Dust explosion

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Dust_explosion 

 

Lab demonstration with burning lycopodium powder

A dust explosion is the rapid combustion of fine particles suspended in the air within an enclosed location. Dust explosions can occur where any dispersed powdered combustible material is present in high-enough concentrations in the atmosphere or other oxidizing gaseous medium, such as pure oxygen. In cases when fuel plays the role of a combustible material, the explosion is known as a fuel-air explosion.

Dust explosions are a frequent hazard in coal mines, grain elevators, and other industrial environments. They are also commonly used by special effects artists, filmmakers, and pyrotechnicians, given their spectacular appearance and ability to be safely contained under certain carefully controlled conditions.

Thermobaric weapons utilize this principle by rapidly saturating an area with an easily combustible material and then igniting it to produce explosive force. These weapons are the most powerful non-nuclear weapons in existence. 

Terminology

If rapid combustion occurs in a confined space, enormous overpressures can build up, causing major structural damage and flying debris. The sudden release of energy from a "detonation" can produce a shockwave, either in open air or in a confined space. If the spread of flame is at subsonic speed, the phenomenon is sometimes called a "deflagration", although looser usage calls both phenomena "explosions".

Dust explosions may be classified as being either "primary" or "secondary" in nature. Primary dust explosions may occur inside process equipment or similar enclosures, and are generally controlled by pressure relief through purpose-built ducting to the external atmosphere. Secondary dust explosions are the result of dust accumulation inside a building being disturbed and ignited by the primary explosion, resulting in a much more dangerous uncontrolled explosion that can affect the entire structure. Historically, fatalities from dust explosions have largely been the result of secondary dust explosions.

Conditions required

Diagram showing the five requirements for a dust explosion

There are five necessary conditions for a dust explosion:

• A combustible dust
• The dust is dispersed in the air at a sufficiently high concentration
• There is an oxidant (typically atmospheric oxygen)
• There is an ignition source
• The area is confined—a building can be an enclosure

Sources of dust

1878 stereograph rendering of the Great Mill Disaster

 

Mount Mulligan mine disaster in Australia 1921. These cable drums were blown 50 feet (15 m) from their foundations following a coal dust explosion.

 

Aftermath of 2008 explosion at Imperial Sugar in Port Wentworth, Georgia, US

Many common materials which are known to burn can generate a dust explosion, such as coal and sawdust. In addition, many otherwise mundane organic materials can also be dispersed into a dangerous dust cloud, such as grain, flour, starch, sugar, powdered milk, cocoa, coffee, and pollen. Powdered metals (such as aluminum, magnesium, and titanium) can form explosive suspensions in air, if finely divided.

Explosive dust can arise from activities such as transporting grain, and grain silos have often been demolished violently. Mining of coal leads to coal dust, and flour mills likewise have large amounts of flour dust as a result of milling. A gigantic explosion of flour dust destroyed a mill in Minnesota on May 2, 1878, killing 18 workers at the Washburn A Mill and another four in adjacent buildings. A similar problem occurs in sawmills and other places dedicated to woodworking.

Since the advent of industrial production–scale metal powder–based additive manufacturing (AM) in the 2010s, there is growing need for more information and experience with preventing dust explosions and fires from the traces of excess metal powder sometimes left over after laser sintering or other fusion methods. For example, in machining operations downstream of the AM build, excess powder liberated from porosities in the support structures can be exposed to sparks from the cutting interface. Efforts are underway not only to build this knowledgebase within the industry but also to share it with local fire departments, who do periodic fire-safety inspections of businesses in their districts and who can expect to answer alarms at shops or plants where AM is now part of the production mix.

Although not strictly a dust, paper particles emitted during processing - especially rolling, unrolling, calendaring/slitting, and sheet-cutting - are also known to pose an explosion hazard. Enclosed paper mill areas subject to such dangers commonly maintain very high air humidities to reduce the chance of airborne paper dust explosions.

In special effects pyrotechnics, lycopodium powder and non-dairy creamer are two common means of producing safe, controlled fire effects.

To support rapid combustion, the dust must consist of very small particles with a high surface area to volume ratio, thereby making the collective or combined surface area of all the particles very large in comparison to a dust of larger particles. Dust is defined as powders with particles less than about 500 micrometres in diameter, but finer dust will present a much greater hazard than coarse particles by virtue of the larger total surface area of all the particles.

