Gelatinized Starch

Starch is the primary energy storage body in cereal grains. It is composed of amylose and amylopectin molecules arranged around a central hilum or growth point. When the starch granule is viewed under polarized light, a black cross passes through the center of the granule. The size of the black cross is relative to the extent of gelatinization of the starch granule. Gelatinization is the irreversible disruption of the crystallinity of the starch granule, and it occurs when starch is heated in the presence of water above around 60°C. The extent of gelatinization depends on the amount of water present and available to the starch granule; when the water runs out, the gelatinization stops. When a starch granule is completely gelatinized, the cross takes up the entire granule, and it appears black or hollow. The pictures at the top are of starch taken from 2 kernels of sorghum that have been steam-flaked, i.e., heated in steam and passed between two rollers so that it is flattened. The sample on the left had less water available in the kernel during steam flaking than the kernel on the right, so the starch is less gelatinized. We know this because the black crosses in the starch granules on the left are smaller and sharper than those on the right. The ESEM images on the bottom reflect the same differences in starch condition. The granules on the left are mostly ungelatinized and physically intact, i.e., they are round and whole. However, we know they have been heated because the centers of the granules are flattened or depressed, so we can tell they've undergone some processing. The starch granules on the right swelled more during processing because they absorbed more water, and are therefore more gelatinized. Highly gelatinized starch granules are often shredded or distorted during processing, as these granules appear to be. (Reference: McDonough, C., Anderson, B.J., and Rooney, L.W. 1997. Structural characteristics of steam flaked sorghum. Cereal Chem. 74(5):542-547.)

Corn Tortilla

When you bite into a corn tortilla, you're biting into a mixture of starch, proteins, lipids, and fiber from corn kernels that have been cooked in an alkaline solution, ground between stones, sheeted and cut into the familiar round shape, and baked in a conveyer oven at high temperatures. The picture is of a cross section of the corn tortilla, with the top of the tortilla at the top of the picture and vice versa. There are small air cells trapped in the interior of the tortilla (arrows), with the mixture of ingredients dried in place during baking. During processing, some of the starch, protein and lipids melted in the heat and formed a continuous starchy phase, which appears smooth in the picture. The amount of continuous phase present in a sample is often an indicator of the extent of processing in that sample. (Reference: Gomez, M.H., McDonough, C.M., Rooney, L.W. and Waniska, R.D. 1989. Changes in corn and sorghum during nixtamalization and tortilla baking. J. Food Sci. 54:(2) 330- 336.)

Wheat Tortilla

A wheat tortilla is similar to a bread dough except that chemical leavening is used instead of yeast. Flour, fat, leavening and water are mixed together to form a dough, which is cut into dough balls, rested, and then flattened in a press and baked in an oven or on a griddle. A picture of fresh tortilla dough is on the left; the round, ungelatinized starch granules are scattered across the scene like pebbles. The starch granules are suspended in gluten, formed from proteins during mixing; gluten gives dough it's elastic texture. On the right is a fully baked tortilla. As the dough heats up in during baking, the water turns to steam and expands rapidly. While doing so, it creates large air tunnels throughout the product, which produces the fluffy texture associated with a bread product. The starch is partially gelatinized and appears thinner and flatter than it did in the dough. (Reference: McDonough, C.M., Seetharaman, K., Waniska, R.D., and Rooney, L.W. 1996. Changes in microstructure during baking of wheat flour tortillas. J. Food Sci. 61(5):995-999.)

Tortilla Chip, fried

The location of oil in a fried product and how it enters the product during frying is easy to illustrate using the ESEM. Air tunnels form in the corn tortilla during baking and frying as water is turned to steam and expands through the interior, and some are open to the exterior of the chip. The oil covers the chip as soon as they make contact in the fryer. Within 10 seconds, some oil has begun to enter the air tunnels into the chip, seen in the ESEM picture on the left. The oil is the smooth material. As frying progresses, the oil enters the interior of the chip and fills the air tunnels, as seen in the ESEM picture on the right. A traditional SEM picture is at the bottom right, showing the air tunnels with the oil removed. When the electron beam is focused on the oil in the product, it can be seen swirling and circulating within the chip, taking along small bit and pieces of loose material as it flows along. Sometimes, when details about the starchy matrix of the chips are needed, it's necessary to use traditional SEM to view the product, since the oil often hides the details. The crunch that you hear when you bite into a fried chip is the collapsing of the various air tunnel walls between your teeth. The more air cells there are, usually the better the crunch. (Reference: McDonough, C., Gomez, M.H., Lee., J.K., Waniska, R.D. and Rooney, L.W. 1993. Environmental scanning electron microscopy evaluation of tortilla chip microstructure during deep-fat frying. J. Food Sci. 58(1):199-203.)

Potato chip, fried

Potato chips are usually full of oil, which you can tell as soon as you put them on a napkin. This picture shows why. The oil completely covers both surfaces. The actual surface of the chip is very irregular, with lots of peaks and valleys, and the oil fills any valley it can stick to. It also flows freely throughout the interior of the chip, which is full of large round air cells that formed during frying. Potato chips tend to form more air cells during frying than tortilla chips because the moisture of the potato is higher than that of the corn masa. The picture is of a cross section of the edge of a potato chip; all of the smooth looking material is the oil. (Reference: McDonough, C.M. 1995. Unpublished material. Cereal Quality Lab, TAMU).

