Types of Dietary Fiber

Finding agreement between various scientific groups and regulatory agencies on a definition for dietary fiber has proven difficult. In a report submitted to the Board of Directors of the American Association of Cereal Chemists (AACC) (AACC International, 2001) , the AACC Dietary Fiber Definition Committee defined dietary fiber as follows:

Dietary fiber is the edible parts of plants and 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 fibers promote beneficial physiological effects including laxation, and/or blood cholesterol attenuation, and/or blood glucose attenuation. (p. 1) As noted in the report, the difficulty in defining dietary fiber is finding a balance between the physiological effect of fiber and the analytical methods used to detect and quantify it in foods. Since the USDA prohibits fiber enrichment of meats, the details of the fiber definition are of limited importance to US meat producers. It is, however, possible to fortify meat products with fiber in other areas of the world.

The more important consideration for the inclusion of fibers in meat products is an understanding of the functional attributes of the various available fiber sources. In order to better grasp the functional properties of dietary fiber it is helpful to categorize the fibers into groups. Historically, dietary fibers have been classified based on their relative solubility in water. Fibers that are composed primarily of cellulose, hemicellulose, and lignin, such as oat fiber and wheat bran, are primarily insoluble. Fibers that include substantial portions of gums, polyfructoses, pectins, and mucilages, such as psyllium, fruits, and oat bran contain significant fractions of soluble fiber. Within the categories of insoluble and soluble fiber it is also helpful to further classify the fiber ingredients as native or refined. This is a more subjective classification and refers to the level of processing or extraction the fibers undergo relative to their starting substrate.
Some common insoluble, native fiber sources used in food products include wheat bran and corn bran. These bran ingredients are produced through the dry milling of cereal grains. Although these fibers find wide use in items such as breakfast cereal and bakery products for fiber enrichment, their use as a functional ingredient in meat is limited. As shown in Table 4.1 , the water and fat absorption of these ingredients is significantly lower compared to the more refined fibers. In addition to the reduced functional attributes, the native, insoluble fibers typically contribute flavor and color components of the raw substrate which may not be acceptable in certain meat products. They can also impart a more gritty texture, owing primarily to the larger particle sizes available in commercial trade.

Powdered Cellulose

One of the first commercially available, insoluble, refined fibers was powdered cellulose. Cellulose, a glucose polymer, is one of the most abundant organic compounds on earth. It is the major structural component of green plants. To manufacture food grade powdered cellulose, organic plant material is cooked in a caustic solution, usually with sulfur compounds, at high temperatures and pressure. This hot caustic solution dissolves the lignin structure and other extractives which are then removed in subsequent filtering and washing steps. The resulting fibrous pulp is bleached to remove color, dried, and ground. Powdered cellulose can be produced from a number of raw material sources. As a result, the Food Chemicals Codex definition for powdered cellulose (Institute of Medicine, 2003) is not specific to a particular substrate. Any plant material which has been processed adequately to meet the purity and quality standards of the Codex can be labeled as powdered cellulose. Due to availability of supply and cost considerations, most powdered cellulose is sourced from either wood-, cotton-, or bamboo-based plant material. Commercially, powdered cellulose is available in a number of variations, the primary differences being fiber length. The absorption capability of the cellulose fiber is largely based on capillary action. The ability to absorb more or less water through the capillary action is at least partially dependent upon fiber length. Longer fibers tend to absorb more water than shorter fibers. In commercial trade water absorption is typically measured using a centrifuge-type method similar to that used to measure protein absorption. In the test, the fibers are over hydrated with water, centrifuged, and decanted.

The mass of water held by the fiber after centrifugation divided by the mass of the starting fiber expressed as a percent of the starting fiber gives the absorption. While this method is useful for comparing the relative absorptions of various insoluble fibers, it is less useful for comparing fibers with high levels of soluble fiber or gel-forming properties. For oil absorption, the same methodology is used, only substituting vegetable oil in place of water. Powdered cellulose is available in fiber lengths ranging from less than 20 μm to over 500 μm. While the longer fibers can impart increased water absorbing capability to meat systems, they can also result in detrimental changes in texture, depending upon the usage level. In practice, the length and water absorption of the fiber must be balanced against the textural changes the fiber causes. Table 4.1 gives an example of the impact of fiber length on absorption. Powdered cellulose is bright white in color and very bland in taste. In addition to its use as an ingredient, cellulose has also been widely used for many years in the production of casings for sausage products. Powdered cellulose is listed in Food Safety and Inspection Service (FSIS) Directive 7120.1 (USDA-FSIS, 2007) as an approved ingredient for use in comminuted poultry, at a level not to exceed 3.5% in various nonstandardized products. Cellulose can also be further processed and modified to produce cellulose ethers such as carboxymethyl cellulose or methyl cellulose.

