Wheat Flour

IntroductionRank_Hovis

White wheat flour is the largest ingredient in nearly all biscuits. It is made from wheat grains by removing the brown surface coating (the bran) and the little yellow embryo and reducing the particle size to a fine powder. Modern milling methods are very complex involving many stages principally because it is difficult to separate the bran from the white central part (known as the endosperm) of the wheat grain.

Wheat varies in its quality as a result of variety, farming practice and climatic conditions. To a certain extent the flour miller can select the wheat to make a flour of desired properties but also the conditions in the mill affect the flour produced. It is possible to make satisfactory biscuits from most types of wheat flour but differences in protein content and moisture, in particular, affect the consistencies of doughs so variations in flour quality are of great concern to biscuit makers. It is not yet possible to define precisely the quality components of flour required for particular biscuits and it is also not possible for flour millers to consistently match, exactly, suggested requirements. Thus, our main aim, in terms of flour quality, is to have flour with consistent character from load to load through the year.

Types of wheat flour

The wheat endosperm is principally a mass of starch grains in a protein matrix. Upon milling this breaks down into a mixture composed of fine protein particles, individual starch grains and conglomerations of both. In order to achieve efficient milling the wheat moisture content must be controlled carefully. The resulting flour normally has a moisture content of between 13 and 15%. 14% moisture is an average moisture content for biscuit flour but it is almost impossible for the flour miller to control the moisture content to better than ±0.5%. When a dough is made with water, both the starch and the protein absorb moisture and mechanical action, in the mixer, on the hydrated protein forms a stretchy and rubbery material, known as gluten. Gluten is essential for the production of leavened baked goods and this is what sets wheat flour apart from nearly all other flours. Gluten quality varies in different flours from being very extensible (it can be pulled out a long way before breaking) to being very inextensible or short, and also being more or less elastic (it will return to its original state after being stretched).

The protein content and the quality of the gluten in flour is dependent on the type of wheat used. Wheat varieties sown in the spring in hotter climates than northern Europe, tend to produce hard grains with high protein contents that give elastic but inextensible gluten. These are known as hard wheats and they produce what is termed strong flour. Strong flour is best for bread making and is also used, in part, for fermented biscuit doughs such as crackers.

Conversely, wheat varieties sown in the autumn/winter, the most common varieties grown in northern Europe, tend to produce softer grains with lower protein contents. The wheat is known as soft wheat; the resulting flour is much more fluffy and soft and the gluten formed from this flour is more extensible and less elastic. The flour is known as medium or weak flour. These are the flours favoured for most biscuit production. They are also less expensive as the price of flour is much related to the protein content.

Domestic flour is normally a medium or weak flour. Self raising flour has baking powder added.

The protein content of strong flours is usually in the range 10.5 to 14.5%. Flour from winter wheat is mostly in a medium category of 8.5 to 10.5% but weak flours with less than 8.5% are found. The protein content of flour is determined by a laboratory test that measures the amount of nitrogen present and a conversion of this value can be made to give an estimation of the amount of protein present. Assessing the quantity of protein from "wet gluten" by washing away the starch in a dough is not a reliable method. However this method can give an idea of how extensible or elastic the gluten is.

As was mentioned above, it is the aim of the miller to remove the bran from the endosperm during milling. There is about 82% endosperm in a wheat berry but in typical milling of a white flour only about 72-74% of the berry can be taken as flour. Even then there are some minute bran particles in the flour. The amount of flour obtained from the wheat is known as the extraction rate. The higher the extraction rate the more bran there will be in it. The bran particles give the flour a grey colour so the higher the extraction rate the less white the flour is.

Bran contains protein that cannot be hydrated and turned into gluten. Bran and the outer layers of the berry also have a mineral (inorganic) and fibre content higher than the main mass of the endosperm. The fibre is indigestible by humans but, particularly in recent years, it has been shown to be useful to our digestive health.

The amount of bran in flour is estimated either by a test to measure the ash, (mineral) content (the ash test) or by a test to measure the reflectance of light from a water slurry made from the flour (the colour grade). The latter test is much quicker and is based on the fact that flours with more bran appear greyer. Assessing the mineral content, the ash test, involves burning away all the carbohydrates and proteins. In Britain legislation demands that a small amount of calcium carbonate, iron salts and vitamins are added to all but wholemeal flours so the ash test is not a reliable indicator of the flour extraction rate. Flours are graded by mineral continent in mainland Europe and in many other countries in the world.

Brown flours have bran deliberately left in the flour, or more usually added back to white flour. Wholemeal flour is composed of 99% of the wheat grain. In order to provide bran which is well graded it is common practice to make brown flours by adding back bran of a particular size range. Thus very large pieces or powdery particles of bran are omitted. Brown flours, except wholemeal, contain bran but no germ.

There are some flours which have the germ (embryo) particles returned to it. The germ is rich in fat and vitamins. The embryo is also rich in lipase, the fat splitting enzyme, so in order that the germ does not become rancid too quickly the germ is normally heat treated to destroy the enzyme before it is returned to the flour. A typical germ enriched flour is Hovis. It is much more expensive than other flours but is nutritionally superior. It will be seen that wholemeal flour that is made by simply grinding the wheat berries into flour will contain the germ in a state that is unstable to fat rancidity. Thus wholemeal flour made like this (for example by stone grinding) have a very limited storage life.