Tortilla Chip, baked

As people become more health-conscious, they are demanding snack foods that are lower in fat and calories than the standards were in the past. The food industry has responded with numerous brands of baked products, including baked tortilla chips. This particular chip was baked from fresh masa (alkaline-cooked corn ground into dough) in an air-impingement oven, which substantially reduces baking time by blowing hot air directly onto the product. The result is a crisp, crunchy chip with no oil. The picture is of a cross section of one of these chips, with the top of the chip at the top of the picture and vice versa. The moisture in the masa turns to steam when heated in the oven and it expands rapidly, forming a large interior air tunnel that courses through the center of the chip. When your teeth bite into the chip, this air cell and other secondary air tunnels provide the crunch. Starch in the interior of the chip has melted during heating and combined with the protein and other ingredients to form a starchy continuous phase of material, and air cells form within that material as well. On the surface, the air blowing on the chip rapidly dehydrates the starch granules in contacts and no gelatinization takes place, leaving the granular appearance visible at the top and bottom. (Reference: Quintero-Fuentes, X., Almeida-Dominguez, A., McDonough, C.M., and Rooney, L.W., 1998. A method to determine ingredient functionality in baked tortilla chips. Paper in review).

Pretzel

Pretzels are made from doughs that are extruded with very low pressure through dies in whatever shape is wanted. The dough is passed through a strong alkaline bath, which chemically gelatinizes the starch granules on the surface. When the doughs are baked in an oven, the layers of gelatinized starch harden into a crispy dense crust. The cross sectional picture on the left shows the sharp edge of this crust on the left. The interior of the pretzel also has a lot of air cells, and the combination of the dense outer layer and these cells results in a very crunchy product. Many pretzels are covered with flavored coatings, usually oil-based in nature. Examples of 2 of these coatings are in the center and the right pictures. The outside edge of the original baked pretzel is indicated by the arrows, with the coatings directly to the left of the arrows. (Reference: McDonough, C.M. 1995. Unpublished material. Cereal Quality Lab, TAMU).

Noodle

There are many kinds of noodles manufactured throughout the world, and these are two of them. The rice noodle on the top is prepared from pure rice starch and is clear white in appearance. The noodle on the bottom is prepared from durum semolina flour and is pale yellow and opaque in appearance. The cross sections give only a basic impression of the texture of the noodles; they break cleanly with polished surfaces when dry. The rice noodle has only gelatinized rice starch in it, but the durum semolina noodle has both wheat starch and protein. At higher magnifications, the durum grits can be resolved. (Reference: C.M. McDonough, C. Kunetz, L.W. Rooney. 1997. Structure and texture of sorghum noodles. (Abstr.) Cereal Foods World 42(8):666.

Flaked Cereals

The pictures at the top are of 2 kernels of sorghum that have been steam-flaked, i.e., heated in steam and passed between two rollers so that each kernel is flattened. The sample on the left had less water available in the kernel during steam flaking than the kernel on the right, so the flake is more opaque and the starch is less gelatinized than the more translucent flake on the right. The two middle pictures are low magnification, traditional SEM pictures of whole flakes. One limitation with the current ESEMs is that low magnification pictures are difficult to obtain. Each flake has the crimp marks from the rollers visible on the surface. At the bottom are two ESEM pictures of fresh flake surfaces, roughly the width between two crimp marks. The opaque flakes with the less gelatinized starch have more air spaces between the intact granules. This allows light to bounce in all directions as it passes through the flake, leaving a whiter, more chalky appearance. On the left, the more gelatinized starch has fewer air spaces, which allows light to pass through it more cleanly. (Reference: McDonough, C., Anderson, B.J., and Rooney, L.W. 1997. Structural characteristics of steam flaked sorghum. Cereal Chem. 74(5):542-547.)

Bread

This bread is a yeast leavened dough prepared from wheat flour mixed with oil, sugar, salt, yeast and water. When the proofed dough is placed in the hot oven, the heat immediately evaporates any water on the surface, so the starch granules are unable to gelatinize, and retain their round granular shape. However, beneath the brown crust area at the top of the picture, there is enough water to allow the starch to partially gelatinize (they appear flatter), and in the center of the loaf, the starch is very moist and fully gelatinized. The air cells are created by the yeast and expanding steam. (Reference: McDonough, C.M., and Rooney, L.W. 1994. Structural profile of the gelatinization of starch in bread. Presented at the annual meeting of the American Association of Cereal Chemists, Nashville, TN, October. Abstract, Cereal Foods World 39(8): 607.)

Couscous, dry

Couscous is prepared by steaming flour and allowing it to agglomerate into larger particle sizes. The steam gelatinizes some of the starch on the outside of the flour particles, and when the steamed flour is sifted, the warm, gelatinized particles stick together. The "glue" that holds the particles together is gelatinized starch, and when it dried, it forms a strong bond and the product is shelf-stable. Starch in the small grits is partially to fully gelatinized; in the ESEM picture on the right, the gelatinized starch granules have depressed centers. (Reference: McDonough, C.M. 1998. Unpublished material. Cereal Quality Lab, TAMU).