Four cooking methods

[Guest guest]

Hi All, The below and attached paper on broccoli cooking methods struck me

as something of interest for comparing the four methods. It seems to me

that steaming is the way to go and I want to try it with the beans as well

as grains. The PDF is available.

Cheers, Al.

Alan Pater, Ph.D.; Faculty of Medicine; Memorial University; St. 's, NF

A1B 3V6 Canada; Tel. No.: (709) 777-6488; Fax No.: (709) 777-7010; email:

apater@...

Alan Pater, Ph.D.; Faculty of Medicine; Memorial University; St. 's, NF

A1B 3V6 Canada; Tel. No.: (709) 777-6488; Fax No.: (709) 777-7010; email:

apater@...

Original paper

Glucosinolates and vitamin C content in edible parts of broccoli florets

after domestic cooking

F. Vallejo, F. A. Tomás-Barberán and C. García-Viguera

Abstract. Total aliphatic and indole/aromatic glucosinolates and vitamin C

content (ascorbic+dehydroascorbic acid) were evaluated in the edible

portions of fresh harvested broccoli (Brassica oleracea L. var. italica)

florets (Marathon cv.) before and after cooking and in the cooking water.

High pressure boiling, steam cooking, microwaving and low pressure boiling

(conventional) were the four domestic cooking processes used in this work.

Results showed great differences among the four cooking processes and their

influence on the content of glucosinolates and vitamin C. Thus, clear

disadvantages were detected when cooking in a microwave due to the high

loss of vitamin C (40%) and total glucosinolates (74%) in comparison with

the rest of treatments. High pressure and conventional boiling had a

significant loss rate of total glucosinolates (33% and 55% respectively)

regarding fresh raw broccoli, due to the leaching into the cooking water.

On the other hand, steaming had minimal effects on glucosinolates and

vitamin C. Therefore, we can conclude that a large quantity of

glucosinolates and vitamin C will be consumed in steamed broccoli when

compared to the other cooking processes.

Keywords. Broccoli (Brassica oleracea L.) - Glucosinolates - Vitamin C -

Domestic cooking - Health-promoting

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Introduction

Over the past 20 years, compelling evidence has been obtained linking

increased consumption of fruits and vegetables, especially cruciferous

vegetables, to reduced incidence of many types of cancer [1, 2]. Brassica

species are very rich in health-promoting phytochemical constituents,

including phenolic compounds, vitamin C and glucosinolates [3, 4, 5, 6].

Epidemiological data show that a diet rich in cruciferous vegetables, such

as broccoli, Brussels sprouts and cauliflower, can reduce the risk from a

number of cancers and that the risk can be significantly reduced by an

intake of as little as 10 g per day [7, 8, 9]. At least some of the cancer

chemoprotective activity of these vegetables is widely believed to be due

to their content of minor dietary components such as glucosinolates [10,

11, 12, 13, 14]. This is a large group of sulphur-containing compounds,

which occur in all the economically important varieties of Brassica

vegetables [5]. It is also remarkable that in both plant and animal

metabolism, the biological functions of ascorbic acid are in relation to

the antioxidant properties of this molecule [15].

Cruciferous vegetables are not always consumed raw, since they are often

exposed to thermal cooking. Therefore, commercial freezing of Brassicas is

preceded by steam cooking, which is known to inactivate catalase,

peroxidase and other enzymes, and thereby prevents or retards development

of off-flavours The glucosinolate content in vegetables depends on the

cooking method, degree of raw material disintegration, and on the raw

material itself [16, 17, 18]. Glucosinolates and some of their hydrolysis

products are water-soluble and on boiling, a proportion of these compounds

will be leached into the cooking water [19]. Therefore, the purpose of the

present work was the quantification of desulphoglucosinolates and vitamin C

in the edible portions of fresh harvested broccoli florets of Marathon cv.

(the most commonly grown variety in southern Europe, representing ~60% of

the production area [20]) before and after cooking and in the cooking water

(sources) in an attempt to identify differences due to domestic cooking

factors (heat treatments). High pressure boiling, steam cooking,

microwaving and conventional boiling were the four domestic cooking methods

used in this work. This report describes glucosinolate and vitamin C levels

in a double interaction (S[times]T) among source (S) and heat treatment (T)

in raw and cooked broccoli florets. Emphasis was placed on broccoli because

of its economic importance and consumer preference for it.

