Grains, such as barley, can be an excellent choice for adding both soluble and insoluble fiber to the diet.

The following article has been condensed and adapted from a thoroughly referenced “Dietary Fiber from Vegetable Products as [a] Source of Functional Ingredients,” by Rocío Rodríguez, Ana Jiménez, Juan Fernández-Bolaños, Rafael Guillén and Antonia Heredia, first published in Trends in Food Science & Technology, January 2006, pp. 3-15. See end of article for more information.—Ed.

Research into dietary fiber (DF) has been ongoing for quite some time. Industry attitudes toward DF range from it being simply a waste by-product of other processes to it being a “universal remedy.” Many aspects about DF properties and functions remain unclear. Differences in attitude toward DF depend, in part, on who is discussing these components. Botanists think of fiber as part of plant organs; chemical analysts define it as a group of chemical compounds; consumers perceive it to be a substance with beneficial effects on human health; and for the dietetic and chemical industries, DF is a subject of marketing. Part of the controversy arises due to the fact that fiber is a combination of chemical substances of distinct composition and structure, such as cellulose, hemicelluloses, lignin, etc.

The most consistently accepted definition for DF is that promulgated by Hugh Trowell, who authored a series of articles in the mid-1970s in Lancet and other journals. According to Trowell, “Dietary fiber consists of remnants of the plant cells resistant to hydrolysis (digestion) by the alimentary enzymes of man,” whose components are hemicelluloses, cellulose, lignin, oligosaccharides, pectins, gums and waxes. However, alternative definitions for fiber are frequently proposed. In 2001, the American Association of Cereal Chemists adopted this definition: “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.”

Although there is no international consensus on the definition of dietary fiber or its analysis, most scientists agree that fiber is an important component of diets. As a consequence, fiber intake is addressed in the official dietary guidelines of many countries.

Dietary Fiber Effects on Health

Although most components of DF cannot be digested by enzymes in the small intestine, they can be partially degraded by bacterial enzymes. The extent depends on the type of bacteria, time of transit through the colon and presence of DF components that limit decomposition.

The bacterial degradation process starts with hydrolysis, which converts polysaccharides to mono- and disaccharides. The next step is anaerobic glycolysis, which creates acetate, propionate and butyrate (short-chain fatty acids).

Various research studies indicate DF consumption is associated with decreased assimilation of proteins (depending on structure and chemical composition of the fiber); decreased lipid absorption; decreased plasma levels of total cholesterol and low-density lipoproteins; and decreased blood levels of glucose (confirmed in diabetic patients). DF may also impact beneficial components such as flavonoids and carotenoids. For example, the main effect associated with viscous polysaccharides such as pectins and gums is a decreased assimilation of certain nutrients by the small intestine.

DF does act as a protective agent against cardiovascular diseases, diverticulosis, constipation, irritable colon, diabetes and colon cancer. The bacterial mass formed from highly fermentable substances (e.g., pectin) and less fermentable polymers (e.g., cellulase and hemicelluloses) and the water retained by them are responsible for increased fecal bulk. And it is believed that low fiber consumption results in very compact feces that may promote oncogenesis due to lengthy intestinal mucous exposure to cancer-risk agents. Several types of fiber have also been shown to have a capacity for adsorbing carcinogenic agents.

The type and quantity of a food’s dietary fiber content depend on factors ranging from its original plant source to processing parameters.

Major Methods of Analysis

Numerous methods have been developed for DF determination, and several can specifically identify precise DF components. Many use highly purified enzymes that selectively release oligo- and polysaccharides, particularly fructans, galactans, mannans, arabinose and beta-glucans.

According to author N.G. Asp, fiber analysis can be classified into two main groups. The first group determines unavailable polysaccharides (except for starch and lignin). The enzymatic-gravimetric methods quantify fiber as the residue remaining after sample treatment with starch and protein degrading enzymes. These procedures quantify total fiber as the sum of soluble and insoluble fiber and determine the chemical and physiological properties of each. (Asp, et al., 1992. Dietary Fiber Analysis in Dietary Fiber. A Component of Food, Springer Verlag).

The second group, also utilizing enzymatic-gravimetric methods, determines non-starch polysaccharides without considering their physiological properties. These tests are based on isolation and fractionation of non-cellulosic polysaccharides, cellulose and lignan, followed by hydrolysis of each fraction and quantification of their sugar composition. Several derivations of these methods exist as well as some quick methods whose results compare to traditional procedures.

In addition, most analyses of prebiotics (e.g., oligofructose, inulin and polydextrose) are based on chromatography techniques. Analysis by gas chromatography is the most complex because a derivatization of samples is first required in order to volatilize the compounds. This, however, allows the direct determination of oligosaccharides of less than 10 degrees of polymerization. Methods based on gel filtration involve a previous enzymatic treatment with inulinase. Molecular biology advances make possible the release of non-digestible oligosaccharides from DF with specific prebiotic, physicochemical, physiological and organoleptic properties. A mixture of fructanase, amyloglucosidase and iso-amylase enzymes is used to release polydextrose from the food matrix.

