Dietary fibers (DF) have captured an immense amount of attention for their health benefits in human and pet nutrition. They have been associated with positive effects on intestinal functions and related health conditions, such as bowel cancer and cholesterol reduction in coronary heart disease (CHD), as well as obesity. The role of DFs in carbohydrate metabolism, glucose release and insulin response, following consumption of carbohydrate-rich foods, has also been much researched and discussed. This article focuses on sources of DF and how processing affects their beneficial functional and physiological properties.
Dietary Fiber Definition and Characteristics
Dietary fibers consist of a) non-digestible carbohydrates and lignins that are intrinsic and intact in plants; and b) added fibers that consist of isolated, non-digestible carbohydrates that have beneficial physiological effects. Total fiber is then a sum of dietary fibers and added fibers that, together or alone, will be referred to as dietary fiber (DF) in this article. There are several definitions of DF going back to 1970s and, more recently, by several organizations worldwide. Since no universal definition is agreed upon, this article will use the definition developed by AACC in 2004. (See definition in sidebar.)
Under this definition, DFs are mainly derived from plant cell wall material. DFs include non-starch polysaccharides, celluloses, lignins, hemicelluloses from plant cell wall materials (such as cereal brans) and non-digestible oligosaccharides (resistant starch, inulin, polydextrose and others). Their common characteristic is that they escape digestion in the small intestine and reach the large intestine, where they undergo fermentation; hence, their effects on metabolism and disease regulation are intrinsically linked to their physicochemical properties, as they pass through the gastrointestinal tract.
Formulation can be complex, as much depends on the DF source; this includes differences within plant varieties and the amounts and compositions of other components (such as protein, starch and fat). These factors, combined with a variety of commercial extraction processes, influence functional properties, such as moisture absorption, viscosity, etc., of the final product.
The physicochemical properties of DFs are dominated by the conformation of the individual polysaccharide chains (e.g., ordered, disordered, random coil chain geometry), the way the polysaccharide chains of different DFs interact with one another and with other food components. These conformations affect their hydration characteristics and, hence, their solubility (Brennan, CS. 2005. Mol Nutr Food Res. 49:560–570).
Functional and physiological characteristics of dietary fibers are largely influenced by their solubility. DFs are categorized as soluble and insoluble fibers. Soluble DFs are used in the food industry to modify or control the viscous properties of liquid and semi-liquid food products and alter their textural characteristics. The majority of soluble DFs have the ability to form gels and alter the viscosity of products. This ability is not only important for their textural impact on foods, but for their nutritional characteristics. For instance, numerous studies have illustrated the potential of these components to increase the viscosity of digesta when consumed. This, in turn, may explain their observed effects on carbohydrate metabolism (Mann J. 2001.Eur J Clin Nutr. 55:919-21).
Properties and Functions in Food Applications
In addition to the complexities in dealing with the DFs just mentioned, the polysaccharides that make up dietary fibers in nature are heterogeneous, and their structure-function relationships are difficult to establish. This means, for example, that the oligosaccharides derived from polysaccharides or manufactured by fermentation, which are often fairly well defined, may be easier to incorporate into food systems.
Processing greatly influences ingredient functionality. The majority of processes involve milling, acid, alkali, enzyme treatments, extrusion and dehydration. Milling processes separate bran, germ and the endosperm from grain. The degree of separation affects particle size and the release of starch from the protein/lipid matrix, which, in turn, affects starch digestibility and glycemic index. The recent trend of formulating foods with whole grains aims to maintain the physical state of starch in these protein/lipid matrixes, thereby controlling digestibility of starch and the glycemic index (GI). Whole grains also provide a range of additional health-promoting components.
Cereal and fruit fibers can be separated from their original food matrix and treated to enhance their functional properties, to improve their usage in different food systems. For example, one technology produces a versatile functional fiber gel (cellulosic fiber gel) from plant fibers that can replace flour and also fat in bakery products (Inglett, GE and Carriere, CJ. 2001.Cereal Chem. 78:471–475). The technology involves a two-stage process with thermal alkaline degradation, along with high shear for the first stage, followed by alkaline peroxide treatment with shear in the second stage.