Concentration

Below a certain value, the lower explosive limit (LEL), there is insufficient dust to support the combustion at the rate required for an explosion. A combustible concentration at or below 25% of the LEL is considered safe. Similarly, if the fuel to air ratio increases above the upper explosive limit (UEL), there is insufficient oxidant to permit combustion to continue at the necessary rate.

Determining the minimum explosive concentration or maximum explosive concentration of dusts in air is difficult, and consulting different sources can lead to quite different results. Typical explosive ranges in air are from few dozens grams/m³ for the minimum limit, to few kg/m³ for the maximum limit. For example, the LEL for sawdust has been determined to be between 40 and 50 grams/m³. It depends on many factors including the type of material used.

Oxidant

Typically, normal atmospheric oxygen can be sufficient to support a dust explosion if the other necessary conditions are also present. High-oxygen or pure oxygen environments are considered to be especially hazardous, as are strong oxidizing gases such as chlorine and fluorine. Also, particulate suspensions of compounds with a high oxidative potential, such as peroxides, chlorates, nitrates, perchlorates, and dichromates, can increase risk of an explosion if combustible materials are also present.

Sources of ignition

There are many sources of ignition, and a naked flame need not be the only one: over one half of the dust explosions in Germany in 2005 were from non-flame sources.^[7] Common sources of ignition include:

• electrostatic discharge (e.g. an improperly installed conveyor belt, which can act like a Van de Graaff generator)
• friction
• electrical arcing from machinery or other equipment
• hot surfaces (e.g. overheated bearings)
• fire
• self-ignition

However, it is often difficult to determine the exact source of ignition when investigating after an explosion. When a source cannot be found, ignition will often be attributed to static electricity. Static charges can be generated by external sources, or can be internally generated by friction at the surfaces of particles themselves as they collide or move past one another.

Mechanism

Dusts have a very large surface area compared to their mass. Since burning can only occur at the surface of a solid or liquid, where it can react with oxygen, this causes dusts to be much more flammable than bulk materials. For example, a 1 kilogram (2.2 lb) sphere of a combustible material with a density of 1 g/cm³ would be about 12.4 centimetres (4.9 in) in diameter, and have a surface area of 0.048 square metres (0.52 sq ft). However, if it were broken up into spherical dust particles 50 µm in diameter (about the size of flour particles) it would have a surface area of 120 square metres (1,300 sq ft). This greatly-increased surface area allows the material to burn much faster, and the extremely small mass of each particle allows them to catch on fire with much less energy than the bulk material, as there is no heat loss to conduction within the material.

When this mixture of fuel and air is ignited, especially in a confined space such as a warehouse or silo, a significant increase in pressure is created, often more than sufficient to demolish the structure. Even materials that are traditionally thought of as nonflammable (such as aluminum), or slow burning (such as wood), can produce a powerful explosion when finely divided, and can be ignited by even a small spark.

• Demonstration of an open-air dust explosion
• 

  Experimental setup

• 

  Finely-ground flour is dispersed

• 

  Cloud of flour is ignited

• 

  Fireball spreads rapidly

• 

  Intense radiant heat has nothing to ignite here

• 

  Fireball and superheated gases rise

• 

  Aftermath of explosion, with unburned flour on the ground

Effects

A dust explosion can cause major damage to structures, equipment, and personnel from violent overpressure or shockwave effects. Flying objects and debris can cause further damage. Intense radiant heat from a fireball can ignite the surroundings, or cause severe skin burns in unprotected persons. In a tightly enclosed space, the sudden depletion of oxygen can cause asphyxiation. Where the dust is carbon based (such as in a coal mine), incomplete combustion may cause large amounts of carbon monoxide (the miners' after-damp) to be created. This can cause more deaths than the original explosion as well as hindering rescue attempts.

Protection and mitigation

This American poster during World War I warned about grain dust explosions

Much research has been carried out in Europe and elsewhere to understand how to control these dangers, but dust explosions still occur. The alternatives for making processes and plants safer depend on the industry.

