Fiber Daryl
content
Schaller,2
and
structure
Ph.D.
ABSTRACT food
Crude
materials,
in the method fraction useful
diet.
nor More
of Van
analysis
it give
accurate
of dietary binding interference
fiber
does
Soest,
in isolating
mineral without
methods
modified fiber.
fiber
studies, from
Journal
of Clinical
Nutrition
not
use with
only for
and bile digestible
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S99/4656130 by University of Minnesota - Twin Cities user on 27 March 2018
dietary was
fiber
are
quick
experimentation.
the
fiber
especially
a simple,
acid binding studies food constituents.
31: OCTOBER
reflect
of dietary
foods,
provided further
accurately
fraction
of estimating for
It not
dietary
does
a constant
Dietary fiber can be defined as those plant components that are resistant to hydrolysis by endogenous enzymes of the human gastrointestinal tract. These components include cellulose and hemicelluloses (used as structural building materials in the plant cell wall), and pectins, gums, and mucilages (involved with plant cell structure and metabolism). Some plant components considered as dietary fiber are not polysaccharides: lignin, plant waxes, sterols, and nonenzymatic browning reaction products that form during cooking or processing. Small amounts of proteins, lipids, and normally digestible carbohydrates intimately associated with the plant cell or cell wall are often indigestible and can be included as dietary fiber. Crude fiber analysis has been the most commonly used method of analyzing for indigestible material in the diet. Unfortunately, this analysis does not reflect the true amount of indigestible material in food products and is primarily useful only for cellulose. About 50 to 90% of cellulose is recovered in crude fiber analysis (1). Only about 20% of the insoluble hemicelluloses are recovered, and only about 10 to 40% of the lignins are measured. Therefore, crude fiber analysis underestimates dietary fiber and does not give a constant fraction when comparing sources of dietary fiber that differ in composition. Table 1 shows the amount of dietary fiber in some plant materials contrasted with crude fiber content. As can be seen from the ratio of dietary to crude fiber, there is little relationship between the two values. The crude The American
in foods1
amount
when compared. useful
of dietary
used The of
Water-holding
in
sources
neutral
in measuring
method
fiber
to compare
detergent the
insoluble
analysis
but
also
capacity
studies,
were run on the insoluble dietary Am. J. Clin. Nutr. 31: S99-S 102,
was trace fiber 1978.
fiber value tells little about the actual amount of dietary fiber present. The content of dietary fiber in foods can be estimated in several ways; of these, biochemical analysis is the most complex. In this method, the components of dietary fiber are separated, and each subgroup is analyzed by hydrolyzing the carbohydrates and measuring the resultant hexoses, pentoses, and uronic acids. The results from all subgroups are totaled, giving an accurate and detailed picture of the dietary fiber. Unfortunately, this approach is laborious, and short-cut methods compromise the detailed information and accuracy that is inherently available. An additional problem associated with biochemical analysis is that it ignores indigestible proteins, fat, and other materials associated with the plant cell wall. Therefore, although this method results in a precise description of all classes of polysaccharides present in dietary fiber, biochemical analysis extracts its price in time, laboratory space, cost, and the need for highly trained technicians. Two other techniques for estimating dietary fiber are based on the principle of using enzymes or chemicals to remove digestible substances and measuring what is left over. Enzymatic methods add enzyme preparations to stimulate those found in the human digestive tract. One problem is that they are rarely iFrom 235 2
1978,
the
Porter Group
pp.
Research
Street, Leader,
S99-S
102.
Department,
Kellogg
Battle Creek, Michigan Cereal Chemistry.
Printed
in U.S.A.
Company, 49016.
S99
SCHALLER
S 100 TABLE I Comparison of crude fiber, dietary capacity of various plant materials
fiber,
Crude fiber
and
water-holding
Insoluble dietary fiber
Hem,cellulose
Cellulose
.
