Steaming is the Most Health-Promoting Form of Cooking Cruciferous Vegetables

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An interesting study was published in August 2009 in the Journal Zhejiang University Science B entitled Effects of different cooking methods on health-promoting compounds of broccoli in which the researchers determined that of the five cooking methods, including:

  • steaming
  • microwaving
  • boiling
  • stir-frying
  • stir-frying followed by boiling (stir-frying/boiling)

steaming retained the highest levels of chlorophyll, vitamin C, soluble proteins and soluble sugars.  1 

In addition, steaming demonstrated less modifications to total aliphatic and indole glucosinolates which led to the lowest loss of total glucosinolates.  Whereas, stir-frying and stir-frying/boiling presented the highest loss of glucosinolates.

The researchers concluded that stir-frying and stir-frying/boiling cause great losses of chlorophyll, soluble protein, soluble sugar, vitamin C, and glucosinolates, but the steaming method appears the best in retention of the nutrients in cooking broccoli.

The researchers determined the main aliphatic glucosinolates in the tested broccoli were:

  • glucoraphanin
  • glucoiberin

The main indole glucosinolates were:

  • glucobrassicin
  • neoglucobrassicin

The chlorophyll content in boiled, stir-fried/boiled, stir-fried, and microwaved broccoli was reduced by 27%, 23%, 18%, and 16%, respectively (P<0.05), while it was almost unchanged in steamed broccoli.  3

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Figure 1:  Each value is mean±SD of three replicate samples. Values not sharing a common letter are significantly different at P<0.05. Cooking methods: 1. Raw; 2. Boiled; 3. Streamed; 4. Microwaved; 5. Stir-fried; 6. Stir-fried/boiled  (Source:  Effects of different cooking methods on health-promoting compounds of broccoli)


 

The highest contents of total soluble proteins and soluble sugars (2.6 m/g FW and 3.5 mg/g FW, respectively) in broccoli were obtained after steaming. The lowest retention of total soluble proteins was observed in broccoli after boiling and stir-frying/boiling, while the lowest retention of total soluble sugars was found in broccoli after stir-frying/boiling and stir-frying.  4

An external file that holds a picture, illustration, etc. Object name is JZUSB10-0580-fig02.jpg

Figure 2:  Each value is mean±SD of three replicate samples. Values not sharing a common letter are significantly different at P<0.05. Cooking methods: 1. Raw; 2. Boiled; 3. Streamed; 4. Microwaved; 5. Stir-fried; 6. Stir-fried/boiled  (Source:  Effects of different cooking methods on health-promoting compounds of broccoli)


 

The greatest loss of vitamin C was observed in broccoli after stir-frying/boiling and boiling (38% and 33%, respectively) treatments, followed by microwaving and stir-frying (16% and 24%, respectively) treatments. In contrast, steaming did not cause any significant loss of vitamin C, compared with the raw sample.  5

An external file that holds a picture, illustration, etc. Object name is JZUSB10-0580-fig03.jpg

Figure 3:  Each value is mean±SD of three replicate samples. Values not sharing a common letter are significantly different at P<0.05. Cooking methods: 1. Raw; 2. Boiled; 3. Streamed; 4. Microwaved; 5. Stir-fried; 6. Stir-fried/boiled  (Source:  Effects of different cooking methods on health-promoting compounds of broccoli)


 

The contents of total aliphatic and indole glucosinolates in broccoli after different cooking treatments are presented in Fig.​Fig.4.4. Total aliphatic glucosinolates were significantly decreased by 55%, 54%, 60%, and 41%, respectively in stir-fried, stir-fried/boiled, microwaved, and boiled broccoli (P<0.05). However, the contents of total aliphatic glucosinolates remained almost unchanged in steamed broccoli.  6

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Figure 4:  Each value is mean±SD of three replicate samples. Values not sharing a common letter are significantly different at P<0.05. DW: dry weight. Cooking methods: 1. Raw; 2. Boiled; 3. Streamed; 4. Microwaved; 5. Stir-fried; 6. Stir-fried/boiled  (Source:  Effects of different cooking methods on health-promoting compounds of broccoli)


 

The main glucosinolate found in broccoli is glucoraphanin.  When raw or steamed broccoli is consumed consumed, the enzyme myrosinase transforms glucoraphanin into raphanin and into sulforaphane, which exhibits anti-cancer and antimicrobial properties in experimental models.   2

Other important glucosinolates include:

  • Glucotropaeolin is the precursor to benzyl isothiocyanate
    • Glucotropaeolin is a phytochemical from Tropaeolum majus, which is commonly known as garden nasturtium, Indian cress or monks cress.
  • Gluconasturtiin is the precursor to phenethyl isothiocyanate
    • Gluconasturtiin is named from its occurrence in watercress (Nasturtium officinale) and horseradish (Armoracia rusticana)
  • Sinigrin is the precursor to allyl isothiocyanate
    • Sinigrin is found in some plants of the Brassicaceae family such as Brussels sprouts, broccoli, and the seeds of black mustard (Brassica nigra)

 


Resources:

Sur La Table® Bamboo Steamers

Bella 7-Liter Multi-Tier Food Steamer

 


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