Wheat and rye flour


1. Raw materials

Wheat flour

The wheat grain consists of three main parts: the endosperm, the bran, which is mostly wall tissue and the wheat's germ. The germ is rich in oil and it contains lots of vitamins. The endosperm is where the starch is stored. It also houses the protein that transforms into gluten during kneading. The wall tissue encapsulates the endosperm. The wall is made from several layers. The aleuron layer separates the endosperm from these adjoining layers. The wall contains a significant amount of fibre.

The aleuron layer is also rich in proteins and vitamins. Wheat is ground to separate the endosperm from these wall layers and the germ. During the grinding the endosperm is reduced to what we know as flour, after grinding the wall layers and germ make up the bran portion.

tarwekorrel The grinding is done in several stages. In order to have complete grinding the wheat kernel passes through 20 sets of rollers. The first 5 or 6 roller pairs are corrugated and serve to break the grains and to turn tiny chunks of endosperm into grist and bran particles. This part of the grinding is called crushing. After every passage the grits is sifted in sifters, where the flour is separated from the larger particles that need more grinding. In the course of the grinding process the more or less pure pieces of endosperm are reduced to flour by smooth rollers. The fraction of the wheat that is converted to flour is determined in this part of the process.

In theory flour is produced in every pass and sifting. Because of the 20 passes, there will be also 20 flour fractions that will differ in many aspects such as : the contents of the proteins, ash, and fibre. The fractions are commonly characterised by their ash content. Flour from the first passage has the lowest ash content. During the manufacture of a specific flour product from a mix of flours, the ash contents are used as a guideline. For instance the flour packaged as Blue Stem has a relatively high ash content. It is not from a species of wheat, but from a mixture of different varieties of wheat.

Whole wheat flour is made of the whole wheat grain, so the ash content is the same as that of wheat, about 1,8%. The ash content drops if the share of the wall layers in the grinding material diminishes. The lowest ash content is found in so called plain flour (0,46%). The inner part of the endosperm is the main constituent of plain flour. A higher degree of grinding goes along with a higher water up take. Most of the fibres end up in the remaining bran. Those fibres can absorb relative large amount of water. The protein content also increases with the degree of grinding. This is quite logical : the outer layers contain much more protein than the more internal layers. Especially in and directly underneath the aleuron layer one finds tissues with a very high protein content. This high protein content does unfortunately not improve the baking properties . The protein from the aleuronic doesnít form gluten during the dough preparation and has consequently no contribution to the baking properties.

The wheat grain consists of about 8 to 15% of protein. The proteins are found enclosed between the starch grains. Wheat protein comes in a large variety of different molecules, with markedly divergent properties. The most import can be distinguished by their specific solvent :

  1. albumin : dissolves in water
  2. globulin : dissolves in salt solutions
  3. gliadin : dissolves in 70 % alcohol
  4. glutenin : dissolves in diluted acids or bases

The first two form about 20% of the total amount of protein, while the glutenine and gliadin contribution is the remaining 80 % . The latter are responsible for the formation of the gluten during kneading. The gliadin and glutenine form a network of molecules capable of holding the carbon-dioxide gas that is a by-product of fermentation. It is this network of lace that causes the bread to rise.

But the amount of protein present doesnít reflect anything about its quality. The baker talks about baking properties of the flour. For example, a flour containing 12% protein, ground from European wheat will, in general, result in a flour with less baking quality, than a similar flour grind from American wheat having the same protein content. This different behaviour is mainly caused by the difference in wheat varieties grown in Europe and America.

In every day practice one always selects those varieties that are most suitable for a certain application. Flours having a low protein content or a bad protein quality are rarely used in bread preparation. These flours result in bread quality that falls below normal standards. They can be used in applications where the gluten development doesnít play such an important role. What comes to mind are cookies or cakes. In those cases the baker often uses flour with a protein content of 10 % and below. For the preparation of most kinds of bread a flour with a protein content between 11 and 13% is used. For special applications like biscuits one uses flours with a protein content in the 15-16% range.

tarwekorrelA number of substances improve the baking properties of the flour. They are called dough conditioners. The use of those dough conditioners makes the dough more substantial, the rise gets larger and the crumb structure gets finer. These substances normally reinforce the gluten network. The improvement of the dough properties is especially important in case of mechanical mixing and kneading. The larger volume and the finer crumb influence mainly the eating properties of the bread. It becomes softer and less crumby.

