Cooked versus raw vegetables
Editor: In response to a reader’s request, we asked the above two experts in the fields of diets and nutrition to explain what changes occur in vegetables when they are cooked. The answer given here shows what changes occur during cooking. Vegetables do however not need to be cooked for birds, because birds are able to digest raw foods much better than we can. The cooking of grains like dried maize, peas is however recommended for bird because they become much more palatable to birds when they are moist and soft and there is less waste. The cooking of beans, especially soy, is also very important because certain undesirable elements like growth inhibitors are destroyed during the cooking process. Now over to the experts:
In answering the reader’s question one has to inevitably look at the make-up of foods and specifically vegetables. Vegetables contain very little protein and usually no fat, so their nutritive value lies mainly in their carbohydrate (starch) content. We will thus have to concentrate on the digestibility of starch to understand why some vegetables are more digestible in their cooked than in their raw state.
Carbohydrates or starches are made up of many glucose molecules bound together and the aim is to break these glucose molecules apart so that they can be absorbed and used by the body for energy. Digestibility therefore depends on how easily these glucose bonds are broken. During cooking, water and heat expand the starch granules to different degrees, some granules actually bursting and freeing the individual starch molecules. This process is termed starch gelatinisation. The swollen granules and free starch molecules are very easy to digest, because the starch digesting enzymes in the small intestine have a greater surface area on which they can act.
There are two sorts of starch in food. They are:
*Amylose: which is a straight chain of glucose molecules, like a string of beads. These tend to line up in rows and clump together so that they are hard to gelatinize and digest.
* Amylopectin: which is a string of glucose molecules with lots of branching points. These are thus larger and more open and the starch is easier to gelatinize and digest.
The starch occurs in small granules that have a core of starch surrounded by a network of protein.This starch forms a crystalline structure within this granule and is resistant to digestion when consumed in the raw state. This is why potatoes will give you a pain in the stomach when eaten raw. The table below shows the effect on cooking on the digestibility percentage of sweet potatoes (Canope et al. 1977)
The table shows that cooking increases the digestibility of both energy and organic matter to the same extent. The biggest increase in digestibility was however noted on nitrogen (i.e. protein) digestibility. This response it observed due to the destruction of heat labile antitypic factors.
Although poultry can digest raw starches, cooking with steam is used universally in the preparation of commercial chickens. In the process, steam is mixed with milled feed and the warm, moist feed is then forced through a metal plate with small holes to form pellets. In comparison with meal, pellets are typically utilized 8% better by poultry.
Other important factors to consider when preparing foods are as follows:
Starch levels: Vegetables may be classified into high, intermediate or low starch foods.
* High levels of starch: Beans, peas, potatoes, pumpkins, sweet potatoes and grains are termed starchy vegetables because of their high starch content. When they are immature they contain sugar, but as they mature the sugar is converted to starch. Cooking these will result in a large increase in digestibility.
* Intermediate levels of starch: Beets, carrots and onions are intermediate in carbohydrate content. Cooking will therefore improve the digestibility only slightly and these may be fed raw.
* Low levels of starch: Vegetables such as tomatoes, lettuce, broccoli, spinach and cauliflower have a low starch content. These vegetables are primarily made up of indigestible fibrous material. Cooking these vegetables has little benefit and they should be fed raw.
Destruction of natural toxins: a number of natural toxins occur in food, the most problematical ones being trypsin inhibitors that occur at high levels in most beans. Lower levels of the trypsin inhibitors also occur in potatoes, rice and barley. The trypsin inhibitors disrupt the digestion of protein and lead to compensatory hypertrophy of the pancreas. Cooking destroys these toxins, resulting in an improvement in the digestion of protein.
Baking versus cooking: During baking heat is applied, while little or no water, sugar and fat is added. Under these conditions starch gelatinisation is not as effective and only about half of the granules are fully gelatinised. Baked foods are thus less digestible than when cooked in water.
Particle size: The particle size of food impacts on starch gelatinisation. Grinding and milling of cereals reduces the particle size and makes it easier for water to be absorbed and enzymes to attack. i.e. Whole wheat grains, when cooked, will be less digestible that the flour made of wheat grains. In poultry, coarsely milled soy beans for example have a lower fat digestibility than the finely milled product. In the case of grains milling is not so important because particles are ground fine in the gizzard.
Seed coat: Whole intact grains and also dried beans have a fibrous coat which acts as a physical barrier to water and enzymes and are thus only digestible when soaked and cooked. Adult birds destroy the seed hulls either by grinding it fine in their gizzard or, as in the case of psittacines, by shelling the seed first.
Anti-nutrients: Some foods such as soy beans contain substances such as phytates and tannins that inhibit the digestion of starch. Most of these anti-nutrients are heat labile and therefore inactivated by cooking. Cooking these ingredients results in big improvements in protein digestibility.
Other nutrients: Vegetables also lose nutrients when cooked:
*Water-soluble vitamins may dissolve into the cooking water, which is usually discarded.
* Certain vitamins are heat sensitive. Notably Thiamine (Vitamin B1) and Ascorbic Acid (Vitamin C) may be altered or lost when a vegetable is cooked.
* Cooking may also denature some amino acids thereby altering the protein quality.
Correct cooking method: The greatest destruction occurs during the first 2-3 minutes of cooking. To minimize the losses, the vegetables should be added to boiling water. For best results only small amounts of cooking water should be used and vegetables should be boiled in the skin where applicable.
It is recommended, that for people high starch vegetables (as listed above) should be cooked, while vegetables with intermediate and low starch levels (see above) do not need to be cooked. Most beans contain high levels of natural toxins and should not be fed or eaten uncooked. To prevent nutrient deficiencies, the diet should be supplemented in order to replenish any nutrients lost during cooking and also to supplement those nutrients that are deficient in the natural ingredients.
Brand Miller J, Foster-Powell K, Colagiuri S. The G.I. Factor. 1996
Charley H, Food Science. 1986
Krause M V, Mahan L K. Food, Nutrition and Diet Therapy. 1984