Bread is a baked food product made from water, flour, and often yeast. It is a staple food across the world, particularly in Europe and the Middle East. Throughout recorded history and around the world, it has been an important part of many cultures' diets. It is one of the oldest human-made foods, having been of significance since the dawn of agriculture, and plays an essential role in both religious rituals and secular culture. Doughs are usually baked, but in some cuisines breads are steamed (e.g., mantou), fried (e.g., puri), or baked on an unoiled frying pan (e.g., tortillas). It may be leavened or unleavened (e.g. matzo). Salt, fat and leavening agents such as yeast and baking soda are common ingredients, though bread may contain other ingredients, such as milk, egg, sugar, spice, fruit (such as raisins), vegetables (such as onion), nuts (such as walnut) or seeds (such as poppy).[41] Methods of processing dough into bread include the straight dough process, the sourdough process, the Chorleywood bread process and the sponge and dough process. Bread may be leavened by naturally occurring microbes (e.g. sourdough), chemicals (e.g. baking soda), industrially produced yeast, or high-pressure aeration, which creates the gas bubbles that fluff up bread. Bread may also be unleavened. In many countries, mass-produced bread often contains additives to improve flavor, texture, color, shelf life, nutrition, and ease of production. Glutenin and gliadin are functional proteins found in wheat bread that contribute to the structure of bread. Glutenin forms interconnected gluten networks within bread through interchain disulfide bonds.[27] Gliadin binds weakly to the gluten network established by glutenin via intrachain disulfide bonds.[27] Structurally, bread can be defined as an elastic-plastic foam (same as styrofoam). The glutenin protein contributes to its elastic nature, as it is able to regain its initial shape after deformation. The gliadin protein contributes to its plastic nature, because it demonstrates non-reversible structural change after a certain amount of applied force. Because air pockets within this gluten network result from carbon dioxide production during leavening, bread can be defined as a foam, or a gas-in-solid solution.[28] Acrylamide, like in other starchy foods that have been heated higher than 120 °C (248 °F), has been found in recent years to occur in bread. Acrylamide is neurotoxic, has adverse effects on male reproduction and developmental toxicity and is carcinogenic. A study has found that more than 99 percent of the acrylamide in bread is found in the crust.[29] A study by the University of Hohenheim found that industrially produced bread typically has a high proportion of FODMAP carbohydrates due to a short rising time (often only one hour). The high proportion of FODMAP carbohydrates in such bread then causes flatulence. This is particularly problematic in intestinal diseases such as irritable bowel syndrome. While in traditional bread making the dough rises for several hours, industrial breads rise for a much shorter time, usually only one hour. However, a sufficiently long rising time is important to break down the indigestible FODMAP carbohydrates. Some flours (for example, spelt, emmer and einkorn) contain fewer FODMAPs, but the difference between grain types is relatively small (between 1 and 2 percent by weight). Instead, 90% of the FODMAPs that cause discomfort can be broken down during a rising time of four hours. In the study, whole-grain yeast doughs were examined after different rising times; the highest level of FODMAPs was present after one hour in each case and decreased thereafter. The study thus shows that it is essentially the baking technique and not the type of grain that determines whether a bread is well tolerated or not. A better tolerance of bread made from original cereals can therefore not be explained by the original cereal itself, but rather by the fact that traditional, artisanal baking techniques are generally used when baking original cereals, which include a long dough process. The study also showed that a long rising time also breaks down undesirable phytates more effectively, flavors develop better, and the finished bread contains more biologically accessible trace elements.