The proteins present in wheat comprise the albumins, globulins, glutelins (glutenins) and the prolamines (gliadins). Glutenins and glia-dins are commonly referred to as the gluten-storage proteins because they combine with water to form the gluten protein network that is crit-ical for the retention of air and carbon dioxide gas in the dough during breadmaking. As well as undergoing hydration, it is necessary to impart energy to a flour–water mix in order to develop a gluten structure.
The transition from (relatively) dry flour proteins to a dough in which gluten has been formed is best appreciated by undertaking mixing of bread dough components by hand. The stages of the process are illus-trated in Fig. 2.1 and may be described as follows:
(1) Place the flour on a flat table and spread it out so that a round, empty
‘bay’ is formed in the centre.
(2) Pour the required amount of water into the bay, add the salt and mix to dissolve it. Disperse the yeast in the salt solution.
(3) Starting on the inside of the bay gradually draw flour into the dough water and mix together.
(4) Continue this process until all the flour has been brought into contact with the dough water. At this stage there will be some dry and some wetter patches but we have not yet formed a dough.
(5) The loose mixture (known as the ‘flock’ stage) now requires the input of energy through the process of kneading. The dough mass requires
(a)
(c) (d)
(b)
Figure 2.1 Stages of handmixing bread doughs.
progressive working backwards and forwards on the table top, fold-ing in any dry or wet patches until a smooth and homogeneous mass has been formed. This may take 15 or 20 min by hand, but is con-siderably quicker with a machine.
By the end of the process, a dough will be formed, but it will still be necessary to modify its rheological character in order to optimise its gas-retaining properties. A number of different processes may be used to achieve the required modification, and these form the basis of the different breadmaking processes that are in use around the world (Cauvain, 2007a).
The gluten proteins present in wheat flour are embedded in the flour particles along with the other flour components, mainly starch granules.
When an excess of water comes into contact with the flour particles, there is a gradual uptake of water. The precise nature of the gluten protein–
water interactions is complex and still unclear. Bernardin and Kasarda (1973) observed that flour particles under the microscope ‘exploded’
when brought into contact with excess water, and strands of protein were rapidly expelled into the aqueous phase. If subjected to linear stresses (e.g. by moving the cover-slip on the microscope slide), the protein strands stretched. The rate of hydration of the protein strands
depends on the ratio of water to protein and proceeds more rapidly when there is an excess of water. In breadmaking, the added water level does not usually exceed the flour weight and so it is unlikely that the features observed by Bernardin and Kasarda are the sole mechanisms by which protein and water come into contact with one another.
Stauffer (2007), in reviewing the principles of dough formation in breadmaking, described the protein in flour as existing ‘as a flinty ma-terial’ which softened during hydration. This description is consistent with the observations of Hoseney et al. (1986), who considered that as water is taken up by the wheat gluten proteins, they pass through a glass transition stage changing from a hard glassy material to a soft rubbery one. Hoseney and Rogers (1990) suggested that for both hand-washed and commercial glutens, this occurs at about 16% moisture con-tent at room temperature (20◦C). It is worth noting that flour and water temperatures at the start of mixing in many breadmaking processes may well be somewhat lower than room temperature, so that the Tg
for gluten will occur at moisture contents above 16% but in most cases probably still within the ‘typical’ water-to-flour ratios that we see in breadmaking.
Starch granules are also embedded in the flour particles and dur-ing mixdur-ing they may become detached. This effect is seen when hand-washing gluten from flour in an excess of water and a milky-white liquid comes out from the dough matrix. This liquid comprises mainly starch granules suspended in water. In normal doughmaking, the water con-tent is not in excess of the flour weight and the process is carried out in some form of container so that any starch granules lost from the softened flour particles will soon be swept up again as mixing continues. In the end, the developed dough matrix essentially comprises a gluten protein network on to which are attached starch granules and hydrated flour particles. Also distributed throughout the dough are the flour lipids, fibre and soluble materials (e.g. any naturally occurring sugars).
As discussed earlier, a proportion of the starch granules present in flour have experienced some mechanical damage as a result of the milling process. The ability of these damaged granules to absorb wa-ter is increased fivefold. In addition, the damaged granules will absorb water faster than those that are undamaged. The absorption of water by starch is necessary for it to undergo the gelatinisation process that occurs on heating (this is discussed in more detail in Chapters 3 and 4). The uptake of water by starch granules provides an element of com-petition for the available water with the flour proteins, although the effect of this on the rate and degree of gluten development is small.
Nevertheless, high levels of damaged starch are known to have adverse effects on bread quality, such as a greying of the crumb and loss of cell structure fineness. Farrand (1969) suggested that there was an optimum
relationship between flour protein content and damaged starch level, and considered that the latter should not exceed the quantity of pro-tein squared and divided by 6. This mathematical relationship is no longer considered completely valid, but there is no doubt that flours with higher protein contents are more able to cope with higher starch damage levels and produce better bread quality, especially with no-time doughmaking processes.
