Understanding Starch Functionality
January 1996 -- Cover Story
By: Scott Hegenbart
*(April 1991 - July 1996)
Corn starch is the primary starch ingredient used by U.S. food companies, But starches from different sources, and even those extracted from less common corn varieties, offer a range of functional properties even before modification. Exploring the unique functionality of various native starches presents several potential advantages.
- Expanded range of functionality. Many starches have properties that aren't so easy to duplicate by modifying another starch.In addition, starting with a raw material closer to the desired functional properties is even desirable in modification. Less extensive modification means...
- Reduced cost. Designers continually require texture ingredients to be more highly functional, yet cost constraints still tighten. In many cases, the less processing a starch receives, the more cost effective it is. Highly functional native corn starches derived from specially developed corn hybrids already are on the market. These can offer greater economy in two ways.
"You will have a starch that won't have to go through modifications, which saves cost," says Ibrahim Abbas, Ph.D., manager of product development, American Maize-Products Co., Hammond, IN. "When these are modified, in some cases the hybrids are more reactive to chemicals; therefore, we can use less. It is more efficient and you can save money."
- Labeling. Although this has not turned out to be the big issue it once was thought to be, modified starches still must carry E numbers in Europe. A more functional native starch will not carry the E number and will appear more natural to European consumers--a concern in the ever-expanding global marketplace.
Relating structure and function Chemically, starches are polysaccharides that consist of repeating glucose units. Starch molecules have one of two molecular structures: a linear structure, known as amylose; and a branched structure, known as amylopectin. Amylose and amylopectin associate through hydrogen bonding and arrange themselves radially in layers to form granules. Starches from different sources vary from one another in the following ways--each of which may affect performance:
- Granule size and shape. Starch granules come in a wide variety of sizes ranging from 3 microns to over 100 microns. With some starches the granule size is polymodal, meaning the granules can be grouped into more than one size range. Wheat starch, for example, has a distribution of both large and small granules. Granule shape also can be diverse. Granule shapes include symmetrical spheres, asymmetrical spheres, symmetrical disks and asymmetrical disks. Some granules exhibit their shape smoothly, while others are polyhedrons with a faceted surface.
- Amylose:amylopectin ratio. All starches are made up of varying proportions of amylose and amylopectin. This ratio varies not only among the different types of starch, but among the many plant varieties within a type. Waxy starches are those that have no more than 10% amylopectin.
- Structure of the amylose and amylopectin molecules. The length of the amylose molecules in a starch - known as its degree of polymerization - can vary tremendously. In amylopectin, the length and number of branches on the molecule are just as variable.
"The length of the amylose molecule varies with type and with cultivar," says Daniel Putnam, senior applications scientist, Grain Processing Corp., Muscatine, IA. "I've seen 200 to 2,000 as the degree of polymerization within a starch type."
- Other variations also exist for starch. These cannot be formed into a single category because they may be unique to one particular starch. In general, however, most such variations consist of the presence of non-starch components in the granule.
The countless varieties of the many starch types couldn't possibly be covered comprehensively in a single article. Consequently, this feature will discuss some general trends among the main types of starch used in the food industry.
- Corn. Four classes of corn starch exist. Common corn starch has 25% amylose, while waxy maize is almost totally made up of amylopectin. The two remaining corn starches are high-amylose corn starches; one has 55% to 55% amylose, while the second has 70% to 75%.
Jay-lin Jane, Ph.D., a professor with the department of food science and human nutrition at Iowa State University, Ames, has been investigating the granule size and shape of many types of starch as part of her ongoing research. Through scanning electron microscopy, Jane and her research team have found that common corn starch has irregular polyhedron-shaped granules. Their size ranges between 5 microns and 20 microns.
Waxy maize starch also has irregularly shaped granules similar in size distribution to those of common corn. However, the individual faces are not as distinct. High-amylose starches also have an irregular shape, but tend to be smooth. Some of these are even rod-shaped. High-amylose starches have a narrower size range: 5 to 15 microns, or even 10 to 15 microns, depending on the variety.
- Potato. Potato starch has about 20% amylose. Like those from many tubers, potato starch granules are large with a smooth round oval shape. Of the starches commonly used for food, potato starch is the largest; its granules range in size from 15 to 75 microns.
- Rice. Common rice starch has an amylose:amylopectin ratio of about 20:80, while waxy rice starch has only about 2% amylose. Both varieties have small granule sizes ranging from 3 to 8 microns. According to Jane, these are irregularly shaped polygons with the waxy rice exhibiting some compound granules.
- Tapioca. Tapioca starch has 15% to 18% amylose. Tapioca granules are smooth, irregular spheres with sizes ranging from 5 to 25 microns.
- Wheat. Wheat starch has an amylose content of around 25%. Its granules are relatively thick at 5 to 15 microns with a smooth, round shape ranging from 22 to 36 microns in diameter. Wheat starch is bimodal in that it also has a group of starch granules of a different size. In this case, these other granules are very small, with diameters of only 2 to 3 microns.
