There, raw sugar crystals (Grade A) are separated from the thick liquid which in turn enters pan B. The same process takes place and again the massecuite from pan B enters the centrifuge and this time the raw sugar obtained is Grade B. The remaining liquid finally enters pan C where after boiling comes out as molasses.During crystallization process there is an increase in the percentage of reducing sugars (glucose, fructose…) at each stage. This is so because there is inversion of sucrose taking place in the stages of crystallization and thus the molasses coming from pan A, B, and C will have an increased reducing sugar concentration.
Crystallization of sugar
Crystallization is the (natural or artificial) process of formation of solid
crystals precipitating from a solution, melt or more rarely deposited directly
from a gas.
The crystallization process consists of two major events, nucleation and
IN SUGAR INDUSTRY…
Crystallisation is the process just after evaporation. During
evaporation the clarified sugar cane juice is boiled in evaporators
which remove most of the water leaving a thick syrup. Then in the
crystallization process the syrup is boiled at low temperatures
under partial vacuum and some seedings are added which causes
the development and growth of sugar crystals and the outcome is
called massecuite (raw sugar crystals mixed with molasses). The
sugar crystals and molasses are then separated in centrifugals.
There are normally 3 vacuum pans namely A, B and C. Syrup
coming from evaporators enters pan A, where boiling takes place
and crystallization begins giving a thicker liquid (massecuite)
which comes out and enters the centrifuge.
Schematic representation of sugar crystallization unit. (1) Syrup (liquor A); (2)
massecuite A; (3) raw sugar (A); (4) mother liquor A used as feed liquor B; (5)
massecuite B; (6) sugar B recycled as magma A; (7) mother liquor B used as feed liquor
C; (8) magma C (prepared in batch seeding crystallizer); (9) massecuite C; (10) sugar C
recycled as magma B; (11) molasses.
During crystallization, it is necessary to initiate the formation of sugar
crystals, in other words, the formation of nuclei through the seeding
The various seeding techniques include :
1) Traditional (secondary, or shock) seeding
2) Full seeding :
a. seeding with slurry
b. seeding with footing magma
In shock method, the syrup entering vacuum pan A is concentrated and a
small charge of powdered sugar is introduced; boiling is continued until a
proof slide appears to have sufficient grain. Supposedly, the sugar by its
mere presence "shocked" the unstable syrup into spontaneous nucleation
to form most of the needed grain. The same procedure is carried out with
the other pans.
The shock seeding method is successfully carried out by seeding the
oversaturated syrup in the pans with a measured amount of standardized
very finely ground sugar slurry.
1. the quality of the crystals is mainly determined by the supersaturation
and pan circulation maintained all over the strike,
2. while the time needed to reach the required product crystal size is
determined by the linear speed of crystal growth
The grainy nature of the crystallized sugar is determined by many factors
which include :
• Degree of super-saturation
• Rate of cooling
• Degree of cooling
• Timing, rigor and length of stirring
• Temperature at stirring
• Blend of sugars present
• The pH
• Presence of crystal growth inhibitors