September 1, 1994
Being on the bottom of a stack isn't always bad. This is definitely the case with the United States Department of Agriculture Food Guide Pyramid, where the bottom level indicates greater significance in overall diet. Grains justifiably occupy this position since they provide carbohydrates, fiber, vitamins and minerals, and are relatively low in fat.
The attention given to grains by the Food Guide Pyramid has consumers looking for ways to increase the amount of grain in their diet, and this has prompted food product designers to focus more on grains.
Science of structure
Modem breeds of grain are members of the grass family and are descended from the grasses originally harvested by hunter-gatherers. Such monocotyledonous plants produce single-seeded fruit. The resulting kernel (or caryopsis) of one type of cereal grain has the same basic structural features as the other ones.
The kernel itself has an endosperm and a germ surrounded by two structural layers. The first outer coating is the pericarp. It clings to the second layer, known as the seed coat. In some grains, the caryopsis is covered by an additional outer hull. This hull begins as a sheath of modified leaves within which the seed develops. Although all grains develop within such a covering, it is particularly close-fitting in rice, barley and oats. This close-fitting covering remains even after threshing.
In addition to the basic similarities, grains also have fundamental differences. Here are some of the differences among the major food grains:
Wheat kernels come in a wide range of sizes, with an average weight ranging from 32 to 38 milligrams. Wheat also has a range of colors and degrees of hardness, depending on the specific variety and where it was grown.
Wheat's endosperm consists of cells filled with starch granules in a protein matrix primarily of gluten. The walls of these cells are made up of hemicelluloses, such as pentosans, and beta-glucans. Once separated and ground, the endosperm forms flour.
Approximately 2.5% to 3.5% of a wheat kernel is germ. A wheat kernel's germ possesses a rudimentary root and shoot along with a storage organ to make it ready for germination. Because it provides nourishment for germination, the germ contains high levels of protein, sugar and oil.
Corn exists in many varieties and colors, but dent corn is the type used for milling. Corn's flat, broad seeds average around 350 milligrams, making it the largest of the cereal grains. As with wheat, the corn caryopsis has the basic structure features of pericarp, seed coat, germ and endosperm. In addition, though, corn kernels frequently have the point of attachment – the tip cap – intact.
Oftentimes, corn's pericarp and seed coat are collectively called the hull. While this terminology is common, corn hulls are not the same as the true hulls of rice or barley. Approximately 5% to 6% of a corn kernel is made up of this outer covering, while 10% to 14% is germ. The rest of the kernel is endosperm.
Rice is harvested with an outer hull intact. De-hulling the just-harvested paddy rice leaves brown rice, which has the basic cereal structural components. Brown rice kernels weigh an average of about 25 milligrams and are about 2% pericarp, 5% seed coat and aleurone, 2% to 3% germ, and 89% to 94% endosperm. The aleurone is the outer layer of the endosperm. When brown rice is polished to form white rice, however, the aleurone is removed along with the seed coat and pericarp to form the bran.
Barley, like rice, retains its husk following harvest. Underneath, the 35-milligram kernels have the four basic grain components of pericarp, seed coat, germ and endosperm.
Rye kernels are harvested hull-free and have the typical grain caryopsis components.
Oats, as with barley and rice, retain the hull formed by a floral envelope. Underneath, the oat kernel (called a groat) is similar to wheat or rye. The germ, however, is much larger and narrower than that of wheat and it extends from 25% to 33% of the length of the groat.
Compared with other grains, oat endosperm contains higher levels of protein and oil. Oat starch is like rice starch in that it exists as compound starch granules, which are large granules made up of smaller individual granules.
The way grain-based ingredients perform in food products is directly related to their composition. The two major constituents of grains are starch and protein.
The amount of starch in grain varies among the different types, but ranges between 60% and 75% by weight. The starch itself is primarily glucose linked into either a linear (amylose) or branched (amylopectin) configuration. (For a more detailed discussion of starch functionality, see "Sifting Through Starches to Maximize Performance" in the September 1993 issue of Food Product Design.)
