Rheological Properties Of Wheat Flour Biology Essay

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Wheat is the most widely produced cereal in the world. Only in the United Kingdom, wheat is the top produced cereal with a total production of 13221000 MT, followed by barley (5079000 MT) and oats (752277 MT) (FAO, 2007).What makes wheat so special in comparison to other cereals is their protein content, 80% of the total protein content is made of glutenins and gliadin they are known to be responsible for the formation of gluten when preparing the dough. Gluten is a sub microscopic network made by intramolecular disulfide bonds, which makes wheat also unique in their ability to retain gas which is essential for breadmaking (Schofield, 1994).

The level and quality of gluten forming proteins depends heavily on the wheat variety, agricultural practices, geographical location, soil fertility and rainfall. The wheat grain has the following average percentage composition: endosperm 85%, bran 12.5% and germ 2.5%. Depending on its geographical origin, the wheat endosperm content may vary from 63 to 73% starch, and 7 to 20% protein. The main types of wheat are classified according to their baking properties (Table 1).

Therefore, the glutenin content influences directly on the quality of bread, The process of developing bread dough brings about changes in the physical properties of the dough and in particular an improvement on its ability to retain the carbon dioxide gas which will later be generated by yeast fermentation.

Before wheat can be used in the food production as wheat flour, it must pass through mechanical and chemical changes by milling. Flour milling involves the following:

Wheat cleaning: is where impurities such husks, mudballs, small friable seed impurities and any insect fragments are removed through by machinery that use sieve apertures and impactor/scourer that breaks the weaker, insect damaged kernels effectively eliminating them.

Conditioning: Conditioning improves milling and baking quality, adjusting the moisture content by warming applying heat or steaming.

Milling: is used to separate the endosperm from bran and germ and also to reduce endosperm to flour particle size.

Although there are many varieties of wheat, there are three basic factors that determine its qualities for bread making they are: hardness, gluten strength and protein content

Hardness

Because hard wheats require more grinding energy to reduce endosperm chunks into flour-sized particles, a considerable number of starch granules become physically damaged during the milling process. Soft wheats, by contrast, produce flours with low levels of damaged starch. Since damaged starch granules absorb more water than intact, undamaged granules, water absorption is normally significantly higher on hard wheat flours than on soft wheat flours at an equivalent level of protein.

Water absorption is an important quality factor to the baker because it is directly related to the amount of bread he can produce from a given weight of flour. It also has a profound influence on crumb softness and bread-keeping characteristics.

Gluten strength

Doughs made from 'strong' flours tend to be less viscous and more elastic than doughs from weak flours, but generally require more mixing time and energy in order to reach an optimum rheological state. Properly developed doughs have the ability to stretch and form thin sheets that will hold gas and promote production of light, well-risen loaves. Moreover, Dough-handling properties during processing may range from soft and sticky for doughs from weak flours to extremely inelastic and tough for doughs produced from flours that are overly strong. Well-balanced dough properties that fall between these two extremes are required for baking. Mixing requirements must also not be too short or too long.

Protein content

Protein content is important in breadmaking because, other things being equal, high-protein flours have higher loaf volume potential, higher water absorption and produce loaves with good keeping quality. A fairly wide range of protein content may be incorporated in bread flour grists, but wheats of less than 11% protein are usually unsuitable for breadmaking if used alone. These type-determining factors are generally controlled genetically by variety although protein content may vary widely for a given variety according to location, soil fertility, rainfall, etc. Due largely to genetic selection, wheats of high protein content tend to be hard, to have strong gluten and to produce good bread, whereas wheats of low-protein content tend to be soft, to have weak gluten and to produce small loaves of inferior crumb structure.

The routine analysis of flour may include the determination of moisture, ash, added chalk, sulphur dioxide, oil, protein, acidity, iron, thiamine and nicotinic acid, an examination for improvers and bleaching agents and a microscopical examination.

Quality control and flour testing

Wheat and flour specifications are communications between buyers and sellers. These specifications are requirements for particular wheat and flour characteristics, in order to meet these specifications, wheat and flour quality testing is necessary. Specifications for moisture content, ash content, protein content, and falling number are determined with basic tests.

The most important chemical tests are moisture, ash and protein, the importance of which one are explained bellow.

Moisture content: Moisture contents are important for several reasons. Yield of flour and test weight are inversely related of the moisture content. Efficient flour milling requires knowledge of the moisture content of the wheat before processing and distribution of water in various parts of the kernel after conditioning and during milling. The amount of water that grain contains is economically significant and may result in certain advantages and disadvantages of various stages of marketing. And finally moisture content is of utmost importance in safe storage of grains. Grains that contains moisture in excess of the critical moisture level is subjected to rapid deterioration from gold growth, heating, insect damage and sprouting. Moisture content can be an indicator of profitability in milling. Flour is sold by weight, grain is bought by weight, and water is added to reach the standard moisture level before milling. The more water added, the more weight and profitability gained from the wheat. Wheat with too low moisture, however, may require special equipment or processes before milling to reach the standard moisture level.

