Does cooking necessarily reduce the amounts of bioactive compounds and antioxidants in vegetables? Is there an ideal way to prepare vegetables so that their functional properties are conserved?
To answer questions like these, several studies supported by FAPESP are under way at the Federal University of São Paulo’s Health & Society Institute (ISS-UNIFESP) in Brazil, with Professor Veridiana Vera de Rosso as principal investigator.
“The conclusion we’ve reached so far is that no single rule applies to all kinds of vegetable,” Rosso said. “It depends on whether the bioactive compounds in a vegetable are water-soluble, as are anthocyanins, for example, or liposoluble, like carotenoids. It depends on how they’re stored in the vegetable, which in the case of carotenoids may be crystalloid, globular-tubular or in liquid-crystal form. It depends on whether they’re bonded to other molecules that are denatured by heat during cooking, such as proteins, so that extraction may be facilitated. It also depends on texture and on water and fiber content. You can’t say a certain type of processing is best for all vegetables or that it’s always best to eat them raw.”
In their latest study, published in the journal Food Chemistry, the group investigated the levels of three classes of antioxidant substances present in kale and red cabbage when these vegetables were cooked by boiling, steaming and stir-frying.
“We chose the three most common cooking techniques in Brazil and aimed at the most palatable texture,” Rosso said. “The antioxidant compounds present in the samples were then extracted in the lab.”
In the case of carotenoids, which are yellow, orange, and red pigments, all three cooking techniques led to a significant reduction in levels of these antioxidants in kale, with the stir-fried sample showing the smallest reduction. The total carotenoid content of raw kale was 155 micrograms per gram (μg/g), and the levels of 19 different carotenoids were measured. Stir-frying reduced the level to 69 μg/g, steaming to 43 μg/g, and boiling to 35 μg/g.
Cooking did not significantly change the level of carotenoids in red cabbage, which naturally contains low levels of these pigments. In the case of anthocyanins (red, purple and blue pigments), however, steaming facilitated extraction in the laboratory and significantly increased their levels.
The total anthocyanin content of raw red cabbage was 23.9 milligrams per 100 grams, and six different anthocyanins were measured. Boiling lowered the level to 14 mg/100 g, whereas stir-frying and steaming raised it to 25 mg/100 g and 28.9 mg/100 g, respectively.
“It’s harder to extract these compounds from raw cabbage, which is rich in fiber,” Rosso explained. “Steaming softens the tissue and facilitates extraction. We can infer that the same thing happens in the human organism: steaming makes these antioxidants in cabbage more bioavailable to the digestive process. Anthocyanins are water-soluble and are largely lost during boiling.”
Phenolic compounds were the third class of antioxidant substances evaluated. In red cabbage, steaming increased the level of phenolic compounds from 49 mg/100 g in the raw vegetable to 91.4 mg/100 g; stir-frying raised it slightly, to 53.3 mg/100 g; and boiling reduced it to 23.6 mg/100 g. In kale, all three cooking techniques reduced the level of phenolic compounds, steaming most of all (from 28.5 mg/100 g in raw kale to 18.6 mg/100 g) and stir-frying least of all (to 26.9 mg/100 g).
Antioxidant action in cells
“We added the compounds extracted from the vegetables to the cellular medium and waited about two hours until they were metabolized,” Rosso said. “We then added a substance that generated free radicals and another that became fluorescent when it reacted with these free radicals. The greater the capacity of the antioxidants in the vegetables to deactivate the free radicals, the less fluorescent the cells became.”
Steaming conserved antioxidant activity the most in both kale and red cabbage, she added, although all three cooking techniques resulted in significant antioxidant activity.
The experiments described in the paper published by Food Chemistry cast fresh light on some issues already mentioned by Rosso and collaborators with insufficient evidence in two reviews of the literature on the subject, one published by the journal Food Research International and the other recently accepted by Critical Reviews in Food Science and Nutrition.
“In these two articles we reviewed 1,000-odd scientific papers and noted a striking disparity in the findings,” Rosso said. “From this it can be concluded that food matrix components are stored differently in each case, which in turn determines whether it’s more or less stable when cooked – hence the different results obtained when the same technique is used to cook different vegetables.”