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Milling By-Products of Cereal Grains
In addition to use as animal feedstuffs, cereal grains are also used for human foodstuffs and industrial products. To produce the food and industrial products, the grains are cleaned and then milled. The main types of milling are dry and wet milling. Dry milling removes the outer fibrous materials from the seed. Dry milling is used to produce such products as flour. Wet milling is used to produce such products as sugar, starch, syrup, and oil. The by-products of cereal grain milling are used as animal feeds. The nutritional values of the by-products vary. Standards stating the minimal nutritional value of each by-product have been established. The Association of American Feed Control Officials (AFCO) publishes official descriptions of each of the milling by-products. Appendix Table I, beginning on page 502 of the text, includes nutritional information on each of the grain milling by-products.
In general, the nutritional value of cereal grain milling by-products varies from the original cereal grain. Generally, as a result in the change in proportions of fibrous and nonfibrous carbohydrates, the energy values for the millfeeds are lower than the parent grains. In contrast, the protein contents of the millfeeds are generally higher than the parent grains.
Many of the terms used to describe the milling by-products are used to describe similar components for multiple grains. For example, hulls refers to the outer covering of the grain and can be used to describe both hulls of oat and rice origin. And some of the terms used to describe the milling by-products are, for the most part, exclusive to one grain. The following sections will discuss the cereal grains milling by-product terms recognized by the Association of American Feed Control Officials.
The first milling by-product is bran. Bran includes the coarse outer covering of the seed and lesser amounts of flour. Nutritionally, bran primarily contains fiber and protein. The most common feed brans are corn, rice, and wheat bran.
Figure 5.1 illustrates wheat bran.
The second milling by-product is flour. Flour is fine in texture. Flour primarily consists of the gluten and starch. The crude fiber content of flour is low. Again, there are many flours with the most common flours being rye and wheat. Generally, flour is fed with other milling by-products. The third by-product is germ meal. Recall, the germ is the embryo of the seed. The germ is high in lipids and protein. Germ meals are classified as protein sources. The most common feed germ meals are corn and wheat germ meal.
The next by-products are gluten feed and gluten meal and they are wet milling by-products. They are most commonly the by-products of the milling of corn and sorghum.
Figure 5.2 illustrates corn gluten meal.
Gluten is the primary substance remaining after removal of the germ and the starch endosperm. Gluten-based feeds are classified as protein sources.
The fifth grain milling by-product is grain screenings. Grain screenings are a mixture of dust, chaff, weed seeds, broken grains, unsound grains, and all other materials separated during cleaning and processing. The nutritional value of grain screenings varies with relative proportion of each of the components. Grain screenings originate from all milled grains. Grain screenings may be marketed as grain screenings, mixed screenings, and chaff and/or dust. Grain screenings must contain a minimum of 70% grain and maximum of 6.5% ash. Mixed screenings must contain a maximum of 27% crude fiber and a maximum of 15% ash.
The sixth milling by-product is groats. Groats are the grain seeds without the hull. The most common groats are oat and rice groats. The nutritional value of groats is greater than the original grain. Groats have a relatively low crude fiber content. The higher quality groats are used for human foodstuffs.
The seventh by-product is corn hominy feed. Hominy feed includes corn bran, germ, and flour. Hominy feed is higher in both crude protein and fiber content compared to corn grain. Compared to other by-products, hominy feed has a relatively low crude fiber content. Hominy feed must contain at least 4% crude fat. Solvent-extracted hominy feed contains a lower fat content.
The eighth by-product is grain hulls. Grain hulls are the outer covering of the grain seed. Grain hulls are most commonly by-products of oat and rice milling. Grain hulls are relatively low in energy, low in crude protein, and high in crude fiber. Hulls are classified as a roughage.
The ninth milling by-product is barley malt sprouts. Malt sprouts consist of roots, sprouts, and malt hulls from the malting industry. Malt sprouts are classified as a protein source. The minimum crude protein content is 24%. The tenth milling by-product is oat meal. Feed oat meal consists of lower quality oat meal portions. The maximum crude fiber content is 4%.
The next milling by-product is middlings or midds. Middlings are the by-products from the production of flour and include bran, shorts, germ, flour, and tailings. Rye and wheat are the most common midds.
Figure 5.3 illustrates wheat middlings.
The maximum levels of crude fiber for rye and wheat middlings are 8.5% and 9.5%, respectively. The twelfth milling by-product is mill run. Mill run is also referred to as mill by-product. Mill run consists of bran, shorts, germ, flour, and tailings. Mill run may originate from all of the cereal grains. The specified requirements are as follows: grain sorghum – minimum 5% crude fat and maximum 6% crude fiber; oat – maximum 25% crude fiber; rice – maximum 32% crude fiber; rye – maximum 9.5% crude fiber; and wheat – maximum 9.5% crude fiber.
The thirteenth milling by-product is rice polishings. Rice polishings are the residue from rice polishing. Polishings are relatively low in crude fiber, high in crude fat, and a good source of thiamin.
