Resistance is not futile (where starch is concerned)

Definitions of the starches and their functionalities were first considered around 1987. Three classes of dietary starch were proposed: RDS (rapidly digested starch, which is digested in the small intestine), SDS (slowly digested starch, which is slowly but completely digested in the small intestine and includes raw cereal starch and cooked pasta) and RS (resistant starch, which resists digestion in the small intestine but may be fermented in the large intestine).

Furthermore, there are four categories of RS, all based on the causes of resistance:

• RS1, physically inaccessible starch due to entrapment of granules within a protein matrix or within a plant cell wall, such as in partially milled grain or legumes after cooking.
  
• RS2, raw starch granules, such as those from potato or green banana, that resist digestion by alpha-amylase, possibly because those granules lack micropores through their surface.
  
• RS3, retrograded amylose formed by heat/moisture treatment of starch or starch foods, such as occurs in cooked/cooled potato and corn flakes.
  
• RS4, which includes chemically modified starches, such as acetylated, hydroxypropylated, or cross-linked starches that resist digestion by alpha-amylase.

Those modified starches would be detected by the in vitro assay of RS. Some RS4 starch may not be fermented in the colon.

RS is counted with the dietary fiber fraction of food and is believed to function as fiber in the human digestive tract. The reduced bioavailability of RS in the human gastrointestinal tract leads to slow glucose release, which reduces blood sugar and blood lipids, including cholesterol and triglycerides.

When RS reaches the colon, it is fermented to hydrogen, methane, carbon dioxide, lactic acid (transient) and short-chain fatty acids (acetate, propionate and butyrate), which may help reduce colon diseases. According to a white paper by Opta Food Ingredients, an early producer of resistant starch, RS assays as an insoluble fiber but behaves physiologically like a soluble fiber.

Bread made with resistant starches retains carbohydrate utility, but reduces carbs to 5 g or so per slice.

How they relate to starch structure

It has been understood that the structure of starch is related to resistant starch, or at least to the amount of resistant starch that can be derived from a starch source. Amylose, the linear fraction of starch, is resistant to the action of enzymes and digesting acids in the stomach and small intestine.

“Normal” starch can be treated to resemble amylose by reacting it with debranching enzymes. The debranching enzyme removes branches, leaving a linear structure that is not easily attacked by digestive enzymes. However, the granular structure of the starch is separate and distinct.

The rheology (texture) of starch solutions is different when the starch is cross-linked as a prevention to digestion, according to James BeMiller, director emeritus of the Whistler Carbohydrate Center of Purdue University. He notes different sources of resistant starch and fiber are included in the AOAC fiber analysis, but that total number didn’t provide good guidance to the consumer. “It’s what we have now, but it’s being studied avidly, and the whole net carb/total fiber puzzle will be solved in the near future.”

Just as there are four designations for resistant starch, there are also several sources for the product. The most common American source is high-amylose corn, a specialty grain grown by a few hundred farmers for one of the two corn wet millers that processes the product in the U.S. Common corn (referred to as No. 2 yellow) contains about 26 percent apparent amylose, the mostly linear polymer of glucose molecules. Through selective breeding of a mutant strain (designated ae, amylase-extender), corn can be produced with 50 to 70 percent amylose, and some strains can consist of 90 percent or more amylase.

Wheat and potatoes also are sources for resistant starch. These resistant starches are made by chemically modifying the starch so it resists digestion. In a patented system, the starches form phosphorylated distarch phosphodiester using a mixture of phosphates, which is not attacked by enzymes in the human digestive system. According to Seib at Kansas State, the chemically modified starches (made in accordance with FDA requirements for food starch modified) swell somewhat during processing, losing some of the resistant starch content. Assays should be done after the food is prepared in order to be accurate.

Market development

In the early 1980s, Opta Food Ingredients began to develop a resistant starch (RS3) by debranching high-amylose corn starch in several steps and allowing it to retrograde. The product, called CrystaLean, is used in diet bars, cookies and other lowfat products. The product is available today from SunOpta Inc. (www.sunopta.com).

But resistant starch really took off with the popularity of low-carb diets. National Starch’s two resistant starches (RS2 versions) are being used in a wide variety of products, says Witwer, and are especially useful in whole grain products that lose some resistant starch during processing. Resistant starches also appear to resist dieting trends. Even if low-carb dieting falls by the wayside, the use of more fiber is likely to continue, especially as the problem of obesity demands attention.

Fiber enhancement is the major thrust of MGP Ingredients’ (www.mgpingredients.com) three-product line. Made by the Kansas State patent, MGP provides a wheat-based RS4 starch called Fibersym 70, a potato-based product called Fibersym 805 (produced by Penford Ingredients) and Fibersym HA, in conjunction with Cargill, made from high-amylose corn. Using the modification method, the products (which can be labeled as food starch modified), achieves very high fiber numbers—70 to 80 percent total dietary fiber.