Lignins
waterholding capacity g !IO/g
Cellulose Pea hulls Beet pulp Corn bran (supplier A) Citrus pulp Corn bran (supplier B) Distiller’s dried grains Defatted wheat bran White wheat bran Rice bran
72.5 36.3 19.8 19.0 14.4 13.1 10.9 10.3 8.7 8.1
94.0 51.8 37.4 88.6 24.8 62.1 45.9 37.3 36.4 21.8
as effective as actual intestinal digestion; therefore, correction factors determined from animal feeding studies are needed to correlate the enzyme digestion results to the actual amount of dietary fiber. Also, enzyme preparations vary in activity from batch to batch, so great care must be taken in standardization. Chemical methods ofisolating dietary fiber have been used in animal nutrition experiments for many years and compare well with actual feeding studies without the need for correlation factors. We use a modification of one of these, the neutral detergent method of Van Soest and Wine (2), to analyze dietary fiber in food products and ingredients. This method uses detergent to solubilize lipids and proteins, EDTA to remove minerals, and heat to gelatinize starch at a neutral pH to prevent hydrolysis of hemicelluhoses. Neutral detergent fiber (NDF) analysis isolates plant cell walls containing insoluble hemicelluloses, cellulose, hignin, and associated components. According to Goering and Van Soest (3), NDF analysis appears to divide the dry matter of vegetable feedstuffs very near the point that separates the nutritively available and soluble constituents from those that are insoluble or incompletely available. NDF analysis was originally designed for forages and for animal nutrition experiments and was later adapted for food products (4). NDF analysis extracts the digestible components, leaving behind the plant cell walls and whatever is tightly bound to them. When this analysis is performed on food products of high starch content, heat-resistant starch is also left behind. The filtered NDF in these
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S99/4656130 by University of Minnesota - Twin Cities user on 27 March 2018
0 11.0 12.0 67.0 0 47.4 23.0 23.0 23.0 11.0
94.0 40.7 23.4 21.4 23.4 14.0 16.0 10.0 10.0 6.9
0 0.1 2.0 0.2 1.4 0.7 6.9 4.3 2.4 3.9
.
dieiarvfi. ber
3.4 8.0 28.1 5.0 28.2 5.0 6.1 6.2 8.5 9.7
cases gives a positive iodine test for starch, so a noncritical enzyme is added to remove this residual starch. An animal-derived enzyme, a-amylase, is used to remove residual starch. This enzyme, obtained from swine pancreatin, also hydrolyzes none of the hemicelluloses as do many other commonly available amylases. Two other analyses can be used to estimate the composition of dietary fiber. One is acid detergent fiber (ADF) analysis, also developed by Van Soest, in which a sample is boiled for 1 hr in a solution that removes the insoluble hemicelluloses in addition to the soluble and digestible components; lignin and cellulose remain. In the other method, cellulose is removed from ADF, giving the amount of hignin and any other materials chemically similar to lignin. Enzyme-modified NDF, ADF, and lignin analyses all enable the estimation of the insoluble hemicelluloses, ccllulose, and lignin in the sample. The components of dietary fiber estimated by these techniques are shown in Table 1. The only disadvantage to these analyses and those based on enzymes is that water-soluble, indigestible materials escape detection. For those foods in which they are important, a simple method pioneered by Southgate (5, 6) enables their rapid estimation. Pectins, gums, mucilages, starch, and other water-soluble pohysaccharides are extracted with water, the starch digested with a-amylase, and the remaining indigestible polysaccharides precipitated with alcohol, hydrolyzed, and colonmetrically estimated with p-hydroxybenzoic acid hydrazide. A comparison of the three basic methods
FIBER
FIG.
CONTENT
1. Aleurone
AND
cells
in dietary
of dietary fiber analysis was made as part of a collaborative study by the Food Fiber Committee of the American Association of Cereal Chemists (4). For white wheat bran, the results were as follows: 1) Van Soest enzymemodified NDF analysis, 36% dietary fiber; 2) Southgate unavailable carbohydrate analysis, 37%; and 3) Saunders in vitro enzymatic digestion method, 35%. Thus the three methods had a maximum variation of only 2%. The Cereal Chemists have been using the Van Soest method because of its ease and reliability. Since dietary fiber is not metabolized, its structure and physical properties are important. One function of dietary fiber is to increase the bulk of the contents of the intestinal tract. That property can be estimated by the material’s water-holding capacity. Aleurone cells (Fig. 1), which make up part of the outer coat or bran of the wheat kernel, are like holes in a sponge, absorbing water and holding it. The particles swell and become soft as they absorb water. If the particles are broken up into smaller pieces, fewer pores and voids remain to hold water, the water-
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S99/4656130 by University of Minnesota - Twin Cities user on 27 March 2018
STRUCTURE
fiber
IN
from
FOODS
wheat
SlOl
bran.
