The evolution of modern flour milling
September 01, 2002
by Emily Wilson
The process of flour milling dates back to Egyptian and earlier times. There are illustrations from ancient drawings showing grain being crushed using a mortar and pestle, with the resulting material being sieved to produce material of greater purity. The development that followed this was the use of millstones, first hand operated, then driven by animals and finally driven by waterpower.
Millstones dominated the process used to produce flour until the 1870s, when rollermills began to supplant them on a large scale because of the superior flour that could be produced using them. However, the gradual reduction system that was introduced at the same time as the widespread adoption of rollermills has its origins in what is now known as the ‘French Process’ (see Figure 1).
The flour milling process, as it is known today, evolved in the period between the years 1830 and 1870. It was a Manchester, U.K., based engineer, Henry Simon, who commissioned the first commercial mill using most of the technologies in use today. The important features of this mill were steel rollermills and the gradual reduction system.
The advantage of the newer, more elaborate process was that higher yields of quality flour could be produced. The older processes employed can only be described as crude and produced typically only 10% high quality flour from the wheat berry compared to more than 70% in rollermilling plants. The remaining flour was of very poor quality and heavily contaminated with the bran and germ constituents of the wheat berry.
The main features of the gradual reduction system were the use of a large number of process stages in an extension of the French process and the exclusive use of rollermills for grinding. The principles of gentle grinding and intermediate sieving were developed upon to give the break, purification and reduction systems that are used today.
RECENT DEVELOPMENTS IN MILLING
It is worth highlighting 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. There are three main categories where advances have been made. These are in machine capacities, machine construction, and new machine technologies.
• Machine Capacities.
The principle 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 millimeters of roll surface per hundred kilograms of wheat processed per 24 hours. This figure has more than halved in the last thirty years.
Typically rolls are operated at double the speed and three times the feed rate that would have been the norm in the 1950s. The advent of new ‘high speed’ rolls have taken this evolution one step further with roll speeds and loadings double those of today’s ‘norms.’
The efficiency of plansifters 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 prior to plansifter sections has also reduced the amount of sieving surface required in some sifter sections.
• Machine Construction.
Significant changes have occurred in the way machines are built. Steel has replaced timber as the material of choice for the construction of plansifters. Rollermills were constructed from cast iron and wood until recently, but rollermill manufacturers now use prefabricated steel sheets for the main frame of the rollermill, with grinding forces being contained in what are known as ‘roll packs.’ This has made the rollermill much lighter than its cast iron predecessors, with resulting implications for machine cost and building design.
The advent of the roll pack also reduced dramatically roll replacement times. This has become increasingly important where flour mills are expected to run for extended periods between maintenance shutdowns.
Developments in rollermill technology are not just limited to the method of fabrication and the way the roll chills are fitted. Significant advances have been made in minimizing the amount of noise generated by rollermills in operation. This includes the replacement of chain and gear drives with timing belt drives on the differential mechanism. Application of exhaust air to the grinding zone of rolls has minimized fluidization, and roll ‘bounce’ is less of a problem.
Sophisticated electronic control systems provide reliable roll engagement and disengagement, and instances of rolls running without feeds have been eliminated. This is a considerable advancement in terms of increasing roll life expectancy and safety since rolls running in contact with each other wear rapidly and pose significant fire risks.
In addition, hygiene considerations are now being taken into account in all new machine generations. Features are being incorporated that minimize stock hold up and ease cleaning, thereby reducing contamination and infestation problems.
• New Machines.
Pin mills have been adapted for use throughout the flour milling process. They have been among the key factors in reducing the amount of grinding equipment required in flour mills. Pin mills are widely used in plants where starch damage production is not an issue or where it is undesirable.
The most notable developments in flour milling have been the double grinding of streams prior to sieving and the debranning of wheat prior to main processing. New machines have been developed to exploit these techniques.
The most radical of recent developments has been the advent of debranning as a unit operation in flour milling. A rationalized process flow is possible with debranning installed. The conventional break system is rendered obsolete with only one or two fluted rolls being required. Because of the purity of the semolina produced in such a grinding system, the requirement for a purification system is significantly reduced. It must be noted that this simplified process is achieved through very careful manipulation of the raw material, namely wheat type, conditioning regime and mill settings.
