Buns can be made with leavened or unleavened dough. Many flatbreads are unleavened, although some are leavened, such as pita bread. The flour is made into flatbreads, including thin, leavened dosa and thicker, unleavened roti. Creamed mixtures are usually further leavened by a chemical leaven like baking soda. It may be leavened or unleavened (e.g. matzo). It is not essential whether the bread be leavened or unleavened. The bread may be either leavened or unleavened (appearing as a wafer), depending on tradition. Fine white loaves were leavened by wild yeasts and sourdough cultures. Aerated bread was leavened by carbon dioxide being forced into dough under pressure. Naan is usually leavened with yeast. Most bread eaten in the West is leavened. Fermentation was also used in this time period to produce leavened bread. The bread is leavened and allowed to rise. According to Professor Philippe Marinval, the local islanders leavened this bread. Election cakes were typically leavened with yeast.