holding capacity becomes lower, and the physiological usefulness is lessened. Therefore, identical amounts of dietary fiber from different sources will not necessarily have the same physiological effect. The water-holding capacities of some sources of dietary fiber are shown in Table 1. In this case, only the water-holding capacity of the dietary fiber was measured; starch, protein, fat, and other digestible materials can affect the measurement of water-holding capacity of some food materials. The enzymemodified NDF analysis was therefore used to isolate the dietary fiber, and the water-holding capacity was measured. Dietary fiber from most sources has the ability to act as an ion exchange resin. Phenolic groups in the hignin and uronic acids in the hemicelluloses and pectins act to adsorb various constituents in the lower gastrointestinal tract. It has been hypothesized that dietary fiber may help lower cholesterol levels by adsorbing bile acids, thus preventing intestinal adsorption and recircuhation. Therefore, new bile acids have to be synthesized from the
SCHALLER
Sl02 TABLE Bile acid
2 binding
of food
ingredients” Glycocholic acid
asrbed %
Wheat bran (whole, ground) Wheat bran diluted with wheat starch 1:1 (whole, ground) Soy protein isolate Corn starch Cellulose Wheat bran (calculated from bile acid binding of dietary fiber only) “Twenty milligrams in 20 ml at pH 8.
of glycocholic
32 31 30 26 0 7
acid
pen
I g sample
body’s cholesterol pool. As shown in Table 2 (4), the bile acid binding ability of wheat bran is fairly good. Diet studies, however, indicate that it is not effective in lowering blood cholesterol levels (7). Protein (soy isolate) and starch also bind bile acids (see Table 2). Both materials are digested, so they will not be important in hindering intestinal absorption. When wheat bran was diluted 1:1 with wheat starch, virtually no change occurred in the ability to bind bile acids, indicating that more than the bile acid binding ability of the dietary fiber was being measured. The dietary fiber was then isolated by the enzyme-modified NDF method, its ability to adsorb bile acids mea-
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S99/4656130 by University of Minnesota - Twin Cities user on 27 March 2018
sured, and the binding ability of the dietary fiber calculated. Dietary fiber in wheat bran was found to be responsible for binding only 7% of the glycochohic acid. This corresponds much more closely to the diet studies. Similarly, the ability of the dietary fiber to bind trace minerals, toxic constituents in the diet, or any other substances of interest have to be measured on dietary fiber alone,
a
References I. VAN S0EST, P. J., AND R. W. MCQUEEN. The chemistry and estimation of fiber. Proc. Nutn. Soc. 32: 123, 1973. 2. VAN S0E5T, P. J., AND R. H. WINE. Use of detergents in the analysis of fibrous foods. J. Assoc. Off. Agric. Chem. 50: 50, 1967. 3. GOERING, H. K., AND P. J. VAN S0EST. Forage Fiber Analyses. Agricultural Handbook 379. Washington, D.C.: U.S. Department of Agriculture, 1973. 4. SCHALLER, D. R. Analysis of cereal products and ingredients. Symposium on Food Fiber. A.A.C.C. Annual Meeting, 1976. S. SOUTHGATE, D. A. T. Determination of carbohydrates in foods. II. Unavailable carbohydrates. J. Sci. Food Agnic. 20: 330, 1969. 6. HUDSON, G. J., P. M. V. JOHN, B. S. BAILEY AND D. A. T. SOUTHGATE. The automated determination of carbohydrate. J. Sci. Food. Agric. 27: 681, 1976. 7. WALTERS, R. L., I. MCLEAN BAIRD, P. S. DAVIES, M. J. HILL, B. S. DRASAR, D. A. T. SOUTHGATE, J. GREEN
AND
B.
dietary
fibre
on
Bnit.
Med.
MORGAN. faecal
J. 2: 536,
Effects steroid
1975.
of and
two lipid
types excretion.
of
content
Schaller,2
and
structure
Ph.D.