The stimulus for development in flour milling technology is the flour miller’s requirement to produce the highest quality products at minimum cost. This has been achieved in the past through investment in new technologies, which are supplied by a small number of equipment manufacturers.
Most recent development work has centered on the optimization of machine design and capacity and the application of these machines to existing processing strategies. The result has been the development of more compact flour mills in recent years, but all employing conventional processing strategies. This kind of optimization can only be pursued to a finite degree. In order to continue optimization of the industry, the manner in which processing itself is carried out must be considered.
The amount of research that has been carried out on flour as a material is quite extensive. However, most of the reported work pertains to the properties of flours and not to the engineering aspects of the process. The engineering work that has been reported tends to be purely experimental in nature with little emphasis on the basic principles employed.
Process automation would seem to promise significant potential for development in the future. Two European researchers performed a survey on the use of Computer Integrated Manufacturing (CIM) and Programmable Automation (PA) in the milling and baking industries. They concluded:
• The milling and baking industries have a number of examples of mature CIM developments, namely automation and manpower reduction;
• The number of such developments is likely to increase, although slowly;
• Much of the industry is unconvinced about the benefits of CIM and PA.
It is possible to demonstrate some specific benefits that CIM and PA will bring to the industry, such as quality improvement and enhanced production control. Many CIM processes also have features that enhance hygiene. In addition, data collection and analysis as well as product traceability are built in features of CIM that are readily exploited by processors to ensure optimum quality for customers and as tools to enhance profitability.
PID (Proportional Integral Differential) feedback control is the accepted control method in flour milling at present. This type of controller employed in flour mills today is used for operations such as water-addition to wheat and micro-ingredient addition to flour. This form of control is not suitable for whole-process optimization; advanced control techniques must be considered for this objective.
Highly integrated processes with many interacting elements, such as the flour mill process, cannot be satisfactorily controlled by basic control systems. Advanced control techniques offer the process operator the tools necessary to control processes accurately using historical and real time process performance information. Improving control improves product consistency and saves energy by ensuring key process variables are more stable. Processes may also be operated closer to optimum values or constraints.
Advanced control techniques principally consist of a computer program and a number of sensors measuring process properties. It is also necessary to have some form of DCS or SCADA/PLC system in place to enact the decisions reached by the advanced control system in terms of the optimal solution to the current set of process conditions and requirements.
OPTIMIZATION OF PROCESSES
Optimization refers to the achievement of optimal conditions within processes, in terms of process and economic performance. Advanced control systems allow optimization to be carried out on an ongoing basis, thereby maximizing process performance. Optimization represents a major opportunity to reduce energy costs and improve quality. For instance, algorithms enacted by computers can identify the most advantageous operating conditions, select the best combinations and loadings for plants, and even determine the best production schedules to meet requirements and minimize costs.
Ideally, optimization should start with the design of the process. This involves looking at the process flow for the specified products and throughputs required. Once hardware is in place, little can be done about the process flow. Therefore, it is important that a logical and informed approach is taken to flow sheet design. Process design is dominated by considerations based on experience, then followed by engineering design considerations. This is a typical food industry approach.
Advanced control techniques require a variety of steady state and dynamic models. These have to be developed for individual unit operations and overall plant performance. Mathematical techniques are then employed to find inputs to the models that satisfy operational objectives while meeting other constraints. These models have to be incorporated into computer packages that are user-friendly enough to be useful to process engineers.
The utilization of high-performance computer systems for automation allows the application of advanced control techniques to overcome existing dynamic limitations in processes, particularly if older mechanical equipment is utilized.
Case studies have demonstrated that steady state optimization modelling provides the basis to increase plant throughput up to 15%. Subsequent dynamic optimization minimizes the effort required for control system design through off-line evaluations and potentially provides a further 10% gain.
To conclude, optimization of flour milling operations to date has been simplistic and focused on macro models of the process. Attempts at modeling the operations performed in flour milling processes are currently being carried out by some milling engineers and in academic establishments, such as the Satake Centre for Grain Process Engineering in the U.K.
This approach to process development is still in its infancy and its general application is some way off yet. However, the technology exists and the need exists for further efficiency improvements. Therefore, it is inevitable that advanced control techniques along with other infant technologies will make their way into commercial milling operations.