Vegetables are rich in vitamins, minerals, antioxidants (such as beta[β]-carotene and lycopene) and fibres yet low in sugar and fat. While most vegetables can be served raw as salads or pickles, cooking them not only softens the cellulose fibres (which aids in chewing and digestion) but also improves taste, texture and food safety. Despite the common belief that raw vegetables are always better than cooked vegetables, this may not always be the case.
Broccoli for example, is rich in glucosinolates (mainly glucoraphanin and glucobrassicin), carotenoids (mainly β-carotene and lutein) and vitamin C. Cooking broccoli (exposure to high temperatures) has been shown to damage the enzyme myrosinase which converts both glucoraphanin and glucobrassicin to sulforaphane and indole-3-carbinol respectively (both have been shown to have anti-carcinogenic activity). This results in less sulforaphane and indole-3-carbinol for absorption. Vitamin C on the other hand is heat sensitive and water soluble, hence cooking methods such as boiling, would result in degradation and leeching of vitamin C into water used to cook broccoli. In fact, the process of stir-frying followed by boiling of broccoli causes the loss of approximately 40% of vitamin C and reduces the carotenoid content by approximately 30%. Conversely, steaming broccoli results in higher carotenoid content compared to raw broccoli while deep frying reduces the carotenoid content by almost 70%.
Comparing the different methods of cooking, gentle steaming (approximately ≤5 minutes) appears to result in better preservation of glucosinolates, vitamin C and carotenoids in broccoli compared to boiling, stir-frying, stir-fry followed by boiling and microwave cooking. In fact, stir-frying and stir fry followed by boiling broccoli which are common methods of preparation in Asia, have been found to result in greatest loss of vitamin C and glucosinolates from broccoli.
Raw carrots are also rich in carotenoids (lutein, α- and β-carotenes) which have been associated with improved visual health. Boiling carrots have been found to increase the carotenoid content but reduces phenolic compounds (mainly chlorogenic acid which is an antioxidant) to undetectable levels – something which can be prevented by steaming or sous vide. Deep fried carrots on the other hand has undetectable amounts of vitamin C with pronounced loss of carotenoids. Therefore, deep frying should be avoided in favour of raw, steamed or sous vide carrots in order to preserve its nutrient content.
On the other hand, red cabbage which is commonly used in pickles, coleslaws, salads, stir frys and stews is rich in anthocynins (antioxidant), vitamin C and glucosinolates. A recent study found that red cabbage loses approximately 80% of glucosinolates, 60% of anthocynin (antioxidants) and vitamin C when it is stir-fried. Red cabbage which was boiled for 5 mins also lost more than 50% of anthocyanins and about 40% of vitamin C while these losses are least pronounced when it is cooked via steaming or microwaving. It appears that preservation of nutrients is best when red cabbage is consumed raw and freshly cut.
Green leafy vegetables such as choy sum and sweet potato leaves can be consumed as salads but it is commonly prepared by stir-frying, steaming and boiling in Asia. A separate study also found that boiled choy sum and sweet potato leaves retained more carotenoids (lutein and β-carotene) compared to stir-frying them.
While eating raw vegetables may be a healthy option, it may not be everyone’s first choice. Certain vegetables can have a more pleasant taste without compromising its nutrient content after cooking, especially if the right methods are used. Having said that, it is always important to have a varied and balanced intake –some raw fruits and salads for heat-sensitive vitamins and cooked vegetables for more antioxidants.
- Miglio et al. (2008) Effects of different cooking methods on nutritional and physicochemical characteristics of selected vegetables. J. Agric. Food.Chem. 56(1): 139 – 147.
- Yuan et al. (2009) Effects of different cooking methods on health-promoting compounds of broccoli. J Zhejiang Univ Sci B. 10(8): 580 – 588.
- Jones et al. (2010) Cooking method significantly effects glucosinolate content and sulforaphane production in broccoli florets. Food Chemistry. 123(2): 237 – 242.
- Xu et al. (2014) Domestic cooking methods affect the nutritional quality of red cabbage. Food chemistry. 161: 162-167.
- Chivaro et al. (2012) Nutritional quality of sous vide cooked carrots and brussels sprouts. J. Agric. Food Chem. 60(23): 6019 – 6025.