Prof. Dr. Farhan Alfin
Independent flour milling consultant

Wheat flour milling is a complex art, requiring precise adjustments at every stage to maintain product quality and operational efficiency. Unlike simpler food processes, wheat milling involves multiple stages and intricate machinery—from breaking and sifting to purifying each intermediate product. This analysis covers essential adjustment methods for maximizing efficiency in flour mills, focusing on techniques for roller mills, sifters, and purifiers. It also highlights the importance of maintaining mill balance to prevent bottlenecks and enhance product consistency.

Wheat flour milling procedure is a sophisticated procedure that differs from other food industry procedures. Typical food production procedures consist of a serial of unit operations. So that it is simple to control production because there is one main intermediate material and one or more by products. While in the wheat flour milling procedure, the raw material, which is wheat, is grounded in the first break mill machine, and the product is divided by plansifter into 4-5 intermediate products, each of which is sent with different intermediate products from another mill machine to a different mill machine. This sophistication in wheat flour mill procedure creates a challenge in controlling the operation of the mill. For that, efficient and effective every machine adjustment is essential in controlling the operation, ensuring optimum product quality, maximum operation efficiency, and yield of the flour produced.

Roller mill machine feed rate and roll gap adjustment, sieve tension and screen size, purifier airflow, sieve size, and divide paddle settings are some of the key adjustments required in wheat flour mill operation.

1. Roll mill adjustment

Roller mills are the main machine of wheat flour milling operation. Roll speed, differential speed, fluting profile, fluting direction, and degree of roll wear are hard to change adjustments. Feed rate and gap between the rolls can be adjusted by the operative miller. Feed rate and gap between the rolls affect the particle size distribution of this machine product and the intermediate stream rate. Adjustment changing of any roller mill machine in the mill can affect the quality and yield of flour. One of the latest innovations in recent years in the roller mill is to equip it with an advanced sensor that enables it to measure roll gaps and with a mechanism to adjust the gap.

2. Sifter adjustment

Another crucial process is sieving, which separates intermediate products according to their particle sizes. Revolutions per minute and the throw are critical factors in the operation of the sifter, which are hard to adjust. The critical factors that the operative miller can adjust in the operation of the sifter are the sieve tension and screen size.

3. Purifier adjustment

Purifiers can remove bran from middlings and classify middlings by size and purity, which are sent to roller mills of reduction system. To ensure that the middlings are properly classified and minimize contamination with bran airflow, sieve size should be fine adjusted by an operative miller.

Knowing the techniques used in milling operations that measure if the operation is well adjusted or not is as important as knowing what parameters to adjust. Break release, granulation curve, distribution table, and cumulative ash curve are the most important techniques of measuring if the mill is well adjusted and working as the operative miller wanted.

MILL BALANCE

Mill balance is defined in “Dictionary of Milling Terms” as “the proper distribution of stock to the various parts of the milling system, as determined by the flow sheet, to ensure proper loading of equipment and high milling efficiencies”. Mill balance must be kept within the system design parameters to maximize milling system performance and efficiency.

When the mill gets out of balance, some of rolls are overloaded and others are underloaded, causing choke up of sifter and dropping out of pneumatic lines. To keep the mill in balance its machines have to be well adjusted. 

1. Break Release

In the previous article in the November 2019, Miller Magazine, the procedure to measure break release was detailed presented. The break system refers to the first passages in a mill diagram. It is the main section of the wheat milling process. The break system is divided to primary (B1, B2, B3) or head end and secondary (B4 and B5) or tail end. The main object of head end of break system is to release as much clean and large pieces of endosperm particles as possible, leaving the bran in large particles and a minimum amount of flour. The break system in flour milling process is the most critical operation. To achieve optimum distribution of stock to various parts of the milling system and optimize the production of high-quality flour, the break system in the flour milling process should be well adjusted. Break release is defined as the percentages of ground material obtained, including sizing, middlings, flour, and fine bran, as a percentage of the original material being tested through a predetermined size sieve. The optimum break release for each individual passage should be defined and scheduled for the mill. It is essential to monitor the balance of break system of the mill and generate a consistent distribution of stock all over the mill. Type of wheat, tempering time, feed rate, roll wear, and roll gap are the factors that affect the break release. Mainly, break releases are used to measure if the roll gaps are properly adjusted.