Stacking the starch rivals With an idea of how starches differ, discussing how these same starches perform should readily divulge how the different elements of starch structure affect performance, right? Far from it. Starch chemists universally agree that starch structure and composition affect performance. However, a direct correlation is not always obvious, and changes in a single trait don't necessarily translate into guidelines.
What follows is a review of what is currently known about how structure and composition affect performance. Keep in mind that this discussion may generate more questions than it answers. But first, here is a brief review of what happens during starch gelatinization:
When starch is dispersed into water and heated, the water penetrates into the starch granule from the outside inward until the granule is fully hydrated. Once hydrated, the hydrogen bonding between the amylose and the amylopectin maintains the integrity of the granule and it begins to swell from the hilum (center). Once gelatinized, the swollen granules may increase the viscosity of the dispersion, and/or associate to form gels and films.
- Granule size and structure. According to many sources, granule size does not, on its own, appear to have a strong effect on starch performance. It is, however, believed to be a contributing factor in how rapidly a starch gelatinizes and its gelatinization temperature. Rice starch and tapioca starch, for example, both have amylose contents in the same range, but tapioca starch granules are much larger and, as a result, swell more easily.
"The larger the granule, the less molecular bonding we have so they swell faster," says Paul Smith, president, Paul Smith Associates, North Plainfield, NJ,. "But they also break down faster."
Large starch granules tend to build higher viscosity, but the viscosity is delicate because the physical size of the granule makes it more sensitive to shear. In spite of such differences, the more compact structure of a smaller molecule doesn't always mean a significant difference in gelatinization. Wheat starch, for example, has a bimodal distribution of both small and large granules. Other than size, these granules have virtually the same composition of amylose and amylopectin, and so on. However, the gelatinization properties of the large and small granules do not show significant performance differences.
"One test showed that the small granules have a 3° higher gelatinization temperature than the large, but the onset temperatures were similar," says Abbas. " I would say that in wheat starch, (granule size is) not a major factor."
- Amylose:amylopectin ratio. Waxy corn and common corn starches both have the same granule size, but waxy corn will swell to a greater degree and each will gelatinize at different temperatures. This is largely due to their differing amylose: amylopectin composition.
"Amylose molecules, because of their linearity, line up more readily and have more extensive hydrogen bonding," says Abbas. "Consequently, it requires more energy to break these bonds and gelatinize the starch."
Generally, the higher the amylose, the higher the gelatinization temperature. This is most noticeable in the two high-amylose corn starches which require such high temperatures for gelatinization that they must be cooked under pressure. The amylose:amylopectin ratio also determines the sort of texture the gelatinized starch will build.
"Generally speaking, the amylose gives you the gel strength and the amylopectin gives you high viscosity," says Abbas. "So the high-amylose starches will give you gelling properties and the waxy starches will give you high viscosity."
Amylose's linear structure also contributes to gel strength. In solution, the linear amylose molecules can more easily align themselves with one another and associate through hydrogen bonding to form gels. The branched amylopectin molecules cannot align as easily and, thus, give weaker hydrogen bonding and gel strength.
Viscosity, on the other hand, is purely a function of molecular weight. The branched structure of amylopectin with all its attached chains yields a much larger molecule than amylose. Consequently, amylopectin is better at building viscosity than amylose.
So, if a product designer wants gelling properties, a high-amylose starch should be selected, while a high-amylopectin (waxy) starch would be the choice if viscosity is needed, right? Not quite. Pure gel strength and viscosity often are useful, but they are not always what product designers need. A starch that is too high in amylose can make a pudding too firm. One that is too high in amylopectin may build the correct viscosity in a diet shake, but it may come across as stringy and "slimy" when consumed. Consequently, the amylose:amylopectin ratio determines not just the basic texture, but the nature of that texture, as well.
Using starch in extruded products illustrates how delicate balancing this ratio can be. As with gel formation, film formation is a function of the association of linear amylose molecules. The higher the amylose, the better the film-forming properties. In an extruded snack, film-forming properties are desired in order to obtain a crisp texture in the finished product. But crispness alone does not make or break a snack.
"The tightly bound nature of the amylose polymer affects crispness," says Jim Zallie, director of food technology, National Starch and Chemical Co., Bridgewater, NJ., "But it is a lower molecular weight material which cannot entrap the air that comes from the water turning to steam during venting."
Using a starch with increasing levels of amylopectin increases expansion accordingly at the expense of crispness. As a result, the amylose:amylopectin ratio must be carefully selected. In some cases, the textural demands of the product require combining starches from different sources.
"Some people are using combinations of different base starches to get either a shorter or longer texture," says Mike Augustine, manager, food ingredient applications, A.E. Staley Manufacturing Co., Decatur, IL. "We've been looking at putting together blends to get a specific texture or finished product quality."
In addition to building texture, starches are used to contribute stability to food products. This often takes the form of holding water. As previously mentioned, gelatinized starch molecules tend to reassociate with one another. This reassociation forces the water out of the molecule, causing the starch to recrystallize. The tendency of a starch to recrystallize, or retrograde, in this fashion determines its suitability for long-term stability.
"The branched amylopectin gives steric hindrance," says Putnam. "This doesn't allow the molecules to reassociate so it doesn't tend to retrograde as easily."
