Joint International Conference on Long-term Experiments ...

The high temperature on the surface leads to heat transfer into the loaf. The water in the
pores inside the loaf contributes to the transfer of heat through the material. This occurs due
to the evaporation of the water at the warm end of a pore and by condensation of the water
at the cold end, known as the heat pipe effect or the Watt principle. This causes a
temperature gradient inside the loaf. The partial water vapour pressure is directly related to
the temperature, and a vapour pressure gradient is built up from the surface to the centre of
the loaf. The results show that the temperature of the surfaces _ in direct contact with the
baking tin Figure 1, curves b. and c is generally lower than the temperature of the top
surface Figure 1, curve a . This is probably why the coldest region is centred slightly below
the geometrical centre of the loaf Figure 1, curves e, f and g.
Water content distribution: From the results it can be seen that the measured water content
in the centre of the loaf decreased up to a centre temperature of approximately 70±5 0 C. This
is probably due to the quantity of dough in the measuring volume in front of the optic fibre
decreasing as the total volume of the loaf increases. Earlier measurements have shown that
there is a good correlation between the rise of the loaf and the decrease in the measured
water content. The true water content is probably constant during this period as the pore
system is not continuous in the dough, which prevents the water from moving. When the
temperature reaches approximately 70 0 C the chemical changes commence, which make the
dough change into bread. The pore structure previously consists of discrete pores in which
the water vapour is captured.
Figure 2: Typical diagram of the changes in water content in the centre of the loaf. The
decrease in water content during the first period in the oven is a measured decrease only
and not an actual decrease. The water content starts to rise in the centre, when the
temperature has reached 70±5 0 C
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