The Drawbacks of Cooking
While a cooked meal is something to savour for most of us, it pays to know which methods to use and which to avoid - and when to go raw, says Peter Dingle PhD
Cooking and processing food is something that is widely accepted, but many of us don't realise there may be nutritional drawbacks. High heat, as well as many of the cooking processes, alters the physical and chemical structure of food, changing how it is digested and the nutrients that are available to be absorbed.
Enzymes are destroyed and nutrients can be lost from almost all forms of cooking, depending upon the type of cooking, temperature, pH, oxygen content and type of food. In addition, the processing of foods may add toxins, which can build up in the body causing a negative effect on health.
Vegetables provide a good example of how cooking reduces the quantity of valuable nutrients. With vegetables, cooking by means of boiling or steaming, produces three composition-changing actions: shrinkage due to the extrusion of vegetable juices; leaching by either boiling water or condensed steam; and hydration.
Leaching produces a higher loss of water-soluble nutrients in vegetables, which increases the longer the vegetable is boiled and the amount of water used. Between 70 and 80% of vitamins and minerals including vitamin C and B group (such as thiamine and folate), and potassium are lost during the boiling process.
Steaming vegetables does not result in the loss of anywhere near as many nutrients into the water as with boiling, but steamed vegetables still may lose up to 30% of their water and water-soluble nutrients. As the nutrients leach into the cooking liquid, the full value of the vegetable can only be maintained if this is consumed as well as the solid food.
In experiments, boiling spinach and broccoli removed between 51% and 56% of folate. Thiamine (B1) has one of the highest losses by cooking - up to 80% - with complete loss from oven-roasted food, while boiled legumes had 50% thiamine loss. Riboflavin (B2) losses tend to be less but still significant; roast chicken retained only 22% of its riboflavin after cooking.
The fat-soluble vitamins, A, D, E and K, are not lost through boiling because they are not water-soluble, though they are still prone to oxidation damage through high heat. Other cooking processes though can certainly have a big impact on the levels of the fat-soluble vitamins.
Oven roasting lamb chops decreased the vitamin A content by 58%. Similarly, baking fish reduced vitamin A content by 37%. Many foods lose between 19 and 57% of the carotenoids, beta-carotene and xanthophylls when cooked...
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