- Molecular structure of amylose and amylopectin. Longer amylose molecules tend to make a product's texture stringy because of the way they associate. The molecular weight of the amylose also affects the elasticity of a gel. Longer molecules tend to associate more strongly and produce stronger, more brittle gels, but there is a limit to this effect.
"Tapioca and potato starch both have amylose, but they produce a cohesive mass rather than a gel like corn starch would," says Peter Trzasko, senior research associate with National Starch and Chemical Co. "The theory behind this is based on molecular weight. The potato and tapioca have a molecular weight so much higher than that of corn that it actually makes it more difficult for the molecules to associate."
Molecular weight does not always provide a direct performance correlation. In 1992, Iowa State's Jane reported on research into the effect of amylose molecular size and amylopectin branch chain length on the pasting properties of starch. Jane found that amylopectin molecules with longer branches not only tended to gel, but that the gel strength increased with branch length. However, the viscosity of amyloses of various lengths did not correlate as well. In fact, the best viscosity was obtained with the intermediate-length amylose, while the largest and smallest amylose molecules both produced similarly low viscosities.
A more clear connection can be made between molecular size and stability. A longer amylose molecule will, to a point, have greater gel strength due to its increased ability to associate through hydrogen bonding. This increased ability to associate increases the molecule's tendency to retrograde. Smaller amylose molecules exhibit weaker association and, thus, are more resistant to retrogradation. Recent information indicates that amylopectin molecules with longer branches also are more susceptible to retrogradation. This is a particular concern to researchers trying to lengthen amylose molecules through cross-breeding.
"When you insert an amylose extender gene, you also end up lengthening the branch chains of the amylopectin," says Pamela J. White, Ph.D., acting director, department of food science and human nutrition, Iowa State University.
- Phosphorus. Starches contain phosphorus in some form or another. The nature of the phosphorus affects starch performance. In most cereal starches, the phosphorus is mainly found as lysophospholipids, which will tend to complex with the amylose of the starch and reduce its water-binding capacity. These complexes also contribute opacity to the starch paste.
The phosphorus in tuber starches, such as potato, is in the form of phosphate monoesters that occur on the starch molecule as negatively charged groups. The ionic repulsion generated by these groups weakens the association between the molecules and increases water-binding capacity, swelling power and paste clarity.
Unraveling the mysteries Understanding native starch functionality not only makes the product designer's job more efficient, it is a vital link to expanding starch functionality through modification. This is true whether the starch is modified through chemical/enzymatic methods, conventional breeding or biotechnology.
As stated previously, the study of starch structure/function relationships generates more questions than it answers. As a result, researchers working in this area have plenty to keep them occupied. Iowa State University is one location where ongoing starch study is occurring.
Since 1987, ISU researchers White and Jane have been searching for starches with unique functional properties to use in the development of new corn hybrids. Working with them is Linda Pollak, Ph.D., a U.S. Dept. of Agriculture-Agriculture Research Service research geneticist working with the ISU Dept. of Agronomy.
Using Pollak's access to the North American library of corn mutant genotypes, the team has been screening the exotic types of corn to determine the nature of the functional property variations.
"It's difficult and time consuming to do a direct structural study," says White. "So our approach has been to start with a quick screening of the starch by extracting it in the lab with as little as one kernel."
This initial screening is done using differential scanning calorimetry (DSC). A sample of the starch is pasted, then scanned on the DSC. After storing the pasted sample for seven days at 4°C (the optimum temperature for starch retrogradation) the sample is re-scanned.
"The scan we get on a fresh and stored sample tells us whether the starch may have unique functional properties," says White. "Once we find something unusual, we verify that that indeed gives us a different DSC another time."
Other information obtained through this DSC analysis includes the gelatinization temperature and the gelatinization range. A low gelatinization temperature may provide energy savings in a large manufacturing operation. A narrow gelatinization range also will make production more efficient by making gelatinization more rapid.
"Those are the key things we begin to look at," says White. "When we see things that differ greatly from the norm when measured by DSC, we then do a structural analysis to determine why they do that and relate the structure to the function."
The first step in doing this requires growing the mutant corn in greater quantities for further analysis. The tests include determining the percent amylose through iodine potentiometric titration and/or gel permeation chromatography; the molecular weight distribution using gel permeation chromatography; and the branch chain length of the amylopectin calculated from the reducing value determined through wet chemistry, or using gel permeation chromatography.
If a sufficient quantity of starch is available, functional tests such as those for viscosity and gel strength also are conducted.
"Another thing we often do is measure the granule size through electron microscopy," says White. "Small granule starch has been shown to be good for a smooth mouthfeel, which is a useful property for fat substitutes to avoid grainy texture."
Eventually, the connection between desired functional properties and the structure of the starch are made. Then plant geneticists take over and try to breed the desired qualities into a variety that can be cultivated.
Expanding the understanding of native starch functionality is useful both for product designers and creators of new starch ingredients. At times, though, it seems that each step in the journey toward this understanding only adds distance to the road. Nevertheless, these efforts must continue because - although the journey may never end - each step closer brings new advances that help improve food products.
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