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).
Figure 2.3 Bread from a dough with less than the optimum water level.
change its shape during moulding, it may not retain its shape and is likely to flow during proof (see Fig. 2.4). When soft doughs are held in pans this may not be too much of disadvantage, but with freestanding, oven-bottom or hearth bread types, the subsequent flow may be to the detriment of final bread quality.
The optimum water level for a given dough varies according to the bread variety required, the breadmaking processes employed, and the
Figure 2.4 Bread from a dough with more than the optimum water level.
methods by which it will be handled and processed, especially moulded.
Reference has already been made to the undesirable flow that can occur with freestanding breads when the added water levels are too high, and it is common to use lower water levels with such breads than those used for pan breads. However, there are exceptions and optimum baguette qualities are obtained only with higher water levels than sometimes seen with pan breads (Collins, 1978a). For example, Collins (1978b) suggested that water levels for the production of baguette by the CBP could be up to 8% higher (based on flour weight) than those used for pan breads.
Such soft doughs may be supported in cradles or shaped pans, at least during proof, if not throughout the whole of the baking process. In the case of baguette, the high water level and the soft dough consistency are an integral part of being able to form the required open cell structure and crisp crust in the baked product, but must be combined with a fully developed dough so that the gluten network can retain the expansion of the dough structure during proof and baking.
During fermentation, bread doughs become softer as the gluten net-work relaxes under the influence of time and temperature, the effects of enzymic activity and the evolution of carbon dioxide gas by the yeast, which decreases dough density. The longer the fermentation time, the softer the dough becomes. The optimum dough water addition is dic-tated largely by the ability of bakers or their equipment to handle the dough, and it is therefore common practice to compensate for increased dough softening by reducing the level of water addition to the dough during mixing. In doughmaking processes that do not have a fermenta-tion period after mixing and before dividing, the dough would have a much firmer consistency, and so it is common practice for levels of water addition with no-time doughs to be higher than those seen with bulk fermentation processes. This additional water in the dough is required to ensure that dough consistencies are the same for all breadmaking processes by the time of dividing (Cauvain and Young, 2006).
The quantity of gas that is occluded in the dough during mixing affects its rheological properties; the more gas that is present in the dough (i.e. the lower its density) the softer it will be. This is one of the factors seen to affect the consistency of dough with increasing fer-mentation time. In the CBP, a partial vacuum may be applied to the dough during mixing to reduce the average size of the gas bubbles that are present (Cauvain and Young, 2006). This same action significantly reduces the overall quantity of gas present in the dough at the end of mixing, often by as much as 50% (Marsh and Cauvain, 2007), and gives a dough that is firmer to the touch. This change in dough rheology is usu-ally offset by increasing the level of water added to the dough in order to produce a dough of suitable consistency for dividing and processing.
Table 2.2 Comparison of added water levels with different doughmaking processes (same flour).
Water addition
Process (% flour weight)
1-h bulk fermentation time 57 4-h bulk fermentation time 55
No-time, spiral mixer 58
CBP – atmospheric pressure 60
CBP with partial vacuum 62
CBP with pressure 58
Typical levels of water addition to the same flour processed by different breadmaking processes are given in Table 2.2.
In those breadmaking processes that require a significant input of en-ergy to the dough during mixing, e.g. the CBP, optimisation of water levels plays a part in the energy transfer mechanism. In particular, the rate of energy transfer is affected by the consistency of the dough. Soft doughs offer less resistance to the action of the mixing tool, and energy transfer rates are therefore lower. However, within the range of dough consistencies that would give doughs suitable for handling and process-ing, the differences in energy transfer rates are small and therefore have only a small effect on mixing times. If doughs are mixed to a specified energy input, as in the CBP (Cauvain and Young, 2006), the impact of changes in dough consistency on dough development is minimal, but when doughs are mixed solely to time, then the impact may be appre-ciable. In the latter case, softer doughs may receive lower energy inputs, yielding doughs with less development and poorer gas retention and ultimately bread with less oven spring and smaller volume and firmer crumb.