Similar new technologies, in combination with or without enzymes, are utilized to produce fiber-rich ingredients that provide options to food formulators. Bleaching by peroxide or hypochlorite is practiced to eliminate dark colors and oxidize tannins and lignins. Grinding to suitable particle size is essential to improve mouthfeel or eliminate grittiness. Grinding can further increase water-holding capacity, which can have positive or negative effects on food texture. Water can be ”balanced” in formulations by the addition of resistant starches or low-viscosity oligosaccharides that build solids. Enzymatic hydrolysis, extrusion, dehydration and roasting are other treatments used to make dietary fiber friendlier to food formulations, depending on the desired food sensory properties.
Overall, the solubility, viscosity, gelation and gel strength, water- and oil-binding capacities of a dietary fiber are greatly impacted by its source and its processing. (See sidebar “Key Properties and Influences, by Source and Processing.”)
The challenge to food formulators is to sift through this information and develop an understanding of DFs that will assist them in the development of good-tasting, stable and economical foods. For example, ones with high-fiber and low-GI characteristics can satisfy internal marketing and, ultimately, consumer needs and preferences. Formulating DF ingredients into food products must be carried out with a “systems approach” similar to the one used to manufacture products with low-calorie, low-sugar or fat replacement properties; these have been on the market for some time.
Nutritional and Physiological Benefits
Research has shown dietary fiber can reduce the gastrointestinal transit time (Harris, PJ, et al. 2000.J Sci Food Agric. 80:2089-2095), and high-viscosity DFs reduce starch digestibility, allowing a portion of the starch to reach the large intestine. In the small intestine, DFs are thought to increase digesta viscosity to strengthen the so-called unstirred water layer in the gut, which potentially leads to a higher diffusion barrier, and also to non-specific binding of enzymes, thus reducing their activity. This, in itself, has a direct influence on the rate of digestion and effectiveness of nutrient absorption. Such effects include moderation of postprandial glucose and insulin response, reduction in total and low-density lipoprotein (LDL) cholesterol and regulation of appetite (Davidson, MH and McDonald, A. 1988. Fiber: Forms and Functions.Nut Res.18:617–624).
In summary, generally accepted physiological benefits of DFs include improved colonic function, including some which function as prebiotics; an ability to lower serum blood glucose and LDL cholesterol levels; appetite control; and the potential to improve calcium, magnesium and iron absorption.
Traditional and Emerging DF Sources
Major sources of DF, such as whole grains, cereal brans and fruit fibers (e.g., apple, orange, etc.) are widely available. Emerging research indicates the whole plant cell wall and the aleuron cells of cereals’ storehouse of complex polymers also have many phenolic compounds and sterols that the refined DF sources, such as bioactive oligosaccharides and hydrocolloids, lack. The cell wall components produce compounds that may be attributed to reducing colon cancer and CHD. These components also ferment similarly to refined DF ingredients, in that they can produce short-chain fatty acids in the colon.
Even as traditional fibers are finding increased use in food and beverage products, constraints in the functionality, process stability, availability and range of benefits necessitates an examination of new fiber sources. Newer DF sources include compounds derived from plant, microbial and marine sources. Examples include whole plant cell wall material derived from whole (cereal) grains, fruits and vegetables; derivatives (such as resistant starches) and extracted material (such a beta glucans); bioactive, non-digestible oligosaccharides; hydrocolloids (mostly soluble polysaccharide gums); and soluble carbohydrate derivatives from plant, microbial and other sources. Here is a brief, but closer, look at some of these.
* Whole plant cell wall material. Many applications exist for these types of fiber, including bakery, low-fat cheese preparations and other low-fat products, beverages and processed meats. Examples include cellulose, a principal component of cell walls of grains, fruits and nuts that are polysaccharides of beta 1-4 linked glucose units, and hemicelluloses that are polysaccharide polymers of sugars other than glucose, of which there are both insoluble and soluble types. Other examples include pectins from fruits and vegetables, which are polysaccharides of galacturonic acids and other sugars, that are soluble in hot water and gel on cooling; beta-glucans that are glucose polymers with mixed 1-3, 1-4, 1-6 linkages; resistant starches of various types and sources; and, lastly, synthetic carbohydrates that are derivatives of cellulose, such as methylcellulose, hydroxymethyl cellulose, polydextrose and microbial polysaccharides.
