In the past, flour was stone ground, a slow milling process that causes less friction and heat, thereby preserving more of the nutrients in the wheat. With the invention of roller milling in Hungary in the mid-nineteenth century, and its spread throughout Europe and the United States in the late nineteenth century, stone-ground flour, especially in the United States, was relegated to a health-food fringe. Massive mills with high-temperature, high-speed steel rollers came to rule the industry. Although this high-speed process creates a much finer flour, it destroys many of the nutrients in the grain. Furthermore, while stone-ground flour is generally aged to improve its baking properties, industrial mills skip the expensive aging process. American mills generally bleach the flour with chemicals, including organic peroxides, nitrogen dioxide, chlorine, chlorine dioxide, and azodicarbonamide. (Japan stopped bleaching flour about 30 years ago, and the use of chlorine, bromates, and peroxides is not permitted in the European Union.) Bleaching has a negative effect on baking, as it toughens the dough, making it brittle, dry, and more difficult to work with.