The proportion of amylose and amylopectin is fairly consistent in the starches derived from different grains (77% amylopectin to 23% amylose, give or take 3%.) Mutant grain varieties, however, deviate from this. Certain mutant barley, corn and rice starches – known as waxy starches – are 100% amylopectin. Other mutants, called amylotypes, have higher-than-typical amylose levels. Such mutations are not universal. Wheat, for example, never varies in its amylose-amylopectin ratio.
Other differences in the starches from various grains occur in size, shape and gelatinization properties.
Barley, rye and wheat have both small spherical granules and large granules shaped like a biconvex lens (lenticular). The chemical composition and functional properties of each are the same, for the most part, but the larger surface area of the smaller granules alters their gelatinization properties. In general, gelatinization (defined as a 50% loss of birefringence) occurs at about 127 degrees F (53 degrees C).
Corn starch gelatinizes at a higher temperature, about 153 degreese F (67 degrees C). The shape of corn starch granules can be anything from polygonal to almost spherical. The different shapes, however, are not thought to function differently.
Not only do oat and rice starch both occur as compound granules, the individual granules of each are quite small with a polygonal shape. The shape of the compound granules of each differ, however, as do their gelatinization properties. Oat starch occurs as large, spherical compound granules and gelatinizes at 131 degrees F (55 degrees C). Rice starch, on the other hand, occurs as small, polygonal compound granules which gelatinize at 158 degrrees F (70 degrees C).
Picturing protein properties
Starch is but one example of how the composition of different grain types varies. Protein content is an area in which grains differ even more significantly. Understanding the content and composition of grain proteins is important for both nutritional and functional reasons. Proteins are most commonly classified into the following four types:
Albumin is soluble in water and coagulated by heat.
Globulin is insoluble in pure water and at high salt concentrations, but is soluble in dilute salt concentrations.
Glutelin dissolves in dilute acids and bases.
Prolamins are soluble in ethyl alcohol. Different grains contain various combinations of these and other proteins that can't be classified. The protein composition as well as the total protein content are believed to be genetically determined to a certain degree. Protein content, however, also is affected by environmental conditions such as weather, available moisture, and the nutrient levels in the soil in which the grain is grown.
Wheat is unique in that the protein in its flour enables the flour to form an elastic dough that can retain leavening gases. The proteins primarily responsible for this effect are the gluten proteins, which are complexes of two protein groups: gliadin, categorized as a prolamin; and glutenin, a glutelin.
Gliadins are single-chained proteins that hydrate into a sticky mass. Because they do not resist extension to any great degree, they contribute to cohesiveness in wheat flour doughs. Glutenins, on the other hand, are multi-chained proteins with significantly higher molecular weights (averaging around 3 million, versus 40,000 for gliadins). Glutenins provide wheat flour doughs with resistance to extension because they are resilient and noncohesive when hydrated.
Other grains lack such a protein combination and, therefore, do not form doughs. Of the other grains, rye is frequently thought of as ranking second to wheat in dough-forming ability, but such doughs are weak and their functional component is believed to be pentosan, rather than protein. Thus, the protein content of non-wheat grains is of interest primarily from a nutritional standpoint.
Most of the major grains have fairly well-balanced amino acid profiles except for one or two limiting amino acids. Many grains, for example, have low levels of lysine. Oats and rye are exceptions. The amino acid profile of oats not only fits well with the Food and Agriculture Organization of the United Nations' established standard protein, but oat groats tend to have much higher overall protein levels compared with other grains. Rye also has a fairly well-balanced amino acid profile, thanks to its high levels of albumins and globulins. Of the protein in rye, 3.5% is lysine, which is a higher level than most of the other major grains.
Grain to the grindstone
While many grains can be made into food products with little processing, most are given further preparation – milling – to allow them to produce more refined and appealing products. Milling, fundamentally a separation process, takes two general forms: dry milling and wet milling.
