Gelatinisation of starches occurs when the starchy food is exposed to liquid and heat (ie cooking). The water binds with the starch in the presence of heat and expands the starch granules. When we boil potatoes, the heat and water expand the hard compact granules, (which make raw potatoes difficult to digest), into easily digestible swollen potato granules. Some granules actually burst and free the individual starch molecules, and this is the reason why potatoes have a high glycemic index – they are easy to digest and absorb.
Intact grains such as whole wheat, barley, whole corn and whole rye have much lower GI values than flours (tiny particles) made from the same grains.
Milling, beating, grinding, mixing, mashing and refining foods raise the GI of that food. For this reason most processed foods will have a higher GI than the same unprocessed food.
Starches, such as rice, can have different types of starch structures which affect their digestibility. Some types of rice such as Basmati rice, have a higher amylose content. Amylose is made up of long straight chains of glucose molecules which are packed closely together, which are more difficult to digest. Other rice, with a higher amylopectin content, is much easier to digest and thus has a higher GI. Amylopectin are branched chains of glucose that do pack closely together and are thus much less dense and easier to digest.
Foods containing soluble fibre, such as oats and legumes, have a lowering effect on the GI because they delay gastric emptying. Insoluble fibre such as that found in digestive bran, on the other hand, has very little effect on the digestability and absorption of the carbohydrate foods. Thus foods containing bran do not necessarily have a lower GI than those foods without the bran. For example South African standard brown bread and white bread both have high GI values.
Sugar may lower the GI of foods that have a very high GI because the sugar competes with the starch for the liquid for gelatinization. For example rice crispies have a high GI. When they are sugar coated, the GI is lower and thus cocopops and strawberry pops have lower GI values than rice crispies! Likewise sugar free Weetbix has a higher GI than the ordinary one with the sugar. As mentioned above, sugar can also lower the GI of baked goods, since it is inclined to bind with the fluid in baking, preventing it from binding with the flour and thereby preventing gelatinisation. However, only small amounts of sugar should be used.
The presence of protein and fat in food may lower the GI. However, it is not advisable to add fat to lower the GI of foods for health reasons. Excess protein tends to wear out the body’s insulin; and fat has the effect of decreasing the effectiveness of insulin. Protein also overtaxes the kidneys and high protein intakes can lead to osteoporosis, arthritis and gout .
Phytates, lectins and polyphenols (tannins) normally slow digestion and thereby decrease the GI. These are found in many vegetables and fruits such as legumes, rhubarb and spinach.
The more acid a food, the lower the GI of that food. For example, beetroot salad with a vinegar dressing will have a lower GI than hot cooked beetroot without the dressing.
Cooking usually increases the digestability of the food, and would thus have the effect of raising the GI of that food.
When starches are cooked and then cooled, the crystalline structure within the food changes to resistant starch which is more difficult to digest. Thus cold cooked starches, (eg boiled, cold potatoes in a potato salad) have a lower GI. This is especially true for mealiemeal. Thus cooked cold maize porridge has a lower GI than the hot freshly prepared porridge.
Studies have shown that blood glucose levels rise less rapidly when eating more slowly.