Cooking method significantly effects glucosinolate content and sulforaphane production in broccoli florets

Food Chemistry 123 (2010) 237–242

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Food Chemistry

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Cooking method significantly effects glucosinolate content and sulforaphaneproduction in broccoli florets

R.B. Jones *, C.L. Frisina, S. Winkler, M. Imsic, R.B. TomkinsKnoxfield Centre, Department of Primary Industries, Victoria, Private Bag 15 Ferntree Gully Delivery Centre VIC 3156, Australia

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 September 2009Received in revised form 23 February 2010Accepted 14 April 2010

Keywords:BrassicaGlucosinolateIsothiocyanateCooking

0308-8146/$ - see front matter Crown Copyright � 2doi:10.1016/j.foodchem.2010.04.016

* Corresponding author. Tel.: +613 9210 9222; fax:E-mail address: [email protected] (R.B. Jone

It is known that glucosinolate levels in Brassica vegetables can be affected during cooking but little is doc-umented about the effect of cooking on isothiocyanate production. In this study, three cooking methodswere evaluated for their effects on the contents in broccoli florets of the glucosinolates, glucoraphanin(GR), glucobrassicin (GB), neoglucobrassicin and progoitrin, as well as on sulforaphane (SF) and sulfora-phane nitrile (SFN) production in broccoli florets. Two broccoli cultivars, ‘Marathon’ and ‘Booster™’, wereanalysed raw and after they were steamed, microwaved (with water) or boiled for 2 or 5 min. Residualcooking water from all treatments was collected and analysed for GR and GB to determine the extentof leaching of intact glucosinolates. Irrespective of time, steaming resulted in significantly greater reten-tion of GR, GB and SF, while boiling and microwave cooking resulted in significant losses of GR, GB and SFin both varieties. Glucosinolate content in the residual cooking water was highest after boiling and micr-owaving. Loss of SF production was primarily due to both leaching of GR into cooking water and thermalinhibition of ESP and myrosinase once internal floret temperatures exceeded 70 �C. Cooking method cansignificantly alter content of potentially beneficial compounds in broccoli florets and optimal SF ingestionmay be obtained by eating raw or lightly steamed broccoli florets.

Crown Copyright � 2010 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Broccoli florets contain comparatively high levels of glucosino-lates, particularly glucoraphanin, which are secondary plantmetabolites containing sulphur and nitrogen, commonly found incruciferous vegetables (Kushad et al., 1999; Rosa, Heaney, Fenwick,& Portas, 1997). Tissue disruption, such as that caused by chewingor cutting, allows glucosinolates to come into contact withmyrosinase (b-thioglucoside glucohydrolase EC 3.2.3.1) whichcauses rapid hydrolysis to form glucose and a range of intermedi-ates, such as isothiocyanates, thiocyanates and nitriles (Halkier &Gershonzen, 2006). Depending on conditions, glucoraphanin (GR)forms the isothiocyanate sulforaphane (1-isothiocyanato-4-meth-ylsulfinylbutane; SF) or sulforaphane nitrile (SFN) upon hydrolysis(Zhang, Talalay, Cho, & Posner, 1992).

At this stage of our understanding, the most bioactive isothio-cyanates found in broccoli are SF, allyl isothiocyanate (derivedfrom sinigrin) and indole-3-carbinol (derived from glucobrassicin).Isothiocyanates have antibacterial and antifungal activity in plantaand provide important protection from insect and herbivore attack(Rosa et al., 1997). In vitro studies have demonstrated that, whenconsumed by humans, isothiocyanates, particularly SF, inhibit both

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Phase I enzymes, responsible for the activation of carcinogens andinduce Phase II detoxification enzyme systems, thereby increasingthe body’s cancer defence mechanisms (Munday & Munday, 2004;Talalay, Fahey, Holtzclaw, Prestera, & Zhang, 1995; Zhang et al.,1992). SF has also recently been demonstrated to interfere with awide range of cancer processes, including the inhibition of cancercell proliferation and induction of apoptosis, the arrest of cancer-ous cell cycle progression, angiogenesis and the inflammationpathway (Juge, Mithen, & Traka, 2007). In addition, SF inhibitsHelicobacter pylori, the bacteria responsible for stomach ulcers(Fahey et al., 2002). SF, derived from broccoli sprouts, has also beenlinked to prevention of cardiovascular disease in an animal modelstudy, using rats (Wu et al., 2004).