Oat Fiber

When considering oat fiber, it is important to differentiate between the dry-milled grain products such as oat bran or oat flour and the refined insoluble oat fiber extracted from oat hulls. The first commercial varieties of refined, insoluble oat fibers appeared on the market in the 1980s 1987 . In two US patents, Gould (1987) and Gould and Dexter (1987) describe an alkaline peroxide treatment of agricultural residues, including oat hulls, which yielded a higher absorbing insoluble fiber. Just a few years later, Ramaswamy (1991) patented the application of a process similar to soda pulping used in paper production to oat hulls, which resulted in a very high absorbing oat fiber product. Oat hulls had long been a low value byproduct of the oat milling industry. Although oat hulls naturally have a high total dietary fiber content (75–80%), they also contain silica. This silica results in an abrasive mouthfeel and texture, which limits the use of oat hulls in food or meat products. In the above patents, the inventors both applied process conditions that partially (Gould) or fully (Ramaswamy) removed the lignin from the oat hulls. As the lignin is removed, the silica is also washed from the oat hulls, resulting in a fiber with a much softer texture. The extraction of the lignin also allows the individual cellulosic fiber strands to separate. This increases the surface area and allows the fibers to swell and absorb larger amounts of water. In the case of the Ramaswamy process, the high temperature and pressure removes most of the lignin and silica from the oat hulls. The rapid decompression process from the high pressure cooking also adds a mechanical shear action which further defibrillates the fiber strands, leading to absorption enhancement.

Comparison of oat fiber structure at 100× magnification (courtesy of J. Rettenmaier & Söhne GmbH + Co KG, Rosenberg, Germany. Reprinted with permission)

Oat fiber absorption levels can range from 250% to over 800%, depending upon the level of extraction applied in the manufacturing process. Once the oat fiber is fully extracted, the absorption can be further manipulated through milling, as with powdered cellulose. Figure 4.1 gives an example of the structure differences between a minimally extracted and highly extracted oat fiber. Oat fiber is available in colors from light tan to white. The more extracted versions have very little taste. Oat fiber is listed as an ingredient in the FSIS Food Standards and Labeling Policy Book (USDA-FSIS, 2005a). In keeping with the USDA policy forbidding nutrient fortification of meat products, the handbook states that oat fiber should be labeled as “Isolated Oat Product” on meat products.

Wheat Fiber

The processes described for the production of powdered cellulose can also be applied to other agricultural materials. In Europe, an insoluble wheat fiber made from wheat straw has been widely used in meat products. This material is produced using a process identical to that used for powdered cellulose. The resultant wheat fiber has very similar characteristics to a fully extracted oat fiber. In fact, in most applications, wheat fiber and oat fiber can be used interchangeably with little formula alteration. Wheat fiber is bright white in color with very little taste. At present, wheat fiber is not listed as an approved ingredient for use in meats by the USDA. It is, however, allowed for use in certain meat products outside the USA.

Soy and Pea Fiber

Soybeans and peas are both legumes and have very similar properties relative to the fibers produced from them. In both cases there are two types of fiber available, either from the outer hulls or from the cotyledon portion. In the case of the hullbased fibers, these can range from simply a dry-milled material to a fully extracted material. A two-step process to produce a fiber from legume hulls has been described in a patent by Vail (1991) . The extracted fibers of the pea and soy hulls are shorter and more cube-like rather than the long thread-like structures seen with oat and wheat fiber. As a result, the absorption characteristics of these materials tend to be lower. The cotyledon-based fibers are typically produced as a by-product of the protein extraction process. The cotyledon-based fibers generally have a higher level of soluble fibers, which can boost the water absorption capability, but they tend not to have the oil/fat-binding capability of the higher insoluble fraction varieties. The extracted versions of the legume hulls are white to off-white in color and carry very little flavor. The cotyledon and minimally extracted hull versions are tan/yellow to white in color and have a definite taste. In some cases, the taste profile may be significant enough to limit application of the material.

