Cereals processing technology

3.4 Recent developments in flour milling
Wheat, corn and coarse grains milling 33
The following discussion highlights the fact that many of the latest
developments are old ideas that have been revisited in recent times because of
advances in manufacturing technology. A prime example is the application of
double grinding without intermediate sieving. 3
There are two principal reasons for the current pace of technological
progress. The first is the small operating margins present in the flour milling
business. These do not allow large quantities of capital to be channelled into
research. The other reason is that flour millers generally consider themselves as
technology ‘consumers’ rather than developers. Thus, they rely almost
exclusively on a small number of ‘milling engineers’ to supply them with state
of the art equipment and process designs. Again, because of their small numbers
and low margins, technological progress by milling engineers is a difficult and
slow process. Despite these limitations, there have been a number of advances in
the application of conventional technologies, which have improved processing
efficiency in flour mills. The following overview details these advances. There
are three main categories where advances have been made. These are in machine
capacities, machine construction and new machine technologies.
3.4.1 Machine capacities
The principal emphasis in this area has been on improving the effectiveness of
existing machines rather than on new types of machine. The so-called ‘short
surface mill’ is now the norm. This refers to the amount of grinding equipment
required to process a specific throughput of wheat. The indicative figure is
known as the available roll surface and is expressed as millimetres of roll
surface per hundred kilograms of wheat processed per twenty-four hours. This
figure has more than halved in the last thirty years (NABIM 1990) from a typical
figure of 18 mm/100 kg/24 h to examples of 6 mm/100 kg/24 h.
The efficiency of plansifters 4 has also been increased significantly. This has
been achieved by making them larger, more space efficient and by increasing
sieving rates. The incorporation of rotary sieving machines into the process flow
before plansifter sections has reduced the amount of sieving surface required in
break sections. This is because these sieving machines are more efficient at
performing coarse separations of large volumes of bulky material.
3 The conventional wisdom in flour milling is that after every grinding step the ground material
should be sieved and the undersize material removed before regrinding. Double grinding technology
successfully counters this philosophy and yields significant process economies.
4 Plansifters are the industrial-scale sieving machines employed in flour milling. These are gyratorytype
sieving machines, which have several compartments capable of handling different materials
simultaneously.