For that extra boost this Christmas, scientists from Italy recommend steaming that broccoli because it will increase the levels of cancer-fighting compounds.
The nutrient-boosting effects are not limited to broccoli, but could be extended to a wide range of vegetables - a result that defies conventional culinary wisdom - report the researchers in the Journal of Agricultural and Food Chemistry.
Researchers from Universities of Parma and Naples looked at the effects of boiling, steaming, and frying on the polyphenol, carotenoid, glucosinolate, and ascorbic acid content of carrots, courgettes, and broccoli.
Such vegetables will be gracing the plates of numerous people worldwide this Christmas. No mention is made however of the unchanged hardy perennial Brussels sprouts - the bane of every child's Christmas dinner - date back to 1587 where they were first cultivated in Belgium.
Lead researcher Nicoletta Pellegrini and colleagues report that an overall increase antioxidant measures after cooking, compared to the raw vegetables. Antioxidants were measured using the Trolox equivalent antioxidant capacity (TEAC), total radical-trapping antioxidant parameter (TRAP), and ferric reducing antioxidant power (FRAP) assays.
This observation is "probably because of matrix softening and increased extractability of compounds, which could be partially converted into more antioxidant chemical species," they said.
However, compared to boiling and steaming frying caused a significantly higher loss of antioxidants.
And for broccoli, they report an increase in the glucosinolates content when the vegetables were steamed, from 71.4 to 93.4 micromoles per gram. These compounds are touted for their cancer-fighting abilities.
"Our findings defy the notion that processed vegetables offer lower nutritional quality and also suggest that for each vegetable a cooking method would be preferred to preserve the nutritional and physicochemical qualities," they wrote.
The findings suggest that it may be possible to select a cooking method for each vegetable that can best preserve or improve its nutritional quality, the researchers say.
Source: Journal of Agricultural and Food Chemistry
Published on-line ahead of print, doi: 10.1021/jf072304b
"Effects of Different Cooking Methods on Nutritional and Physicochemical Characteristics of Selected Vegetables"
Authors: C. Miglio, E. Chiavaro, A. Visconti, V. Fogliano, N. Pellegrini