Carrot Fiber

Carrot fiber is a relatively new fiber to find application in meats. A recent US patent describes a process for producing a carrot fiber from the cuttings and peelings of carrots (Roney & Lang, 2003) . This process uses benzoyl peroxide as bleaching agent to reduce color and flavor. The resulting fiber from this process is off-white in color and most of the typical carrot flavor has been removed. The high-waterabsorption capability (1500%) of this fiber makes it useful for many meat applications, but like many mixtures of soluble and insoluble fibers, the oil absorption is relatively modest at 300%. Carrot fiber is listed in the FSIS Food Standards and Labeling Policy Book (USDA-FSIS, 2005a) and is approved for use in meat products in the USA. The specified labeling for carrot fiber in meat products would be as “isolated carrot product”.

Citrus and Fruit Fibers

There are a number of variations of citrus and fruit fibers available in the market. The source of these materials is usually the by-products of juice and pectin manufacturing. Fibers sourced from the juicing process, like apple pomace, tend to contribute significant color and flavor properties that can limit their application. Fibers derived from pectin manufacturing are normally higher in fiber content and more consistent in their nutritional profile. As seen in Table 4.1 the absorption of a citrus fiber is actually extremely high, likely based on the high soluble fiber content. An important consideration when formulating with citrus fibers is the taste profile. Some of these fibers have a very low pH which can cause an acidic/bitter taste when applied in meat products. Citrus fiber is not specifically listed in the FSIS Food Standards and Labeling Policy Book but would be covered under the vegetable extract guidelines.

Potato Fiber

Potato fiber is manufactured from the cuttings and peeling by-products of the potato processing industry. The cuttings and peelings are washed in a water solution, which may or may not include other extraction chemicals, to remove residual sugars and other solubles. The resulting fiber is a mixture of fiber and starch. It is interesting to note that potato fiber contains a portion (12%) of resistant starch. Resistant starch is the starch fraction that is resistant to digestion in the small intestine, but which is available for bacterial fermentation in the large intestine. In potatoes, the resistant starch is largely due to the high amylose starch content. The tightly bound nature of the amylose starch granule that provides the resistance to digestion also results in a low water absorption capability. In the case of the potato fiber, the low absorption nature of the resistant starch fraction is offset by the nonresistant starch content (16%). This results in a fiber with good water absorption capability (1500%), but relatively low oil binding capability (250%). The residual starch content should also be taken into consideration when formulating products as well. While in the initial cooking phase the gelatinization of the starch granules will increase viscosity and absorption, these granules can retrograde upon cooling and storage, leading to syneresis. Potato fiber is tan to off-white in color and has some residual potato flavor. The water-only-extracted potato fiber would meet the guidelines described in the FSIS Food Standards and Labeling Policy Book for “vegetable extract” and be labeled as “potato extract”.

Sugar Beet Fiber

Sugar beet fiber is derived from the fibrous pulp remaining after the extraction of sucrose from sugar beets. During the sugar refining process, the beets are thinly sliced and washed to solubilize and remove the sugars. In the most common process, the leftover pulp is washed, dried, and milled to form sugar beet fiber. In other processes, more complicated washing steps, including the use of further extraction and bleaching chemicals, can be used. Sugar beet fiber has a high level of soluble fibers. The water absorption and oil absorption of sugar beet fiber is low compared to other fiber sources, limiting its use in meat product. Sugar beet fiber also has a flavor, best described as “earthy,” which can also limit its application in food products. The color of sugar beet fiber ranges from tan/gray to off-white. Like potato fiber, sugar beet fiber would be labeled as “sugar beet extract” in meat products.

