The dry heat of baking changes the form of starches in the food and causes its outer surfaces to brown, giving it an attractive appearance and taste. The browning is caused by caramelization of sugars and the Maillard reaction. Maillard browning occurs when "sugars break down in the presence of proteins. Because foods contain many different types of sugars and proteins, Maillard browning contributes to the flavour of a wide range of foods, including nuts, roast beef and baked bread." The moisture is never entirely "sealed in"; over time, an item being baked will become dry. This is often an advantage, especially in situations where drying is the desired outcome, like drying herbs or roasting certain types of vegetables.