The molecular weights and degree of branching of these starch molecules vary by plant source. Reports for amylopectin molecular weight range from 5.0x107 to 5.0x108. Amylose molecular weight can range from 1.6x103 to 1x106.
When starch granules are put into solution with heating, they swell irreversibly in a process called gelatinization. In gelatinization, intermolecular bonds are broken down, which allows hydrogen-bonding sites (between hydroxyl hydrogen and oxygen on a molecule) to “engage” more water. In solution, after gelatinization, the linear amylose chain has a tendency to re-associate or realign, forming gelled structures upon cooling. This re-alignment of the amylose chains is referred to as retrogradation.
Starch CharacteristicsRegular wheat starch has an approximate composition of 70% to 75% amylopectin and 25% to 30% amylose, which is similar to corn starch composition. Starch characteristics can be influenced by chemical or physical modification. Many commercial starches have been modified in some way to change swelling characteristics, gelatinization temperatures and/or retrogradation tendencies.
Through plant breeding, traditional starch can be altered to adjust the amount of amylopectin and amylose in the starch granules. Waxy and high amylose starches are results of plant breeding. Typically, when a starch is termed waxy, regular or high amylose, this refers to the relative ratio of amylose and amylopectin within the plant starch granules. Waxy starch is higher in amylopectin; near 97% with 2.5% amylose. Some waxy starches reach up to 99.99% amylopectin.
“High amylose starches reverse the tendencies [toward higher amylopectin content],” states Joe Lombardi, business manager for a well-known supplier of starch ingredients. These ingredients are typically 50% amylose or higher. Some hybrids now reach 60%, 70% and even 80% amylose, he adds.
Waxy StarchAccording to USDA evaluations of wheat starch, in order for waxy wheat to have great commercial potential, the starch should be full waxy and consistently exhibit greater thickening power than the non-waxy counterpart. Waxy wheat starch has distinct characteristics that may increase its commercial use and provide a means to obtain variations in specific food products.
With little to no amylose present, waxy wheat starch thickens upon gelatinization but does not gel or retrograde upon cooling due to its branched amylopectin chains. The phenomenon known as syneresis, where water seeps out of a gel, is also due to amylose chains which are re-establishing bonds. Therefore, waxy starch tends not to cause syneresis.
Waxy starch forms a paste at lower temperatures and holds more water than regular wheat starches. Waxy starch gels do not tend to lose water when exposed to freezing and thawing (syneresis). Waxy wheat starch is more stable in solution over time and under stress conditions such as cold temperature storage and temperature cycling. “While waxy wheat starch granules gelatinize at lower temperatures (typically 45°C), the gels are not as strong as regular starch gels,” states Robin Guy, Ph.D., manager of cereal processing and bakery science at U.K.-based Campden & Chorleywood Food Research Association.
“High amylose starches can be extremely hard to cook, but once gelatinized and cooled, they form very firm and rigid gels and films,” advises Lombardi. “High amylose starches also form strong complexes through the amylose fraction with emulsifiers and several flavor components. These complexes can reduce the formation of resistant starch from the amylose. Normally higher levels of flavor should be used to overcome the losses in the starch,” advises Guy. “The high amylose starch functions better at high temperatures and is more stable in an acid environment, whereas the waxy wheat starch breaks down easily under mild cooking and shearing conditions,” he adds.
Functionality“The primary function of a waxy wheat flour is similar to standard flour: to deliver rich, indulgent textures and appearance and authentic, culinary quality taste. The key difference is that it offers the same positive attributes of traditional flour while eliminating one of the key negatives—textural instability. Textural instability limits traditional flours used in a wide variety of value added and rapidly growing segments, such as refrigerated soups,” contributes Lombardi.
As mentioned, the inherent properties of native starch materials can be leveraged for advantage in processing and/or texture. If, for example, a product needs to be very stable to textural changes over time, it would be best to stay away from amylose-containing materials. However, if a rigid gel structure is needed, such as in confections or inflexible films for coatings, a higher amylose-containing product might be utilized.
“Cold temperatures and temperature cycling tend to hasten the re-association or retrogradation of the amylose chain, contributing to textural instability when using regular starches. In low-moisture systems like breads and cakes, retrogradation can contribute to and exacerbate the staling process. Waxy wheat, due to inherent stability, will contribute to maintaining good texture. As with all starches and flour thickeners, time, temperature, pH and mechanical shear in the process need to be considered when choosing the appropriate ingredients,” adds Lombardi.
“Enzymes can sometimes be an issue. Enzymes found in fruits and spices can cause a breakdown of the flour or starch, resulting in a thin, watery texture. To avoid this issue, food manufacturers need to ensure that the ingredients they use are properly processed to deactivate the enzymes. Additionally, to get the best functionality from flour, it needs to be suitably cooked. Undercooking can contribute to off-flavor (raw flour flavor) and a less stable product,” states Leslie Drew, food scientist at a popular starch supplier.
All starches can be degraded by amylases after they are gelatinized if the moisture levels are above about 14% w/w. This would reduce viscosity or consistency in the food, which is why amylase should be removed by heat treatment (heating to > 90°C).
ApplicationsWaxy wheat flours can be used to thicken soups, sauces and gravies, but they also offer excellent textural stability upon cooling, freezing and reconstitution, which is a great tool for high-quality refrigerated soups, frozen meals and shelf-stable sauces. Both native waxy and high amylose starches and flours are in development stages for baked goods, so their uses are still under investigation.
“Waxy starch would be useful in extruded snacks, because the granules are easily dispersed, and it is the dispersed starch polymers that are functional in most extruded products. In bread and cakes, waxy starch fails to stabilize the structure but can be used in small amounts to give a softer crumb. However, waxy maize is used as a thickener in sauces, soups and gravies after chemically cross-linking or high temperature heat treatment, and waxy wheat starch would also be suitable for these products after the same treatment,” Guy advises.
“At present, high amylose starches are not used in bakery products to any extent; because they are still in the development stages, their functional performance is not known except for small lab-scale trials and they are mostly from genetically modified crops,” says Guy. “However, high amylose starch has created interest for breadmaking, because it can carry the wheat proteins with it to make the dough while giving a change in digestibility and lowering the glycemic index,” he says.
“The high amylose starch would probably be more unstable than normal starch and retrograde to form resistant starch in bread or cakes, which could be regarded as a positive effect that might be used to increase fiber in bread and improve its nutrition,” contributes Guy. “But high amylose starches do not swell to create structure very well in bread or cakes. They can be used in extruded products where the conditions are more severe to gelatinize and disperse the granules and, in some batters, at the 10%-15% levels to give crispiness where gelatinization of the granules is not required,” he offers.
Wheat starch may be preferred over other starch sources in certain baked goods due to the lack of chemicals used in production. “We are only now beginning to explore the various benefits waxy wheat flour or starch can bring to baked goods, particularly in extending textural quality in frozen and refrigerated doughs and [baked] goods. In addition, we are exploring the benefits in shelflife for standard fresh baked items,” says Lombardi.
“The waxy and high amylose maize starches are priced much higher than normal maize starch, even though ‘invented’ in the 1960s, because of the production costs and functional benefits. The new wheat starches are likely to be much more expensive than normal wheat flour or separated wheat starch initially and possibly for the foreseeable future,” Guy predicts.
http://food.oregonstate.edu/learn/starch.html— Information on the chemistry of starch molecules
www.nationalstarch.com— Searchable commercial starch ingredients site with application-oriented information and technical support
www.starch.dk— Information on research, engineering, manufacture and application of starch (warning, loud tune with site)