Soluble Fibers: Inulin and Hydrolyzed Oat Flour

The functionality and application of soluble fibers in meats encompasses a wide array of ingredients. Modified cellulose ethers like methylcellulose and carboxymethyl cellulose, along with hydrocolloid ingredients like xanthan and acacia, can also be considered soluble fibers under some definitions. Since a thorough review of hydrocolloids is beyond the scope of this chapter (see Chap. 3 for an in-depth discussion), the present discussion will be limited to those soluble fibers that are also typically used as fiber sources. Inulin is a soluble fiber extracted by a washing process from chicory roots. It contains both oligo and polysaccharides. The polymer is composed of fructose connected by β -(2,1) links and usually terminates with a glucose molecule. The degree of polymerization (chain length) ranges from 2 to about 60 (Orafti Active Food Ingredients, 2006b) . The use of inulin in meat products has been especially focused on fat replacement. Inulin has the ability to form a stable gel network which can be used to mimic some textural properties of fat when applied to low-fat meat products. By substituting the inulin gel for fat processed meat applications, it is possible to achieve acceptable textural and sensory properties in low-fat products (Orafti Active Food Ingredients, 2006a) . Inulin is a fine off-white to white powder with little flavor or odor and is listed in the FSIS Food Standards and Labeling Policy Book (USDA-FSIS, 2005a) as approved for use in meat products.

Another soluble fiber that has been used in meat products is hydrolyzed oat flour. In two US patents, Inglett (1991, 1992) describes a method for producing a hydrolyzed cereal flour with increased content of soluble fiber in the form of β -glucan. This product, developed by the USDA Research Labs in Peoria, IL, was licensed under the trade names “Oatrim” and “TrimChoice.” β -Glucan is another glucose polymer (like cellulose) but soluble in nature, forming thick gels. β -Glucan is widely associated with its cholesterol-lowering benefits. The FDA currently permits a “reduced risk of coronary heart disease” health claim for food products which contain soluble fiber from oats at a level of at least 0.75 g per serving. In addition to its use as a soluble fiber supplement, hydrolyzed cereal flours are also useful in meat applications for water absorption and their impact on texture. In particular, the gel-forming capability of the hydrolyzed cereal flours can be used to mimic the textural properties of fat in meat products. A 1994 US patent (Jenkins & Wild, 1994) describes a food composition comprising hydrolyzed cereal flour, a hydrocolloid, and a comminuted meat product. The patent further points out that the high-water-binding and gel-forming characteristics of hydrolyzed cereal flour, when used alone, contribute to a weak or mushy texture. In fact this effect is seen when applying many of the very high absorbing fibers previously discussed. Hydrolyzed oat flour is listed in the FSIS Food Standards and Labeling Policy Book (USDA-FSIS, 2005a) and must be labeled as “hydrolyzed oat flour.”

Colloidal Fibers

Like the soluble fiber β -glucan, mentioned above, colloidal fibers also form gels. In this case, however, the gel is not due to soluble fiber, but to the formation of a colloidal dispersion of very small insoluble fibers. When insoluble fibers are wetmilled to an extremely small fiber diameter, they can form a stable thixotropic gel. As early as 1961, Battista, Hill, and Smith (1961) , in a US patent, describe a method for producing an insoluble gel from microcrystalline cellulose. In that case, hydrolyzed cellulose was subjected to shearing in a blender, forming a stable gel. It is also possible to produce this fibrous gel via microbial fermentation. The technology for this type of material, called Cellulon, was developed by the Weyerhauser Company using Acetobacter xylinum (Deis, 1997) . The small microreticulated fibers form an extremely stable gel which exhibit reversible shear thinning. The wet milling technology used to produce insoluble fiber gels can be applied to a number of substrates, including oat fiber, wheat fiber, cellulose, and corn bran. From a logistics point of view it is impractical to ship and store these very high water content gels. As a result, the gels are normally dried for commercial sale. In these cases it is necessary to include a dispersing agent (e.g., hydrocolloid, protein, polysaccharide, etc.) to coat the fibers and prevent re-agglomeration upon drying. Although high shear is still necessary to re-activate the colloidal gel, the coating reduces the needed activation force. Activation in a high-shear mixer or bowl chopper is usually adequate to re-form the gel. These colloidal gels exhibit many of the same fat replacement properties seen with the soluble fibers, including smooth texture and water-holding capability. Approval for use in meats of the colloidal gel products would be dependent upon, and the same as, the fiber substrate from which they were derived.