The Milling Process
First in a Series of Articles
by
Thom Leonard *
Thom Leonard is a baker and writer living in Lawrence, Kansas and working with Farm to Market Bread Company in Kansas City, Missouri. Virtually all of the white flour used in North America is produced by roller mills. What follows is a greatly abbreviated description of that process. While this information may not directly help you make better bread, the better under-standing we, as bakers, have of the whole process of bread-making - including wheat breeding, farming, and milling - the better bakers we will be.
Before wheat can be milled into flour, it must first be cleaned and conditioned to remove foreign material and poor quality or infested kernels. Cleaning is done with an assortment of machines that utilize air currents, magnets and screens to separate the wheat from the chaff- and from other undesirable contaminants. Kernel size, shape, density, dimension and friability under impact are all qualities that can be exploited to effect separation.
Conditioning is the adjustment of the moisture level of the grain to facilitate maximum separation of bran from endosperm. For most milling systems, wheat is conditioned to approximately 16% moisture. An optimum toughness of bran and "mellowness" of endosperm is the goal. The bran should flatten into large flakes when passing between the break rolls in the milling process. If the wheat is too dry and brittle, the bran wilt be ground rather than flaked and will be difficult, if not impossible, to separate completely from the endosperm, resulting in a relatively dark flour with a high ash content. If the wheat is too wet, the endosperm will adhere to the bran, reducing the yield of flour.
Essentially, there are only two things that happen in a mill: grinding and separating. But that apparent simplicity belies the complicated nature of the modern milling process, as well as the degree of skill required to produce good flour of consistent quality from a variable supply of wheat.
The process used to produce white flour in a modern roller mill is referred to as a gradual reduction. The endosperm is gradually reduced in particle size by running it between a series of pairs of rotating hardened steel rollers, either corrugated or smooth, and is separated from the bran and germ by running it over sieves. All pairs of rollers in a mill have one slow mill and one fast one. The rolls turn in opposite directions, toward each other, pulling the stock (material being ground) between them. The cut, depth, and spiral of the corrugation, together with the rotation differential, determine the aggressiveness of the milling at any particular step.
There are five roll systems in a flour mill: break, sizing, midds (for middlings), low grade, and residue.
In the break system, the kernel is opened, the bran flattened and the endosperm broken into large chunks. Al-though some flour is produced here, the goal in the break system is not to produce a lot of flour but to maximize separation of bran from endosperm. Because the break rolls are at the beginning of the milling process, the quality of the work done here effects each subsequent step, thus determining both the yield of flour and the quality of that flour. If the rolls are too aggressive, portions of the bran may be torn or ground into dust that will be impossible to separate from the endosperm in later steps.
In the sizing system, rolls are used to further flatten and separate bran and germ from the endosperm. Sizing rolls are either finely corrugated or smooth. Coarse and fine sizings are produced on these rolls.
Most of the high quality flour is produced in the midds reduction system. Here the rolls are either very finely corrugated or smooth.
Each time stock passes between a pair of rolls, the resulting milled stock is run through a purifier, primarily gyratory bolters with stacks of sieves with different screen and cloth meshes. Vibration and air flow contribute to stratification and separation of the material. The material which will pass through ("thrus") the finest (bottom) sieve cloth in the purifier is flour. Each set of rolls thus has its own flour "stream," identified by the roll the stock came from before arriving at the purifier: 1st Break, 2nd Break, 1st Midds, Sizings, etc.
The "overs" of each sieve (particles not fine enough to pass through) are directed to another set of rolls for further reduction, or to one of the residue streams: bran, germ, shorts, or red dog. None of these end up in the flour. Indeed, any part of the wheat that does not enter one of the flour streams will be one of these four "by-products." These materials, unless there is a specialty market for them, are generally sold as feedstock.
The separation by size, grade, etc. at each stage of the milling process creates many dozens of "streams" which wind their way through the mill. In a multi-story circle, the stock is repeatedly gravity-fed down through the systems and mechanically or pneumatically brought back up to the top floor. The results can look very complex and confusing to anyone seeing a mill for the first time. Nevertheless, a visit to a mill is highly recommended and is perhaps the only way to truly understand what is described in this article.
In the end, the various streams are blended and mixed to make various grades of flour, then treated with the addition of malted barley, bleaching agents, enrichments, etc. before packaging. If all the flour streams are combined and blended, the resulting flour is "Straight Grade." 'Patent" is the flour from those streams containing the least bran and germ particles, thereby the whitest and lowest in ash. "Clear" flour, on the other hand, is from the "dirtier" flour streams. While straight and clear flours will have more protein than the patent flour from the same wheat on the same mill run, this additional protein is from the aleurone and germ, not gluten from the endosperm.
A word about extraction: if a miller starts with 100 pounds of wheat and yields 72 pounds of flour, that flour is said to be a 72% extraction. Whole wheat flour is 100% extraction. Extraction rate can be an indicator of both efficiency of milling and of quality of flour. The more flour a miller extracts from the wheat, the better, it would seem. But beyond a certain point, even with the best milling equipment and expertise, the ash content, an indicator of bran and germ in the flour, will rise , bringing a change in quality, reflected in performance, color, and flavor.
The complexities of milling have many quality and performance implications for artisan bakers. These will be the subject of the next article in this series.
*The Artisan thanks Greg Mistell, Executive Director of the Bread Bakers Guild of America for permission to reproduce this series of article by Mr. Leonard.
Last Updated on08/04/1999 10:37:45 PM