New Fibers For Foods

Fibers in foods are forms of carbohydrates and lignin that are not digested in the small intestine. They play a role in human health by providing a feeling of fullness while minimizing calories, reducing constipation, and supplying an energy source for friendly bacterial colonies in the digestive tract. Diets high in fiber have also been linked to reduced risk of diseases such as diabetes and colon cancer.

Yet adult Americans, on average, eat only 15 g of fiber per day, according to data from the National Health & Nutrition Examination Survey. Thats about half of the amount recommended by health authorities. To try to address the disparity, researchers and food manufacturers are developing digestion-resistant starches as ingredients to add additional fiber to foods such as bread, pasta, and cereals in a way that consumers will accept.

“Every major health organization agrees that people need to consume more dietary fiber,” says Roger Clemens, who is associate director for regulatory science at the University of Southern California, president-elect of the Institute of Food Technologists, and a member of the 2010 Dietary Guidelines Advisory Committee. Getting consumers to eat more fiber, however, can be difficult. “People do not consume enough fruits and vegetables to do it alone, and for that matter, agriculture can’t produce enough fruits and vegetables to meet the need,” Clemens says.

The obvious source of increased fiber in foods is whole grains, but there’s a catch: Many people have trouble adjusting to the taste, texture, and color of a whole-grain, high-fiber pasta, for example. “Every consumer says that taste is their number one priority,” Clemens says. “If it doesn’t taste good, then people won’t eat it, and if you don’t eat it, you don’t get the nutrition.” New ingredients aim to increase fiber with minimal agitation to the palate.

Historically, food fiber has been divided into two classes: insoluble and soluble. Insoluble fiber is primarily cellulose. Sources of insoluble fiber include whole grains, nuts, seeds, and vegetables such as green beans and zucchini. Insoluble fiber plays a role in digestion largely as a bulking agent that reduces intestinal transit time. Reduced transit time promotes bowel regularity and likely plays a role in cancer reduction by lessening exposure to potential carcinogens.

Soluble fiber, which absorbs water, includes some β-glucans, fructans, and gums. Sources of soluble fiber include legumes, oats, and root vegetables. Soluble fibers are fermented by bacteria in the colon. Fermentation produces short-chain fatty acids such as butyrate, which is known to promote healthy colon epithelial cells and could inhibit some cancer cells.

The lack of fiber digestibility gives people a feeling of satiety while adding minimal calories to the diet. High-fiber foods overall also take longer to digest, which means that glucose is released more slowly to the bloodstream, which in turn moderates the body’s insulin response. Keeping insulin levels on an even keel might help avoid development of insulin resistance (a condition in which a person’s cells become resistant to the hormone and therefore need more of it to metabolize a given amount of sugar) and of type 2 diabetes.

Food researchers and manufacturers are now developing a new class of food fiber, called resistant starch—“resistant” because the normally digestible starch resists digestive enzymes. Starch itself is a polysaccharide made up of two polymers of glucose: linear amylose and branched amylopectin. In plants, starch molecules are arranged into semicrystalline granules that range from about 2 μm in diameter in rice to 100 μm in diameter in potatoes. Resistant starches could play a role similar to other food fibers by increasing satiety, moderating blood glucose levels, reducing digestive tract transit time, and promoting colon fermentation. Informally, resistant starches form five classes—RS1 to RS5—based on the mechanism of enzyme resistance.

RS1 is starch that is trapped or encapsulated in a shielding matrix; examples of RS1 sources include beans or whole or coarsely ground grains.

RS2 is a starch with a crystalline structure that resists enzymatic digestion, such as the starch found in a raw potato or green banana. Donald B. Thompson, a food science professor at Pennsylvania State University, is experimenting with ways of heating starch granules from various sources under a variety of conditions to see how their crystal structures change and how their resistance to digestion can be modulated. It’s not just about making more of the starch resistant to digestion, he says, but also about influencing the rate of breakdown of the digestible portion and the rate of fermentation of the resistant portion.

RS3 is starch from certain strains of corn that contain a high percentage of amylose. In a process called retrogradation, the high-amylose starch is hydrated and heated, and the amylose and linear parts of amylopectin align into a more crystalline structure that makes the starch harder to digest. When it occurs on a countertop, retrogradation is also what hardens stale bread.

Some researchers are looking at engineering other plants to produce high-amylose starch. Yong-Cheng Shi, a professor of grain science and industry at Kansas State University, is developing high-amylose wheat and rice. Having digestion-resistant starch inherent to plants that will be used in recipes, without the need to isolate it and add it to recipes as an ingredient, would be a benefit to food manufacturers, Shi says.

RS4 is starch that has been chemically treated to encourage branching and cross-linking of the starch polymers. Atchison, Kan.-based MGP Ingredients, for example, takes starch from wheat and treats it with sodium tripolyphosphate (Na₅P₃O₁₀) to promote branching and with sodium trimetaphosphate (Na₃P₃O₉) to promote cross-linking, says Clodualdo (Ody) Maningat, MGP’s vice president of applications technology. Cross-linking is what prevents digestion of the starch, but the blend of the two treatments helps the company meet Food & Drug Administration requirements for residual phosphorus.

One of MGP’s products is the treated starch, and another is a heated version that mimics fat when it’s hydrated. “It has a dual benefit of adding fiber and at the same time replacing fat,” Maningat says.

RS5 is a starch-lipid complex, in which helical amylose chains serve as host molecules to guest lipids in the center. The complexes can be amorphous or crystalline, with crystalline complexes being more enzyme resistant than their amorphous counterparts. Granules of the starch-lipid complexes resist swelling during cooking, and the lack of swelling seems to reduce the ability of amylase to access and hydrolyze the starch, says Jay-lin Jane, a food science professor at Iowa State University.

Jane has been studying complexes of high-amylose starch with various fatty acids and has found that when mice are fed a diet including RS5, the complexes turn up in the feces intact.

Jane is also collaborating with Iowa State nutrition professor Suzanne Hendrich on human feeding studies of RS5. In one investigation, they found that plasma glucose and insulin levels in the blood of 20 men were reduced by 55% and 43%, respectively, when they ate bread prepared with RS5, compared with standard white bread containing the same amount of total carbohydrates (Cereal Chem., DOI: 10.1094/cchem-87-4-0257).

Even while acknowledging the potential benefits of new fiber ingredients, researchers and nutritionists say that they won’t replace the need for a balanced diet. “Each of the fiber components has a different function in plants and a different function in the body,” USC’s Clemens says. “We encourage people to consume a continuum of dietary fiber components,” including, of course, those from whole grains, fruits, and vegetables.