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Materials and methods

...........

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Results and discussion

Desulphoglucosinolates and vitamin C in broccoli were evaluated in order to

determine variation in amounts and types among edible parts and cooking

water under four different heat treatments and using the same analytical

procedure. The profile of 11 glucosinolates found in broccoli

inflorescences, shown in Table 1, is in agreement with previous reports [4,

6, 24, 25]. These compounds were identified by their chromatographic

behaviour and UV spectra, HPLC-MS-MS (see Table 1) and chromatographic

comparisons with markers (samples supplied by R. and B. Holst, IFR,

Norwich). However, there were significant differences in glucosinolates and

vitamin C concentrations due to cooking conditions. The predominant

glucosinolates were glucoraphanin in aliphatic glucosinolates as well as

glucobrassicin and neoglucobrassicin in indolyl glucosinolates.

A typical glucosinolate chromatogram from broccoli inflorescences is

presented in Fig. 1. The glucosinolate pattern of the cultivars was similar

to that described by other authors [4, 6, 20, 24]. Glucoraphanin,

glucobrassicin, 4-meo-glucobrassicin and neoglucobrassicin (Table 2,

Table 3) were common to edible parts of all treatments. The rest of the

glucosinolates (except glucobrassicanapin) leached into the cooking water.

On the other hand, vitamin C was present in all the edible parts and water,

except in the cooking water of steam cooking in which this vitamin was not

detected (Table 4).

Total glucosinolates

Total glucosinolate levels were significantly affected by source,

treatments and their interaction (Table 3). In our work, the highest total

glucosinolate value [28.6 µmol/g dry weight (d.w.)] was obtained in the

edible part after the steam process and the lowest (0.2 µmol/g d.w.) was

observed in the cooking water of those microwaved (Table 3). Steaming led

to the highest total glucosinolate levels (edible part) while microwaving

presented the lowest (Table 3).

Domestic cooking reduced the total glucosinolate content of the edible part

of broccoli by over 50% (except steaming), which is similar to effects

reported by and Heaney (1993) [16] in cooked cabbage. According to

this, it seems that microwaving caused a great loss (about 74%) in total

glucosinolate content due to the high cooking water evaporation that

contained leached compounds. This treatment also led to low levels in the

cooking water (about 1%); the remaining 25% was found in the cooked edible

part (Fig. 2). The loss rate in the other treatments varied between 2% in

steam cooking and 55% in conventional boiling (Fig. 2). Thus,

glucosinolates remained in the edible part of broccoli during steaming, due

to inactivation of myrosinase, in accordance with the previous results of

et al. [5]. Conventional boiling had led to glucosinolate levels in

the edible part similar to those found after microwave treatment (Fig. 2).

Almost half of total glucosinolates were retained in the edible part under

high pressure treatment, while the leaching rate was similar to that which

was found after conventional cooking.

Thermal degradation could be due to a glucosinolate loss in volatiles as

previously reported by Sones et al. [26]. Also, in our case, glucosinolate

breakdown products (sulphoraphane, sulphoraphane nitrile,

cyanohydroxybutene, etc) determination was not achieved. Therefore we can

not ascertain that the high loss rate in the microwave treatment is due to

the presence of these compounds in cooking water, as previously reported by

Slominski and [27] who described glucosinolate breakdown products

of over 50%, in cabbage cooking water.

We can conclude that a greater quantity of glucosinolates will be present

for consumption after a steam cooking process, in agreement with other

authors [27].

Aliphatic glucosinolates

Steam cooking led to the highest total aliphatic glucosinolate level

(edible part) while microwaving presented the lowest (Table 2). Domestic

cooking reduced the total aliphatic glucosinolate content of the edible

part of broccoli by 50% (except steaming) as reported by and Heaney

[16] for cooked cabbage. During microwave heating the same occured for

aliphatic glucosinolates as has previously been reported for total

glucosinolates. This treatment caused a great loss (80%) in total aliphatic

glucosinolate content (Fig. 2). The loss rate in the other treatments

varied between 2% in steam cooking and 57.5% in high pressure cooking

(Fig. 2).