The postharvest process that has arguably the most effect onglucosinolate and other phytochemical contents is cooking. Gluco-sinolates are primarily lost from broccoli tissue through leachinginto the cooking water but the rate and extent of loss dependson the amount of water, the type of cooking used and the cookingtime (Jones, Faragher, & Winkler, 2006). Several studies haveshown that microwaving and boiling are the cooking methods thatcause the largest losses of glucosinolates from broccoli (Conawayet al., 2000; Dekker, Verkerk, & Jongen, 2000; Howard, Jeffery,Wallig, & Klein, 1997; Vallejo, Tomas-Barberan, & Garcia-Viguera,2002). In comparison, steaming appeared to cause the smallest lossof glucosinolates, although the degree of loss varied (Conaway

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238 R.B. Jones et al. / Food Chemistry 123 (2010) 237–242

et al., 2000; Howard et al., 1997; Rosa et al., 1997). Along withmany other Brassica vegetables, there is a significant variety-dependent variation in the decline in glucosinolates after cookingin broccoli (Rosa et al., 1997).

Cooking may also be an important factor in determiningwhether isothiocyanate production dominates over nitrile produc-tion. When the florets of a number of broccoli cultivars where mac-erated raw at room temperature, sulforaphane nitrile (SFN) wasthe principal breakdown product of glucoraphanin, due to the ac-tion of the epithiospecifier protein (ESP) (Howard et al., 1997;Matusheski & Jeffery, 2001; Matusheski, Juvik, & Jeffery, 2004;Mithen et al., 2003). SF production increased, however, after broc-coli was heated to 60 �C for 5 or 10 min, as temperatures of 50–60 �C were sufficient to inactivate ESP activity (Matusheski et al.,2004). Effect of genotype is also important in determining theamount of SF produced. The SF to SFN ratio in tissue maceratedat room temperature varied widely between broccoli varietiesand appeared to be genetically determined (Matusheski & Jeffery,2001; Matusheski et al., 2004; Mithen et al., 2003).

Most studies investigating the effect of cooking on broccoli haveconcentrated on glucosinolate content but it is the isothiocyanates,particularly SF, that are biologically active in humans. Howardet al. (1997) reported that steaming broccoli for 2 min significantlyinhibited SF production but other cooking methods were not testedin this study. Matusheski et al. (2006) showed that denaturing ESPwith mild heat treatments can significantly increase SF productionin several broccoli cultivars but the heating method used did notmimic common household cooking techniques, such as microwa-ving or boiling. A more recent study by Rungapamestry, Duncan,Fuller, and Ratcliffe (2007) showed that absorption of SF in humans(measured as the mercaptic acid metabolite in the urine) wasapproximately three times higher after consumption of broccolimicrowaved for 2 min than for 5 min, indicating that the longercooking time may have inhibited SF production. Vermeulen, Klop-ping-Ketelaars, van den Berg, and Vaes (2008) demonstrated thateating raw broccoli resulted in significantly higher SF in the bloodand urine than eating broccoli microwaved for an unspecified time.No SF production was evident in ‘Marathon’ florets boiled orsteamed for 15 and 23 min, respectively but pressure cooking fora much shorter time (2 min) resulted in only a 17% loss (Galgano,Favati, Caruso, Pietrafesa, & Natella, 2007). In the light of thesestudies, we have investigated the effects of three common domes-tic cooking techniques, steaming, microwaving and boiling, for 2 or5 min, on glucosinolate content and SF production in two broccolicultivars.

