Cereals processing technology

Breakfast cereals 165
In continuous cooking or extrusion cooking processes, continuous liquid mix
systems can be used. As noted earlier, loss-in-weight liquid feed systems, as well
as good liquid meter systems adapt themselves to this unit operation.
8.3.3 Developments in cooking
The cooking operation has seen great changes in the last two decades. The
general trend has been toward extrusion cooking and away from batch cooking.
Significant upgrades have been made in batch cookers and their control,
however.
First, has been the total redesign of the shape of the pressure vessel itself. The
old style perfect, straight-sided cylinder, has now been replaced by a ‘cylinderlike’
vessel whose center horizontal diameter is approximately 15% larger than
the diameter at the end of the cylinder. The ends of the cylinder are no longer
flat, and at 90º to the horizontal sides, but are shaped more like a flattened
parabola. These two design changes have obviated the need for the four internal
lifting flights that were welded at an angle on the internal surface of the old
cylinder shape cookers. These flights were needed to ensure the cooking grain
could mix while being rotated during cook time, and also, more importantly,
cause the cooked grain to dump from the cooker hatch when cooking was
complete. Elimination of the internal flights has eliminated contamination in
product flow from overcooked grain which stuck to the flights, and was cooked
repeatedly before finally breaking loose, and dumping.
The next feature incorporated in these new designs is automated hatch cover
closing and opening mechanisms. This has greatly increased operator safety and
reduction in steam burns which occurred with older style hand operated covers.
The third upgrade comes in the control of steam injection and exhaust. These
changes are the direct application of PLC and computer control operation. In
some designs, injection and exhaust are alternated from one end of the cooking
vessel to the other on a preprogrammed cycle basis. The advantage is that the
steam exhaust port screens are constantly cleaned when inlet and exhaust steam
is changed from one end to the other. PLC and computer control, and
automation of hatch cover opening and closing, have greatly improved operator
safety of the batch cooking unit operations. Accuracy of cooking times is
controlled totally and uniformity of cooked grain improved.
The move to extrusion cooking has been sparked to a great extent by the
perfection of twin screw extruders. This development redirected and brought
under control the tremendous excesses in shear imparted to the grain formula
during extrusion cooking in single screw units. This excessive shear was
apparent in the finished products as a ‘dead’ gray, unappetizing appearance. The
bowl-life of the finished product was also shortened by the over gelatinization of
starch caused by mechanical overworking.
The flexibility in set-up of the screw elements in a twin screw, along with
greater flexibility in screw speed and heat input, have brought the extrusion
cooking process under very exacting control. These control constraints coupled