Indolyl glucosinolates

The degradation of these compounds followed the same trend as observed for

aliphatic glucosinolates, and, as a general rule, steam cooking preserved

the highest indolyl glucosinolate levels while microwaving preserved the

lowest (Table 3). Domestic cooking reduced the total indolyl glucosinolate

content of the edible part of broccoli by 70% (Fig. 2) (except steam

cooking and high pressure boiling), similar to results reported by

Slominski and [27] for cooked cabbage.

Higher losses of indole glucosinolates than aliphatic glucosinolates in all

treatments were detected (Fig. 2). This results from a higher

thermolability of those compounds as their leaching into the water was poor

and the rate of loss was very high, in contrast to what was previously

reported for cooked cabbage [16, 28].

Individual glucosinolates

When studying aliphatic glucosinolates, glucoraphanin was the only compound

that remained in the edible part in quantifiable amounts, being more stable

during steaming and conventional cooking (Table 2). On the other hand,

within indolyl glucosinolates, neoglucobrassicin presented the highest

stability, particularly when cooked under steam or high pressure (Table 3).

According to and Heaney [16], in cooked cabbage glucoiberin is more

thermolabile than other glucosinolates like neoglucobrassicin, which is in

disagreement with our results for cooked broccoli, showing similar

percentage losses for both glucosinolates (Fig. 3). Michajlovskij et al.

[29, 30] also found glucoiberin to be more thermolabile than

glucobrassicin, also in disagreement with our results for cooked broccoli

that showed a higher loss rate for glucobrassicin in all treatments (except

microwaving) (Fig. 3). It should be taken into consideration that

glucoiberin was found in very small amounts in the broccoli studied here,

but its stability was similar to that presented by the other compounds.

According to Ciska and Kozlowska [28] different crop varieties of the same

species can yield different losses of glucosinolate contents in cooked

cabbage, therefore larger differences could be found between different

species.

Vitamin C

Vitamin C levels were significantly affected by source, treatment and their

interaction (Table 4). The highest vitamin C value (117.3 mg/100 g f.w.) in

the edible part was obtained after steam cooking (Table 4).

Domestic cooking reduced vitamin C (AA and DHAA) content in the edible part

of broccoli, by between 20% and 46% (Fig. 4) (except steaming), presenting

similar results to those reported by Gil et al. [31] in cooked spinach.

Microwaving caused a great loss (about 38%) in vitamin C content (Fig. 4).

According to Lisiewska and Kmiecik [32] steaming of broccoli caused a

decrease in vitamin C content ([cong]40%) after a short heat treatment that

may not inactivate the enzymes such as ascorbic acid oxidase and phenolase.

However, in our case, no losses due to leaching or insufficient cooking

time were observed, in contrast to the above reports.

In summary, the results obtained in this work for all treatments showed

significant differences in unifactorial and multifactorial analysis in

total/individual aliphatic and indolyl glucosinolates, and AA/DHAA and

vitamin C, for the various treatments and sources. This clearly indicates

incidence of the double interaction in the changes in glucosinolate and

vitamin C contents.

Cooked broccoli exhibited a larger content (more than 3 times larger) in

total indolyl glucosinolates, when compared to total aliphatic

glucosinolates. The loss rates were slightly higher in indolyl

glucosinolates than in aliphatic glucosinolates due to the higher

thermolability of those compounds, as diffusion into the cooking water was

quite similar. The loss rate of glucobrassicin and neoglucobrassicin

suggests that these glucosinolates are more thermolabile than glucoiberin

and glucoraphanin. Slight AA and vitamin C losses and leaching rates

occurred in all treatments, except microwaving.

Clear advantages were detected in the analysis of steamed broccoli versus

microwaved broccoli, mainly due to the higher water evaporation presented

by this last system. High pressure and conventional boiling are interesting

treatments, as typical domestic cooking methods, for their good retention

levels of total glucosinolates and vitamin C, with a slight advantage for

the first.

Therefore, it can be concluded that time and pressure differences among

heat treatments might explain the different levels of glucosinolates and

vitamin C reported here. Finally, this variation in levels among different

treatments suggests differences in their health-promoting properties.