Cereals processing technology

220 Cereals processing technology
structure. The two other gases present in the dough after mixing are oxygen and
nitrogen. The residence time for oxygen is relatively short since it is quickly
used up by the yeast cells within the dough (Chamberlain 1979). Indeed so
successful is yeast at scavenging oxygen that no oxygen remains in the dough by
the end of the mixing cycle. With the removal of oxygen the only gas which
remains entrapped is nitrogen and this plays a major role by providing bubble
nuclei into which the carbon dioxide gas can diffuse as the latter comes out of
solution.
The numbers and sizes of gas bubbles in the dough at the end of mixing are
strongly influenced by the mechanism of dough formation and the mixing
conditions in a particular machine. Recent work to measure bubble distributions
in CBP bread doughs (Cauvain et al., 1999) has confirmed that different mixing
machines do yield different bubble sizes, numbers and distributions. However,
in one CBP-compatible mixing machine variation of impeller design had a small
effect on the gas bubble population. This lack of difference in the characteristics
of the dough bubble populations was confirmed by the absence of discernible
differences in the subsequent bread cell structures.
The modification of bubble populations through the control of mixer
headspace atmospheric conditions has been known for many years, commonly
through the application of partial vacuum to CBP-compatible mixers (Pickles,
1968). This control was useful in the creation of the fine and uniform cell
structures typically required for UK sandwich breads but was unsuited to the
production of open cell structure breads. In more recently developed CBPcompatible
mixers which are able to work sequentially at pressures above and
below atmospheric it has become possible to obtain a wider range of cell
structures in the baked product. When the dough is mixed under pressure larger
quantities of air are occluded which give improved ascorbic acid oxidation but
more open cell structures. In contrast, dough bubble size becomes smaller as the
pressure in the mixer headspace reduces and ascorbic acid oxidation decreases
as the pressure decreases. The greater control of dough bubble populations
realised in these mixers allows a wide range of bubble structures to be created in
the dough (Cauvain et al., 1999). In addition to the fine and uniform structure
created from the application of partial vacuum open cell structure for baguette
and similar products can take place in the mixing bowl by mixing at above
atmospheric pressure (Cauvain, 1994; Cauvain, 1995).
Similar considerations to those discussed above apply to the horizontal bar
mixers which are typically used with sponge and dough processes. Air is
incorporated in the sponge during the mixing stage and oxygen is lost because of
the yeast activity leaving only nitrogen gas bubble nuclei. When the sponge is
mixed with the other ingredients at the dough mixing stage quantities of these
gas bubbles may be lost as the dough matrix ruptures. However, at the same time
fresh air bubbles are incorporated and the process of oxygen depletion by yeast
action again takes place. At the end of mixing the gas bubble population will be
dominated by nitrogen though carbon dioxide will be present in larger quantities
compared with CBP doughs. Nevertheless the same principle applies after