Cereals processing technology

164 Cereals processing technology
8.3.2 Developments in blending
For batch cereal operations, batch ribbon blenders are still the industry
workhorse for blending. They are easily loaded, unloaded, and cleaned; by
hand, by screw conveyor, or by pneumatic conveyer. Most are double ribbon
blenders; that is one ribbon pushes the material toward one end, while the
opposing ribbon pushes the material to the other end. The level of fill in these
units is somewhat critical. For optimum blending in the shortest time, the
blenders should be filled between 50–75%. In no case, however, should they
be filled over the level of the ribbons. If this is done, the top part of the load is
in a dead mixing zone.
Microprocessors can again come into play in this unit operation in several
ways. The whole unit can be mounted on load cells and serve as a receiver and
weigh hopper vessel. Blending times and speeds, of course, can be controlled via
programmable logic controllers (PLCs), as well as load and discharge opening
and closing functions. Cycle time is always the sum of loading time, blending
time, and discharge time. In order to make best use of the blending time, it pays
to keep the openings for loading and discharge as large as possible. Also, as an
alternative to using the blender itself as a weigh hopper, time is saved by using a
weigh hopper/receiver above the blender to preweigh all dry ingredients, and a
receiver below the blender big enough to accommodate several batches of dry
blended ingredients. In addition to standard ribbon blenders, conical blenders
can also be used.
There are in addition many ways that continuous blend systems can be set up
to preblend the dry ingredients, and then even add the liquid flavor mix before
cooking in batches. In extrusion cooking it is customary to add the preblended
drys to the extruder, and liquids to the extruder, via separate ports – more on this
in Section 8.3.3.
Any dry blend set up should have as a consideration the pros and cons of
clean-in-place (CIP). These systems can be added easily if engineered into the
initial design and layout. CIP systems also run off PLC control systems. We will
limit our discussion here on liquids blending to the four basic flavor materials;
water, sugar, salt and malt. The workhorse mixing vessel for these has been, and
still is, the steam jacketed kettle. Liquid sucrose, dry salt, and liquid malt extract
are slurried in water in a kettle equipped with agitation, and a steam jacket
sufficient in capacity to heat the mix to 125ºF (52ºC). This is a high enough
temperature to make a good useable slurry of such viscosity for ease of handling.
If dry sugar and dry malt flour are used, these can be incorporated in the dry
blend with the other dry ingredients.
In making liquid flavor mixes in batches it is common to use a kettle large
enough to accommodate flavor aliquots for multiple batches of dry materials.
When doing so it is best to weigh the aliquots of liquid flavor for each batch of
drys. Metering of liquid flavor batches introduces more chances for batch-tobatch
variability than weighing the increments due to the on/off withdrawal
cycles from the kettle. Slight drops in temperature can cause significant
viscosity and flow character changes.