diffusion to the center of the loaf of about 4 hours, far too long. The
viscosity of the dough is far too high to allow significant convection.
However, the dough contains a high volume fraction (about 0.8) of fairly
large gas cells, for the most part containing CO 2 , and in these cells
convection is possible. The value ofcpof the gas is too small to allow
transport of sufficient heat, but water will evaporate at the hot side of a gas
cell (consuming heat) and condense at the cool side (giving up heat). In this
way, sufficient heat can be transported. It implies that also mass is
transported; indeed, the water content in the center of freshly baked bread is
higher by about 4%water than that in the outer layer. (This difference is in
fact one of the quality marks of fresh bread; it disappears—by distillation
and diffusion—when keeping the bread for a day or so in a plastic bag.)
Question
It has been observed by polarized light microscopy that some emulsion droplets
(diameter about 5mm), containing a triglyceride oil that can partly crystallize at
room temperature, have crystalline fat in their outer layer. This has been ascribed to
the cooling of the droplets occurring from the outside, so that crystallization would
start there. Is this a reasonable explanation?
Answer
No. Applying Eq. (5.21) with x^0 ¼ 1 mm, we obtain a halving time for the
temperature difference of 10^5 s. This implies that it would need a cooling rate of
order 10^5 K?s^1 to achieve a temperature difference of only 1 K. This rate is widely
outside the range attainable in everyday life. Even if such fast cooling could be
achieved by special apparatus, the temperature gradient would not be from the
outside to the inside of a droplet, since that would imply thatDHis much smaller in
oil than in water, whereas the two values are about equal.
5.3 TRANSPORT IN COMPOSITE MATERIALS
A liquid food of not very high viscosity can be stirred to speed up transport
of heat or mass. Even if it contains dispersed particles, these mostly are
small enough to allow rapid diffusion in or out of them (cf. Table 5.3).
Many foods, however, are solidlike, and there are even some that contain a
lot of water (cucumbers, for example, contain about 97%water); transport
generally is by diffusion and in some cases by—greatly hindered—flow.
Some examples are