Cereals processing technology

166 Cereals processing technology
with rapid product gelatinization measurements while running have greatly
enhanced extrusion cooking acceptance. While all these features would seem to
weigh in favor of extrusion cooking over batch, it must be pointed out that the
two methods do not yet produce identical products. They have drawn closer and
closer, but small differences still exist, and preference for one system over the
other is a matter of finished product character choice.
8.3.4 Developments in drying
‘Drying’ unit operation in RTE processing is a necessary step between cooking
and forming, i.e. flaking, shredding, gun puffing, etc. Most cooked grain recipes
exit the cooker or extruder cooker at 25–30% moisture content. They are then
dried to ranges of moisture between 9 and 17% for the forming operation. The
units used for this step are still predominantly flat belt conveyor dryers – either
single pass or multiple pass.
The two developments that stand out are in the areas of humidity control and
unit sanitation. Humidity control impinges heavily in the area of dryer
efficiency, and, just as importantly or even to a greater extent, affect condition
quality of product exiting the dryer. Use of humidity increases efficiency of the
dryer heat use, since exhaust air carrying the entrained moisture from the
product can be reduced to the bare minimum. Reducing exhaust air naturally
reduces the need for heated make-up air thus cutting down on fuel usage on the
supply air side of the dryer.
The quality of product exiting a dryer with humidity control is greatly
improved. The improvement is noted in that ‘case hardening’ of product
particles during drying is much less severe. Product particles dry because the
drying air sweeps away the microscopic layer of moisture from the particle
surface. This causes a migration of moisture from the interior of the particle to
the surface, when another, and another layer after layer of moisture are taken
away. If, however, the drying air is too dry or too hot, a dry film forms on the
particle surface which acts as a barrier to further moisture migration from the
interior. This dried surface condition is referred to as ‘case hardened’.
8.3.5 Developments in tempering
The next step after drying is tempering, a step used to allow equilibration of
moisture within the particles and from one particle to another. If the particles are
case hardened, temper times must be much longer to allow equilibration to take
place. Use of humidity greatly reduces the amount of in-process material, and
equipment needed to hold it, for this temper to take place.
It is hard to believe that advances can be made in a unit operation in a process
that merely allows the product-in-process to just sit. Great light, however, has
been shed on the tempering process in the last two decades. Early breakfast
cereal scientists wondered in the very early years after World War II whether the
physical and chemical changes that took place in breakfast cereal tempering