2. Materials and methods

2.1. Plant material

Broccoli cvs ‘Marathon’ and ‘Booster™’ seedlings were trans-planted 6 weeks after germination in early March, 2005 and2006. Heads were harvested at commercial maturity approxi-mately 70 days after transplanting in mid May. Crops were grownusing commercial fertiliser and irrigation protocols at Temple-stowe, Victoria, Australia (approx. Lat. �37.7; Lon. 145.05). Headswere transported back to the laboratory in an air-conditioned vehi-cle within 1 h of harvest and immediately placed at 4 �C under highhumidity conditions where they were stored overnight.

2.2. Cooking treatments

Cooking trials were conducted in 2005 and 2006. Data pre-sented are the combined means of both years. Broccoli heads wereprocessed into florets on the day after harvest. Five outer florets

were cut from 20 heads of broccoli and combined to form one rep-licate of 100 florets (n = 4). Ten florets were then randomly se-lected for each treatment, with each treatment replicated fourtimes. The cooking methods used were: microwaving (LG Intello-wave sensor model no. MS-324SCE at full power = 1100 W), steam-ing and boiling. In addition, an uncooked sample was collected foranalysis. For each method, the cooking times were 2 or 5 min. Thevolume of water used in microwaving and steaming was equal tothat of the floret FW (approx. 150 ml) and for boiling it was twicethat of the floret FW (approx. 300 ml). Cooking water was boilingat the commencement of the cooking time in both the boilingand steaming treatments. At the end of each cooking treatment,broccoli florets were immediately immersed in liquid nitrogen toprevent any further cooking. They were then placed in storage at�20 �C prior to analysis. The residual cooking water was also col-lected immediately after cooking and frozen at �20 �C.

Internal floret temperatures were measured during steamingand boiling by inserting the probe of a Tinytag Ultra 2 K tempera-ture data logger into the stem during cooking and recording tem-peratures every 10 s. The probe was inserted as far as practical(approx. 3–4 cm), so as not to pierce completely through the floret.The temperature of microwave-cooked broccoli florets was mea-sured, using a handheld digital temperature probe. The microwa-ving was stopped every 30 s and the temperature measuredimmediately. A new batch of florets was cooked for each time mea-surement (n = 5).

2.3. Phytochemical analysis

2.3.1. GeneralFrozen samples were freeze-dried and ground to a fine powder.

Powdered material was stored in airtight containers at room tem-perature prior to analysis. No loss of glucosinolate was observedunder these conditions (results not shown).

2.3.2. Glucosinolate analysisGlucosinolates were separated on a Prevail C-18 column (All-

tech Associates), as described by West, Tsui, and Haas (2002), usinga binary gradient of 50 mM ammonium acetate (mobile phase A)and 20% methanol in 50 mM ammonium acetate (v/v) (mobilephase B) at a flow rate of 1 ml/min. Individual glucosinolates werecharacterised using LCMS as by Rochfort, Trenerry, Imsic, Panozzo,and Jones (2008). Powdered broccoli (ca. 800 mg) was heated in awater bath set at 100 �C in capped polypropylene Oakridge centri-fuge tubes. 20 ml of boiling microfiltered deionized water wereadded and the tubes heated for a further 10 min. Samples werecentrifuged (20,000g, 10 min) and the supernatants collected. Pel-lets were resuspended in 20 ml of microfiltered deionized water,centrifuged (20,000g, 10 min), the supernatants combined andthe volume adjusted to 50 ml. Samples were filtered (0.45 lm,regenerated cellulose) prior to analysis. HPLC analysis was per-formed on an HPLC (GBC, Australia) equipped with an LC5100UV–vis diode array detector. Gradient was maintained at 0% B for10 min, then increased to 40% B over the next 35 min, increasedto 100% B over 5 min, maintained at 100% B for 10 min before re-equilibrating the column with 100% A for 15 min.