ABSTRACT food
Crude
materials,
in the method fraction useful
diet.
nor More
of Van
analysis
it give
accurate
of dietary binding interference
fiber
does
Soest,
in isolating
mineral without
methods
modified fiber.
fiber
studies, from
Journal
of Clinical
Nutrition
not
use with
only for
and bile digestible
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S99/4656130 by University of Minnesota - Twin Cities user on 27 March 2018
dietary was
fiber
are
quick
experimentation.
the
fiber
especially
a simple,
acid binding studies food constituents.
31: OCTOBER
reflect
of dietary
foods,
provided further
accurately
fraction
of estimating for
It not
dietary
does
a constant
Dietary fiber can be defined as those plant components that are resistant to hydrolysis by endogenous enzymes of the human gastrointestinal tract. These components include cellulose and hemicelluloses (used as structural building materials in the plant cell wall), and pectins, gums, and mucilages (involved with plant cell structure and metabolism). Some plant components considered as dietary fiber are not polysaccharides: lignin, plant waxes, sterols, and nonenzymatic browning reaction products that form during cooking or processing. Small amounts of proteins, lipids, and normally digestible carbohydrates intimately associated with the plant cell or cell wall are often indigestible and can be included as dietary fiber. Crude fiber analysis has been the most commonly used method of analyzing for indigestible material in the diet. Unfortunately, this analysis does not reflect the true amount of indigestible material in food products and is primarily useful only for cellulose. About 50 to 90% of cellulose is recovered in crude fiber analysis (1). Only about 20% of the insoluble hemicelluloses are recovered, and only about 10 to 40% of the lignins are measured. Therefore, crude fiber analysis underestimates dietary fiber and does not give a constant fraction when comparing sources of dietary fiber that differ in composition. Table 1 shows the amount of dietary fiber in some plant materials contrasted with crude fiber content. As can be seen from the ratio of dietary to crude fiber, there is little relationship between the two values. The crude The American
in foods1
amount
when compared. useful
of dietary
used The of
Water-holding
in
sources
neutral
in measuring
method
fiber
to compare
detergent the
insoluble
analysis
but
also
capacity
studies,
were run on the insoluble dietary Am. J. Clin. Nutr. 31: S99-S 102,
was trace fiber 1978.
fiber value tells little about the actual amount of dietary fiber present. The content of dietary fiber in foods can be estimated in several ways; of these, biochemical analysis is the most complex. In this method, the components of dietary fiber are separated, and each subgroup is analyzed by hydrolyzing the carbohydrates and measuring the resultant hexoses, pentoses, and uronic acids. The results from all subgroups are totaled, giving an accurate and detailed picture of the dietary fiber. Unfortunately, this approach is laborious, and short-cut methods compromise the detailed information and accuracy that is inherently available. An additional problem associated with biochemical analysis is that it ignores indigestible proteins, fat, and other materials associated with the plant cell wall. Therefore, although this method results in a precise description of all classes of polysaccharides present in dietary fiber, biochemical analysis extracts its price in time, laboratory space, cost, and the need for highly trained technicians. Two other techniques for estimating dietary fiber are based on the principle of using enzymes or chemicals to remove digestible substances and measuring what is left over. Enzymatic methods add enzyme preparations to stimulate those found in the human digestive tract. One problem is that they are rarely iFrom 235 2
1978,
the
Porter Group
pp.
Research
Street, Leader,
S99-S
102.
Department,
Kellogg
Battle Creek, Michigan Cereal Chemistry.
Printed
in U.S.A.
Company, 49016.
S99
SCHALLER
S 100 TABLE I Comparison of crude fiber, dietary capacity of various plant materials
fiber,
Crude fiber
and
water-holding
Insoluble dietary fiber
Hem,cellulose
Cellulose
.