Dry milling separates a grain kernel into its anatomical parts. It generally involves removing the bran (usually the pericarp, the seed coat and an outer layer or two of the endosperm) and the germ. Because bran contains a significant percentage of insoluble cellulose, removing the bran increases the palatability of the remaining endosperm. Germ is removed because it has a high content of oil, which tends to become rancid quickly, thus shortening the shelf-life of products that contain the germ.
Another function of dry milling involves sizing the grain. Although rice and barley are more desirable in one piece, this isn't the case with most other grains. Rye and wheat, for instance, are usually ground into a fine flour, while a somewhat larger particle (grit) is typical for corn.
Wet milling accomplishes the same fundamental separation as dry milling, but continues the separation process further. The endosperm, for example, is broken down into starch and protein, while the bran and germ are processed into fiber and oil. As with dry milling, the exact treatment varies with the grain itself and the designated application.
Barley is harvested with an intact hull which must be removed as the first step of dry milling. A process called pearling accomplishes this by rubbing off the outer layers of the kernel with an abrasive surface to leave the endosperm. Food processors most often use whole, pearled barley in soups, but the ingredient also can add texture and interest to baked products, breakfast cereals and snack foods. Similar applications exist for pearled barley that has been either rolled into flakes, or reduced to grits or meal through hammer milling.
Hammer milling also can reduce barley to flour. As previously mentioned, the composition of non-wheat grain flours can't build structure in a leavened baked product. Still, barley flour can be used to add flavor and texture to baked products and breakfast cereals.
For the most part, barley isn't wet-milled. It is, however, the grain most frequently malted. Malting is controlled germination of the grain kernel to enhance its flavor and enzyme activity. Malting occurs over three basic stages: steeping, germination and kilning.
For steeping, grain is allowed to soak in water to absorb equilibrium moisture for germination. Germination takes place in a special bed over a period of four to five days. Moisture, temperature and air flow are adjusted to control growth. Kilning takes moisture out of the "green" (undried) malt and promotes the browning reaction that produces the malt flavor. The more this reaction is allowed to proceed, the stronger the malt flavor.
The most widely known use of malt is in brewing. Here, malt affects flavor and color, and provides a source of fermentable carbohydrates. More importantly, the malt enzymes convert starch to simple sugars so they can be fermented by the yeast into ethanol and carbon dioxide. In breakfast cereals and bakery foods, malt primarily contributes flavor. Malt enzymes also are useful for improving dough consistency in baked products.
Corn is challenging to dry mill because of its caryopsis shape and large germ. Two specialized machines – the degerminator and the entoleter – are typically used to remove the germ, then to remove the hull. Roller mills subsequently reduce the endosperm to grits or, in some cases, flour. Corn meal is simply stone-ground with the germ intact. While corn meal and grits are primarily sold for home use, product designers can use dry-milled corn ingredients to enhance the flavor and texture of bakery products and breakfast cereals. More often, though, dry-milled corn is subjected to specialized processing for consumer food products such as ready-to-eat breakfast cereals and snacks.
Corn flakes are made from dry-milled corn that has had the bran and germ removed and the endosperm split in half. Such a large grit is necessary since each grit produces an individual flake. The grits are first cooked in water with added sugar, salt and malt. After partially drying in a counter-current air stream, the grits are tempered to equilibrate the remaining moisture. Once this step is complete, the grits are flaked on smooth rolls under extremely high pressure, then briefly toasted.
Corn is the basis for many snack products. Popcorn, for example, is sold for home use and is popped commercially for packaged snacks. Dry-milled corn flour is used to make a dough that is then processed in a cooker-extruder into corn curls and related products.
Corn chips, taco shells and tortillas require corn to be subjected to a unique, traditional process to form masa. Masa production starts by heating corn with calcium hydroxide (lime). After cooling, the resulting nixtamal is stone-ground into masa. The finished masa is somewhat cohesive when compressed. It serves as the starting material for a variety of products. Compressed, flattened and baked, it becomes a corn tortilla. If shaped and fried, rather than baked, the masa becomes a taco shell. For corn chips, the masa can either be extruded or rolled and cut before frying.
