The term fermentation sometimes refers specifically to the chemical conversion of sugars into ethanol, producing alcoholic drinks such as wine, beer, and cider. However, similar processes take place in the leavening of bread (CO2 produced by yeast activity), and in the preservation of sour foods with the production of lactic acid, such as in sauerkraut and yogurt.
Other widely consumed fermented foods include vinegar, olives, and cheese.
More localised foods prepared by fermentation may also be based on
beans, grain, vegetables, fruit, honey, dairy products, and fish.
History and prehistory
Conical loaves of bread left as grave goods, exactly as laid out in the Great Tomb at Gebelein, Egypt, 2435-2305 BC.
Natural fermentation precedes human history. Since ancient times,
humans have exploited the fermentation process. The earliest
archaeological evidence of fermentation is 13,000-year-old residues of a
beer, with the consistency of gruel, found in a cave near Haifa in Israel. Another early alcoholic drink, made from fruit, rice, and honey, dates from 7000-6600 BC, in the Neolithic Chinese village of Jiahu, and winemaking dates from ca. 6000 BC, in Georgia, in the Caucasus area.
Seven-thousand-year-old jars containing the remains of wine, now on
display at the University of Pennsylvania, were excavated in the Zagros Mountains in Iran. There is strong evidence that people were fermenting alcoholic drinks in Babylon ca. 3000 BC, ancient Egypt ca. 3150 BC, pre-Hispanic Mexico ca. 2000 BC, and Sudan ca. 1500 BC.
The French chemist Louis Pasteur founded zymology, when in 1856 he connected yeast to fermentation.
When studying the fermentation of sugar to alcohol by yeast, Pasteur concluded that the fermentation was catalyzed by a vital force, called "ferments",
within the yeast cells. The "ferments" were thought to function only
within living organisms. "Alcoholic fermentation is an act correlated
with the life and organization of the yeast cells, not with the death or
putrefaction of the cells", he wrote.
Nevertheless, it was known that yeast extracts can ferment sugar
even in the absence of living yeast cells. While studying this process
in 1897, the German chemist and zymologist Eduard Buchner of Humboldt University of Berlin, Germany, found that sugar was fermented even when there were no living yeast cells in the mixture, by an enzyme complex secreted by yeast that he termed zymase. In 1907 he received the Nobel Prize in Chemistry for his research and discovery of "cell-free fermentation".
Beer and bread, two major uses of fermentation in food
Food fermentation is the conversion of sugars and other carbohydrates into alcohol or preservative organic acids and carbon dioxide. All three products have found human uses. The production of alcohol is made use of when fruit juices are converted to wine, when grains are made into beer, and when foods rich in starch, such as potatoes, are fermented and then distilled to make spirits such as gin and vodka. The production of carbon dioxide is used to leaven bread. The production of organic acids is exploited to preserve and flavor vegetables and dairy products.
Food fermentation serves five main purposes: to enrich the diet
through development of a diversity of flavors, aromas, and textures in
food substrates; to preserve substantial amounts of food through lactic acid, alcohol, acetic acid, and alkaline fermentations; to enrich food substrates with protein, essential amino acids, and vitamins; to eliminate antinutrients; and to reduce cooking time and the associated use of fuel.
Chin som mok is a northern Thai speciality made with grilled, banana leaf-wrapped pork (both skin and meat) that has been fermented with glutinous rice
Sterilization is an important factor to consider during the fermentation of foods. Failing to completely remove any microbes
from equipment and storing vessels may result in the multiplication of
harmful organisms within the ferment, potentially increasing the risks
of food borne illnesses like botulism. The production of off smells and
discoloration may be indications that harmful bacteria may have been
introduced to the food.
Alaska has witnessed a steady increase of cases of botulism since 1985. It has more cases of botulism than any other state in the United States of America. This is caused by the traditional Eskimo practice of allowing animal products such as whole fish, fish heads, walrus, sea lion, and whale flippers, beaver tails, seal oil, and birds, to ferment for an extended period of time before being consumed. The risk is exacerbated when a plastic container is used for this purpose instead of the old-fashioned, traditional method, a grass-lined hole, as the Clostridium botulinum bacteria thrive in the anaerobic conditions created by the air-tight enclosure in plastic.
The World Health Organization has classified pickled foods as possibly carcinogenic, based on epidemiological studies. Other research found that fermented food contains a carcinogenic by-product, ethyl carbamate (urethane).
