Some issues have been raised by a recent post in bread-bakers that reported
on temperature measurements and pan thicknesses for several bread machines.
In a bread machine, the heating element transfers heat to the pan by
heating the air outside the pan. Air convection in the space between the
pan and the surrounding wall distributes the hot air around the pan. The
air heats the pan, which heats the dough inside the pan. The heating of
the dough occurs by conduction from the pan to the wet dough directly
contacting the pan.
If the pan is empty and the temperature probe is inside the pan and not in
contact with it, the transfer of heat to the probe depends on convection
currents in the air inside the pan. Convection occurs when a liquid or gas
is heated in one place and cooled in another, like water heating in a pan,
or the air outside the pan in contact with the heating element and the top
cover of the machine. The larger the temperature difference, the stronger
the convective flow. But inside the pan, the heat is spread out by the pan
and the temperature of the pan is much lower than the temperature of the
heating element, so the convection currents will be weak. Any cold air
leak through the lid seal, especially where the probe wires exit, will cool
the air inside the pan much more than the air outside.
When bread is baked in a pan, whether in an oven or a bread machine, the
pan is quite thin. The dough contains water, some of which remains when
the bread is fully baked. The temperature of the bread cannot exceed the
boiling point of water; the final temperature of properly baked bread is 30
degrees F or more below boiling. The pan conducts heat well, so the
outside of the pan is close to the temperature of the surface of the baking
bread. The air in the oven is hotter by 200 degrees F or more. Heat
transfers from the air to the pan and onward to the bread. The air cools
and circulates down to the heating element to be heated again.
A thick baking pan has no advantage over a thin pan. A thick pan will take
longer to heat because the hot air must heat more metal. This will result
in the top of the dough which is exposed to the air baking faster than the
bottom. Frying pans are very different. The temperature difference across
the bottom of a frying pan from where it is in contact with the flame to
places where it is not is very large - many hundreds of degrees F. A thick
pan conducts significantly from side to side, smoothing the temperature
distribution on the cooking surface. I suspect that the thickness of bread
machine pans is determined by the mechanical requirements of the kneading
process and not by thermal considerations.
A bread machine is a very small oven. The distance between the pan and the
wall is very much smaller than the distance from side to side or top to
bottom. There is significant heat loss through the walls of the bread
machine and out the lid and top seal. These areas are close to the pan and
to the exposed top of the bread. All this is very different from a
conventional oven which is a closer to a large cube with the bread far from
the walls and door.
In my experience with several different bread machines, there are
significant differences between them. Clearly some are from the mixing,
kneading and rising as can easily be seen by observing the dough. Likely
the baking adds more differences. Differences in dimensions, lid design
and insulation are certain to cause differences in the air temperature
distribution around the pan from one machine to another. Even though two
machines are both at the same temperature (as measured in a similar way in
both machines), I suspect (but have no proof) that the bread will bake
differently. I believe that temperature settings on bread machines are
useful as relative indications only, so the actual numbers are not very
important.
Jeff