August 2, 2007
Resistant starch (RS) has been a part of the human diet for thousands of years. Although we consume a wide variety of nutrient-rich foods today, our current consumption of resistant starch is low, due to the amount of processed foods in our diet. “People in less-developed countries will consume 15 to 20 grams of resistant starch per day, while most of us only consume 3 to 6 grams of resistant starch per day,” explains Rhonda Witwer, business development manager of nutrition, National Starch Food Innovation, Bridgewater, NJ. Several sources of resistant starch exist, including whole and partially milled grains, legumes, bread, green bananas, cooked pasta and potatoes, and extruded corn cereals. Consumption of these foods helps to increase dietary fiber intake, but resistant starch can also be added to foods as an ingredient. And, in fact, the benefits of adding resistant starches to foods go beyond just adding dietary fiber.
Defining moments for RS
Most of us think of starch as a quickly-digested carbohydrate, something that became undesirable in the low-carb days. Freshly cooked starchy foods fall into this category of rapid digestion. However, most raw cereals are slowly, but entirely, digested in the small intestine. Most starches are digested in the small intestine, but some resist digestion and are later fermented in the large intestine.
“Definitions of the starches and their functionalities were first considered around 1987,” says Dorothy Peterson, starch development lead—North America, Cargill Texturizing Solutions, Hammond, IN. “Three classes of dietary starch were proposed: RDS (rapidly digested starch, which is digested in the small intestine), SDS (slowly digested starch) and RS (resistant starch), which resists digestion in the small intestine but is fermented in the large intestine.”
Because RS is often used as a fiber source, it is important to show how it compares with other fibers. For many years we thought there were only two types of fiber: soluble and insoluble. Soluble fibers include pectins, gums, mucillages and some hemicelluloses found in fresh and dried fruit, vegetables, oats, legumes and seeds. Soluble fibers can be fermented in the large intestine by bacteria and act as prebiotics. Insoluble fibers, like cellulose, lignan and hemicellulose, found in whole grain breads and cereals, fruits, vegetables, and unprocessed bran and wheat germ, provide bulk and help regularity.
Resistant starches have been defined as “the sum of starch and products of starch digestion not absorbed in the small intestine of healthy individuals” (1992, European Journal of Clinical Nutrition, 46 Suppl. 2 S1).
Several types of RS have been classified according to their sources. RS1 is the physically inaccessible form found in partially milled grains and seeds. RS2 is in the form of resistant granules from raw potatoes, green bananas, legumes and highamylose corn starch. RS3 is from cooked and cooled starchy foods, such as bread, cornflakes, potatoes or retrograded high-amylose corn. They can also be produced by enzymatic debranching of starch followed by retrogradation.
Starch retrogradation occurs when starch granules are dispersed in water and fully gelatinized by cooking. The granules lose their crystalline structure and the starch polymers become soluble. When the solution cools and ages, the polymers once again form a partially crystalline structure. During retrogradation (recrystallization), slightly hydrolyzed amylose forms doublehelical, crystalline structures called crystallites. RS4 comprises chemically modified starches, such as starch ethers, esters and cross-bonded starches. This type of RS can have a wide variety of structures and functional properties.
The early years of resistant starch technology began in the late 1950s with the introduction of high-amylose corn starch, an RS2 type. “Its use as a resistant starch has been studied for nearly 27 years,” says Peterson. “More recently, both physical and chemical methods have been developed to increase the TDF (total dietary fiber) content, expand functionality and include other starch sources.” Initially, interest in RS ingredients focused on the physiological benefits of gut fermentation, insulin patterns and reducing serum cholesterol. The development of low-carbohydrate foods increased the popularity of resistant starches to help reduce baked products’ carbohydrate content. “Today, there is more focus on both fiber enhancement and calorie reductions; however, the physiological benefits are still of high interest,” she says.
Development of the RS3 and RS4 types allowed the use of starch sources other than highamylose corn. “RS4 types are some of the highest in total dietary fiber, lowest in calories and most stable to thermal processing,” says Peterson. “They also can be added at higher levels than highamylose products, due to the extremely low water-binding properties.” Resistant-starch labeling differs for each type, with RS2 typically labeled starch or corn starch, RS3 as maltodextrin, and RS4 as modified food starch.