In the coal mining industry, a methane explosion can initiate a coal dust explosion, which can then engulf an entire mine pit. As a precaution, incombustible stone dust may be spread along mine roadways, or stored in trays hanging from the roof, to dilute the coal dust stirred up by a shockwave to the point where it cannot burn. Mines may also be sprayed with water to inhibit ignition.

Some industries exclude oxygen from dust-raising processes, a precaution known as "inerting". Typically this uses nitrogen, carbon dioxide, or argon, which are incombustible gases which can displace oxygen. The same method is also used in large storage tanks where flammable vapors can accumulate. However, use of oxygen-free gases brings a risk of asphyxiation of the workers. Workers who need illumination in enclosed spaces where a dust explosion is a high risk often use lamps designed for underwater divers, as they have no risk of producing an open spark due to their sealed waterproof design.

Good housekeeping practices, such as eliminating build-up of combustible dust deposits that could be disturbed and lead to a secondary explosion, also help mitigate the problem.

Best engineering control measures which can be found in the National Fire Protection Association (NFPA) Combustible Dust Standards include:

• Wetting
• Oxidant concentration reduction
• Deflagration venting
• Deflagration pressure containment
• Deflagration suppression
• Deflagration venting through a dust retention and flame-arresting device

Notable incidents

Dust clouds are a common source of explosions, causing an estimated 2,000 explosions annually in Europe. The table lists notable incidents worldwide.

Event	Date	Location	Country	Source material	Fatalities	Injuries	Notes
Washburn "A" Mill explosion	May 2, 1878	Minneapolis, Minnesota	United States	grain dust	22		Destroyed the largest grain mill in the world and leveled five other mills, effectively reducing the milling capacity of Minneapolis by one-third to one-half. Prompted mills throughout the country to install better ventilation systems to prevent dust build-up.
Milwaukee Works explosion	May 20, 1919	Milwaukee, Wisconsin	United States	Feed grinding plant	3	4	The blast was felt for miles around and completely leveled the plant owned by the company.
Douglas Starch Works explosion	May 22, 1919	Cedar Rapids, Iowa	United States	corn starch	43	30	The blast was felt for miles around and completely leveled the plant owned by the company.
Port Colborne explosion	August 9, 1919	Port Colborne	Canada	grain	10	16	Blast also destroyed the steamer Quebec, which was near the grain elevator
Large terminal grain elevator in Kansas City	September 13, 1919	Kansas City, Missouri	United States		14	10	Originated in basement of elevator, during a cleanup period, and travelled up through the elevator shaft
Mount Mulligan mine disaster	September 19, 1921	Mount Mulligan, Queensland	Australia	coal dust	75		The series of coal dust explosions within a mine rocked the close-knit township and was audible as far as 30 kilometres (19 mi) away.
Benxihu Colliery explosion	April 26, 1942	Benxi, Liaoning	Manchukuo (now  China)	coal dust and gas	1,549		34% of the miners working that day were killed. This is the world's worst-ever coal-mining accident.
Westwego grain elevator explosion	December 22, 1977	Westwego, Louisiana	United States	grain dust	36	13
Galveston grain elevator explosion	December 27, 1977	Galveston, Texas	United States	grain dust	20
Bird's Custard factory explosion	November 18, 1981	Banbury	United Kingdom	corn starch		9
Metz malt factory explosion	October 18, 1982	Metz	France	barley dust	12	1
Harbin textile factory explosion	March 17, 1987	Harbin	China	flax dust	58	177
Blaye grain explosion	August 1997	Blaye	France	grain dust	11	1	Explosion in a grain storage facility at the Société d’Exploitation Maritime Blayaise killed 11 people in nearby offices and injured one.
West Pharmaceutical Services explosion	January 29, 2003	Kinston, North Carolina	United States	polyethylene dust	6	38
Imperial Sugar explosion	February 7, 2008	Port Wentworth, Georgia	United States	sugar dust	14	42
2014 Kunshan explosion	August 2, 2014	Kunshan	China	metal powder	146	114
Formosa Fun Coast explosion	June 27, 2015	New Taipei	Taiwan	colored starch powder	15	498	Explosion when Holi-like colored powder was released at an outdoor music and color festival at the Formosa Fun Coast.
Bosley wood flour mill explosion	July 17, 2015	Bosley, Cheshire	United Kingdom	wood flour	4	4

Bread

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Bread 

Bread

Various leavened breads
Main ingredients	Flour, water

Bread is a staple food prepared from a dough of flour and water, usually by baking. Throughout recorded history, it has been a prominent food in large parts of the world. It is one of the oldest human-made foods, having been of significant importance since the dawn of agriculture, and plays an essential role in both religious rituals and secular culture.