"A 2009 review of the existing studies conducted across Asia concluded
that regularly eating pickled vegetables roughly doubles a person's risk
for esophageal squamous cell carcinoma."
Three
different kinds of wheat and rye flour. From left to right: wheat flour
Type 550 (all purpose flour), wheat flour Type 1050 (first clear
flour), rye flour Type 1150
Flour is a powder made by grinding raw grains, roots, beans, nuts, or seeds. Flours are used to make many different foods. Cereal flour, particularly wheat flour, is the main ingredient of bread, which is a staple food for some cultures. Corn flour has been important in Mesoamerican cuisine since ancient times and remains a staple in the Americas. Rye flour is a constituent of bread in central and northern Europe.
Cereal flour consists either of the endosperm, germ, and bran together (whole-grain flour) or of the endosperm alone (refined flour). Meal is either differentiable from flour as having slightly coarser particle size (degree of comminution) or is synonymous with flour; the word is used both ways. For example, the word cornmeal often connotes a grittier texture whereas corn flour connotes fine powder, although there is no codified dividing line.
Etymology
The English word flour is originally a variant of the word flower, and both words derive from the Old Frenchfleur or flour, which had the literal meaning "blossom", and a figurative meaning "the finest". The phrase fleur de farine
meant "the finest part of the meal", since flour resulted from the
elimination of coarse and unwanted matter from the grain during milling.
History
A field of unripe wheat
The earliest archaeological evidence for wheat seeds crushed between simple millstones to make flour dates to 6000 BC. The Romans were the first to grind seeds on cone mills. In 1779, at the beginning of the Industrial Era, the first steam mill was erected in London.
In the 1930s, some flour began to be enriched with iron, niacin,
thiamine and riboflavin. In the 1940s, mills started to enrich flour and
folic acid was added to the list in the 1990s.
Degermed and heat-processed flour
An important problem of the industrial revolution was the preservation of flour. Transportation distances and a relatively slow distribution system collided with natural shelf life. The reason for the limited shelf life is the fatty acids of the germ,
which react from the moment they are exposed to oxygen. This occurs
when grain is milled; the fatty acids oxidize and flour starts to become
rancid.
Depending on climate and grain quality, this process takes six to nine
months. In the late 19th century, this process was too short for an
industrial production and distribution cycle. As vitamins, micronutrients and amino acids
were completely or relatively unknown in the late 19th century,
removing the germ was an effective solution. Without the germ, flour
cannot become rancid. Degermed flour became standard. Degermation
started in densely populated areas and took approximately one generation
to reach the countryside.
Heat-processed flour is flour where the germ is first separated from the
endosperm and bran, then processed with steam, dry heat or microwave and blended into flour again.
Milling of flour is accomplished by grinding grain between stones or steel wheels.
Today, "stone-ground" usually means that the grain has been ground in a
mill in which a revolving stone wheel turns over a stationary stone
wheel, vertically or horizontally with the grain in between.
Modern mills
Roller mills soon replaced stone grist mills as the production of flour has historically driven technological development, as attempts to make gristmills more productive and less labor-intensive led to the watermill and windmill. These terms are now applied more broadly to uses of water and wind power for purposes other than milling.
More recently, the Unifine mill, an impact-type mill, was developed in the mid-20th century.
Modern farm equipment allows livestock farmers to do some or all
of their own milling when it comes time to convert their own grain crops
to coarse meal for livestock feed.
This capability is economically important because the profit margins
are often thin enough in commercial farming that saving expenses is
vital to staying in business.
Flour contains a high proportion of starches, which are a subset of complex carbohydrates also known as polysaccharides.
The kinds of flour used in cooking include all-purpose flour (known as
plain outside North America), self-rising flour, and cake flour
including bleached flour. The higher the protein content the harder and
stronger the flour, and the more it will produce crusty or chewy breads.
The lower the protein the softer the flour, which is better for cakes,
cookies, and pie crusts.
Bleached flour
"Bleached flour" is "refined" flour with a chemical whitening
(bleaching) agent added. "Refined" flour has had the germ and bran,
containing much of the nutritional fibre and vitamins, removed and is
often referred to as "white flour".
Bleached flour is artificially aged using a "bleaching" agent, a
"maturing" agent, or both. A bleaching agent affects the carotenoids
responsible for the natural colour of the flour; a "maturing" agent also
affects gluten development. A maturing agent may either strengthen or weaken gluten development.
Additives
The four most common additives used as bleaching/maturing agents in the US are:
Potassium bromate, listed as an ingredient, is a maturing agent that strengthens gluten development. It does not bleach.