Some of these procedures have already been adopted as official analytical methods by the Association of Official Analytical Chemists (see chart “AOAC Methods”), and it is predicted that others will be accepted following validation.

Most analysis of prebiotics (e.g., oligofructose, inulin and polydextrose) is based on chromatography techniques.

Fiber Content and Consumption

Dietary fiber is naturally present in cereals, vegetables, fruits and nuts. Among them, cereals are the main source of fiber. Several non-starch foods provide up to 20g to 35g of fiber/100g dry weight, and some starch-containing foods (no cereals) provide about 10g/100g of dry weight.

Although recommended levels for fiber consumption vary among countries, cereals contribute about 50% of the fiber intake in Western countries, vegetables 30% to 40%, fruits about 16% and the remaining 3% come from other sources. (See chart “Recommended Consumption.”)

The amount of DF from cereals differs depending on source and processing. DF in wheat flour, for example, varies from 2.5g/100g in refined flour to 12g/100g in unrefined flour obtained from wheat bran. DF content of vegetables can be 28% to 30% of dry matter, although some products such as white and red beans have much higher values. Fruits contain a significant amount of water which results in DF content of only 1% to 3.5% of dry matter.

DF is an important ingredient in the formulation of functional foods because it increases fecal bulk volume, decreases intestinal transit time as well as cholesterol and glycemic levels, traps mutagenic and carcinogenic agents and stimulates intestinal flora proliferation and so on.

The most fiber-rich foods consumed are breakfast cereals and bakery products. Traditionally, bakery products were enriched by adding unrefined cereals. But the use of other DF sources, mainly fruits, can result in better nutritional quality, higher amounts of total and soluble fiber, lower caloric content, stronger antioxidant capacity and a greater degree of fermentability and water retention. Isolated fiber components such as resistant starch and b-glucans are also used to increase fiber content in applications such as pastries and breakfast cereals.

The addition of DF to beverages generally increases their viscosity and stability. Soluble fiber (SF) is used most often because it is more dispersible in water than insoluble fiber. Examples of SF include fractions of grains and multi-fruits, pectins, ß-glucans, cellulose beet-root fiber and polydextrose.

Certain SFs, such as pectins, inulin, guar gum and carboxymethyl-cellulose, are utilized as functional ingredients in milk products. Guar gum, pectins and inulin are added during cheese processing to decrease the percent of fat content without loss of organoleptic characteristics, such as texture and flavor. On the other hand, the addition of DF to yogurt and ice cream improves emulsion stability.

DFs based on pectins, cellulose, soy, wheat, maize or rice isolates and beet fiber can improve the texture of meat products, such as sausages, pâtés and salami and may also be used in the formulation of low-fat products, such as “dietetic hamburgers.” Since they can increase water retention, their inclusion in the meat matrix contributes juiciness, which implies that volatile flavor compounds are released more slowly.

Pectins with different degrees of esterification are the most commonly added fibers to jams and marmalades. Fruits are the primary source, and they assist in the stabilization of the end product. In low-calorie chocolates and related products, fiber compounds such as inulin and oligofructose are used as sugar substitutes.

Dietary Fiber Isolation

The DF market is highly competitive, and new fibers are continuously being introduced. Many are residual substances that remain after the desired component of a product has been isolated. For example, fiber-containing by-products result from the juice extraction of oranges, apples, peaches and olives. Pepper, artichoke, onion and asparagus also create waste during their processing which consists of both soluble and insoluble fiber compounds.

Orange and lemon by-products are very rich in pectins. The processing of grapes, apples, bananas, mango, guava and other fruits also creates large amounts of by-products such as peels, “bones” and seeds. Utilizing this material offers significant financial benefits that can reduce the cost of the primary processed fruit product.

Among many bioactive compounds, significant amounts of pectins and polyphenols can be recovered from apple by-products. Different types of fibers can also be isolated from grapes after juice extraction. Fibers rich in highly branched pectins that can be isolated from the mango skin are also of interest. The fruit and vegetable fibers linked to “soluble compounds” during the isolation process are of interest not only for their physico-chemical properties but also for the functional characteristics that those compounds, mainly antioxidants, confer to the fibers.

Gak’s Snacks has introduced a line of peanut-free, tree nut-free, egg-free and dairy-free products that are also whole grain and contain no trans fat. For many nutritionally oriented baked goods, the addition of DF can decrease overall fat content by using DF as a fat substitute without loss of quality.

Food Processing's Influence

Fermentative and heating processes may alter the chemical composition and physico-chemical, nutritional and functional properties of a food’s fiber fraction. This in turn affects its physiological effects on the human body.

Fermentation, such as that which occurs during the production of sauerkraut and olives, modifies the composition and structure of DF. Amylase, proteinase, polygalacturonase, cellulase and b-galactosidase are the main enzymes found in fermentation brines. These enzymes selectively degrade cell wall polysaccharides and thus decrease fiber content. They primarily release soluble compounds such as neutral polysaccharides and pectins, although some insoluble fiber, such as hemicelluloses and cellulose, can be released as well.