The next by-product is wheat red dog. Red dog is a by-product of wheat milling for flour and includes tailings with some bran, germ, and flour. The maximum amount of crude fiber is 4%. The final cereal grain milling by-product is wheat shorts. Wheat shorts are also a product of flour milling and consist of bran, germ, flour and tailings. The maximum crude fiber content for shorts is 7%.
In the U.S., the majority of wheat-based products fed to livestock are wheat milling by-products. The common by-product feeds of wheat milling are wheat bran, middlings, mill run, shorts, red dog, screenings, germ meal, and germ oil. Table 5.15 on page 73 of the text describes the composition specifications of wheat millfeeds. Wheat bran is a common wheat by-product. Nutritionally, in comparison to the other wheat millfeeds wheat bran contains the highest fiber content. Another relatively common wheat milling by-product is wheat middlings or wheat midds. Wheat middlings have a higher digestible energy content than wheat bran. Wheat shorts have a similar appearance to wheat middlings. Red dog is a minor millfeed. The following is a discussion of the nutritional value of wheat milling by-products. The energy values of wheat milling by-products are lower than the energy value of the original grain. With this being stated, the specific energy values of the by-products are not well defined. The protein quantity and quality are greater than the original grain. However, the by-products still remain deficient in the essential amino acids. The by-products are good sources of most of the water soluble vitamins. In terms of minerals, by-product feeds are relatively high in phosphorus, magnesium, and manganese. The majority of the phosphorus is bound in the phytate complex. Similar to other grains, wheat millfeeds are low in calcium. Similar to other grain-based products, wheat milling by-products may be further processed to improve the nutritional value of the feed. Wheat milling by-products are palatable when fed at moderate levels. Wheat milling by-products are bulky feeds; low mass per unit volume. Animal fill may limit intake, therefore limiting energy consumption. Finally, wheat millfeeds will also absorb and retain water. Therefore, wheat millfeeds have a moderate laxative effect. Wheat milling by-products are fed to poultry, swine, horses, and cattle.
The two main corn dry milling processes are the degerming and the nondegerming processes. The degerming process separates the hull, germ, and endosperm prior to milling. The degerming process produces the majority of the corn milling by-products. The common corn by-products of dry milling are hominy feed, corn flour, and corn bran. Hominy feed is an important corn milling by-product feed. For both monogastrics and ruminants, hominy feed is a good source of energy. Corn flour is another milling by-product feed.
Wet milling of corn is a more extensive process than dry milling.
Figure 5.4 illustrates an overview of the wet corn milling process.
In brief, wet milling involves cleaning, steeping, grinding, extraction, other separation processes, and drying of the products. The common feed by-products of corn wet milling are starch molasses, liquefied corn product, gluten feed, gluten meal, germ meal, condensed fermented corn extractives, and hydrolyzed corn protein. The by-products are classified as either liquid feeds or protein supplements.
Sorghum is processed similar to corn. Sorghum is processed both by dry and wet milling processes. The common sorghum milling by-products are grain sorghum grits and sorghum gluten feed.
Barley may be milled similar to other grains. Similar to oats, the primary processing is to remove the hull of the grain. The common milling by-products are hulls, pearl by-product, mill by-product, and screenings. In addition to grain milling, barley is also used in the malting and brewing and distilling industries. Malting, brewing, and distilling by-products used as animal feeds include malt cleanings, malt hulls, malt sprouts, malted barley, brewer's grains, distiller's solubles, spent hops, and brewer's yeast. A number of these by-products are classified as protein sources.
Following cleaning, oats are roasted to facilitate hull removal. The hull accounts for approximately 30% of the mass of the intact oat grain. The common oat milling by-products used as animal feedstuffs are hulls, groats, feeding meal, mill by-product, and clipped by-product. Oat groats are used for human foodstuffs and animal feedstuffs. The maximum allowed crude fiber content for oat groats is 22%.
Rice is primarily produced and processed for human consumption. Therefore, the primary rice-based feedstuffs are rice milling by-products. In brief, rice milling includes cleaning, hulling, pearling, and polishing. An objective of rice milling is to remove the hull. Rough or paddy rice is rice with the hull. Brown rice is rice without the hull. Brown rice will be processed to remove the bran layers. The milling by-products used as feedstuffs include hulls, bran, polishings, groats, and mill by-products. Rice hulls are high in silica. Silica is an indigestible compound. Therefore, the feeding value of rice hulls is relatively low. Processing the rice hulls may improve feeding value. Rice bran consists of the fibrous outer coats and the germ with lesser amounts of hulls, rice pieces, and calcium carbonate. The maximum crude fiber level for rice bran is 13%. Nutritionally, the energy value of rice bran is similar to the original grain. Rice bran has a high oil content and is a good source of B-complex vitamins, protein, and amino acids. As a result of the high oil content, rice bran that has not been stabilized is susceptible to rancidity. Rice polishings are also relatively high in fat. Both solvent extracted bran and polishings are marketed.
Rye is also milled and the by-products are used as animal feedstuffs. The common rye milling by-products are millrun and middlings.
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