Inside the milling process
A report written under the guidance of the American Association of Cereal Chemists International Whole Grain Working Group (W.G.W.G.) asserted existing data comparing the two principal means of manufacturing whole wheat flours and meals — single-stream milling and multiple-stream milling with recombination — do not show any strong advantage for either milling method with regard to maintaining the nutritional value of the whole grain.
“If recommendations for increased consumption of whole grains are to be met, consumers need to be encouraged to consume whole grain foods produced by all milling methods,” the report’s authors stated.
The report, titled “Nutritional Impacts of Different Whole Grain Milling Techniques: A Review of Milling Practices and Existing Data,” was published in the May-June edition of Cereal Foods World.
Federal dietary guidance for consumers encourages consumption of whole grain foods for their nutritional benefits. Those positive health benefits largely have been identified and documented in epidemiological studies on whole grain foods manufactured from whole grain flours and meals. Those meals and flours, in turn, mostly were manufactured by mills using a multiple-stream with recombination process.
Yet there are those who maintain that same milling process in some way diminishes the nutrient content of the whole grain in the process of separating and then recombining the principal components of the original grain kernel — germ, bran and endosperm. The suggestion is only products of relatively low-volume single-stream milling in which all parts of the original kernels stay together from the beginning to the end of the milling process deserve the appellation whole grain foods. The comprehensive report by the W.G.W.G. writers drew on a wealth of research data and contested that notion.
The report began with a review of the A.A.C.C.I. whole grain definition, which has served as the basis of other definitions for whole grain, including that employed by the U.S. Food and Drug Administration. The report proceeded to examine the science forming the basis of dietary recommendations to increase consumption of while grain foods, review the history of milling and current milling practices, and assess whether techniques used to create whole grain foods result in measurable and practical differences in the nutritional quality of whole grain flours and foods.
“Some sources have voiced concerns that milling processes that separate and combine millstreams may capture fewer whole grain components and their nutrients, fiber and micro-constituents than milling processes that never separate millstreams,” the report said. “Such concerns have been expressed despite the fact that separation and recombination of millstreams have been practiced for much of the history of grain milling. Interestingly, the food intake data linking numerous health benefits and whole grain foods are based on foods using flours produced through the recombination of millstreams.”
The report also noted there are those who are concerned about whole grain foods produced by manufacturers who purchase separate grain components and combine them at the food processing plant (not at the mill).
“The fear is that they can potentially produce a product that fails to deliver all the whole grain components because manufacturers or millers may not mix the millstreams in the correct proportions, either though error or through an attempt to lessen some of the negative sensory or baking properties imparted by the bran or germ,” the report said. “Thus, the resulting products would contain components that do not comply completely with the definition of a whole grain ingredient.”
Those concerns motivated the W.G.W.G. authors to undertake their study and write their review.
The A.A.C.C.I.’s definition of whole grains is: “Whole grains shall consist of the intact, ground, cracked or flaked caryopsis, whose principal anatomical components — the starch endosperm, germ and bran — are present in the same relative proportions as they exist in the intact caryopsis.” The report noted all other whole grain definitions also state that whole grain must contain the bran, germ and endosperm in the same proportions as the original grain.
The report indicated in nearly all studies reviewed, whole grains consumed would have been kernels processed to remove the hull and outer hull (if a hulled variety). The grains, meals and flours then would have been treated with one of the following methods:
Whole grains minimally processed or milled. The grain kernel is left intact or is minimally processed by breaking or flattening the kernel. Think steel-cut oats used in making oatmeal or cracked wheat or bulgur.
Whole grain flours or meals produced at the mill by means of single-stream milling or multiple-stream milling with recombination. In single-stream milling, the grain is crushed between steel rollers or millstones. All parts of the kernel stay together from the beginning to the end of the milling process. In multiple-stream milling with recombination, the grain is crushed, and different grain fractions are channeled into separate millstreams. The millstreams may be sifted and separated by particle size. Large particles are often returned to the mill for further grinding to attain flours and meals with desired and uniform particle sizes. The last step in the milling process reunites all the flour streams at the mill so that they have the original proportions of bran, germ and endosperm in the whole grain.
Whole grain flours or meals produced away from the mill — reconstitution. When crushed and separated products of milling, specifically the bran, germ, endosperm, and minor milling fractions (from the same grain), are reunited at the point of use by the food manufacturer, the process is called reconstitution.
The authors observed, “The contribution of single-stream milling to whole grain foods in Western diets is minimal, an even tinier fraction comes from stone mills. This is due, in part, to differences in the efficiency of stone milling versus roller milling.” The authors noted an average stone mill grinds about 2,000 lbs of flour per hour while a steel-roller mill may have an output of around 3 million lbs of flour per day.
The authors said most whole grain flours are made by recombining millstreams at the mill. The authors pointed out that multiple-stream milling with recombination was not a new milling technology. It has been practiced in various forms to produce several types of flour over the long history of flour milling. Its use in the production of whole grain flour is long-standing. The authors noted the practice of multiple-stream milling with recombination in the production of whole wheat flour was recognized by the F.D.A. as early as 1941.
To ensure nothing is lost in the multiple-stream milling and recombination process, grain millers employ the concept of “mass balance,” which means the mass of the grain at the start and end of the milling process is the same (minus a minor correction for moisture loss that occurs during processing).
“Existing data comparing single-stream milling and multiple-stream milling with recombination do not show any strong advantage for either milling method,” the authors indicated.
The authors challenged the notion that only whole, intact or unprocessed grains can deliver health and nutritional benefits. “Studies show that the nutritional value of whole grains may actually be improved by milling and processing, which increase digestibility and the availability of amino acids, minerals and vitamins,” they explained.
The authors also asserted data do not support the notion that stone milling produces whole grain flour more nutritious, or with less nutrient loss, than that produced by mills employing steel roller mills.
“The alleged high-temperature destruction of nutrients with steel rollers has not been documented, and the high throughput rate (of roller mills) means the shorter contact time between the grain and the rollers results in less loss of unsaturated lipids and greater enzyme activity and nutrient retention,” the report noted.
The authors said manufacturers reconstituting whole grain flour in the manufacture of their whole grain food products must exercise diligence to ensure that the proportions of the various components meet the standards set by the A.A.C.C.I. whole grain definition.
“Some have suggested that safeguards are needed to ensure that all the fractions are present and that the ratios are not ‘tweaked’ to improve bread quality or other sensory of functional characteristics,” the authors noted.
The authors said in summary, “The A.A.C.C.I. Whole Grains Working Group supports increasing whole grain intake through the use of intact whole grains and grains milled with the use of stones and steel rollers in single-stream milling, multiple-stream milling where streams are recombined at the mill, and multiple-stream milling in which whole grains are responsibly reconstituted ‘at the mixing bowl.’