Benzoyl peroxide bleaches, but does not act as a maturing agent. It has no effect on gluten.
Ascorbic acid
is listed as an ingredient, either as an indication that the flour was
matured using ascorbic acid or that a small amount is added as a dough
enhancer. It is a maturing agent that strengthens gluten development,
but does not bleach.
Chlorine
gas is used as both a bleaching agent and a maturing agent. It weakens
gluten development and oxidizes starches, making it easier for the flour
to absorb water and swell, resulting in thicker batters and stiffer
doughs. The retarded gluten formation is desirable in cakes, cookies,
and biscuits, as it would otherwise make them tougher and bread-like.
The modification of starches in the flour allows the use of wetter
doughs (making for a moister end product) without destroying the
structure necessary for light, fluffy cakes and biscuits.
Chlorinated flour allows cakes and other baked goods to set faster and
rise better, and the fat to be distributed more evenly, with less
vulnerability to collapse.
Some other chemicals used as flour treatment agents to modify color and baking properties include:
"Cake flour" in particular is nearly always chlorinated. At least one flour labeled "unbleached cake flour blend" (marketed by King Arthur Flour)
is not bleached, and the protein content is much higher than typical
cake flour at about 9.4% protein (cake flour is usually around 6% to
8%). According to King Arthur, this flour is a blend of a more finely
milled unbleached wheat flour and cornstarch, which makes a better end
result than unbleached wheat flour alone (cornstarch blended with
all-purpose flour is commonly substituted for cake flour when the latter
is unavailable). The end product, however, is denser than would result
from lower-protein, chlorinated cake flour.
All bleaching and maturing agents (with the possible exception of ascorbic acid) have been banned in the United Kingdom.
Bromination of flour in the US has fallen out of favor, and while
it is not yet actually banned anywhere, few retail flours available to
the home baker are bromated anymore.
Many varieties of flour packaged specifically for commercial
bakeries are still bromated. Retail bleached flour marketed to the home
baker is now treated mostly with either peroxidation or chlorine gas.
Current information from Pillsbury is that their varieties of bleached
flour are treated both with benzoyl peroxide and chlorine gas. Gold
Medal states that their bleached flour is treated either with benzoyl
peroxide or chlorine gas, but no way exists to tell which process has
been used when buying the flour at the grocery store.
Enriched flour
During the process of making flour, specifically as a result of the
bleaching process, nutrients are lost. Some of these nutrients may be
replaced during refining – the result is known as enriched flour.
Cake flour
Cake flour is the lowest in gluten protein content, with 6-7% (5-8% from second source) protein to produce minimal binding so the cake "crumbles" easily.
Pastry flour
Pastry flour has the second-lowest gluten protein content, with 7.5-9.5% (8-9% from second source) protein to hold together with a bit more strength than cakes, but still produce flaky crusts rather than hard or crisp ones.
Plain or all-purpose flour
All-purpose, or "AP flour", or plain flour is medium in gluten protein content at 9.5-11.5%(10-12% from second source)
protein content. It has adequate protein content for many bread and
pizza bases, though bread flour and special 00 grade Italian flour are
often preferred for these purposes, respectively, especially by artisan
bakers. Some biscuits are also prepared using this type of flour.
"Plain" refers not only to AP flour's middling gluten content but also
to its lack of any added leavening agent (as in self-rising flour).
Bread flour
Bread flour, or strong flour is high in gluten protein, with 11.5-13.5%(12-14% from second source) protein. The increased protein binds to the flour to entrap carbon dioxide released by the yeast fermentation process, resulting in a stronger rise and more chewy crumb. Bread flour may be made with a hard spring wheat.
Hard flour
Hard is a general term for flours with high gluten protein content, commonly refers to extra strong flour, with 13.5-16% (or 14-15% from some sources) protein (16% is a theoretically possible protein content).
This flour may be used where a recipe adds ingredients that require
the dough to be extra strong to hold together in their presence, or when
strength is needed for constructions of bread (e.g., some centerpiece
displays).
Gluten flour
Gluten
flour is refined gluten protein, or a theoretical 100% protein (though
practical refining never achieves a full 100%). It is used to strengthen
flour as needed. For example, adding approximately one teaspoon per
cup of AP flour gives the resulting mix the protein content of bread
flour. It is commonly added to whole grain flour recipes to overcome
the tendency of greater fiber content to interfere with gluten
development, needed to give the bread better rising (gas holding)
qualities and chew.