The damaged starch granules found in flour are susceptible to at-tack from alpha-amylase enzymes, which may be naturally present in the flour (cereal) or added by the miller or baker (fungal, maltogenic or bacterial). Breakdown products from this enzymic action are dex-trins, which may lead to problems with stickiness and collapse in bread (Cauvain and Young, 2006); maltose, which provides a substrate for the yeast; and water. The degree to which alpha-amylase activity oc-curs in bread doughs depends on the level of initial activity present, the availability of substrate (i.e. the damaged starch level), the tempera-ture of the dough and the time available for the reaction. In general, the timescale of mixing is too short for significant alpha-amylase action to oc-cur, even with the higher dough temperatures that are encountered with no-time doughmaking processes. When longer timescales are involved in breadmaking, for example during bulk fermentation (floortime) or proof, or where temperatures are raised, i.e. during proof and baking, then the effects of amylase activity on dough viscosity are more pro-nounced. Such effects are considered later in this chapter and again in Chapter 4.
Approximately 2–3% of the weight of flours comprises a mixture of water-soluble proteins and pentosans. Their roles in breadmaking are unclear and complex. The water-soluble pentosans have a significant water-absorbing capacity, which Stauffer (2007) estimated to be around seven times their own weight, and are able to form viscous solutions.
The latter effect is of particular value in the formation of rye bread structures, where the gluten-forming potential of the flour is limited (see below).
The fibrous components of the wheat grain, which derive from the bran skins, also absorb water during mixing but more slowly than other flour components. This slower hydration is evident when wholemeal flour doughs are taken from the mixer and are then processed. Initially such doughs have a surface which is sticky to the touch, but this stick-iness is gradually lost as the character of the dough changes with time after mixing. A practical consequence of this change is that the doughs become more viscous, or ‘stiffer’, as if the doughs are lacking sufficient water. This adverse effect on dough rheology can lead to sub-optimal bread quality following the interaction of the dough with any handling or moulding equipment (see below).
Marsh and Cauvain (2007) summarised the requirements of mixing as:
r
to disperse uniformly the recipe ingredients;r
to encourage the dissolution and hydration of those ingredients;r
to contribute energy to the development of a gluten structure in the dough;r
to incorporate air bubbles within the dough;r
to provide a dough suitable for processing.Water addition has an impact on all these aspects of mixing, but most importantly on the first, second and last requirements of those listed.
Dispersion, dissolution and hydration have already been discussed when considering the hand-mixing process, and the formation of a dough suitable for processing is discussed in subsequent sections of this chapter.
The direct role that water plays in energy contributions and air incor-poration is limited. Most of the energy associated with the dough mixing process comes from the mechanical interactions with ingredients. The rate of transfer of energy to the dough is to some extent related to the viscosity (consistency of the dough). Within limits, energy is transferred to the dough faster when the viscosity is higher, that is when less water has been added. Such changes are recognised as an increase in temper-ature rise during mixing, especially when doughs are mixed to a fixed time, e.g. with a spiral mixer. When doughs are mixed to a fixed en-ergy expenditure, as is the case with the Chorleywood Bread Process (CBP) (Cauvain and Young, 2006), the faster transfer of energy with stiff doughs is seen as a shortening of the mixing time, though only by a few seconds in the 3 or 4 min that are normally required. In most breadmaking processes, using stiffer doughs to shorten mixing times or increase the rate of energy transfer has a little practical value because dough development depends to a significant extent on optimum water additions (see below). The more viscous doughs that are obtained with sub-optimal water additions are less suited to subsequent processing and may yield impaired final product qualities, though there may be a case for using lower water levels in free-standing breads to preserve the required shape (e.g. bloomers and cottage loaves, see below).
The incorporation of air and the creation of small gas bubbles are critical to the development of bread cell structures; a detailed discussion of the processes involved is outside the scope of this book, but can be obtained elsewhere (Cauvain, 2007a). The role that water plays in these aspects of the mixing process is largely that of facilitating the development of a suitable gluten network in the dough. In summary, the processes involved are:
AIR
N2
O2
N2 + CO2
Yeast action
Figure 2.2 Changes in gas composition with mixing.
r
the entrapment of air as small bubbles by the developing gluten struc-ture, 5–300m in size;r
the loss of oxygen from the air bubbles mainly because of the action of the yeast;r
the retention of the nitrogen gas bubbles in the gluten matrix;r
the release of carbon dioxide from solution in the dough water to expand the remaining nitrogen gas bubbles (see Fig. 2.2);r
the expansion of the nitrogen gas bubbles by carbon dioxide.Much of the expansion of the gas bubble structures in bread doughs oc-curs post-mixing, especially in the proof and baking stages, and depends on the gas retention properties of the dough (Cauvain, 2007a), which in turn depends on the dough development that has been achieved. A gluten structure with the ‘correct’ rheological properties is a critical el-ement in the retention of gas and the expansion of fermented doughs (see below).