* Newly popular whole grains, seeds and dietary fibers. A list of these ingredients would be lengthy. Examples include chia and quinoa seeds originally from Latin America. Chia is rich in DF, omega-3s, phytosterols and minerals. Quinoa is said to have laxation effects, is a prebiotic and lowers cholesterol, among other benefits. Sorghum, an insoluble dietary fiber, again has similar benefits, as do fibers from sugar beets, sweet potato skins, peanut hulls and pineapple cores.
* Bioactive oligosaccharides. These non-digestible oligosaccharides (NDOs) are a class of compounds obtained from plants, or are produced by fermentation or enzymatic processes. NDOs generally consist of 3-10 glucose units that form viscous networks and are important for their physiological and processing properties in foods. Some publications include oligosaccharides with up to 19 glucose units. There is also potential of these components to increase the viscosity of digesta when ingested. This, in turn, may explain the effects observed on carbohydrate metabolism, such as lowering cholesterol and low-GI properties. They can be obtained by direct extraction from natural sources, or produced by chemical processes hydrolyzing polysaccharides, or by enzymatic and chemical synthesis from disaccharides. Their properties include water dispersibility and solubility, viscosity effects, bulk, absorption and fermentability, and binding of other compounds. The oligosaccharides are prebiotic, contributing to improved gut functions, such as boosting beneficial bacterial populations, and positive biochemical and physiological effects.
* Hydrocolloids. These ingredients influence the physical properties of foods with water, and many have physiological properties similar to DFs. In particular, hydrocolloids swell and produce a viscous solution or dispersion when exposed to water; there are reports that indicate some have prebiotic contributions. Commonly known hydrocolloids include functional proteins, such as gelatin, myosin, albumens and globulins, but the largest range is polysaccharide-based, macromolecular entities, such as cellulosics, glucans and starches, including such polymers as galactomannans, glucomannans, pectinaceous materials, arabinogalactans, seaweed-based extracts, the microbial (gellan and xanthan) and others.
The future of dietary ingredients will continue to evolve, as more is learned about human physiology and the benefits of such ingredients. In view of insufficient consumption of DFs, especially in Western diets, great opportunities remain in the development and marketing of foods that can enhance the levels in diets worldwide. pf
Sakharam K. Patil, Ph.D, is president of Munster, Indiana-based S. K. Patil and Associates, a group that consults for both small and Fortune 500 food companies. Patil has held positions in India, Central Caribbean and Europe. These have included assignments as director of QA, vice president commercial development, R&D, marketing and vice president of technology transfer in Europe with American Maize/Cerestar. Patil has a Ph.D. in food/cereal science from Kansas State University and is currently an adjunct professor at the Whistler Center for Carbohydrate Research, Purdue University. Contact info: firstname.lastname@example.org, 219-922-1033, www.skpatilassociartes.com.
“The edible parts of plants or analogous carbohydrates that are resistant to digestion and absorption in the human small intestine, with complete or partial fermentation in the large intestine. Dietary fiber includes polysaccharides, oligosaccharides, lignin and associated plant substances. Dietary fiber promotes beneficial physiological effects, including laxation, and/or cholesterol attenuation, and/or blood glucose attenuation.”
--AACC International board, of which Sakharam Patil was a member from 2003-2006, when the definition of DF was developed
Key Properties and Influences, by Source and Processing
* Solubility: Influenced by natural variation. Branching of polymer networks enhances solubility, by preventing association of polymer chains. Esterification, such as with pectins, decreases ionizing groups and reduced solubility. Carboxylation enhances hydration and polymer chains combination of alpha and beta linkages, etc.
* Viscosity: Greatly influenced by polymer characteristics, such as degree of branching, molecular weight and so on. Hydrocolloids offer wide range of viscosities for many food applications.
* Gelation and gel strengths: Influenced by polymer chain association. High amylose starches form rigid gels and are used in gum candies and have DF characteristics. Resistant starches are available that are marketed to bakery and other food segments. Several fibers from wheat and oats, among other sources, form gels.
* Water-binding capacity: This property is influenced by fiber particle size, fiber length and fiber porosity. This property is very important in all food formulations.
* Oil-binding capacity: Fiber network and porosity influence this property, and it is used in batters/breadings to reduce oil pick-up and to enhance meat emulsification.