Bread may be leavened by naturally occurring microbes, chemicals, industrially produced yeast, or high-pressure aeration. In many countries, commercial bread often contains additives to improve flavor, texture, color, shelf life, nutrition, and ease of production.

Etymology

The Old English word for bread was hlaf (hlaifs in Gothic: modern English loaf), which appears to be the oldest Teutonic name. Old High German hleib and modern German Laib derive from this Proto-Germanic word, which was borrowed into some Slavic (Czech chléb, Polish bochen chleba, Russian khleb) and Finnic (Finnish leipä, Estonian leib) languages as well. The Middle and Modern English word bread appears in Germanic languages, such as West Frisian brea, Dutch brood, German Brot, Swedish bröd, and Norwegian and Danish brød; it may be related to brew or perhaps to break, originally meaning "broken piece", "morsel".

History

Bread shop, Tacuinum Sanitatis from Northern Italy, beginning of the 15th century

Bread is one of the oldest prepared foods. Evidence from 30,000 years ago in Europe and Australia revealed starch residue on rocks used for pounding plants. It is possible that during this time, starch extract from the roots of plants, such as cattails and ferns, was spread on a flat rock, placed over a fire and cooked into a primitive form of flatbread. The world's oldest evidence of bread-making has been found in a 14,500-year-old Natufian site in Jordan's northeastern desert. Around 10,000 BC, with the dawn of the Neolithic age and the spread of agriculture, grains became the mainstay of making bread. Yeast spores are ubiquitous, including on the surface of cereal grains, so any dough left to rest leavens naturally.

There were multiple sources of leavening available for early bread. Airborne yeasts could be harnessed by leaving uncooked dough exposed to air for some time before cooking. Pliny the Elder reported that the Gauls and Iberians used the foam skimmed from beer, called barm, to produce "a lighter kind of bread than other peoples" such as barm cake. Parts of the ancient world that drank wine instead of beer used a paste composed of grape juice and flour that was allowed to begin fermenting, or wheat bran steeped in wine, as a source for yeast. The most common source of leavening was to retain a piece of dough from the previous day to use as a form of sourdough starter, as Pliny also reported.

The Chorleywood bread process was developed in 1961; it uses the intense mechanical working of dough to dramatically reduce the fermentation period and the time taken to produce a loaf. The process, whose high-energy mixing allows for the use of grain with a lower protein content, is now widely used around the world in large factories. As a result, bread can be produced very quickly and at low costs to the manufacturer and the consumer. However, there has been some criticism of the effect on nutritional value.

Types

Brown bread (left) and whole grain bread

 

Dark sprouted bread

 

Ruisreikäleipä, a flat rye flour loaf with a hole

Bread is the staple food of the Middle East, Central Asia, North Africa, Europe, and in European-derived cultures such as those in the Americas, Australia, and Southern Africa. This is in contrast to parts of South and East Asia, where rice or noodle is the staple. Bread is usually made from a wheat-flour dough that is cultured with yeast, allowed to rise, and finally baked in an oven. The addition of yeast to the bread explains the air pockets commonly found in bread. Owing to its high levels of gluten (which give the dough sponginess and elasticity), common or bread wheat is the most common grain used for the preparation of bread, which makes the largest single contribution to the world's food supply of any food.

Sanggak

 

Strucia — a type of European sweet bread

Bread is also made from the flour of other wheat species (including spelt, emmer, einkorn and kamut). Non-wheat cereals including rye, barley, maize (corn), oats, sorghum, millet and rice have been used to make bread, but, with the exception of rye, usually in combination with wheat flour as they have less gluten.

Gluten-free breads are made using ground flours from a variety of ingredients such as almonds, rice, sorghum, corn, or legumes such as beans, and tubers such as cassava, but since these flours lack gluten they may not hold their shape as they rise and their crumb may be dense with little aeration. Additives such as xanthan gum, guar gum, hydroxypropyl methylcellulose (HPMC), corn starch, or eggs are used to compensate for the lack of gluten.