A character of flour that is of great importance to bakers is the water absorption. This is a measure of the amount of water that is needed to produce a flour and water dough of a given consistency. There are a number of factors that determine the water absorption of a flour. These include,

  • the flour moisture content; the damper the flour the less extra water that is needed to make a dough with the required consistency.
  • the protein content; the higher the protein the more water is required.
  • the damaged starch content. The starch in flour is present mostly as minute oval grains with a structural film around them to maintain their shape and integrity. During milling some of these grains become mechanically damaged and in this state the grains will absorb much more water than the undamaged ones. There is typically much more damaged starch in flour milled from hard wheat than from soft wheat. It is possible for the miller to change the amount of starch that is damaged by adjusting the pressure of the grinding but it is very difficult for him to increase, significantly, the amount of damaged starch in soft wheat flours.
  • the flour particle size distribution. The water absorption is higher in flour with a fine particle size than a coarse flour just because of the surface area factor. It should be mentioned, however, that it is a complicated procedure for a miller to change the particle size range of the flour he makes so it is normal that all the flour from a particular mill has a similar particle size range irrespective of the types of wheat milled.

Thus, the mill products with which biscuit makers may be concerned are,

Strong flour 10.5-14.5% protein

Medium flour 8.5-10.5% protein. This is the type of flour often referred to a Biscuit Flour

Weak flour less than 8.5% protein

Wholemeal flour, 99% extraction

Wheatmeal flour, more than 80% extraction but of any chosen value

Bran-thumbWheat bran, of various particle sizes, known as fine, medium or coarse bran. Bran is always handled in bags.

wheat-germ-gluten-thumbGerm enriched flours. These are occasionally used for nutritional, flavour or marketing reasons in biscuits. Some germ (which will always include some bran) is added to white flour. Because of the higher fat content the storage life is much shorter than white flour.

Flour treatment

Wheat flours may be treated at the mill for special purposes.

  • Heat treated flour. Heating the flour partially or totally destroys the ability of protein to form gluten when it is hydrated and mixed. This treatment is made under very controlled conditions at the mill. The heat treatment also increases somewhat the water absorption of the flour. Use of this flour results in softer eating textures in short dough biscuits and reduces spread during baking. Heat treated flour has no value for crackers or semisweet biscuits but may be used, to a very limited extent, as a filler in creams for sandwiched biscuits.
  • Chlorinated flour. Treatment of flour with gaseous chlorine has a similar effect to heat treatment in that it destroys the gluten forming property of the protein. It also swells the starch grains and increases the water absorption character of the flour. This flour used to be widely used for cake flours in the UK but is now banned throughout Europe. In the USA by chlorine treatment of flour the amount of spread produced during the baking of short dough biscuits and cookies is adjusted.
  • Air classified flour. By a technique of centrifugal separation it is possible to collect fractions of a flour with different particle size ranges. The principal reason for this is to collect the fine particles which are rich in protein fragments however it is also of interest to bakers because the coarser fractions have excellent free flowing properties. The coarse fraction of air classified flour produces a good dusting flour, flour that is sprinkled in a fine and uniform manner onto the dough surface to reduce sticking onto gauge rollers etc.
  • Vital wheat gluten. In order to boost the gluten or protein content of flour a technique has been developed to extract gluten and then to dry it in a manner that does not denature the viscoelastic properties. The powder is called vital wheat gluten. It is used principally to improve bread flours but it may be useful for strengthening flour to be fermented for crackers. When using vital wheat gluten to fortify a flour it is most important that the powder should be well dispersed into flour before it is wetted and a dough is mixed. If vital gluten is wetted undiluted it forms tough rubbery balls that cannot subsequently be dispersed properly through a dough.

Flour enrichment

Current UK legislation requires that all flours for human consumption are enriched. The additions are; 24g thiamine, 16g nicotinic acid, 16.5g iron (ferric ammonium citrate or ferrous sulphate) and 2.35 to 3.9kg calcium carbonate per 1000 kg flour. (Calcium carbonate is not added to wholemeal flour.)

As mentioned above this enrichment precludes the use of the ash test for estimating the bran content in flour.

Uses and functions of wheat flours for biscuits

Wheat (or oat) flour is a component of all biscuits, crackers and cookies. Baked products that are similar to biscuits but which do not have flour are called bakery confections, examples are meringues and macaroons.

  • Fermented doughs for Cream Crackers and Soda Crackers use medium to strong flour to produce a dough capable of producing a good open texture during baking after a period of fermentation with yeast.

Bran is sometimes included in the recipe to give flavour and appearance. The finer the particles of bran the less it is seen and the more it interrupts the dough structure giving sheeting problems and reduction of thickness development during baking.