2.3.3. Endogenous sulforaphane and sulforaphane nitrile productionSF and SFN were analysed using LCMS as by Agrawal et al.

(2006): 15 ml of 0.1 M HCl were added to 1 g of freeze-dried broc-coli powder, mixed and left to stand at room temperature over-night. The mixture was extracted (with gentle shaking), threetimes, with 30 ml of dichloromethane. The dichloromethane solu-tions were combined and reduced to near dryness (1 ml) with a ro-tary evaporator on full vacuum, the remaining 1 ml was reduced todryness with a reduced vacuum. The residue was reconstituted in

Table 2GB contents (lmol/g DW) of raw and cooked ‘Booster™’ and ‘Marathon’ broccoliflorets. Cooked florets were steamed, microwaved (MW) or boiled for 2 or 5 min.

Cooking method Cultivar

Booster™ Marathon

Raw 4.07a 3.63a

Steam 2 min 4.50a 4.33a


Microwave 2 min 4.33a 4.31a


Boil 2 min 3.14b 3.54a

Boil 5 min 2.90b 2.99b

lsd (P = 0.05) 0.74 0.54

Means with different superscripts within the same column are significantlydifferent at P < 0.05.

R.B. Jones et al. / Food Chemistry 123 (2010) 237–242 239

5 ml of 5% acetonitrile/95% water, with vortexing; the solution wasfiltered and assayed by LC–tandem mass spectrometry. For LC–MS/MS analysis, an Agilent 1100 series HPLC (Waldbronn, Germany),equipped with a quaternary gradient pump, autosampler withsample cooler (maintained at 4 �C), and diode array detector, wascoupled with a Thermo Electron LTQ ion trap mass spectrometer(Waltham, MA, USA). Five microlitres of extract were injected ontoa 150 � 2.1 mm, i.d. 3 lm, Thermo BDS Hypersil C-18 column(Waltham, MA). The mobile phase consisted of three components,A (distilled water), B (acetonitrile) and C (10% aqueous formicacid), and followed the gradients at a flow rate of 0.2 ml/min withC maintained at 1% throughout: gradient 1, 0–5 min, 95% A; 5–40 min, 30% A; 45–46 min, 95% A; 46–50 min (0.3 ml/min), 95%A; gradient 2, 0–15 min, 91% A; 15–27 min, 87% A; 27–35 min,54% A; 35–53 min, 9% A; 53–54 min, 91% A; 54–58 min (0.3 ml/min), 91% A.

2.3.4. ChemicalsGlucoraphanin standard was purchased from www.glucosino-

lates.com. All other chemicals used were of analytical grade andpurchased from either Crown Scientific P/L (Australia) or LombScientific P/L (Australia).

3. Results and discussion

In this study, the three most abundant glucosinolates found inthe broccoli cultivars ‘Booster™’ and ‘Marathon’ were GR, GB andneoglucobrassicin (data not shown). This agrees with Kushadet al. (1999) who listed the most common glucosinolates foundin broccoli as GR, the indoles GB and neoglucobrassicin, sinigrin,progoitrin, and gluconapin. Content varied significantly betweencultivars, with ‘Booster™’ having higher levels of GR and GB than‘Marathon’ (see values for raw florets in Tables 1 and 2). HighestGB content was also found in ‘Booster™’ (Table 2).

Boiling and microwave cooking significantly reduced both GRand GB contents within broccoli florets, regardless of cooking time,and the effect varied between cultivars (Tables 1 and 2). However,steaming for 2 or 5 min had no significant effect on GR contentcompared with raw tissue, regardless of variety (Table 1). GR, analiphatic glucosinolate, was significantly reduced in ‘Booster™’and ‘Marathon’ by boiling for either 2 or 5 min, or by cooking ina microwave oven for 5 min (Table 1). Similarly, the indole gluco-sinolate, GB, was also not affected by steaming time but was signif-icantly reduced by boiling for 2 or 5 min in ‘Booster™’ and by5 min boiling in ‘Marathon’, compared with raw florets (Table 2).Microwave cooking, however, did not significantly affect GB con-tent in either variety.