Lignins
waterholding capacity g !IO/g
Cellulose Pea hulls Beet pulp Corn bran (supplier A) Citrus pulp Corn bran (supplier B) Distiller’s dried grains Defatted wheat bran White wheat bran Rice bran
72.5 36.3 19.8 19.0 14.4 13.1 10.9 10.3 8.7 8.1
94.0 51.8 37.4 88.6 24.8 62.1 45.9 37.3 36.4 21.8
as effective as actual intestinal digestion; therefore, correction factors determined from animal feeding studies are needed to correlate the enzyme digestion results to the actual amount of dietary fiber. Also, enzyme preparations vary in activity from batch to batch, so great care must be taken in standardization. Chemical methods ofisolating dietary fiber have been used in animal nutrition experiments for many years and compare well with actual feeding studies without the need for correlation factors. We use a modification of one of these, the neutral detergent method of Van Soest and Wine (2), to analyze dietary fiber in food products and ingredients. This method uses detergent to solubilize lipids and proteins, EDTA to remove minerals, and heat to gelatinize starch at a neutral pH to prevent hydrolysis of hemicelluhoses. Neutral detergent fiber (NDF) analysis isolates plant cell walls containing insoluble hemicelluloses, cellulose, hignin, and associated components. According to Goering and Van Soest (3), NDF analysis appears to divide the dry matter of vegetable feedstuffs very near the point that separates the nutritively available and soluble constituents from those that are insoluble or incompletely available. NDF analysis was originally designed for forages and for animal nutrition experiments and was later adapted for food products (4). NDF analysis extracts the digestible components, leaving behind the plant cell walls and whatever is tightly bound to them. When this analysis is performed on food products of high starch content, heat-resistant starch is also left behind. The filtered NDF in these
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S99/4656130 by University of Minnesota - Twin Cities user on 27 March 2018
0 11.0 12.0 67.0 0 47.4 23.0 23.0 23.0 11.0
94.0 40.7 23.4 21.4 23.4 14.0 16.0 10.0 10.0 6.9
0 0.1 2.0 0.2 1.4 0.7 6.9 4.3 2.4 3.9
.
dieiarvfi. ber
3.4 8.0 28.1 5.0 28.2 5.0 6.1 6.2 8.5 9.7
cases gives a positive iodine test for starch, so a noncritical enzyme is added to remove this residual starch. An animal-derived enzyme, a-amylase, is used to remove residual starch. This enzyme, obtained from swine pancreatin, also hydrolyzes none of the hemicelluloses as do many other commonly available amylases. Two other analyses can be used to estimate the composition of dietary fiber. One is acid detergent fiber (ADF) analysis, also developed by Van Soest, in which a sample is boiled for 1 hr in a solution that removes the insoluble hemicelluloses in addition to the soluble and digestible components; lignin and cellulose remain. In the other method, cellulose is removed from ADF, giving the amount of hignin and any other materials chemically similar to lignin. Enzyme-modified NDF, ADF, and lignin analyses all enable the estimation of the insoluble hemicelluloses, ccllulose, and lignin in the sample. The components of dietary fiber estimated by these techniques are shown in Table 1. The only disadvantage to these analyses and those based on enzymes is that water-soluble, indigestible materials escape detection. For those foods in which they are important, a simple method pioneered by Southgate (5, 6) enables their rapid estimation. Pectins, gums, mucilages, starch, and other water-soluble pohysaccharides are extracted with water, the starch digested with a-amylase, and the remaining indigestible polysaccharides precipitated with alcohol, hydrolyzed, and colonmetrically estimated with p-hydroxybenzoic acid hydrazide. A comparison of the three basic methods
FIBER
FIG.
CONTENT
1. Aleurone
AND
cells
in dietary
of dietary fiber analysis was made as part of a collaborative study by the Food Fiber Committee of the American Association of Cereal Chemists (4). For white wheat bran, the results were as follows: 1) Van Soest enzymemodified NDF analysis, 36% dietary fiber; 2) Southgate unavailable carbohydrate analysis, 37%; and 3) Saunders in vitro enzymatic digestion method, 35%. Thus the three methods had a maximum variation of only 2%. The Cereal Chemists have been using the Van Soest method because of its ease and reliability. Since dietary fiber is not metabolized, its structure and physical properties are important. One function of dietary fiber is to increase the bulk of the contents of the intestinal tract. That property can be estimated by the material’s water-holding capacity. Aleurone cells (Fig. 1), which make up part of the outer coat or bran of the wheat kernel, are like holes in a sponge, absorbing water and holding it. The particles swell and become soft as they absorb water. If the particles are broken up into smaller pieces, fewer pores and voids remain to hold water, the water-
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S99/4656130 by University of Minnesota - Twin Cities user on 27 March 2018
STRUCTURE
fiber
IN
from
FOODS
wheat
SlOl
bran.