  • Puff doughs also use medium to strong flour for the same reason but these doughs are not fermented.
  • Savoury crackers require a medium strength flour for their texture.
  • Semisweet types require only a medium or weak strength flour preferably with an extensible and not very elastic gluten quality. However, the gluten quality can be modified, to a certain extent, chemically during mixing (refer to the problems sections) or by enzyme treatment after the dough has been mixed (refer to the problems sections). Flour high in protein or giving a gluten that is not sufficiently extensible will tend to give doughs that are difficult to sheet and produce biscuits with poor shape because the dough shrinks after cutting and during baking. There is increasing difficulty in obtaining good flour for semisweet biscuit doughs because wheat breeding centres on production of flour suitable for bread production. This type of flour commands a higher price than the best biscuit flours. Also the wheat used for this flour usually has a higher protein content which makes it more valuable for animal feed usage.
  • Short doughs are not so specific in their flour requirements. There is little water in the dough so only a little gluten development occurs. The mixing technique is designed to ensure that minimum gluten forms as the protein in the flour hydrates.

Brown flours or bran is commonly used in short doughs. The bran contributes to the eating texture and flavour. In nutritional terms it also adds dietary fibre. The larger the bran particles the more it affects the appearance of the product.

  • Wafers require medium to low protein flours. If the protein is too high or too strong there will be a tendency to bake hard wafers and if too low the wafers may be very fragile.

Several types of biscuit require brown flours. The basic strength of the flour is as stated above but there are significant levels of bran in the flour. Biscuits made with brown flours do not develop to the same thickness during baking as do similar recipes with "white" flour. This is because the bran particles interrupt the structure and impair the formation of fine gas bubbles that are the basis of the structural development.

Brown flours are less satisfactory to handle in bulk, so it is common practice to add an appropriate quantity of bran to white flour as a separate ingredient.

In this way the majority of the flour can be bulk handled and the bran that is added can be of the desired particle size.

Handling of flour

Handling of flour requires attention to health hazards to both factory employees and to customers who will eat biscuits made from it.

Firstly, the health of factory employees. Flour is dusty and it is not desirable that too much flour dust is breathed. Some people are allergic to flour dust and develop respiratory diseases. When handling flour, keep the amount of dust produced to a minimum and make sure it is contained. Mixtures of flour dust and air can, in rare circumstances, form explosive mixtures. A spark or flame in such an area will have disastrous results. Such dust mixtures will only occur if the flour handling area has not been kept clean, especially in terms of flour lodged on overhead beams or shelves, or within pneumatic handling equipment. It is essential that all pneumatic handling pipes etc. are well earthed (grounded) to eliminate the possibility of sparks arising from static electric discharges. In case an explosion occurs within a silo, explosion relief doors are designed into the top of the silo. The doors will open and prevent damage to the silo itself. Fortunately such explosions are extremely rare.

Hazards can arise from unwanted inclusions in the flour. These may include, pieces of paper, string, hair, metal etc. Flour emptied from bags should be put through a sieve and particular care should be taken to place string and labels, released when the bags are opened, into special waste bins and not on the floor. Material on the floor can easily adhere to the bag surface and fall into the flour as the bag is tipped.

Bulk handled flour is normally pneumatically handled from silo to weigher. Flour is taken from the bottom of a silo, or after a sieving system, via a rotary seal into a stream of air and is blown to a hopper which is mounted on a weighing system, probably above a mixer. The rotary seal is necessary to prevent the air in the conveying pipe blowing up into the silo. The flour is taken out of the air at the hopper because the hopper is designed as a cyclone. The flour/air mixture is fed in at the edge of the conical shape and as it swirls round the flour falls out and more or less clear air is blown out at the centre of the top. The cyclone separates the flour from the air by centrifugal forces and also because the air speed is suddenly much lower in the larger space of the hopper. Separation of the flour is very effective. The air is taken from the cyclone by a pipe that either returns to the silo and passes through the filter there or into a special filter. It is common to return the air to the silo because in this way, using a diverter valve, excess flour can also be returned. This allows the hopper to be filled quickly and, at the moment that weight is made, the valve closes the feed and sends any excess flour into the line back to the silo. If a signal to stop the feed of flour from the silo only stopped the rotary seal under the silo there would be a long delay while the flour in the line cleared and the system shut down. Also the weight arriving at the hopper would be much less precise. In a system where several hoppers are filled from a silo, or more than one silo is involved, there are more diverter valves all programmed to open or close as appropriate.

In most systems a sieve is included in the pneumatic transfer system so that lumps of flour or any other unwanted matter is removed. In these cases the discharge from the silo is usually by a worm conveyor into the sieve and the flour which passes through the sieve is fed via a rotary seal into the conveying pipeline. A magnet will be located somewhere in the system, usually before the sieve, to collect any tramp iron and steel particles.

Discharge from the silo is aided either by a vibrator attached to the base of the silo or by means of a fluid bed at the base of the silo. The fluid bed operates by compressed air being blown through porous tiles up into the silo thus lifting and moving the flour so that it falls into the feeder which takes it away. If the silo is poorly designed or the flour has compacted in the silo discharge may be impaired and through "bridging" the flour will not fall into the feeder and the silo may seem to be empty before it really is.