Carboxymethylcellulose, methylcellulose and hydroxymethylcellulose products are available for controlling viscosity and oil use or absorption in foods (5,15). Although these cellulose derivatives are considered fiber molecules, they have some solubility in aqueous ethanol solutions, thus they may not be detected by analytical procedures that employ an ethanol precipitation step.
Brans, fruits, and other dark or highly colored products may be difficult to use in those foods where lightness in color is important. Bleaching may be used to lighten fiber materials, but chlorine-based bleaching agents should be avoided. Hydrogen peroxide treatment of wheat bran resulted in lighter, less red, and more yellow product (9). Wheat straw bleached with hydrogen peroxide could be incorporated into cakes without affecting flavor or producing gritty mouthfeel (16). Rats fed bleached oat hulls showed no abnormalities (17).
Peroxide bleaching frees carbohydrates bound to lignin. Bleaching increased soluble fiber neutral sugars in oat hulls by over 3%, while lignin decreased over 6% compared to non-bleached hulls (18). Health benefits due to phenolic compounds and other phytochemicals associated with fiber in foods may be lost during bleaching (19). As more consumers appreciate the importance of fiber in their diets, there may be less need to bleach fiber-rich materials to enhance marketability.
Oat beta-glucans and guar gum have been enzymatically hydrolyzed to improve functionality. The primary change after enzyme treatment is a reduction in molecular weight, which in turn leads to lower viscosities when the gums are dispersed in water. While a fiber-enriched mineral water may appeal to some consumers, there may not be a nutritional advantage to such supplementation. Bile acid binding and delayed glycemic response (20) are two health benefits that are dependent upon the viscous gels formed in the intestine. Further study is needed on the changes this procedure has on physiological effects.
Enzymes naturally present in fiber-rich materials can reduce food quality. Lipase inactivation is necessary for rice bran use. Although most processors employ some type of thermal treatment to destroy the enzyme, USDA researchers have found solvent extractions to be effective as well. Ethanol at room tempera-
378 Camire ture and hexane at 68°C inhibited lipid hydrolysis in rice kernels and flour during storage while producing slightly higher total dietary fiber contents (21).
Food processors utilize a variety of methods. Foods in microwavable frozen dinners may have been subjected to boiling, steaming, chemical peeling, baking, dehydration, or extrusion in addition to freezing and microwaving. Knowledge of the effects of these methods on fiber composition and properties is limited. Changes in fiber analytical methodology have made older reports less useful since many reported only total fiber or soluble plus insoluble fiber.
Solubilization of sugarbeet fiber was temperature- and pressure-dependent (22). No changes were observed at 10-minute intervals when sugarbeet pulp was auto-claved at 50° or 100°C and atmospheric pressure; soluble fiber content doubled after 10 minutes at 121°C and 1.2 bar.
Cooking foods in an excess volume of water leads to loss of water-soluble materials into the cooking water. Despite the potential nutrition impairment, this method of cooking remains common in households and institutions. Seaweed boiled for one hour lost up to 40% of insoluble fiber (23). Soaked and boiled chickpeas and kidney beans retained more dietary fiber than when soaked and pressure-cooked, presumably due to losses of soluble fiber into the cooking water (24).
The combination of time and temperature required to produce commercially sterile foods can severely affect fiber composition. Retorted (121 °C, 60 minutes) green bean and carrot purees lost significant amounts of pectic material (25). No changes were found in the neutral sugar fractions of the vegetables. Since this type of processing is used to prepare fruits and vegetables for infants and elderly persons, the health implications of such changes warrant further study. However, the vegetables in this study were subjected to many processes (freezing, freeze-drying, milling) prior to analysis. Fiber changes at each step were not evaluated.
Although blanching produced minor changes in carrot fiber compared to a frozen standard, boiling, microwaving and canning carrots caused increases in soluble fiber molecules (26). Canning also resulted in larger changes in molecular weight distribution of nonstarch polysaccharides in green beans, Brussels sprouts, and green peas compared with boiling and microwaving (27).
Modification of Dietary Fiber
More fiber should be retained in steamed vegetables since the food is not immersed in the cooking medium and leaching is minimized. Steam-cooking wheat bran increased IDF, but boiling and autoclaving increased SDF (28). Oil absorption, but not water-binding capacity, was increased by steaming and autoclaving; boiling significantly increased both properties. The processed brans also exhibited different farinogram effects when mixed with flour. These changes were attributed to physical, not chemical, modifications of fiber during processing.
Vegetables vary in their changes due to microwave cooking. Green beans and Brussels sprouts showed little change in molecular weight of fiber molecules, but green peas showed significant loss of higher molecular weight compounds (MW > 40000) with increased recovery of middle-sized polymers (MW 1000040000) (27). Microwaved (previously blanched and frozen) carrots elicited higher postprandial glucose values in volunteers fed a mixed meal than did raw carrots (29). Total dietary fiber was lower in raw carrots during the first year of the study, and a significantly higher satiety score was obtained for raw carrots that year.