Checking the break release is a simple procedure that requires taking a uniform representative sample that has been taken equally from the left side and right side of the roll. Then sifting the sample in a large enough sifter and for uniform sifting time. Break release should be checked at least once a day or at a change of wheat mix to measure and monitor the mill balance.

Break system extraction is another useful concept calculated from break release. It is the percentage of stock removed in each passage compared to the total quantity of wheat delivered that went to the next passage. The cumulative break system extraction is the total amount of stock removed in the combined break system passages. Table 1, illustrate cumulative extraction calculation.

GRANULATION CURVES

A granulation curve is a graph illustrating the measured percentage of cumulative material held over a given sieve size of ground product for roll passage better than break release. The break release provides the percentage of ground material through a specific sifter but not the distribution of fine product and middlings.

A granulation curve can be used to estimate the expected distribution of ground stock and fluted roll wear according to changes in product distribution and to monitor changes in intermediate stream flow rate to purification and sizing systems and may be done weekly.

Figure 1

Creating a granulation curve involves sifting the stock sample using a test sifter with screens similar in micron size to the sieves in the corresponding sifter flow. Each fraction held on the sieves and in the pan is weighed and recorded (Table 2 illustrates an example of granulation curve calculations). The percentage held on each sieve based on the total sample weight is calculated. Then summing the percentage held over each sieve to the next smaller sieve fraction percentage. The cumulative percent held over the pan would be 100%. The granulation curve is then plotted as a scatter plot and line graph on X-Y axis, with the X-axis representing sieve size in microns and the Y-axis showing percent held over the sieve. Figure 1. shows the granulation curve of Table 1 data.

Figure 2 

The percent held over a given sieve can be calculated from calculation table data or from the plot; it is the difference between the cumulative percent held over that sieve and the specified larger one. Let’s look at an example where the plansifter of the first break has sieves as shown in Figure 2. What is the percentage load of purifier 2 (S2)? It is difficult to calculate it from the calculation table. It can be calculated by mathematical interpolation. The material sent to purifier 2 (S2) is the material through the 475 μm but held over 265 μm. To estimate the cumulative held over a given sieve size, it is made by drawing a vertical line from the desired size until it intersects the granulation curve. Then drawing a horizontal line from the interception point to Y axis.

Graphically, it looks like Figure 3, where the estimated cumulative percentage held over the 475 μm sieve is 73% and the estimated cumulative percentage held on the 265 μm sieve is 86.5%. Thus, the load to Purifier 2 (S2) is the difference between the percentage held over 265 μm, which is 86.5%, and the next larger sieve, 475 μm, which is 73% (86.5-73 = 13.5%).

Figure 3

About the author

Prof. Dr. Farhan Alfin is a distinguished independent consultant for production and quality control at wheat flour mills. In 2000, he received his Ph.D. from the Department of Food Engineering at Ege University in Izmir, Turkey. From 2015 to 2019, he was employed by Avrasya University in Trabzon, Turkey, where he served as the department’s head for food engineering. Between 2006 and 2010, he served as the department’s chairman at Albaath University in Homs City, Syria, where he worked in the food engineering department from 2000 to 2015. He has provided consulting and education services to various flour mills in Syria, contributing to the improvement of their operations. Furthermore, he served as the executive manager of Alakhras Mill in Homs, where he successfully oversaw its operations from 2009 to 2015. His expertise and knowledge are further demonstrated through his authorship of “Cereal Milling Technology,” a book written in Arabic. Prof. Dr. Farhan Alfin’s unwavering dedication to the wheat flour milling industry, coupled with his extensive experience, makes him an invaluable asset and a respected authority in the field.