All flour improvers are oxidants i.e. they produce oxygen when mixed with water and when energy is added to the system (by the mixing). The oxygen in its turn reacts with protein in the flour. All the fine details of this chain reaction are still unknown. The action of the flour conditioners can be compared with what happens during storage of flour. It is a well-known fact that the baking properties of the flour improve when it is stored for some time after grinding. This is attributed to the oxidation of proteins by oxygen from the air. Nowadays flour is already used within one week after grinding, so it doesnít get enough time to ripen. Moreover, the effect of natural oxidation is quite small compared to the action of dough conditioners. However, a too large dose causes a deeply advanced oxidation which makes the dough to stiff. It becomes overly unworkable.

In the past and especially in the USA potassium bromate (KBrO3) was used as oxidizer. It is the most effective substances of all the dough conditioners known. It gives the dough good working up properties and the bread a fine, soft and regular crumb. And it has a slow reaction. After kneading and even after the final prove a portion of the KBrO3 still not has reacted. Only during the baking is the bromate converted to bromide (KBr). This means that the potassium bromate is doing a part of its job during the ovenspring. Continuing to soften and elasticise the proteins. Stiff doughs have a good use in bakeries where Pullman style or rectangular loaves are produced. Ovenspring is assured by the use of the dough conditioners. Interestingly, it is mostly seen in use in the Netherlands and the Anglo-Saxon or English speaking countries. During the 1980s bromate and many of its compounds became suspected as health threatening and prohibition of the use of potassium-bromate followed.

In countries where hearth baked breads is the favourite kind of bread the baker uses ascorbic acid (vitamin C) as dough conditioner. It is totally harmless. Heat decomposes the ascorbic acid completely. During the baking of the bread all vitamin C is lost. The function of ascorbic acid is the same as any other, oxidizing the dough.

The gluten lace is formed through sulphur bridges. Too many S-S bridges makes the dough stiff and sturdy. In the dough thiol groups (SH) influence of the mobility of the protein molecules with respect to each other. This can be demonstrated by adding extra thiol groups in the form of cystein to the dough. The dough gets much more flabby. Oxidation removes thiol groups, two SH groups are converted to one SS group plus water. The mobility of the proteins decreases, thereby making the dough less pliable.

Ascorbic acid differs in one aspect from other dough conditioners. In itself is it not an oxidizer but a reducer. Its activity stems from a preceding reaction during kneading with oxygen in the air. It is than converted to dehydro-ascorbic acid and that is an oxidizer. When this substance oxidises the proteins it is reduced back to ascorbic acid. The formation of dehydro-ascorbic acid is only possible during the kneading. This is the only moment that oxygen from the air can be beaten into the dough. After the kneading the yeast consumes all the remaining oxygen, leaving none in the dough during the rise.

The damaged starch content of wheat flour is an important aspect of the flour quality. The action of Ŗ-amylase on undamaged starch and ungelatinised starch granules is very slow. a-Amylase does attack granules with no visible damage at an appreciable rate. Both enzymes attack gelatinised starch very rapidly but this reaction cannot be of much importance in the bread making process because the starch in dough does not become gelatinised until virtually all of the enzyme activity has been destroyed by heat. Starch granules which have been mechanically damaged during milling are also broken down by both enzymes. Perhaps 3 or 4 % of the starch granules is visibly damaged. This mechanical damage is due to shearing forces and pressures encountered during the milling process. Consequently the proportion of damaged granules is a function of the milling conditions and may vary not only from mill to mill but also between flours of different extraction rates. However flours of similar extraction made from the same types of wheat (hardness of the wheat kernel) and ground at the same mill should contain a rather constant proportion of damaged starch.

Rye flour

Rye flour is milled much like wheat flour. Light rye extracted from the inner part of the kernel, with a very fine texture and a high starch percentage but little protein, corresponds to patent flour. Medium rye is straight flour, milled from the whole rye grain after the bran has been removed. Dark rye is like clear flour, which is milled from the part of the rye grain closest to the bran. Rye meal, or pumpernickel flour, is a dark coarse meal made from the entire rye grain. Rye blend is a mixture of rye flour (generally about 25 - 40%) and strong wheat flour, such as clear flour, for making a lighter rye bread.

