Raw vegetables are often believed to be more beneficial than cooked ones because cooking causes significant changes to the chemical composition of vegetables; however, recent studies have shown that cooking can also have positive effects on the amount of nutrients that can be absorbed and utilized by the body. In fact, a study published last year in the Journal of Agricultural and Food Chemistry reported that cooking is beneficial for some vegetables by promoting the production antioxidant compounds.
In this study, Nicoletta Pellegrini and her co-authors investigated the effects of domestic cooking methods (boiling, microwaving, basket-steaming, oven-steaming) on the nutritional quality of broccoli, Brussels sprouts and cauliflower. In particular, they compared the levels of carotenoids, chlorphyll, glucosinolates, phenols, and ascorbic acid of cooked vegetables to raw vegetables.
Broccoli, Brussels sprouts, and cauliflower are known as Brassica vegetables (part of the Brassicaceae family) and are almost the sole source of glucosinolates in the human diet. Glucosinolates are beneficial as they increase the activity of enzymes that deactivate carcinogens.
The results of their study are shown in below:
1. Data show statistical importance but may not be representative of physiological importance
2. NC indicates there are no changes in the nutritional quality after cooking
3. + and – indicate an increase or decrease in content and the number of +’s and -’s indicate the relative increase/decrease
What these compounds are good for
The compounds listed above are considered phytochemicals, which are substances found in plant foods that may have health-promoting properties but are not essential. Specifically, carotenoids are phytochemicals that have antioxidative properties. Some are converted to vitamin A while others can decrease the risk of macular degeneration (i.e. lutein and zeaxanthin). Macular degeneration occurs from oxidative damage to cells in the macula, resulting in a decline of visual acuity and ultimately leads to blindness. Chlorophyll have potential antioxidative properties but have yet to be studied in detailed. Glucosinolates as discussed above have anticarcinogenic properties. Polyphenols (phenol compounds) have antioxidative effects and protect against the oxidation of low-density lipoprotein (LDL ) cholesterol. Antioxidative substances can reduce oxidative damage by neutralizing reactive oxygen molecules.
What it all means
The researchers of this study found that overall, cooking allows more nutrients to be released from within the vegetables. Furthermore, steam-cooking was found to be the best method for preserving or enhancing the nutritional quality of Brassica vegetables. Generally speaking steaming inactivates oxidative enzymes. Furthermore, nondirect contact with water to prevents solubilization of compounds. Thus more of the nutrients are retained in the vegetables and not leached into the cooking water. Regardless of cooking method, the bottom line is to incorporate vegetables into your diet–raw or cooked.
L. A. Smolin and M. B. Grosvenor, Nutrition Science & Applications. USA: John Wiley & Sons, Inc., 2008
Pellegrini, N., Chiavaro, E., Gardana, C., Mazzeo, T., Contino, D., Gallo, M., Riso, P., Fogliano, V., & Porrini, M. (2010). Effect of Different Cooking Methods on Color, Phytochemical Concentration, and Antioxidant Capacity of Raw and Frozen Brassica Vegetables Journal of Agricultural and Food Chemistry, 58 (7), 4310-4321 DOI: 10.1021/jf904306r