In spite of the many uses for dry-milled corn, much of it is wet milled to recover starch, oil and protein. Starch, in particular, is further processed into specialty ingredients such as modified starches, fat mimetics and corn sweeteners.
Wet milling begins by steeping the corn in a 0. 1% to 0.2% solution of sulfur dioxide, After steeping, the softened grain is ground in an attrition mill to free the germ. This is then separated from the rest of the kernel by density in a liquid cyclonic separator called a hydroclone. The germ is processed to recover the oil it contains. The rest of the kernels are sieved and reground to separate the remaining portions from each other. This process leaves the bran in larger pieces that can be screened out. Finally, a hydroclone separates the starch from the protein, based on their different densities. (For more details on these further processed corn-based ingredients, read "Grain Based Ingredients: No Longer Run-of-the-Mill" in the July 1992 issue of Food Product Design.
Oats are mostly grown for their value as animal feed, but current dietary trends have increased interest in oats as human food.
Dry milling oats begins with a heat treatment to inactivate indigenous enzymes in the grain. This process also develops a desirable flavor. Following heat treatment, the oats are de-hulled to obtain the groat itself. Most groats are steamed and rolled into traditional rolled oats, or cut, steamed and rolled into quick-cooking oats. These are packaged for consumer use and are used by product designers to enhance texture and flavor in bakery products and snacks such as granola bars.
Although not a specific goal of most oat dry milling, rolling oats produces oat flour as a byproduct. This flour is finding use in the newer, oat-based breakfast cereals and in multi-grain bakery foods.
Traditionally, oats have not been subject to wet milling. Consumer demands for lower fat products, however, have changed this somewhat with the creation of Oatrim. Developed by the United States Department of Agriculture's National Center for Agriculture Utilization Research, Oatrim has been called both oat maltodextrin and modified oat flour. It can function as a fat mimetic in a wide variety of food products. Oatrim is made by treating hydrated oat flour with enzymes to hydrolyze the oat starch.
Rice is used in the United States primarily as a whole grain that has been dry milled to remove the outer bull and the bran. Product designers have many options when using rice, depending on the rice type and exact processing method.
Rice varieties can be divided into long, medium and short grain. Long grain rice has a long and slender shape. When cooked, the grains tend to remain separate and are light and fluffy. Medium grain rice is plump, but not round. When cooked, these grains are more moist and tender than those of long grain and have a greater tendency to cling together. Short grain rice is almost round in shape, and the grains tend to cling together when cooked. It also is a little softer than medium grain.
The way a rice is processed after harvesting yields still more performance choices.
Parboiled rice is made using a steam-pressure process that improves the nutritional content of the rice by encouraging nutrients to migrate to the kernel from the eventually removed bran. Parboiled rice is made by soaking, steaming and drying rice while it is still in the husk. Whether parboiled or not, the rough rice, still in its inedible husk, is then dried and moved through the regular milling sequence.
Brown rice is the product after the first milling stage, when the rough rice passes through shelter machines that remove the outer hull. Brown rice consists of whole or broken kernels that have a slightly chewy texture and a nut-like flavor when cooked. The light brown color is caused by the presence of seven bran layers that are rich in vitamins and minerals – especially the B-Complex group.
Regular-milled white rice is left after machines further mill brown rice by rubbing the grains together under pressure to remove the bran layers by abrasion.
Precooked rice is made from regular-milled white rice that has been completely cooked and dehydrated after milling. In the dry form, this rice is more porous so boiling water can penetrate the grains more easily, reducing the necessary cooking time.
Rice can add flavor and texture to many food products. Medium and short grain rice are good choices for foods requiring a creamy, clingy texture, such as molds and desserts. The separate, distinct grains of long grain rice make it desirable in pilaf or other rice-based side dishes.
