If run below 40 degrees, or the point where water begins to expand, all
cooling below that point lessens the difference in specific gravity
between the water and the fat globules, and operates diametrically in
the opposite direction to what is desired. The aim is to condense the
water, which is a good conductor, and leave the fat globules, which are
poor conductors, unchanged or but slightly contracted. In this way,
the heavier fluid settles and drives the light particles of fat upward
to rest on the surface. But, if we go below 40 degrees, we produce
the directly opposite effect and retard the rising of the cream. For
quality, we prefer the slower cooling in water, and think the longer
time given will secure all the cream available and in a purer condition.