Since solubilized DF is released into sauerkraut brine, the overall DF content of sauerkraut is similar to raw cabbage. In contrast, olive brine is not consumed, and thus the amount of DF available from fermented olives is less than raw olives.

Boiling, cooking and canning can considerably change the texture of plant tissues. Their effects are distinct in that texture changes depend on temperature and time of heating. Modifications also depend on the composition and structure of the fiber components under consideration. Boiling (an implied heating with water at 100°C, steam water or in a microwave) inactivates practically all enzymes and negatively affects the organoleptic properties of the final products through excessive softening of the plant tissues, loss of color and flavor and/or development of odd colors and flavors. When cooking takes place at temperatures lower than boiling in conventional or microwave ovens, the process will take longer. Canning is usually carried out by the application of high temperature and pressure during short periods of time.

Despite the large amounts of heated plant foods that are consumed, there are relatively few published studies on DF modifications during thermal processing. It is not clear which fibers experience the greatest changes, although it has been reported that hemicelluloses and pectic substances seem to be the most affected components.

Zoe’s O’s cold cereal is one of the many products entering the market today that touts its fiber content on the front label.

In some cases, fiber content appears to increase after thermal processing. This is due to the formation of complexes between polysaccharides and other components of the food, such as proteins and phenolic compounds, which are analyzed as fiber. For example, boiling, cooking or roasting increases total fiber in wheat bran through formation of fiber-protein complexes that are resistant to heating and are quantified as DF.

The pH level of cooked vegetables, notably potatoes and cauliflower, also plays an important role in texture changes. The solubilization of fiber components, which results in a softer texture, is more pronounced under higher pH (i.e., basic) conditions. In general, final pH effects depend on fiber composition, time of treatment, size of the plant food and grade of penetration of the processing liquids.

Changes in fiber quantity and quality can also occur in fruits and vegetables, depending upon post-harvest storage conditions. For example, apples stored in a controlled atmosphere show no changes to their fiber content. Another study revealed that onions, which are usually stored in less restrictive conditions, had a general increase in fiber components, particularly the uronic acids that constitute the pectic polysaccharides.

There are still many aspects within this field of research that need further investigation. For example, few studies have been conducted on the impact of freezing vegetables, and of those studies done, contradictory results have been obtained. Although further scientific research is essential, what has been clearly established is that DF possesses great potential as a functional ingredient that may produce distinct human health benefits.

The original article, with over 150 references, has been condensed and adapted by Claudia D. O’Donnell, chief editor. It is reprinted from Trends in Food Science and Technology, Vol 17, Issue 1, Rocío Rodrígues, et al., “Dietary Fiber from Vegetable Products as Source of Functional Ingredients,” pp 3-15, copyright 2006, with permission from Elsevier.

Sidebar: AOAC Methods

  • Fiber (Crude) Animal Feed and Pet Food. AOAC Official Method 962.09.
  • Acid Detergent Fiber and Lignin in Feed. AOAC Official Method 973.18.
  • Fiber (Crude) in Animal Feed and Pet Food. AOAC Official Method 978.10.
  • Total Dietary Fiber in Foods (Enzymatic-gravimetric Method). AOAC Official Method 985.29.
  • Fiber Acid–detergent and Protein Crude in Forages. Near Infrared Reflectance Spectroscopic Method. AOAC Official Method 989.03.
  • Insoluble Dietary Fiber in Food and Food Products. Enzymatic-gravimetric Method, Phosphate Buffer. AOAC Official Methods 991.42.
  • Total Soluble and Insoluble Dietary Fiber in Foods. Enzymatic-gravimetric Method. MES-TRIS Buffer. AOAC Official Methods 991.43.
  • Total Dietary Fiber. Enzymatic-gravimetric Method. AOAC Official Methods 992.16.
  • Soluble Dietary Fiber in Food and Food Products. Enzymatic-gravimetric Method (Phosphate Buffer). AOAC Official Methods 993.19.
  • Total Dietary Fiber in Foods and Foods Products with ≤2% Starch. Non-Enzymatic-gravimetric Method. AOAC Official Method 993.21.
  • Total Dietary Fiber Determined as Neutral Sugar Residues, Uronic Acid Residues and Klason Lignin. Gas Chromatographic–colorimetric–gravimetric Method. AOAC Official Methods 994.13.
  • Fructans in Food Products. Ion Exchange Chromatographic Method. AOAC Official Methods 997.08.
  • Food Polydextrose (ref. 2000.11).

Website Resources:— Scroll down to access abstract of this article, then click to January 2006 issue, where the pdf of this article may be accessed in full— Shortcut to Elsevier Ltd.’s Trends in Food Science and Technology— 2001 American Association of Cereal Chemists definition of dietary fiber— H.C. Trowell’s definition and benefits of dietary fiber