尽管大多数人坚信生吃蔬菜比煮熟它更有营养价值，但实际上越来越多的证据却显示，煮熟蔬菜的过程，可能释放了某种营养成分。但是，科学家正在寻找更充足的数据，来分析煮熟蔬菜的营养成分。
在一项新研究中，研究人员用不同的方式烹饪意大利人常吃的三种蔬菜——胡萝卜、南瓜和椰菜，分别计算煮、蒸、炸时，它们的营养价值。煮和蒸的方式，让蔬菜保持了更多的抗氧化成分，相反，和前两种烹饪方式相比，油炸则让蔬菜中的抗氧化成分大量流失。以椰菜为例，蒸的方式实际上增强了其中硫代葡萄糖苷的作用 ——这种化合物存在于植物中，具有抗癌作用。科学家们的新发现显示，我们应该为每一种蔬菜选择适当的烹饪方式，以便最大程度地保持它们的营养成分，甚至提升蔬菜的营养品质。
烹调可以提高绿叶蔬菜和橙黄色蔬菜中维生素K和类胡萝卜素的利用率。这两类物质只喜欢溶于油脂，热烹调使细胞壁软化，促进胡萝卜素、番茄红素的溶出，提高吸收率。烹调可以提高蔬菜中钙镁元素的利用率。很多人只知道钙来自牛奶，镁来自香蕉，却不知道绿叶蔬菜也是这些营养素的好来源。这是因为，大部分绿叶蔬菜中存在着草酸，它不利于钙和镁的吸收。然而，在烹调加工当中，只要经过焯烫步骤，再行炒制或凉拌，即可除去绝大部分草酸。
煮熟的蔬菜比生吃能更好地保护身体免于心脏病及癌症的侵袭。蔬菜中所含的类胡萝卜素在煮熟的情况下更易为人体吸收。英国食品研究中心的苏·索森说：“从生胡萝卜中吸收的类胡萝卜素大约为3%到4%，而把它们煮熟或捣碎后，类胡萝卜素的吸收可增加四、五倍。人身体能否吸收类胡罗卜素的一个关键是食品的结构，尤其是胡萝卜里的那些厚壁细胞，烹饪可帮助溶解它们。”胡萝卜富含类胡萝卜素，绿色蔬菜、黄色蔬菜、西红柿及西瓜中的类胡萝卜素也相当丰富。
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Y: What do you have in your lunch box there, Don? Is that a raw tomato?
D: Sure is. I've been eating raw tomatoes since I was a kid. My father always eats them that way. And he is the healthiest man you ever met. Of course, everyone knows that vegetables and fruits are more nutritious when raw. Cooking them reduces the amount of vitamins and minerals, the very stuff that makes vegetables and fruits so good for us.
Y: There is some truth to that, but some research shows that tomatoes in particular are actually better for you cooked rather than raw. You're right that cooking tomatoes reduces their vitamin content. Tomatoes, known for being rich in vitamin C, can lose ten to thirty percent of that vitamin C through the cooking process. On the bright side, though, cooking tomatoes greatly increases their levels of a compound called lycopene, which gives tomatoes their red color. Lycopene is an antioxidant. Antioxidants protect the body from cell and tissue damage which can lead to cancer. Lycopene specifically is known for decreasing the risk of heart-disease. Thus, lycopene, as well as antioxidants in general, are an important part of the diet.
D: Hmm. Does this mean that other fruits and veggies are also more healthful when cooked?
Y: That isn't clear yet, but it's quite possible. Further research might greatly change the way we think about and consume fruits and vegetables. We might discover reasons to feel just as good about ourselves for eating cooked fruits and vegetables as we do for eating raw ones.
D: So we should enjoy tomatoes raw on our salads and sandwiches, but also enjoy them cooked as a sauce for pasta.
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. Reason enough to shun tomato sauce and chomp on raw salads at the risk of ingesting the odd parasite egg? Not really.
Apply heat for extra nutrition
Lycopene molecules come in several shapes. Cooking actually increases the kind your body can absorb more efficiently. Adding olive oil makes yet more lycopene available. Unfortunately, research shows sunflower oil doesn’t.
Heat also breaks up the cell matrix that traps nutrients such as beta-carotene and lycopene (carotenoid pigments, precursors of vitamin A), and folate (vitamin B9) in spinach, carrots and tomatoes.
Cooking also neutralizes antinutrients:
• Thiaminases, which render thiamin (vitamin B1) inactive, and are found in Brussels sprouts and red cabbage, are destroyed by cooking.
• Goitrogens stop your body from using iodine. They are found in cabbage-like veggies, turnips, tapioca, sweet potatoes, and beans. Cooking deactivates them.
Also Read Manidipa Mandal’s earlier columns
• Phytates interfere with mineral and niacin (vitamin B3) absorption. They are found in cereal grains and beans, especially the red ones. Beans and dals also have bitter tannins in the skin, and insoluble carbohydrates within that cause flatulence. Soaking and draining helps. Cooking (especially pressure cooking) and then draining improves things further. Adding a pinch of soda while soaking the beans makes for a more effective alkaline bath.
• People prone to kidney stones are often advised to avoid oxalate-rich foods such as leafy greens, colocasia, potatoes, beet, tomatoes, lady’s fingers, citrus fruits, berries, mangoes, nuts and seeds and soyabean. These also hamper the absorption of calcium. Drain the water after cooking and you will absorb the calcium in your meal normally.
Pick ‘processed’ for better health
As with cooking, “processing” isn’t always a pejorative:
• Sprouts, for instance, are more nutritious—providing nutrients such as zinc and iron—as is fermenting dal and grain batter used to make idlis and dosas.
• Hand-pounded rice, millet and maize are left with less phytates, yet have more vitamin than milled grain.
• Pasteurizing milk prevents curdling and destroys a host of bacteria.
• Many people who are lactose intolerant can digest yoghurt easily.
• Fish bones are full of calcium but make a prickly mouthful—unless you get yours from a can. Processing softens the bones, making extra calcium available.
The raw and the cooked: eat both
Water-soluble nutrients lost in cooking can be replaced (say, by eating fruits). At the same time, the nutrients are not totally lost in cooking. A study by Punjab Agricultural University in 2006 analysed 10 common vegetable dishes (from sarson ka saag to aloo-capsicum) and found they provided 269.9% and 77.5% of the vitamin C and beta-carotene you need daily.
Manidipa Mandal is deputy features editor at Mint.
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