Neoglucobrassicin and progoitrin levels were not affected bycooking method in either of the cultivars studied (data not shown).

Table 1GR content (lmol/g DW) of raw and cooked ‘Booster™’ and ‘Marathon’ broccoliflorets. Cooked florets were steamed, microwaved (MW) or boiled for 2 or 5 min.


Booster™ Marathon

Raw 28.6a 13.1a


Steam 5 min 28.6a 12.3a,b


Microwave 5 min 23.9b 11.1b

Boil 2 min 23.8b 11.2b

Boil 5 min 20.1c 9.96d

lsd (P = 0.05) 2.54 1.17


Cooking has been widely reported to cause a decline in gluco-sinolates within broccoli tissue but results have been variable. Ingeneral, microwaving and boiling resulted in the largest losses ofglucosinolates in broccoli (Jones et al., 2006). Steaming, on theother hand, appeared to minimise the loss of glucosinolates,although the degree of loss varied (Conaway et al., 2000; Rosaet al., 1997). For example, microwave cooking (1000 W for 5 min)caused a significant reduction in total glucosinolates and an 80%decline in GR and GB contained in ‘Marathon’ (Vallejo et al.,2002). Our results indicated a 15% reduction of GR in ‘Marathon’,and a 17% decline of GR content in ‘Booster™’ after 5 min of micr-owaving at 1100 W but no significant change was recorded in GBcontent after microwave cooking (Table 2). Boiling is widely re-ported to result in significant declines in glucosinolates. Sones,Heaney, and Fenwick (1984) found that the total glucosinolatecontent of broccoli boiled for 10 min was approximately 40% lessthan that for fresh broccoli. Vallejo et al. (2002) reported 36%and 84% declines in GR and GB content, respectively, in ‘Marathon’florets, while Rosa and Heaney (1993) reported losses exceeding50%. Glucosinolate losses during boiling increased when cookingtime and water volume increased (Dekker et al., 2000; Joneset al., 2006), indicating that leaching of glucosinolates into thecooking water is a major cause of loss. In the present study, highestlosses of both GR and GB of between 18% and 30% (depending onvariety) were reported after the longest boiling time of 5 min (Ta-bles 1 and 2).

Steaming for 2 or 5 min resulted in the greatest retention ofglucosinolates, with no significant loss of GR or GB compared withraw florets (Tables 1 and 2). Previous studies have also found thisto be the case. Steaming for up to 20 min had no significant effecton glucosinolate content in cabbage (Rungapamestry, Duncan, Ful-ler, & Ratcliffe, 2006) or broccoli (Song & Thornalley, 2007). Good-rich, Anderson, and Stoewsand (1989), however, reported asignificant decline in GR and GB after steaming for 4 to 5 minand Vallejo et al. (2002) reported no loss in GR but a 41% loss ofGB after steaming for 3.5 min.

GR content increased significantly in ‘Booster™’ cooking waterafter all cooking methods, with the highest content being foundin water after microwaving or boiling for 5 min (Table 3). Steamingdid not result in a significant increase in GR in water after cooking‘Marathon’ florets. However, boiling or microwaving for 2 or 5 minresulted in significantly higher GR contents in cooking water inboth cultivars compared to steaming (Table 3). Leaching of gluco-sinolates into the cooking water is considered to be the mainway that glucosinolates are lost from tissue (Conaway et al.,2000; Vallejo et al., 2002). Boiling appears to result in more effec-tive leaching of glucosinolates as water volumes used tend to begreater and the vigorous action of boiling water could result in amore effective leaching action.

http://www.glucosinolates.com

http://www.glucosinolates.com

Table 3GR (lmol/ml) in cooking water before (raw) and after ‘Booster™’ and ‘Marathon’broccoli florets were steamed, microwaved or boiled for 2 or 5 min.


