Elieser S. Posner, ESP International, Savyon, Israel
WHEAT: Chemistry and Technology, Fourth Edition
Pages 119-152
DOI: https://doi.org/10.1094/9781891127557.005
ISBN: 978-1-891127-55-7
Abstract
This chapter is, to a large extent, a continuation of the wheat flour milling chapter contributed by Bass (1988) and is not intended to repeat all the information documented previously. Although the basic principles regarding wheat milling are described, the reader and student of milling should regard the information as additional information based on new research and development generated since the last edition. This chapter also includes new ideas regarding methods and technology, which are generated continuously in research institutions and in practice by flour millers.
Since the dawn of civilization, wheat has been ground to flour for baking using various technologies. The common denominator or objective of all “wheat processors” or millers was to crush the kernels to such a form that, when mixed with water, it would create dough to be leavened and baked by heat. As human taste evolved and therefore the products demanded from millers changed, objectives such as the separation of endosperm, bran, and germ became important. Various manuscripts describing the history of human civilization deal with the development of milling, from crushing wheat kernels by stones to the modern roller milling technology (Bass 1988). Since the end of the eighteenth century, when Oliver Evans set up the continuous flour milling process, many different control and automation levels have been implemented in the system, from wheat blending, cleaning, and grinding to flour blending and warehouse stock inventory.
However, most of the control and automation devices do not control the technological parameters of milling, which are based on the miller's knowledge, experience, and intervention skills. The twenty-first-century concepts of mill adjustments and fine tuning of equipment in the modern high-capacity flour mill are very precise, expressed in micrometers (μm) and milligrams (mg). New wheat varieties with different characteristics, even very minute ones, are reaching the mill, and the miller needs to observe the changes and make the necessary adjustments in the technical aspects of the operation. Since wheat is a commodity, the standards for it relate only to appearance and purity, neither of which affects its processing characteristics. Present and future trends of increased plant capacities, automation, and sophistication require a change in the wheat marketing system so that it is based also on processing quality characteristics. To create a market for wheat based on milling quality, proper evaluation and segregation must start at the first collection point, at the country elevator. Around the world, differences in wheat trading specifications still exist that differ in methods for grade evaluation and require different test results, such as for moisture content. Many countries report wheat and flour at dry basis. Canadians report wheat at 13.5% and flour and semolina at 14%, and the United States reports wheat at 12% and flour and semolina at 14%. Similar quality classification and reporting systems should be agreed upon among countries and will be an advantage to traders and processors.
The technological challenge of separating the three main parts of the wheat kernel (endosperm, germ, and bran) as efficiently as possible is complex because of the kernel shape, the crease, and the aleurone layer (the latter is considered botanically as endosperm but is difficult to separate from the bran). The efficiency of separation is estimated by calculating the amounts of the various end products. The amount of flour recovered while its qualities are kept at specified levels is usually expressed as “flour extraction” and can be expressed in a number of ways. The more common methods of calculating flour extraction are based on dirty wheat to the mill, wheat to first break, or the total products from the milling operation. All values can be expressed on an “as is” moisture basis or corrected to a dry moisture basis. Periodically, flour extraction is calculated based on measurements with a full accounting of all stocks on the premises, “all wheat in,” and “all mill products out” during that period. Each of the methods has its merits and advantages in flour milling and generates information for different purposes. Determination of total product extraction is advantageous in controlling and evaluating technical adjustments and changes in the process.
With the dry milling process, it is not possible to achieve optimum separation of the endosperm, bran, and germ parts. Accordingly, the objective remains to get as close as possible to the optimum. The miller needs to consider wheat shape, size, texture, density, and chemical content among the characteristics that dictate the adjustments needed in the milling process in order to achieve the best possible results. The milling is also substantially different when different kinds of wheat (e.g., soft, hard, or durum) are ground, and the process is designed and adjusted differently for each of the wheat varieties. In the last few decades, new white and especially hard white wheat varieties have been grown and have gained popularity because they provide millers with a higher extraction rate than red wheat does.
Globalization of wheat milling technology is developing and is exemplified in the similar design concepts of new mills around the world. Machine designs from different engineering companies vary mainly in hygiene, special adjustment and control features, ease of access and maintenance, noise level, and durability. This chapter does not include milling equipment pictures but only schematic drawings of machines and processes where it is necessary to understand their function or a process that needs elaboration. The reader is encouraged to use the websites listed at the end of this chapter to get firsthand information and pictures of the equipment from various mill engineering companies.
