DIETARY FIBER

DIETARY FIBER

Dietary fibre has been considered to be composed of non-starch polysaccharides plus lignin plus resistant oligosaccharides plus resistant starch. Since lignin is not a carbohydrate, it will not be discussed. Dietary fibre occurring in foods and food products can be considered to consist of cellulose, hemicelluloses, pectic substances, hydrocolloids (gums and mucilages), resistant starches, and resistant oligosaccharides.

Cellulose, the major cell wall structural component in plants, is an unbranched linear chain of several thousand glucose units with b -D-(1->4)-glucosidic linkages. Cellulose's mechanical strength, resistance to biological degradation, low aqueous solubility, and resistance to acid hydrolysis result from hydrogen bonding within the microfibrils. There is a portion (10-15%) of the total cellulose, referred to as "amorphous," that is more readily acid hydrolyzed. Controlled acid hydrolysis of the amorphous fraction yields microcrystalline cellulose. Cellulose has been used as a bulking agent in food due to its water-absorbing ability and low solubility. Some of the early dietary fibre ingredient sources were based on cellulose powders or microcrystalline cellulose. Cellulose is not digested to any extent by the enzymes of the human gastrointestinal system.

Hemicelluloses may be present in soluble and insoluble forms and are comprised of a number of branched and linear pentose- and hexose-containing polysaccharides. In cereal grains, soluble hemicelluloses are termed "pentosans." Hemicelluloses are of much lower molecular weight than cellulose. Component monosaccharide units may include xylose, arabinose, galactose, mannose, glucose, glucuronic acid, and galacturonic acid.

Mixed linkage b -glucans, the (1->3)(1->4)- b -D-glucans, have generated considerable interest in recent years due to their physiological response as soluble dietary fibre. While these glucans are found in relatively small quantities in wheat, they are a major component of cell-call material in barley and oats. These glucans form viscous aqueous solutions and have been shown to be effective in reducing serum cholesterol concentrations. Oat bran, a rich source of b -D-glucan, has been incorporated into many food products, particularly cereals, as a source of the soluble fibre that has been touted for cholesterol reduction.

Both soluble and insoluble hemicelluloses play important roles in food products, the former functioning as soluble and the latter as insoluble fibre. They are characterized by their ability to bind water and hence serve as bulking agents. The presence of acidic components in some hemicelluloses impart the capacity to bind cations. Hemicelluloses are fermented to a greater extent than cellulose in the colon.

Pectins find widespread use in foods such as jams and jellies because of their ability to form stable gels. Completely esterified pectins do not require the addition of acid or electrolyte to form gels. The presence of calcium salts enhances the gelling capacity and decreases the dependence on pH and sugar concentration. Pectic substances are of importance as a component of dietary fibre because of their ion-exchange properties, due to the presence of the galacturonic acid units, and gelling (viscosity enhancing) properties.

 Categories of Dietary Fiber

• Cellulose, found in bran, legumes, peas, root vegetables, cabbage family, outer covering of seeds, and apples
• Hemicellulose, found in bran and whole grains
• Polyfructoses (Inulin and Oligofructans)
• Galactooligosaccharides
• Gums, found oatmeal, barley, and legumes.
• Mucilages
• Pectins, found in apples, strawberries, and citrus fruits
• Lignin, found in root vegetables, wheat, fruits with edible seeds (such as strawberries)
• Resistant Starches, found in ripe bananas, potatoes
  

 Cellulose

The major component in the rigid cell walls in plants is cellulose. Cellulose is a linear polysaccharide polymer with many glucose monosaccharide units. The acetal linkage is beta which makes it different from starch. This peculiar difference in acetal linkages results in a major difference in digestibility in humans. Humans are unable to digest cellulose because the appropriate enzymes to breakdown the beta acetal linkages are lacking. Undigestible cellulose is the fiber which aids in the smooth working of the intestinal tract.

Hemicellulose

A hemicellulose is any of several heteropolymers (matrix polysaccharides), such as arabinoxylans present along with cellolose in almost all plant cell walls. While cellulose is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength. It is easily hydrolyzed by dilute acid or base as well as myriad hemicellulose enzymes

These polysaccharide contain many different sugar monomers. In contrast, cellulose contains only anhydrous glucose. For instance, besides glucose, sugar monomers in hemicellulose can include xylose, mannose, rhamnose and arabinose. Hemicelluloses contain most of the D-pentose sugars, and occasionally small amounts of L-sugars as well. Xylose is always the sugar monomer present in the largest amount, but mannuronic and galactouronic acid also tend to be present.

 Pectin

Pectins are another complex group of polysaccharides that are abundant in primary cell walls and in the middle lamella between all plant cells. Like hemicellulose, pectin polymers are chemically diverse molecules. Pectins are acidic and contain a high proportion of D-galacturonic acid residues joined by a-1,4 glycosidic linkages. Some of the carboxylic acids of the galacturonates are esterified to methanol. L-rhamnose (a 6-deoxyhexose) residues are usually interspersed throughout the chain. The linkage of D-galacturonic acid to L-rhamnose is a-1,2 and the linkage from D-galacturonic acid to the next galacturonic acid in the chain is a-1,4. Side chains are often attached to these rhamnose residues. The side chains appear to be neutral sugars polymers containing monomers such as L-arabinose or D-galactose. These types of pectins are known as rhamnogalacturonans. Neutral pectins are also known which appear to be large branches of the acidic pectins. The 3 main types are the arabinans, the arabinogalactans and the galactans. Arabinans are highly branched consisting of a core of a-1,5 arabinosyl (a pentose in the furanose ring form) residues containing a-1,3- and a-1,2-linked arabinosyl side chains. The arabinogalactans contain ß-1,4-linked galactose chains carrying arabinose residues at the 3 and 6 positions that are further substituted. Galactans are mostly linear ß-1,4-linked D-galactose polymers with occasional single L-arabinose branches.

 Β-glucan

β-glucan, a kind of soluble dietary fiber, is a polymer made of a large number of glucose molecules bound by β-connection. Depending on the binding mode, it is categorized as β-1,3-1,4-, β-1,3-1,6-, etc. showing the binding position of glucose.
Mushroom, baker's yeast, and black yeast (Aureobasidium pullulans)-derived foods contain β-1,3-1,6-glucan.
Barley and oats contain 3-8% β-1,3-1,4-glucan, which is a primary constituent of soluble dietary fiber. Based on the molecular structure, "Barley Beta-Glucan" is β-1,3-1,4-glucan, and "Fermented Beta-Glucan" is β-1,3-1,6-glucan.


 Sources of Fiber

Dietary fiber is found only in plant foods: fruits, vegetables, nuts and grains. Meat, milk and eggs do not contain fiber. The form of food may or may not affect its fiber content. Canned and frozen fruits and vegetables contain just as much fiber as raw ones. Other types of processing, though, may reduce fiber content. Drying and crushing, for example, destroy the water-holding qualities of fiber.

The removal of seeds, peels or hulls also reduces fiber content. Whole tomatoes have more fiber than peeled tomatoes, which have more than tomato juice. Likewise, whole wheat bread contains more fiber than white bread.

Functional fiber is a newer term that includes food that consist of isolated, non-digestible carbohydrates that have beneficial physiological effects in humans. Cellulose, chitin, beta glucans, gums, inulin, oligofructose, fructoligosaccharides, lignin, pectins, psyllium, and resistant starches are forms of functional fiber when added to foods. Inulin or chicory inulin is one functional fiber that is receiving more attention in the news as it is being added into many food products and is known to cause gas.