Properties

Physical-chemical composition

In wheat, phenolic compounds are mainly found in hulls in the form of insoluble bound ferulic acid, where it is relevant to wheat resistance to fungal diseases.

Rye bread contains phenolic acids and ferulic acid dehydrodimers.

Three natural phenolic glucosides, secoisolariciresinol diglucoside, p-coumaric acid glucoside and ferulic acid glucoside, can be found in commercial breads containing flaxseed.

Small home made bread with pumpkin and sunflower seeds

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. Gliadin binds weakly to the gluten network established by glutenin via intrachain disulfide bonds. 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.

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.

Culinary uses

Bread pudding

Bread can be served at many temperatures; once baked, it can subsequently be toasted. It is most commonly eaten with the hands, either by itself or as a carrier for other foods. Bread can be spread with butter, dipped into liquids such as gravy, olive oil, or soup; it can be topped with various sweet and savory spreads, or used to make sandwiches containing meats, cheeses, vegetables, and condiments.

Bread is used as an ingredient in other culinary preparations, such as the use of breadcrumbs to provide crunchy crusts or thicken sauces; toasted cubes of bread, called croutons, are used as a salad topping; seasoned bread is used as stuffing inside roasted turkey; sweet or savoury bread puddings are made with bread and various liquids; egg and milk-soaked bread is fried as French toast; and bread is used as a binding agent in sausages, meatballs and other ground meat products.

Nutritional significance

Nutritionally, bread is categorized as a source of grains in the food pyramid. Further, it is a good source of carbohydrates and nutrients such as magnesium, iron, selenium, B vitamins, and dietary fiber.

Crust

Crust of a cut bread made of whole-grain rye with crust crack (half right at the top)

Bread crust is formed from surface dough during the cooking process. It is hardened and browned through the Maillard reaction using the sugars and amino acids due to the intense heat at the bread surface. The crust of most breads is harder, and more complexly and intensely flavored, than the rest. Old wives' tales suggest that eating the bread crust makes a person's hair curlier. Additionally, the crust is rumored to be healthier than the remainder of the bread. Some studies have shown that this is true as the crust has more dietary fiber and antioxidants such as pronyl-lysine, which is being researched for its potential colorectal cancer inhibitory properties.

Preparation

Steps in bread making, here for an unleavened Chilean tortilla

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).

Methods of processing dough into bread include the straight dough process, the sourdough process, the Chorleywood bread process and the sponge and dough process.

Baking bread in East Timor

Formulation

Professional bread recipes are stated using the baker's percentage notation. The amount of flour is denoted to be 100%, and the other ingredients are expressed as a percentage of that amount by weight. Measurement by weight is more accurate and consistent than measurement by volume, particularly for dry ingredients. The proportion of water to flour is the most important measurement in a bread recipe, as it affects texture and crumb the most. Hard wheat flours absorb about 62% water, while softer wheat flours absorb about 56%. Common table breads made from these doughs result in a finely textured, light bread. Most artisan bread formulas contain anywhere from 60 to 75% water. In yeast breads, the higher water percentages result in more CO₂ bubbles and a coarser bread crumb. 500 grams (1 pound) of flour yields a standard loaf of bread or two baguettes.

Calcium propionate is commonly added by commercial bakeries to retard the growth of molds.

Flour

Flour is grain ground to a powdery consistency. Flour provides the primary structure, starch and protein to the final baked bread. The protein content of the flour is the best indicator of the quality of the bread dough and the finished bread. While bread can be made from all-purpose wheat flour, a specialty bread flour, containing more protein (12–14%), is recommended for high-quality bread. If one uses a flour with a lower protein content (9–11%) to produce bread, a shorter mixing time is required to develop gluten strength properly. An extended mixing time leads to oxidization of the dough, which gives the finished product a whiter crumb, instead of the cream color preferred by most artisan bakers.

Wheat flour, in addition to its starch, contains three water-soluble protein groups (albumin, globulin, and proteoses) and two water-insoluble protein groups (glutenin and gliadin). When flour is mixed with water, the water-soluble proteins dissolve, leaving the glutenin and gliadin to form the structure of the resulting bread. When relatively dry dough is worked by kneading, or wet dough is allowed to rise for a long time (see no-knead bread), the glutenin forms strands of long, thin, chainlike molecules, while the shorter gliadin forms bridges between the strands of glutenin. The resulting networks of strands produced by these two proteins are known as gluten. Gluten development improves if the dough is allowed to autolyse.