Booster™ Marathon

Water prior to cooking 0.000a 0.000a

Steam 2 min 0.193b 0.096a


Microwave 2 min 0.648c 0.235b

Microwave 5 min 1.510d 0.584c

Boil 2 min 0.879c 0.364d

Boil 5 min 1.450d 0.596c

lsd (P = 0.05) 0.182 0.120


Table 4Sulforaphane production (mg/kg DW) in raw and cooked ‘Booster™’ and ‘Marathon’broccoli florets. Cooked florets were steamed, microwaved or boiled for 2 or 5 min.


Booster™ Marathon

Raw 51.9a 19.5a


Steam 5 min 5.0c 0.2b



Boil 2 min 0.4c 1.0b

Boil 5 min 0.7c 0.2b

lsd (P = 0.05) 18.7 9.25


Table 5Sulforaphane nitrile production (mg/kg DW) in raw and cooked ‘Booster™’ and‘Marathon’ broccoli florets. Cooked florets were steamed, microwaved or boiled for 2or 5 min.


Booster™ Marathon

Not cooked 149a 76.2a

Steam 2 min 60.9b 49.5b

Steam 5 min 48.3b 32.7b

Microwave 2 min 28.8b 19.3c

Microwave 5 min 91.8b 39.5b

Boil 2 min 23.9b 17.5c

Boil 5 min 51.6b 19.0c

lsd (P = 0.05) 41.4 17.7



Endogenous SF production was significantly higher in raw‘Booster™’ than in ‘Marathon’ (Table 4) as was sulforaphane nitrileproduction (SFN; Table 5). All cooking methods, however, signifi-cantly reduced endogenous SF and SFN production in both culti-vars. Steaming for 2 min resulted in a loss of SF production ofapproximately 50% in ‘Booster™’ and 17% in ‘Marathon’ (Table 4),while microwaving or boiling (for either 2 or 5 min) resulted inonly residual SF production in the florets of both cultivars. In ourstudy, SF production in ‘Booster™’ florets steamed for 2 min wasapproximately five times higher than in florets steamed for 5 minand >25 times higher than in microwaved or boiled florets. Simi-larly, ‘Marathon’ florets subjected to cooking by steaming, micro-wave or boiling for 5 min, all displayed very low SF production.SFN production also declined significantly in floret tissue after allthree cooking methods but the degree of loss was not as markedafter microwave cooking or boiling as was seen with SF (Table 5).Howard et al. (1997) reported a decline in SF production in broccoli‘Arcadia’ of 65% after 2 min of steaming, similar to our result in‘Booster™’ (Table 4). Similarly, Rungapamestry et al. (2007) re-ported that SF uptake in humans (measured as SF mercaptic acidsin the urine) was three times higher after subjects consumed broc-coli microwaved for 2 min than for 5.5 min, indicating a higher SFproduction in lightly cooked broccoli. Cooking times >5 min re-sulted in complete loss of SF in ‘Marathon’ when boiled, steamedor microwaved (Galgano et al., 2007). These studies, and our data,demonstrated that cooking times must be kept to 2 min or less ifconsumers wish to maximise SF ingestion in these broccolivarieties.

Internal floret temperature was measured in ‘Booster™’ floretsduring steaming, microwaving and boiling for up to 5 min(Fig. 1). Internal temperatures rose significantly faster duringmicrowave cooking, with florets reaching 70 �C after 50 s, com-pared to 110 s during boiling and 150 s during steaming (Fig. 1).