The basic composition of the rye grain is similar to that of wheat (see table below). Compared to wheat, rye generally contains less starch and crude protein but more dietary fibre. The dietary fibre. content of rye is actually the highest among the common cereals. The major dietary fibre. constituents of rye are arabinoxylan, β-glucan, cellulose, fructan, and lignin (Åman and others 2010). Rye contains more fructan, mixed-linked β-glucan, and arabinoxylan but similar amounts of cellulose and lignin as compared to wheat (Nilsson and others 1997a). The range of dietary fibre. content in different rye varieties in the EU-Healthgrain project variety screen was 20,4–25,2% (Nyström and others 2008). The dietary fibre. in cereal grains is located mainly in the outer layers of the kernel, especially in the bran. Wheat and rye have a similar bran content, but rye contains more cells within the endosperm, and thus has a higher dietary fibre. content (Nyman and others 1984; Åman and others 1997). The distribution of rye proteins in the Osborne solubility classes indicates that in comparison to wheat, rye contains less glutelins. Rye prolamins are not considered to play a role in rye baking either because of their low concentration or their interactions with other components (Shewry and Bechtel 2001). Rye starch is similar in its structure and properties to starches from wheat and other cereals. It consists of two types of polymers, amylose and amylopectin, with amylose accounting for 24–26% of total (Shewry and Bechtel 2001).

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Average composition of wheat and rye

%

wheat

rye

ash

2

2

fat

3

2 - 3

protein

12 -15

8 - 13

starch

65 - 70

55 - 65

dietary fibre

13 - 17

19 - 21

beta-arabinoxylans

6

8 - 10

glucan

0,8

2 - 3

cellulose

2,5

1 - 2

fructan

1,5 - 2,5

4 - 6

Arabinoxylans are the major fraction (85–90%) of the so-called pentosans. Different cereal species contain different amounts of arabinoxylans. The highest contents are present in rye (6–8%), whereas wheat contains only 1,5–2,0 % arabinoxylans. On the basis of solubility arabinoxylans can be subdivided into a water-extractable and a water-unextractable fraction. The former makes up 25 – 30% of total arabinoxylans in wheat and 15 – 25% in rye. In particular water extractable arabinoxylans have considerable functionality in bread-making.

The unique technological properties of arabinoxylans are attributable to the fact that arabinoxylans are able to absorb 15 – 20 times more water than their own weight and, thus, form highly viscous solutions, which may increase gas holding capacity of wheat doughs via stabilisation of the gas bubbles. This is the basic reason why it is a good idea to add 5 - 10 % of rye flour to a bread made from wheat flour. In total, water extractable arabinoxylans (or pentosans) bind up to 25% of the added water in wheat doughs . Under oxidizing conditions, in particular under acidic pH, the so-called “oxidative gelation” leads to arabinoxylans gel formation by inducing di- and oligoferulic acid cross-links . This is thought to be one major structure-forming reaction in rye sourdoughs. Because of covalent crosslinks to the cell wall structure water unextractable arabinoxylans do not dissolve in water. Although they have high water-holding capacity and assist in water binding during dough mixing they are considered to have a negative impact on wheat bread making as they form physical barriers against the gluten network and, thus, destabilise the gas bubbles.

However, the baking performance can be affected by adding endoxylanases, which preferentially hydrolyse water unextractable arabinoxylans. This produces solubilised water unextractable arabinoxylans, which have techno-functional effects comparable to water extractable arabinoxylans. Beside arabinoxylans the pentosan fraction contains a small part of a water-soluble, highly branched arabinogalactan peptide. Unlike arabinoxylans, arabinogalactan peptides have no significant effects in cereal processing.

Baking with rye is different from baking with wheat in many respects. The main difference between rye and wheat flours is that rye protein cannot form a continuous network and an elastic dough. Instead rye pentosans (mainly arabinoxylans) are able to bind water during mixing to produce a dough that can be baked into bread.



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NoŽl Haegens

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