Nutritional value in vegetable preparation
Dear Donovan,
How much nutritional value is lost from cooking vegetables? Would it be better to eat raw vegetables? What about vegetables in a can? I know it’s not advised to eat vegetables like kale raw, so how can I have it without losing the nutrients? What vegetables should never be eaten raw?
Vegetables are packed with large amounts of fibre, vitamins, minerals, as well as beneficial plant substances. Vegetables are therefore perfect for meeting nutritional needs without a large number of extra calories. Some studies have shown that some vitamins, especially the B vitamins, as well as vitamin C, either lose or reduce value significantly during cooking in water. These nutrients are usually leached into the water. The higher the temperature used and the longer the cook time, the more nutrients might be lost from the vegetables. It is suspected that at least 30 per cent of water soluble vitamins can be lost by cooking the vegetables. In addition, studies have shown that many minerals — potassium, sodium, calcium, magnesium, phosphorus, iron, zinc, manganese and copper — have been decreased up to 70 per cent by cooking vegetables. The biggest loss of these minerals happens when the vegetables are squeezed out after boiling.
In addition, some enzymes as well as other helpful chemicals present in vegetables are also destroyed during cooking. For example, allicin, the antibacterial chemical found in garlic, can be destroyed by cooking. I always say that cooked vegetables act like a mop and raw vegetables act like a broom.
It would be good if the same water used to cook the vegetables is kept and used as a base for soups and stews.
However, cooking some vegetables actually increases the bioavailability of some nutrients like Vitamin A, calcium, iron and some antioxidants. Cooking actually helps to break down the vegetables, allowing the nutrients to be more readily absorbed. Also, from a food safety standpoint, vegetables can sometimes become contaminated with microorganisms and cooking will kill the microorganisms.
Vegetables like kale contain many healthy and beneficial nutrients. However, when eaten raw, kale can inhibit the uptake of iodine by the thyroid gland. This can lead to a hormone imbalance. In addition, kale also contains oxalic acid which can bind with minerals such as calcium and magnesium in the body and cause them to crystalise. This can result in tissue damage. The best way to avoid this is to boil the kale for about seven minutes in water and squeeze the water out.
There are some other vegetables that should not be eaten raw. These include brussels sprouts, broccoli, cauliflower, collard greens, turnip and mustard greens.
You have also asked me about canned vegetables. It should be noted that freshly picked vegetables typically start with a lot of minerals and vitamins. However, depending on how long it takes to reach the consumer, some of the nutrients may be lost. Canned vegetables are usually protected against this. However, how they are canned must be taken into consideration — whether it is done with preservatives, etc. But fresh vegetables lose nutrients much faster than canned vegetables.
Good luck.
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