Liquids

Water, or some other liquid, is used to form the flour into a paste or dough. The weight or ratio of liquid required varies between recipes, but a ratio of three parts liquid to five parts flour is common for yeast breads. Recipes that use steam as the primary leavening method may have a liquid content in excess of one part liquid to one part flour. Instead of water, recipes may use liquids such as milk or other dairy products (including buttermilk or yoghurt), fruit juice, or eggs. These contribute additional sweeteners, fats, or leavening components, as well as water.

Fats or shortenings

Fats, such as butter, vegetable oils, lard, or that contained in eggs, affect the development of gluten in breads by coating and lubricating the individual strands of protein. They also help to hold the structure together. If too much fat is included in a bread dough, the lubrication effect causes the protein structures to divide. A fat content of approximately 3% by weight is the concentration that produces the greatest leavening action. In addition to their effects on leavening, fats also serve to tenderize breads and preserve freshness.

Bread improvers

Bread improvers and dough conditioners are often used in producing commercial breads to reduce the time needed for rising and to improve texture and volume. The substances used may be oxidising agents to strengthen the dough or reducing agents to develop gluten and reduce mixing time, emulsifiers to strengthen the dough or to provide other properties such as making slicing easier, or enzymes to increase gas production.

Salt

Salt (sodium chloride) is very often added to enhance flavor and restrict yeast activity. It also affects the crumb and the overall texture by stabilizing and strengthening the gluten. Some artisan bakers forego early addition of salt to the dough, whether wholemeal or refined, and wait until after a 20-minute rest to allow the dough to autolyse.

Mixtures of salts are sometimes employed, such as employing potassium chloride to reduce the sodium level, and monosodium glutamate to give flavor (umami).

Leavening

A dough trough, located in Aberdour Castle, once used for leavening bread.

Leavening is the process of adding gas to a dough before or during baking to produce a lighter, more easily chewed bread. Most bread eaten in the West is leavened.

Chemicals

A simple technique for leavening bread is the use of gas-producing chemicals. There are two common methods. The first is to use baking powder or a self-raising flour that includes baking powder. The second is to include an acidic ingredient such as buttermilk and add baking soda; the reaction of the acid with the soda produces gas. Chemically leavened breads are called quick breads and soda breads. This method is commonly used to make muffins, pancakes, American-style biscuits, and quick breads such as banana bread.

Yeast

Compressed fresh yeast

Many breads are leavened by yeast. The yeast most commonly used for leavening bread is Saccharomyces cerevisiae, the same species used for brewing alcoholic beverages. This yeast ferments some of the carbohydrates in the flour, including any sugar, producing carbon dioxide. Commercial bakers often leaven their dough with commercially produced baker's yeast. Baker's yeast has the advantage of producing uniform, quick, and reliable results, because it is obtained from a pure culture. Many artisan bakers produce their own yeast with a growth culture. If kept in the right conditions, it provides leavening for many years.

The baker's yeast and sourdough methods follow the same pattern. Water is mixed with flour, salt and the leavening agent. Other additions (spices, herbs, fats, seeds, fruit, etc.) are not needed to bake bread, but are often used. The mixed dough is then allowed to rise one or more times (a longer rising time results in more flavor, so bakers often "punch down" the dough and let it rise again), loaves are formed, and (after an optional final rising time) the bread is baked in an oven.

Many breads are made from a "straight dough", which means that all of the ingredients are combined in one step, and the dough is baked after the rising time; others are made from a "pre-ferment" in which the leavening agent is combined with some of the flour and water a day or so ahead of baking and allowed to ferment overnight. On the day of baking, the rest of the ingredients are added, and the process continues as with straight dough. This produces a more flavorful bread with better texture. Many bakers see the starter method as a compromise between the reliable results of baker's yeast and the flavor and complexity of a longer fermentation. It also allows the baker to use only a minimal amount of baker's yeast, which was scarce and expensive when it first became available. Most yeasted pre-ferments fall into one of three categories: "poolish" or "pouliche", a loose-textured mixture composed of roughly equal amounts of flour and water (by weight); "biga", a stiff mixture with a higher proportion of flour; and "pâte fermentée", which is simply a portion of dough reserved from a previous batch.