Tapioca-based RS provides a bland flavor profile and smooth mouthfeel, which makes it easy to formulate into foods. The RS3 types are nongelling and less viscous than high-amylose derived products. They are heat stable up to 120°C, depending on the application, stable at low pH and have a low water-binding capacity. They are produced by enzymatic debranching of tapioca maltodextrin and retrogradation. The amylose crystallites have a particle size of 10 ìm, which contributes to a smoother mouthfeel.
“RS is an invisible fiber when used in grain-based products such as breads, bars and pasta,” says Peterson. “In addition, very few changes are required in terms of processing or formulation adjustment.” In most applications, RS is used as a replacement for flour. The RS3 type has an RS content of 54.0% and a digestible carbohydrate fraction of 40.3%, with 2.8 kcal per gram. Its potential applications are breads, cereal bars, cookies, biscuits, muesli, low-fat fermented milks, UHTflavored milk drinks and dry mixes, such as instant soups and chocolate drinks. The RS4 type, a modified resistant starch, has one of the highest total dietary-fiber contents of any resistant starch, which allows for lower usage levels to achieve the target TDF in an application. This RS has only 0.25 kcal per gram, making it ideal for the formulation of reduced-calorie foods.
“Resistant starches analyze as dietary fiber and are believed to be metabolized as fiber in the digestive tract,” says Peterson. With that in mind, a wide range of applications would benefit from the high fiber content, including breads, rolls, buns, pizza crust, tortillas, pancakes, biscuits, cookies, cakes, muffins, sweet breads and crackers. “As an extension of our resistant starch offerings, we have introduced a product called Fiber Krunch,” she says. “This is a high-fiber inclusion (35%) that resembles crisped rice. The primary fiber source is resistant starch, but it also includes inulin. It was designed to be a convenient way to add fiber to bars and cereals, but it also works very well in chocolate bars and coatings to add both fiber and a crispy texture.”
Beyond adding dietary fiber, these tapioca-based RS ingredients have potential health benefits. They are fermented more slowly than oligosaccharides, which means some fermentation occurs lower in the colon. This usually translates into a higher tolerance (up to 120 grams per day of the RS ingredient) and less gastrointestinal distress. They might stimulate the growth of beneficial bifidobacteria and lactobacilli. They provide an excellent substrate for the bacterial flora in the colon and produce a 60% yield of short-chain fatty acids (SCFA) with over 20% of n-butyrate, a colon-cell nutrient. This butyrate production can help maintain a healthy colon and overall good digestive health. Tapioca RS contributes to bileacid reduction and increases stool weight. After consumption, it also reduces blood glucose and insulin levels, resulting in a low glycemic response.
“We have developed two RS4- type resistant starches derived from wheat with very high TDF contents,” says Steve Ham, director of marketing, specialty ingredients, MGP Ingredients, Atchison, KS. The plant-based starches resist amylase breakdown during digestion. Both RS4 ingredients have a higher TDF content on a dry basis (75% and 85%) than RS2 or RS3 types, which are both in the 60% TDF range. “The higher levels of TDF can significantly improve the nutritional profile of foods and make it easier to achieve the targets of 2.5 grams of fiber per serving for a ‘good source of fiber,’ or the 5 grams per serving necessary to achieve an ‘excellent source of fiber,’” he says. The 85%-TDF product, introduced in 2003, has a white color, low water-holding ability (0.7 grams water per gram of ingredient) and contributes less than 1 kcal per gram. According to the U.S. Dietary Guidelines Advisory Council, the average adult should consume 28 grams of fiber daily per 2,000 calories. Most Americans typically consume 4 to 6 grams of fiber daily.
Some hesitancy about adding fiber to foods still exists, as traditional fibers often bind three to four times their weight in water in a formulation, and are often dark in color, with a gritty texture and bitter flavor. “Our wheatbased RS4 resistant starches are a natural fit for wheat-based products, because they don’t change the appearance or sensory properties of the finished product,” says Steve Pickman, vice president, corporate relations, specialty ingredients, MGP Ingredients. “We recommend that you replace flour with our resistant starches in bakery applications.”
