Cereals processing technology

Breadmaking 225
that a point which starts at the centre at the beginning of proof may end up about
three-quarters of the way up the dough piece at the end of proof (Whitworth and
Alava, 1999).
After proof the dough must be heat-set, that is baked. The process is one of
conversion of a foam to a sponge. In the former case the gas bubbles are discrete
and separated from one another by gluten films, in the latter the cells of the
crumb structure are ruptured and interconnected to one another. Baking
temperatures will vary from oven to oven and with product but typically they lie
in the region of 220–250ºC. A key parameter of loaf quality is to achieve a core
temperature of about 92–96ºC by the end of baking to ensure that the product
structure is fully set.
For the centre of the dough piece, the move from prover to the oven has little
impact because it is so well insulated by surrounding dough. This means that the
centre of the dough gets additional proof. The driving force for heat transfer is
the temperature gradient from regions near the crusts, where the temperature is
limited to the boiling point of water, to the centre. The heat transfer mechanism
is conduction along the cell walls and the centre temperature will rise
independently of the oven temperature and approach boiling point asymptotically.
There is no significant movement of moisture and the moisture content
will be the same at the end of baking as at the beginning.
As dough warms up it goes through a complex progression of physical,
chemical and biochemical changes. Yeast activity decreases from 43º and ceases
by 55ºC. Structural stability is maintained by the expansion of the trapped gases.
Gelatinization of the starch starts at about 60ºC and initially the starch granules
absorb any free water in the dough. -amylase activity converts the starch into
dextrins and then sugars and reaches its maximum activity between 60 and 70ºC.
Too little amylase activity restricts loaf volume, because the starch structure
becomes rigid too soon, while too much may cause the dough structure to
become so fluid that the loaf collapses completely.
The formation of a crust provides much of the strength of the finished loaf
and the greater part of the flavour. Condensation on the surface of the loaf at the
start of baking is essential for the formation of gloss, but quite soon the
temperature of the surface rises above the local dew point temperature and
evaporation starts. Soon after that the surface reaches the boiling point of the
free liquid and the rate of moisture loss accelerates. The heat transfer
mechanisms at the evaporation front are complex. There is conduction within
the cell walls and water evaporates at the hot end of the cell. Some is lost to the
outside but the rest moves across the cell towards the centre and condenses at the
cold end of the cell. In doing so it transfers its latent heat before diffusing along
the cell wall to evaporate again at the hot end. The evaporation front will
develop at different rates depending on the bread types. The crust is outside the
evaporation front and here the temperature rises towards the air temperature in
the oven. As water is driven off and the crust acquires its characteristic crispness
and colour, flavour and aroma develop from the Maillard reactions, which start
at temperatures above 150ºC.