Unbleached flour
Unbleached
flour is simply flour that has not undergone bleaching and therefore
does not have the color of "white" flour. An example is graham flour, whose namesake, Sylvester Graham, was against using bleaching agents, which he considered unhealthy.
Self-raising flour
In the English-speaking countries, self-raising (or self-rising) flour is commercially available with chemical leavening agents already in the mix.
In America, it is also likely to be pre-salted; in Britain this is not
the case. The added ingredients are evenly distributed throughout the
flour, which aids a consistent rise in baked goods. This flour is
generally used for preparing sponge cakes, scones, muffins, etc. It was
invented by Henry Jones
and patented in 1845. If a recipe calls for self-raising flour, and
this is not available, the following substitution is possible:
(US recipes) a pinch to 1⁄4 teaspoon (1 g or less) salt
Types
Gluten-containing flours
Wheat flour
Wheat is the grain most commonly used to make flour. Certain varieties may be referred to as "clean" or "white". Flours contain differing levels of the protein gluten.
"Strong flour" or "hard flour" has a higher gluten content than "weak"
or "soft" flour. "Brown" and wholemeal flours may be made of hard or
soft wheat.
Atta flour is a whole-grain wheat flour important in Indian and Pakistani cuisine, used for a range of breads such as roti and chapati. It is usually stone-ground to coarse granules, which gives it a texture not easily found in other flatbreads.
Common wheat flour (T. aestivum) is the flour most often used for making bread. Durum wheat flour (T. durum) is the second most used.
Maida flour
is a finely milled wheat flour used to make a wide variety of Indian
breads such as paratha and naan. Maida is widely used not only in Indian
cuisine but also in Central Asian and Southeast Asian cuisine. Though
sometimes referred to as "all-purpose flour" by Indian chefs, it more
closely resembles cake flour or even pure starch. In India, maida flour
is used to make pastries and other bakery items such as bread, biscuits
and toast.
Noodle flour is a special blend of flour used for the making of Asian-style noodles, made from wheat or rice.
Semolina is the coarse, purified wheat middlings of durum wheat used in making pasta, breakfast cereals, puddings, and couscous.
Spelt, an ancient grain, is a hexaploid species of wheat. Spelt dough needs less kneading than common wheat or durum wheat dough.
Compared to hard-wheat flours, spelt flour has a relatively low (six to
nine percent) protein count, just a little higher than pastry flour.
That means that plain spelt flour works well in creating dough for soft
foods such as cookies or pancakes. Crackers turn out well because they
are made from dough that does not need to rise when baked.
Other varieties
A variety of types of flour and cereals sold at a bazaar in Bishkek, Kyrgyzstan
When flours do not contain gluten, they are suitable for people with gluten-related disorders, such as coeliac disease, non-celiac gluten sensitivity or wheat allergy sufferers, among others.
Contamination with gluten-containing cereals can occur during grain
harvesting, transporting, milling, storing, processing, handling and/or
cooking.
Acorn
flour is made from ground acorns and can be used as a substitute for
wheat flour. It was used by Native Americans. Koreans also use acorn
flour to make dotorimuk.
Amaranth flour is a flour produced from ground amaranth
grain. It was commonly used in pre-Columbian meso-American cuisine and
was originally cultivated by the Aztecs. It is becoming more and more
available in speciality food shops.
Apple flour is made from milling apple pomace, the solid remains of juiced apples.
Banana flour
has been traditionally made of green bananas for thousands of years and
is currently popular both as a gluten-free replacement for wheat flour
and as a source of resistant starch.
Bean flour is a flour produced from pulverized dried or ripe beans. Garbanzo and fava bean flour is a flour mixture with a high nutritional value and strong aftertaste.
Buckwheat flour is used as an ingredient in many pancakes in the United States. In Japan, it is used to make a popular noodle called soba. In Russia, buckwheat flour is added to the batter for pancakes called blinis which are frequently eaten with caviar. Buckwheat flour is also used to make crêpes bretonnes in Brittany. On Hindu fasting days (Navaratri mainly, also Maha Shivaratri), people eat food made with buckwheat flour. The preparation varies across India. The most famous dishes are kuttu ki puri and kuttu pakora. In most northern and western states the usual term is kuttu ka atta.
Cassava flour is made from the root of the cassava plant. In a purified form (pure starch), it is called tapioca flour (see in list below).