In white pan bread “you can add a significant amount of fiber, replace high levels of flour and then add a wheat protein isolate to replace the gluten in the flour and maintain dough strength,” adds Ham. Due to the low waterholding ability of this RS4 type, it allows addition of significant fiber levels to unique applications, such as pretzels, tortillas, bagels and snack foods. Other popular applications include cookies, muffins, breakfast cereal, bakery mixes, noodles, pancakes and waffles.
“Another RS4 starch introduced in 2006, containing 75% TDF on a dry basis, is designed to have higher water-holding ability, which provides a creamy texture and fatlike mouthfeel,” says Ham. “It can provide partial fat replacement with reduced calories in icings, creme fillings, yogurt smoothies, frozen desserts, salad dressings and bakery products.”
Studies are ongoing to evaluate the health benefits associated with these wheat-derived resistant starches. Resistant starch doesn’t carry a health claim, although it has many potential health benefits, such as reducing glycemic load, increasing insulin sensitivity, reducing blood levels of low-density lipoprotein (LDL) cholesterol while increasing blood levels of beneficial high-density lipoprotein (HDL) cholesterol, and promoting good colon health due to butyrate production.
“High-amylose-corn resistant starch became available commercially in 1994,” says Witwer. A fine, white starch with a small particle size, it is derived from a natural variety of non-GMO corn and processed with mild heat and moisture to align the amylose chains within the starch granule. It is classified as an RS2 because it retains its natural granule structure. RS3 types can also be made from high-amylose corn by combining an enzyme treatment with mild heat and moisture to align the amylose chains outside the starch granule. Retrogradation following the starch granule’s disruption classifies it as an RS3.
The RS2’s calorie content is 2 to 3 kcal per gram, and it has some similarities and advantages that the RS3 and RS4 types share. It is easy to use as a flour replacement in baked products like bread, crackers, cookies, cereal, pasta and snacks, with minimal changes to finished-product taste or texture. It is also fermented slowly in the large intestine, so it can be consumed at higher levels than inulin and fructo-oligosaccharides without digestive discomfort. “Inulin is a soluble fiber, so consumption of 10 to 15 grams per day will usually create bloating and gas for most people,” Witwer says. “Most people can tolerate up to 45 grams per day of high-amylose-corn resistant starch, because it is insoluble and it takes time for the bacteria to digest it. As a result, this RS is fermented lower in the colon and doesn’t produce gas.”
The real differences arise between the different resistant-starch sources in the area of health benefits. “High-amylosecorn resistant starch has the advantage of accumulating over 15 years of research and 120 nutritional studies related to its health benefits,” says Witwer. Many of the human studies have focused on glucose and insulin response, increased insulin sensitivity, and even weight management. “In one study, consumption of high-amylose corn increased lipid oxidation by 23%, which increased fat-burning ability,” she says. “Another study using a control diet compared to two restricted-calorie but isocaloric diets showed that there was an increase in satiety hormones with the diet containing high-amylose corn but not the diet using cellulose as a fiber source.” Substitution of flour with high-amylose corn in high-carbohydrate products, like bread, resulted in a lower glycemic response and improved moderation of blood sugar and insulin levels in other human trials. Researchers suggest the fermentation byproducts may be responsible for the increase in insulin sensitivity.
The majority of the human studies have focused on colon health and positive effects on biomarkers, such as production of SCFAs, lower pathogenic compounds like ammonia and phenolics, lower pH, and decreased bile acids. “High-amylose-corn consumption results in a high butyrate production, as compared to other fibers,” Witwer says. Animal trials have also shown healing of intestinal ulcers and prevention of damage from toxins and high protein diets.
Whether you are interested in a more functional fiber or a functional food, resistant starches have benefits that can contribute to the foods you develop today and in the future.
Kimberlee J. Burrington has worked in the area of food product development for 20 years. She is the dairy ingredient applications coordinator for the Wisconsin Center for Dairy Research in Madison, WI, where she explores the use of dairy ingredients, with an emphasis on whey proteins in beverages, bars, dairy products, confections and baked products. She received a B.S. and M.S. in Food Chemistry from the University of Wisconsin–Madison. She has been a contributing editor for Food Product Design since 1997. K. J. can be reached at [email protected].
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