Generally, the less processed a carbohydrate, the more likely it is to have a low GI score (for example, pumpernickel bread, which contains whole grains, has a low GI rating), but it is not always that simple. The following guidelines should help you to understand the GI of carbohydrates.
Cooking and processing increases the GI number of a food, because it starts the process of breaking down the carbohydrate that would otherwise take place in the stomach. In cornflakes, for example, the corn grain has been ground down, mixed with sugar and baked, resulting in a high GI score. However, sugar-free muesli, which is made from whole grains of flaked cereals, still requires the digestive system to do a lot of work to break down the carbohydrate, so it has a low GI rating.
If the tough outer skin of a grain or pulse is still in place, it is harder for the digestive system to break it down. This means that multigrain bread has a lower GI number than white bread, which has a high GI score because the wheat husks have been removed by the milling process.
The type of starch in a carbohydrate also affects its GI rating. Rice contains two types of starch: amylose and amylopectin. Basmati rice is high in amylose starch, which has a complex structure that is difficult to digest. Standard white rice is high in amylopectin, which has a less complex structure and is easy to digest. This explains why basmati is a medium-GI carbohydrate food, while standard white rice is a high-GI food.
Tightly packed starch grains are harder for the digestive enzymes to process than fluffy starch grains. This is why pasta, new potatoes and pitta bread all have lower GI ratings than gnocchi, baked potatoes and white bread.