. Timeȱ(min) 4ȬPakȱchoi 60100 1ȬRedȱcabbage Rungapamestry V., Duncan A.J., Fuller Z., Rat- ) 5ȬChineseȱcabbage cliffe B. (2006): Changes in glucosinolate con- (% 2ȬBroccoli ȱ 4080 3ȬBrusselsȱsprouts centrations, myrosinase activity, and production of 41ȬȬPakRedȱȱchoicabbage metabolites of glucosinolates in cabbage (Brassica Methoxyglucobrassicin 100 Ȭ ) 20 4 60 5ȬChineseȱcabbage oleracea var. capitata) cooked for different dura- (% 2ȬBroccoli ȱ tions. Journal of Agricultural and Food Chemistry, 080 3ȬBrusselsȱsprouts 40 54: 7628–7634. 0 20 40 60 80 1004ȬPakȱ 120choi 60 Rungapamestry V., Duncan A.J., Fuller Z., Rat- Methoxyglucobrassicin Ȭ 20 Timeȱ(min) 5ȬChineseȱcabbage 4 cliffe B. (2007): Effect of cooking brassica vegetables on the subsequent hydrolysis and metabolic fate of Figure40 1. Glucosinolate content during incubation at 100°C 0 glucosinolates. Proceedings of the