Cereals processing technology

32 Cereals processing technology
purification system is passed on to the reduction system. The remaining material
is sent to the break system. Because the aim of this block is the purification of
milling streams, almost no flour is produced in the purification system.
The reduction block is the main flour producing block. It is also the area
where the other desirable property in flour is manipulated, that is mechanical
starch damage. 1 The reduction block consists of a series of roller mills and
sifters in sequence. Material is transferred from the break and purification blocks
to these roller mills principally for size reduction, although the sieving apparatus
removes some remaining impurities. The roller mills used in the reduction
system differ from the roller mills used in the other blocks in that smooth
surfaced rollers are usually used and lower differential speeds are employed
(Scanlon and Dexter 1986). Material that is not sufficiently reduced in size in a
particular grinding pass is sieved out and ground again in a subsequent grinding
stage. This process is repeated up to eleven times in what are termed long
surface mills.
The starch damage mentioned earlier is achieved through the application of
shear and pressure to the starch granules that constitute the endosperm. The
shear stresses are applied by virtue of the differential speeds employed by the
grinding rolls. The mechanical linkage that supports the grinding rolls applies
the pressure to the particles.
The result of the above iterative grinding and sieving process in the three
processing blocks is the cumulative release of endosperm from the wheat berry,
followed by the cumulative release of flour from this material. The actual
quantities released at each stage vary widely between particular examples of
mills. The operational settings depend on factors as diverse as wheat type, plant
operator, customer demands, equipment supplier, geographical location, and
even tradition. Typical releases 2 for a flour mill in the British Isles are (NABIM
1990):
Cumulative release from the break system: 88%
Rejection from the purification and reduction systems: 10%
Cumulative flour release: 78%
While no two milling plants are the same, the differences between them occur in
the intensity with which the processes described above are applied and particular
machine configurations. This same process has been applied for more than one
hundred years with only minimal changes to processing strategy.
1 Mechanical starch damage is induced in order to increase the water absorbing capacity of flours,
which in turn improves bread yield for bakers.
2 Release is a term widely used in flour milling. It is used in association with the break system to
describe the amount of material passed out of the break system to the reduction and purification
systems.