Chestnut flour is popular in Corsica, the Périgord, and Lunigiana for breads, cakes and pastas. It is the original ingredient for polenta, still used as such in Corsica and other Mediterranean locations. Chestnut bread keeps fresh for as long as two weeks. In other parts of Italy it is mainly used for desserts.
Chuño flour is made from dried potatoes in various countries of South America.
Coconut
flour is made from ground coconut meat and has the highest fiber
content of any flour, having a very low concentration of digestible
carbohydrates and thus making an excellent choice for those looking to
restrict their carbohydrate intake. It also has a high fat content of
about 60 percent.
Corn (maize) flour is popular in the Southern and Southwestern US, Mexico, Central America, and Punjab regions of India and Pakistan, where it is called makai ka atta. Coarse whole-grain corn flour is usually called cornmeal. Finely ground corn flour that has been treated with food-grade lime is called masa harina (see masa) and is used to make tortillas and tamales in Mexican cooking. Corn flour should never be confused with corn starch, which is known as "cornflour" in British English.
Cornmeal is very similar to corn flour (see above) except in a coarser grind.
Corn starch is starch extracted from endosperm of the corn kernel.
Glutinous rice flour or sticky rice flour is used in east and southeast Asian cuisines for making tangyuan, etc.
Hemp
flour is produced by pressing the oil from the hemp seed and milling the
residue. Hemp seed is approximately 30 percent oil and 70 percent
residue. Hemp flour does not rise, and is best mixed with other flours.
Added to any flour by about 15-20 percent, it gives a spongy nutty
texture and flavor with a green hue.
Mesquite flour is made from the dried and ground pods of the mesquite
tree, which grows throughout North America in arid climates. The flour
has a sweet, slightly nutty flavor and can be used in a wide variety of
applications.
Nut flours are grated from oily nuts—most commonly almonds and hazelnuts—and
are used instead of or in addition to wheat flour to produce more dry
and flavorful pastries and cakes. Cakes made with nut flours are usually
called tortes and most originated in Central Europe, in countries such as Hungary and Austria.
Peasemeal or pea flour is a flour produced from roasted and pulverized yellow field peas.
Peanut flour made from shelled cooked peanuts is a high-protein alternative to regular flour.
Potato starch
flour is obtained by grinding the tubers to a pulp and removing the
fibre and protein by water-washing. Potato starch (flour) is very white
starch powder used as a thickening agent. Standard (native) potato
starch needs boiling, to thicken in water, giving a transparent gel.
Because the flour is made from neither grains nor legumes, it is used as
a substitute for wheat flour in cooking by Jews during Passover, when grains are not eaten.
Potato flour, often confused with potato starch, is a peeled, cooked potato powder of mashed, mostly drum-dried
and ground potato flakes using the whole potato and thus containing the
protein and some of the fibres of the potato. It has an off-white
slight yellowish color. These dehydrated, dried, potatoes, also called instant mashed potatoes can also be granules or flakes. Potato flour is cold-water-soluble; however, it is not used often as it tends to be heavy.
Rice flour
is ground kernels of rice. It is widely used in Western countries
especially for people who suffer from gluten-related disorders. Brown
rice flour has higher nutritional value than white rice flour.
Sorghum flour is made from grinding whole grains of the sorghum plant. It is called jowar in India.
Tapioca flour, produced from the root of the cassava plant, is used to make breads, pancakes, tapioca pudding, a savoury porridge called fufu in Africa, and is used as a starch.
Teff
flour is made from the grain teff, and is of considerable importance in
eastern Africa (particularly around the horn of Africa). Notably, it is
the chief ingredient in the bread injera, an important component of Ethiopian cuisine.
In
some markets, the different available flour varieties are labeled
according to the ash mass that remains after a sample is incinerated in a
laboratory oven (typically at 550 °C (1,022 °F) or 900 °C (1,650 °F), see international standardsISO 2171 and ICC 104/1).
This is an easily verified indicator for the fraction of the whole
grain remains in the flour, because the mineral content of the starchy
endosperm is much lower than that of the outer parts of the grain. Flour
made from all parts of the grain (extraction rate:
100%) leaves about 2 grams (0.071 oz) ash or more per 100 grams
(3.5 oz) dry flour. Plain white flour with an extraction rate of 50–60%
leaves about 0.4 grams (0.014 oz).
German flour type numbers (Mehltypen)
indicate the amount of ash (measured in milligrams) obtained from 100 g
of the dry mass of this flour. Standard wheat flours (defined in DIN
10355) range from type 405 for normal white wheat flour for baking, to
strong bread flour types 550, 812, and the darker types 1050 and 1600
for wholegrain breads.
