This operation is the next in course, and is performed in what is known
at the sugar factory as the strike pan, a large air tight iron vessel
from which the air and vapor are almost exhausted by means of a suitable
pump and condensing apparatus. As is the case with the saccharine juices
of other plants, the sugar from sorghum crystallizes best at medium
temperature.
The process of boiling to grain may be described as follows: A portion
of the sirup is taken into the pan, and boiled rapidly _in vacuo_ to the
crystallizing density. If in a sirup the molecules of sugar are brought
sufficiently near to each other through concentration--the removal of
the dissolving liquid--these molecules attract each other so strongly as
to overcome the separating power of the solvent, and they unite to form
crystals. Sugar is much more soluble at high than at low temperatures,
the heat acting in this as in almost all cases as a repulsive force
among the molecules. It is therefore necessary to maintain a high vacuum
in order to boil at a low temperature, in boiling to grain. When the
proper density is reached the crystals sometimes fail to appear, and a
fresh portion of cold sirup is allowed to enter the pan. This must not
be sufficient in amount to reduce the density of the contents of the pan
below that at which crystallization may take place.
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