Carbohydrates came in various forms, and each form reacts differently wen exposed to heat. The two forms of carbohydrates that are of interest from a basic food science perspective are sugar and starch.
When exposed to heat, sugar will at first melt into a thick syrup. As the temperature continues to rise, the sugar syrup changes color, from clear to light yellow to a progressively deepening brown. This browning process is called caramelization. It is a complicated chemical reaction, and in addition to color change, it also causes the flavor of the sugar to evolve and take on the rich complexity that we know to be characteristic of caramel. Different types of sugar caramelize at different temperatures. Granulated white sugar melts at 320 F/160 C and begins to caramelize at 338 F/170 C.
In foods that are not primarily sugar or starch, a different reaction, known as the Maillard reaction, is responsible for browning. This reaction involves sugars and amino acids (the building blocks of protein). When heated, these components react and produce numerous chemical by-products, resulting in a brown color and intense flavor and aroma. It is this reaction that gives coffee, chocolate, baked goods, dark beer, and roasted meats and nuts much of their rich flavor and color.
Though the Maillard reaction can happen at room temperature, both caramelization and the Maillard reaction typically require relatively high heat (above 300 F/149 C) to occur rapidly enough to make an appreciable difference in foods. Because water cannot be heated about 212 F/100 C unless it is under pressure foods cooked with moist hear (boiling, steaming and poaching) will not brown. Foods cooked using dry-heat methods (sauteing, grilling, or roasting) will brown. It is for this reason that many stewed and braised dishes being with an initial browning of ingredients before liquid is added.
Starch, a complex carbohydrate, has powerful thickening properties. When starch is combined with water or another liquid and is heated, individual starch granules absorb the liquid and swell. this process, known as gelatinization, is what causes the liquid to thicken. Gelatinization occurs at different temperatures for different types of starch. As a general rule of thumb, root-based starches (potato and arrowroot for instance) thicken at lower temperatures but break down more quickly, whereas cereal-based starches (corn and wheat for example) thicken at higher temperatures but break down more slowly. High levels of sugar or acid can inhibit gelatinization, while the presence of salt can promote it.