French flour type numbers (type de farine)
are a factor of 10 smaller than those used in Germany, because they
indicate the ash content (in milligrams) per 10 g flour. Type 55 is the
standard, hard-wheat white flour for baking, including puff pastries (pâte feuilletée). Type 45 is often called pastry flour, and is generally from a softer wheat (this corresponds to what older French texts call farine de gruau). Some recipes use type 45 for croissants, for instance, although many French bakers use type 55 or a combination of types 45 and 55. Types 65, 80, and 110 are strong bread flours of increasing darkness, and type 150 is a wholemeal flour.
Czech flour types describes roughness of milling instead of amount
of ash, though sometimes a numbering system is used, it is not a rule.
Czechs determine following four basic types of mill: extra soft wheat
flour (výběrová hladká mouka, 00), soft wheat flour (hladká mouka, T650), fine wheat flour (polohrubá mouka), rough wheat flour (hrubá mouka) and farina wheat flour (pšeničná krupice)
Latin American flour uses roughness of milling as well, being 0, 00,
000 and 0000, where the number of zeroes indicates its refinement.
Polish flour type numbers, as is the case in Germany, indicate the
amount of ash in 100 g of the dry mass of the flour. Standard wheat
flours (defined by the PKN in PN-A-74022:2003) range from type 450 to 2000.
In the United States and the United Kingdom, no numbered standardized
flour types are defined, and the ash mass is only rarely given on the
label by flour manufacturers. However, the legally required standard
nutrition label specifies the protein content of the flour, which is
also a way for comparing the extraction rates of different available
flour types.
In general, as the extraction rate of the flour increases, so do
both the protein and the ash content. However, as the extraction rate
approaches 100% (whole meal), the protein content drops slightly, while
the ash content continues to rise.
The following table shows some typical examples of how protein and ash content relate to each other in wheat flour:
Residual ash mass
Protein
Wheat flour type
US
UK
German
French
Italian
Czech/Slovak
Polish
Argentine
Japanese
Chinese
~0.4%
~9%
pastry flour
soft flour
405
45
00
Hladká mouka výběrová 00
tortowa
0000
Hakurikiko 薄力粉
DiJinMianFen低筋麵粉
~0.55%
~11%
all-purpose flour
plain flour
550
55
0
Hladká mouka
luksusowa
000
Churikiko 中力粉
ZhongJinMianFen中筋麵粉
~0.8%
~14%
bread flour or "high gluten flour"
strong or hard
812
80
1
Polohrubá mouka
chlebowa
00
Kyorikiko 強力粉
GaoJinMianFen高筋麵粉
~1.1%
~15%
first clear flour
very strong or hard
1050
110
2
Hrubá mouka
sitkowa
0
kyorikimatsufun 強力末粉
TeGaoJinMianFen特高筋麵粉
>1.5%
~13%
white whole wheat
wholemeal
1600
150
Farina integrale di grano tenero
Celozrnná mouka
graham, razowa
½ 0
Zenryufun 全粒粉
QuanMaiMianFen全麥麵粉
This table is only a rough guideline for converting bread recipes.
Since flour types are not standardized in many countries, the numbers
may differ between manufacturers. There is no French type corresponding
to the lowest ash residue in the table. The closest is French Type 45.
There is no official Chinese name corresponding to the highest
ash residue in the table. Usually such products are imported from Japan
and the Japanese name Zenryufun (全粒粉) is used, or it is called
QuanMaiMianFen (全麥麵粉).
It is possible to determine ash content from some US
manufacturers. However, US measurements are based on wheat with a 14%
moisture content. Thus, a US flour with 0.48% ash would approximate a
French Type 55.
Other measurable properties of flour as used in baking can be determined using a variety of specialized instruments, such as the farinograph.
Flammability
Flour dust suspended in air is explosive—as is any mixture of a finely powdered flammable substance with air. Some devastating explosions have occurred at flour mills, including an explosion in 1878 at the Washburn "A" Mill in Minneapolis that killed 22 people.
Products
Bread, pasta, crackers, many cakes, and many other foods are made using flour. Wheat flour is also used to make a roux as a base for thickening gravy and sauces.
It can also be used as an ingredient in papier-mâchéglue.
Cornstarch is a principal ingredient used to thicken many puddings or desserts, and is the main ingredient in packaged custard.
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:
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.
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:
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.
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.
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.
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