holding capacity becomes lower, and the physiological usefulness is lessened. Therefore, identical amounts of dietary fiber from different sources will not necessarily have the same physiological effect. The water-holding capacities of some sources of dietary fiber are shown in Table 1. In this case, only the water-holding capacity of the dietary fiber was measured; starch, protein, fat, and other digestible materials can affect the measurement of water-holding capacity of some food materials. The enzymemodified NDF analysis was therefore used to isolate the dietary fiber, and the water-holding capacity was measured. Dietary fiber from most sources has the ability to act as an ion exchange resin. Phenolic groups in the hignin and uronic acids in the hemicelluloses and pectins act to adsorb various constituents in the lower gastrointestinal tract. It has been hypothesized that dietary fiber may help lower cholesterol levels by adsorbing bile acids, thus preventing intestinal adsorption and recircuhation. Therefore, new bile acids have to be synthesized from the
SCHALLER
Sl02 TABLE Bile acid
2 binding
of food
ingredients” Glycocholic acid
asrbed %
Wheat bran (whole, ground) Wheat bran diluted with wheat starch 1:1 (whole, ground) Soy protein isolate Corn starch Cellulose Wheat bran (calculated from bile acid binding of dietary fiber only) “Twenty milligrams in 20 ml at pH 8.
of glycocholic
32 31 30 26 0 7
acid
pen
I g sample
body’s cholesterol pool. As shown in Table 2 (4), the bile acid binding ability of wheat bran is fairly good. Diet studies, however, indicate that it is not effective in lowering blood cholesterol levels (7). Protein (soy isolate) and starch also bind bile acids (see Table 2). Both materials are digested, so they will not be important in hindering intestinal absorption. When wheat bran was diluted 1:1 with wheat starch, virtually no change occurred in the ability to bind bile acids, indicating that more than the bile acid binding ability of the dietary fiber was being measured. The dietary fiber was then isolated by the enzyme-modified NDF method, its ability to adsorb bile acids mea-
Downloaded from https://academic.oup.com/ajcn/article-abstract/31/10/S99/4656130 by University of Minnesota - Twin Cities user on 27 March 2018
sured, and the binding ability of the dietary fiber calculated. Dietary fiber in wheat bran was found to be responsible for binding only 7% of the glycochohic acid. This corresponds much more closely to the diet studies. Similarly, the ability of the dietary fiber to bind trace minerals, toxic constituents in the diet, or any other substances of interest have to be measured on dietary fiber alone,
a
References I. VAN S0EST, P. J., AND R. W. MCQUEEN. The chemistry and estimation of fiber. Proc. Nutn. Soc. 32: 123, 1973. 2. VAN S0E5T, P. J., AND R. H. WINE. Use of detergents in the analysis of fibrous foods. J. Assoc. Off. Agric. Chem. 50: 50, 1967. 3. GOERING, H. K., AND P. J. VAN S0EST. Forage Fiber Analyses. Agricultural Handbook 379. Washington, D.C.: U.S. Department of Agriculture, 1973. 4. SCHALLER, D. R. Analysis of cereal products and ingredients. Symposium on Food Fiber. A.A.C.C. Annual Meeting, 1976. S. SOUTHGATE, D. A. T. Determination of carbohydrates in foods. II. Unavailable carbohydrates. J. Sci. Food Agnic. 20: 330, 1969. 6. HUDSON, G. J., P. M. V. JOHN, B. S. BAILEY AND D. A. T. SOUTHGATE. The automated determination of carbohydrate. J. Sci. Food. Agric. 27: 681, 1976. 7. WALTERS, R. L., I. MCLEAN BAIRD, P. S. DAVIES, M. J. HILL, B. S. DRASAR, D. A. T. SOUTHGATE, J. GREEN
AND
B.
dietary
fibre
on
Bnit.
Med.
MORGAN. faecal
J. 2: 536,
Effects steroid
1975.
of and
two lipid
types excretion.
of