These results indicate that florets, that were microwave-cookedfor 2 min, reached a temperature high enough to inhibit SF produc-tion (i.e. >70 �C according to Matusheski et al., 2004) for approxi-mately 58% of the cooking time, while florets boiled for 2 minwould exceed 70 �C for only 8% of the cooking time. Steamed flo-rets, on the other hand, would not exceed 70 �C at all if cookedfor only 2 min as it took 150 s to reach this temperature (Fig. 1).Matusheski et al. (2004) demonstrated that a heat treatment of5 min at 60 �C maximised SF production in broccoli florets but pro-duction dramatically declined at temperatures of 70 �C or above.Internal tissue temperatures, however, were not measured. Thiscould explain, in part, the low SF production in florets cooked bymicrowave or boiling, as internal temperatures were above thethreshold (70 �C) where SF production was inhibited for a signifi-cant percentage of the total cooking time (Fig. 1). While it is likelythat ESP was inactivated during microwaving and boiling, and SFproduction was depressed, it is unlikely that this was the solecause of the dramatic decline in SF production with these cookingmethods. More likely is that microwaving and boiling also partiallyor fully inactivated myrosinase, depending on the treatment time,and this, combined with glucosinolate leakage into cooking water,resulted in almost complete inhibition of SF production in micro-waved and boiled florets. Tissue temperatures in florets steamedfor 2 min, however, did not exceed 60 �C (Fig. 1) and myrosinaseactivity could have remained intact, yielding SF production similarto that of uncooked samples (Table 4). While SF production in flo-rets steamed for 2 min was significantly higher, in both varieties,than in those boiled or microwaved, there was also a significant de-cline compared to uncooked florets, most likely due to glucosino-late leaching.

Maximum temperature (95.4 �C) was reached within 120 s dur-ing microwave cooking, while steamed florets and boiled floretsreached the same maximum temperature of 96.8 �C after 240 s(Fig. 1). The maximum temperatures reached within the floret tis-sues are not considered high enough to cause thermal degradationof glucosinolates. Oerlemans, Barrett, Bosch-Suades, Verkerk, andDekker (2006) calculated that, while thermal degradation can oc-cur in cabbage at temperature exceeding 80 �C, very little actuallyoccurs below 110 �C. Indole glucosinolates were shown to degradeat lower temperatures, so it is possible that some of the GB lossesafter 5 min of cooking could be caused by thermal degradation.However, it is considered unlikely that thermal degradation is amajor cause of glucosinolate decline when Brassicas are cookedfor less than 10 min (Rosa et al., 1997).

As SF had a far more potent effect on human Phase I and II en-zymes than had SFN (Matusheski & Jeffery, 2001), the potentialhealth benefits of optimising SF ingestion could be significant.The present study suggests that this could be achieved by preferen-tial ingestion of high SF-producing varieties that are raw or lightly

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40

50

60

70

80

90

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0 50 100 150 200 250 300 350

Time (s)

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Fig. 1. Tissue temperature (�C) recorded between 3 and 4 cm within ‘Booster™’ broccoli florets during steaming (�), microwaving (N) or boiling (j) for up to 300 s.

R.B. Jones et al. / Food Chemistry 123 (2010) 237–242 241

steamed. In this study, raw ‘Booster™’ florets were shown to pro-duce over 2.5 times more SF than did ‘Marathon’ and must there-fore be considered a good source of SF. When raw broccoli tissueis consumed, glucosinolates can be hydrolysed to isothiocyanates,such as SF, by myrosinase during chewing or in the stomach andthe isothiocyanates are then rapidly absorbed in the small intestine(Holst & Williamson, 2004). Intact glucosinolates that reach thelarge bowel may also be hydrolysed to isothiocyanates by the bo-wel microflora (Combourieu, Elfoul, Delort, & Rabot, 2001; Krulet al., 2002). Cooking broccoli florets, even by the most mild meansof steaming, can significantly reduce SF intake and absorption.

Acknowledgments

This is a publication from Vital Vegetables, a Trans Tasman re-search project jointly funded and supported by Horticulture Aus-tralia Ltd., New Zealand Institute for Crop and Food ResearchLtd., the New Zealand Foundation for Research Science and Tech-nology, the Australian Vegetable and Potato Growers FederationInc., New Zealand Vegetable and Potato Growers Federation Inc.and the Victorian Department of Primary Industries.

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Cooking method significantly effects glucosinolate content and sulforaphane production in broccoli florets