Dry Heat Treatments
Breads and other baked grain products are popular world-wide, yet relatively little is known about the effects of baking on dietary fiber composition. Baking for one hour at 135°C had no effect on the dietary fiber composition or hydration capacity of cornmeal or oatmeal, but for baked potato peels, insoluble nonstarch polysaccharides increased and hydration capacity decreased (30). Baking bread transforms wheat starch into resistant starch that is digested and fermented like dietary fiber (31). More resistant starch was formed in the crumb than in the crust. These chemical changes are difficult to predict. Estimation of fiber content "S
after baking from the total dietary fiber content of ingredients was inadequate g
(32). Formation of Klason lignin during baking cannot be predicted reliably, but j content of other fiber components correlated well with predicted values. Some baked foods have increased soluble fiber and prediction of this change in solubil- ^
ity was also limited.
Several commodities such as nuts are heated at high temperatures (>170°C) to produce characteristic colors and flavors via Maillard reactions. Roasting/toasting significantly increased the lignin content of wheat bran (25) and cocoa beans (33). Although total fiber values were unaffected, both neutral sugars and uronic acids decreased in the insoluble fiber fraction of cocoa beans after roasting. The a a
Camire involvement of these carbohydrates in the formation of Maillard polymers is most likely responsible for the higher apparent lignin formation.
Many foods and feeds are now processed by extrusion cooking. This unique process performs several unit operations simultaneously. Foods are subjected to heat and shear during transportation along the extruder barrel. Fiber-rich materials may be extrusion-cooked to modify functional properties. Extruded bran mills into smaller pieces with reduced grittiness and improved water absorption (34). Increased screw speed during extrusion increased soluble fiber and enzyme-susceptible starch contents for whole wheat and wheat bran (35).
Branched molecules are more susceptible to shear during extrusion than is cellulose. Extrusion significantly increased the water solubility of sugar beet pulp fiber by decreasing the molecular weight of pectin and hemicelluloses molecules (36). No difference in X-ray diffraction patterns was found after extrusion of corn fiber-corn starch blends, suggesting than semi-crystalline cellulose is more resistant to degradation under typical extrusion conditions (37).
Prior processing can influence fiber changes during extrusion. Both acid and alkaline treatments increased soluble fiber somewhat in corn bran (38). Although grinding doubled the soluble fiber of pea hulls to 8% (dry basis), all extruded hulls contained over 10% soluble fiber (39). Total fiber content after extrusion was lower, perhaps due to losses of smaller molecular weight compounds formed from fiber during the process.
Soluble fiber created during extrusion is chemically different from naturally soluble fiber compounds such as pectin and gums, and thus should be expected to have different nutritional properties. The soluble fiber content of potato peels was about doubled by extrusion, and barrel temperature significantly increased in vitro binding of cholic acid and deoxycholic acid (40). Those findings agreed with a rat feeding study. Total serum and liver cholesterol levels were lower in young rats fed extruded oats, barley or wheat than in rats fed a control diet or feeds containing unextruded grains (41). Soluble fiber was higher in all extruded feeds, and soluble P-glucans increased slightly in extruded oats and barley. j
Higher viscosities of aqueous suspensions of extruded grains were also found.
Extruded citrus peels had higher levels of soluble fiber and increased in vitro viscosity (42). Starch digestion and glucose diffusion were not different from those of non-extruded products, however. Additional research could lead to extruded products with improved glycemic properties designed for diabetic consumers.
Extrusion-induced molecular changes could influence the ability of fiber to bind carcinogens. Few studies have been published on this subject. Extrusion conditions, with one exception (110°C, 30% feed moisture), did not affect the
Modification of Dietary Fiber
ability of potato peels to bind benzo[a]pyrene, a polycyclic aromatic hydrocarbon (43). Ready-to-eat breakfast cereals are often produced by extrusion. Sixteen commercial cereals bound at least 40% of the benzo[a]pyrene added during in vitro digestion, but carcinogen binding was not correlated with a specific dietary fiber fraction (44).
Many vegetables are blanched prior to freezing to inactivate enzymes, and this brief treatment may produce slight fiber solubilization (45). Fruits are generally not blanched to maintain texture and prevent juice loss. Pectinase activity is only slowed at freezer temperatures. Pectin solubilization and degradation may occur during frozen storage since consumers may keep frozen items for a year or more. Starch retrogradation may also occur in frozen foods; the formation of resistant starch by this mechanism is possible.
Insoluble dietary fiber increased after fermentation in three legumes (Bengal gram, cow pea, and green gram) (46). Since many microorganisms can use soluble fiber as an energy source, the increase in insoluble fiber was probably due to removal of soluble molecules.
Germination produced slight increases in legume IDF (46). Rolled flakes made from malted barley contained only 1.2% soluble beta-glucans, compared with 2.9% in flakes made from untreated barley (47). Total dietary fiber was also lower in the malted flakes.
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