Ash: Objectives of milling are to separate endosperm from bran and germ and subsequently to reduce endosperm particles to flour. The efficiency of separation can be judged by several empirical, indirect methods based on measuring any constituent that is concentrated to a larger extent in the bran or germ than in endosperm. Since the mineral content of the bran is about 2o time that of the endosperm, the ash test fundamentally indicates the purity of the flour or thoroughness of the separation of bran and germ from the rest of the wheat kernel. The ash test assumed greater importance in the milling trade than any other test for the control of the milling operation. The ash content in wheat and flour has significance for milling. Millers need to know the overall mineral content of the wheat to achieve desired or specified ash levels in flour. Since ash is primarily concentrated in the bran, ash content in flour is an indication of the yield that can be expected during milling. Ash content also indicates milling performance by indirectly revealing the amount of bran contamination in flour. Ash in flour can affect color, imparting a darker color to finished products. Some specialty products requiring particularly white flour call for low ash content while other products, such as whole wheat flour, have high ash contentA weighted sample is incinerated in a muffle furnace at 550C.

Determination of protein: The protein content of wheat and flour are extremely important because almost all flour properties (water absorption, mixing requirements, mixing tolerance, handling characteristics, oxidation requirements and even bread crumb grain) are highly correlated with protein content. Highest protein content is required for wheat flour used in bread making, intermediate for family all-purpose flour and low for cookies and lowest for cakes.

Another important wheat quality factor influencing baking quality is the level of enzyme activity (α-amylase) which is defined sound wheat. Sound wheat contains very low levels of an enzyme that attacks and liquefies starch. Yeast in bread dough, for example, requires sugars to develop properly and therefore needs some level of enzyme activity in the dough. Too much enzyme activity, however, means that too much sugar and too little starch are present. Since starch provides the supporting structure of bread, too much activity results in sticky dough during processing and poor texture in the finished product. If the falling number is too high, enzymes can be added to the flour in various ways to compensate. If the falling number is too low, enzymes cannot be removed from the flour or wheat, which results in a serious problem that makes the flour unusable.

The ultimate test of wheat's suitability for breadmaking is to see if flour milled is capable of producing good bread. However, in selecting a suitable test baking procedure one must first make some difficult decisions: what end-product what formula and what process to use for testing? For the flour miller these decisions may be relatively straight forward since he wants his flour to do well using the formulae and processes used by his customers to make an acceptable end-product as judged by local standards.

Rheological Properties

The main function of ingredients incorporated into the formulation is to modify gluten swelling, starch gelatinization, and interactions between constituents. However, when comparing wheat from various genetic origins or grown in different agroclimatical environments, the rheological properties of formulated dough generally still reflect those of the simpler flour-NaCl-water dough: this is the reason why many experimental studies are devoted to this "simple" dough. Four main parameters are particularly relevant: water content, because water is a plastifying agent, kneading conditions, temperature, and rest time before measurement (BUSHUK, 1998).

Two requirements were outlined by Bloksma (1990) for producing high quality loaves: "1) the dough must have sufficiently large viscosity to prevent the ascent of gas cells, and 2) it must remain extensible for a long enough time during baking to avoid premature rupture of membranes between gas cells." The consluion was that the first condition is met by virtually all doughs while the second can serve as a discriminator for flours of different baking performance.

Standard tests to check the dough quality regarding to its rheological properties are:

Alvenograph: The alveograph determines the gluten strength of a dough by measuring the force required to blow and break a bubble of dough. The alveograph test provides results that are common specifications used by flour millers and processors to ensure a more consistent process and product. The alveograph is well suited for measuring the dough characteristics of weak gluten wheats

Extensograph: The extensograph determines the resistance and extensibility of a dough by measuring the force required to stretch the dough with a hook until it breaks. Extensibility indicates the amount of elasticity in the dough and its ability to stretch without breaking.

In conclusion, bread wheat quality can be expressed on different perspectives: The consumer wants a high quality product on the shelf that suits his personal taste and he will expect the same taste every time he buys that same product with no surprises. As for the baker, he will consider quality wheat flour the flour that gives him the best bulk production using the same recipe without having to change it every time he gets a new batch of flour. The key to bread eating qualities lies in the manipulation of the ingredients and processing methods which are used in bread production. The numbers of potential interactions are very large, and it remains difficult to predict the end result with confidence.

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