Brown rice is well suited to good-for-you products because it is slightly more nutritious than white rice. It contains more protein, calcium, phosphorus, potassium, niacin, fiber and vitamin E.
Although not practiced in the United States to the extent it is in Europe and Asia, wet milling of rice can produce starch. This starch can be used to make rice syrup and a variety of modified starch-based ingredients such as fat mimetics. None of the whole-grain varieties are typically wet milled. Rather, waxy rice is selected for its high amylose content.
Rye is, perhaps, the one major grain with the fewest traditional applications. It isn't subjected to wet milling at all. It is almost exclusively dry milled into a flour used primarily for baked goods. Even in this application, rye is limited. Because rye doesn't have gluten proteins like wheat for gas retention, rye-based breads tend to be dense and heavy. Consequently, rye is typically used to flavor a primarily wheat-flour dough.
While rye can be rather dark, it is not responsible for the traditional strong flavor of rye bread. Much of this flavor, in fact, comes from the molasses and caraway seeds usually included in the formula. Industry sources believe further exploration of rye applications is ignored because many product designers associate the strong flavors with the grain itself.
Nevertheless, this limitation may lead to unique opportunities for product designers to explore applications using whole rye kernels or larger dry-milled pieces to add interest and flavor to food products outside the baking industry.
Rye is available pearled like barley, flaked like corn and steel-cut like oats. Having a protein content second only to oats in its amino acid balance, rye also can be a perfect ingredient far the trend toward more healthful food products. Finally, rye's high pentosan content might someday be exploited for its functional benefits.
Wheat is probably the most widely used grain, thanks to its unique ability to form baked products. As a result, a significant portion of wheat is dry milled to flour. How a flour performs in baked products depends on a combination of many factors, such as the type of wheat, how the wheat is milled, and how the flour is treated after milling. (For more information about the flour milling process and its effect on bakery foods, see "Designing Bakery Products: Merging Art and Science," in the January 1994 issue of Food Product Design.)
Besides being the main structural component of baked products, dry-milled wheat can be used in a variety of whole-grain forms.
Like corn, wheat can be processed into flakes that can either be sold as breakfast cereal or used to enhance texture in other products such as confections. Pearled wheat might be used as an alternative to rice side dishes in frozen entrees, or like barley to enhance the texture of soup. Steel-cut wheat might be added to baked products for visual and textural interest, or even be mixed with oats for a unique hot breakfast cereal. Larger grits of wheat endosperm already are the basis for many hot breakfast cereals.
While corn reigns as the grain of choice for wet milling, a significant amount of wheat starch also is processed for use as a specialty ingredient. For the most part, wheat starch is collected as a byproduct when gluten is separated for use as a dough improver in baking. Because the gluten is the primary product in this case, the wet-milling procedure for starch recovery differs from that of corn.
Instead of using the whole endosperm, low-grade flour usually is the raw material for wheat starch. This is mixed with water to form a soft dough. Starch is water-soluble and the gluten hydrates and sticks together so the two are easily separated through water washings and individually recovered and purified.
As long as grain occupies an important position at the bottom of the food pyramid, it should be relatively easy for product designers to help consumers increase their intake. With all the grain varieties available and the many forms in which they can be processed, almost any product can be enhanced in some way through the addition of grain. With a little creative formulation, product designers can use grain ingredients for unique texture and taste experiences. By increasing consumer appeal from both an organoleptic and nutritional standpoint, grains ultimately become ingredients for product success.
You May Also Like
CRN petition to FTC: RCTs aren’t required to substantiate ‘health-benefit’ claimsSep 22, 2023
Collagen peptide ingredient solutions for seniors’ changing needs – infographicSep 19, 2023
Radicle Insights—Covid Eris and dietary supplements: separating fact from fictionSep 21, 2023
More bioavailable hyaluronic acid complex for today’s beauty-from-within consumer – snapshotSep 18, 2023