• 

  Before first rising

• 

  After first rising

• 

  After proofing, ready to bake

Sourdough

Sourdough loaves

Sourdough is a type of bread produced by a long fermentation of dough using naturally occurring yeasts and lactobacilli. It usually has a mildly sour taste because of the lactic acid produced during anaerobic fermentation by the lactobacilli.

Sourdough breads are made with a sourdough starter. The starter cultivates yeast and lactobacilli in a mixture of flour and water, making use of the microorganisms already present on flour; it does not need any added yeast. A starter may be maintained indefinitely by regular additions of flour and water. Some bakers have starters many generations old, which are said to have a special taste or texture. At one time, all yeast-leavened breads were sourdoughs. Recently there has been a revival of sourdough bread in artisan bakeries.

Traditionally, peasant families throughout Europe baked on a fixed schedule, perhaps once a week. The starter was saved from the previous week's dough. The starter was mixed with the new ingredients, the dough was left to rise, and then a piece of it was saved (to be the starter for next week's bread).

Steam

The rapid expansion of steam produced during baking leavens the bread, which is as simple as it is unpredictable. Steam-leavening is unpredictable since the steam is not produced until the bread is baked. Steam leavening happens regardless of the raising agents (baking soda, yeast, baking powder, sour dough, beaten egg white) included in the mix. The leavening agent either contains air bubbles or generates carbon dioxide. The heat vaporises the water from the inner surface of the bubbles within the dough. The steam expands and makes the bread rise. This is the main factor in the rising of bread once it has been put in the oven. CO₂ generation, on its own, is too small to account for the rise. Heat kills bacteria or yeast at an early stage, so the CO₂ generation is stopped.

Bacteria

Salt-rising bread employs a form of bacterial leavening that does not require yeast. Although the leavening action is inconsistent, and requires close attention to the incubating conditions, this bread is making a comeback for its cheese-like flavor and fine texture.

Aeration

Aerated bread was leavened by carbon dioxide being forced into dough under pressure. From the mid-19th to mid-20th centuries, bread made this way was somewhat popular in the United Kingdom, made by the Aerated Bread Company and sold in its high-street tearooms. The company was founded in 1862, and ceased independent operations in 1955.

The Pressure-Vacuum mixer was later developed by the Flour Milling and Baking Research Association for the Chorleywood bread process. It manipulates the gas bubble size and optionally the composition of gases in the dough via the gas applied to the headspace.

Cultural significance

A Ukrainian woman in national dress welcoming with bread and salt

Bread has a significance beyond mere nutrition in many cultures because of its history and contemporary importance. Bread is also significant in Christianity as one of the elements (alongside wine) of the Eucharist, and in other religions including Paganism.

In many cultures, bread is a metaphor for basic necessities and living conditions in general. For example, a "bread-winner" is a household's main economic contributor and has little to do with actual bread-provision. This is also seen in the phrase "putting bread on the table". The Roman poet Juvenal satirized superficial politicians and the public as caring only for "panem et circenses" (bread and circuses). In Russia in 1917, the Bolsheviks promised "peace, land, and bread." The term "breadbasket" denotes an agriculturally productive region. In parts of Northern, Central, Southern and Eastern Europe bread and salt is offered as a welcome to guests. In India, life's basic necessities are often referred to as "roti, kapra aur makan" (bread, cloth, and house).

Words for bread, including "dough" and "bread" itself, are used in English-speaking countries as synonyms for money. A remarkable or revolutionary innovation may be called the best thing since "sliced bread". The expression "to break bread with someone" means "to share a meal with someone". The English word "lord" comes from the Anglo-Saxon hlāfweard, meaning "bread keeper."

Bread is sometimes referred to as "the staff of life", although this term can refer to other staple foods in different cultures: the Oxford English Dictionary defines it as "bread (or similar staple food)". This is sometimes thought to be a biblical reference, but the nearest wording is in Leviticus 26 "when I have broken the staff of your bread". The term has been adopted in the names of bakery firms.