KANSAS CITY, MISSOURI, US — About 780 million tonnes of wheat are produced annually worldwide, most of which humans consume. The growing acreage for wheat is not increasing substantially, and new varieties hardly increase production per acre.
A research paper authored by S. Asseng (see reference table) estimated that global wheat production falls by 6% for each 1 degree C of further temperature increase and becomes more variable over space and time. With the global population increasing by 0.9% per year, a shortage of wheat flour is becoming increasingly possible. Based on the estimated usage of about 600 million tonnes of all kinds of wheat for human consumption, an average increase of 1% or more of flour production in the world’s operating mills would result in millions of additional tonnes of flour globally.
With the increase in fully automated operations and increasing milling capacities, it was recognized that the emphasis on milling technology is reduced in many operations. This article highlights some practical means by which the efficiency of flour milling will increase, causing maximal flour extraction and qualities from the existing processes.
Numbers and load, wherever stated, are relative and estimated, and this is because of two main variables — the variation in wheat characteristics and the environment within and outside of the mill building. There are hundreds of wheat varieties around the world and even those change from year to year based on land and growing conditions.
The wheat flour milling process is based on careful grinding and scraping steps of the wheat kernel between steel rolls. Following each roll’s action, the material is transferred to a stack of sieves where it is graded according to particle size. The different streams flowing from the sifters are different in granulation distribution, as well as their content of the three main parts of the original kernel, namely bran, endosperm and germ.
The routine grinding and sifting process is described by a flowsheet where all the details of the successive grinding operation with subsequent sifting stages are shown in detail. Sifting is performed with a stack of sieve frames with screens of different apertures stretched upon them.
Even with the newest automated milling equipment, reaching peak efficiency and quality depends on professional millers understanding the flow of the mill and continuously making the necessary changes with each shipment of wheat that enters the mill. The large variations in affecting parameters require continuous learning about and exposure to new technological methods.
The following are practical suggestions to optimize the milling technology to increase production at acceptable qualities.
Distribution table
A mill flowsheet describes the equipment and flow of materials from one stage to the next. The distribution table describes the system’s quality, material characteristics and efficiency.
A research paper by R. Lopes and E. Posner (see reference table) showed examples of distribution tables, which include qualities, quantities and the direction of material flow in the system. The miller lays out samples from all streams in the system according to the distribution table arrangement. This allows the miller to observe similarities or differences in material quality flowing to a certain processing stage.
In the optimized mill, a gradual increase of cumulative ash content of streams occurs as we go down the breaks, sizing and reduction stages.
Credit: ©SOSLAND PUBLISHING CO.Effective break roll adjustment
After reviewing many publications and instructions from different entities and mill construction companies, it can be stated that there is no effective method to determine the optimized percent break release of the head breaks in commercial milling. It depends on the roll’s characteristics, type of wheat, and kernel characteristics before entering the mill.
The following method, Break Release Factor (BRF), can give the operative miller a conceptual target to optimize the process for his specific variables. Lopes and Posner in their 2023 research paper described the way to generate and optimize BRF for an operating mill (see reference table).
Effective load on purifiers
Effective purifier operation can be achieved by the selection of optimized sieves aperture and accurate air distribution to allow particle size segregation based on their specific weight.
While the estimated floating velocities of mill materials to purifiers are in the range of 2.5 to 5 meters per second, 1.5 to 3 m/sec, and up to 0.7 m/sec for coarse sizings, fine sizings, and bran, respectively. The air velocities should be set lower than required for the sizings but higher than that of the bran to have a “boiling” effect on the products moving over the sieves.
The load factor is a significant parameter that the operative miller should control by adjusting the sifter sieve aperture.
The following table shows the ranges of the different granulations and their suggested quantities that might be sent to purifiers for purification.
Optimization of sifter sieves
Optimization of the sifters’ action can be achieved by regulating the loads to rolls and purifiers. Optimization should also be achieved by the loads on the flour sieves.
Periodically or with significant change in wheat used, the miller should determine the content of flour in the overs from the flour sieves in each sifter section. If the material contains more than 5% or less than 2% of flour, it should be of concern to the miller.
Those stated percentages can vary because of changes in the type of wheat used, grinding effect, or environmental conditions. The miller should collect information on the subject and act upon the problematic sifter section.
One way to control material load on the flour sieves and optimize apertures was featured in a US Patent (see reference table). Using a quantitative infrared chemical imaging method can aid in determining the concentration of a desired high-value product in a milling process.
Determination is instantaneous and decisions about necessary sieve changes apertures can be made while the sieves of the sifter section are taken out. The method determines the concentration of purified endosperm within heterogeneous solid particulate mixtures containing endosperm and no endosperm botanical parts. Problematic sifter sections can be improved within a short time.
Another approach is to collect data from the sifter sections, and at the first opportunity for a mill shutdown under full load, go through the problematic sifter section. Carefully collected samples of materials from over and under each of the flour sieves are sent to the laboratory for ash and color determination. This method results in more time and testing efforts but is another way to improve sifters’ efficiency by selecting the appropriate aperture.
A “frequent opportunity” for sample collections in designated sifter sections is an unscheduled electrical shutdown, where the mill is shut down under full load.
The process of reaching a high optimization level of the wheat milling system is slow, and it requires learning, testing, and decision-making by the responsible miller. Changes should be made gradually. After each change, the miller should evaluate the effect during subsequent mill runs before applying the next change.
Dr. Elieser Posner is an international milling consultant with more than 60 years in operations, research and teaching of wheat milling. He may be reached at Elieser.posner@gmail.com.
