10 Unconventional Sources of Dietary Fiber Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 07/09/18. For personal use only.
Some in Vitro and in Vivo Properties of Dietary Fibers from Noncereal Sources P. VAN SOEST, P. HORVATH, M. McBURNEY, J. JERACI, and M. ALLEN Cornell University, Department of Animal Science, Ithaca,NY14853 Non-cereal dietary fiber sources are compared and evaluated for cation exchange, water holding capacity and in v i t r o fermentabi1ity by gut microflora. F i b e r sources s t u d i e d i n c l u d e Psyllium gum, alfalfa, p u r i f i e d wood c e l l u l o s e , p e c t i n , cabbage, synthetic M a i l l a r d product, soybean hulls and propol. Sources are compared to the standard AACC wheat bran. Fermentability of the substrates a l f a l f a , wood cellulose and wheat bran show the largest unfermentable residue and the greatest differences between inocula sources (rumen inocula greater than human fecal inocula in all cases). Water-soluble sources (pectins) are completely fermentable, the exception of Psyllium gum which contains about 44% of an unfermentable residue. Cabbage pectin is unusual in having considerable cold water (50%) solubility. Cation exchange binding measured with copper are the highest for p e c t i n , cabbage, bran, M a i l l a r d product = alfalfa and soybean hulls descending in that order. Wood cellulose, propol and Psyllium gum have e s s e n t i a l l y no metal binding capacity. Dietary fibers of high prophylactic value appear to have substantial metal binding capacity and are divisible into two c l a s s e s b a s e d on f e r m e n t a b i l i t y . Human feeding studies indicate that the fiber sources containing unfermentable residues w i l l contribute to fecal bulk and, therefore, t r a n s i t and passage, while the more fermentable have t h e i r major effects upon gut microflora. These two aspects are complementary relative to the effectiveness of dietary fiber.
The identification of dietary fibers as an overlooked factor in human health and disease has promoted a great deal of interest 0097-6156/83/0214-0135$06.00/0 © 1983 American Chemical Society
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i n the c h a r a c t e r and q u a l i t y o f food f i b e r s o u r c e s * While c o n s i d e r a b l e a t t e n t i o n has been g i v e n to c e r e a l b r a n s , o t h e r sources of d i e t a r y f i b e r e x i s t that o f f e r a v a r i e t y o f q u a l i t i e s * These i n c l u d e v e g e t a b l e s , gums, wood p r o d u c t s and s y n t h e t i c polymers, D i e t a r y s t u d i e s w i t h humans and o t h e r a n i m a l models have emphasized the v a r i e d n a t u r e o f r e s p o n s e s to d i f f e r e n t f i b e r sources. T r a n s i t and passage depend on the b u l k o f c o l o n i c c o n t e n t s which i n t u r n a p p e a r s to depend upon l i g n i f i e d unfermentable residues as found i n wheat bran. The p a r t i c l e s i z e o f b r a n i s an i m p o r t a n t f a c t o r i n f l u e n c i n g i t s b u l k i n g e f f e c t , coarser residues having more e f f e c t (O* In general u n l i g n i f i e d s o u r c e s such as v e g e t a b l e s and p e c t i n have l e s s e f f e c t upon t r a n s i t and i n f l u e n c e gut m i c r o f l o r a more because of t h e i r high f e r m e n t a b i l i t y . These residues being l e s s r i g i d s t r u c t u r a l l y may have greater s w e l l i n g and h y d r a t i o n c h a r a c t e r i s t i c s (2, 3). Hypotheses to account f o r the e f f e c t s of d i e t a r y f i b e r have e m p h a s i z e d the b u l k i n g and b i n d i n g c a p a c i t i e s , i n the argument t h a t f a s t e r t r a n s i t r e d u c e s exposure to h a z a r d o u s s u b s t a n c e s , while the b i n d i n g e f f e c t might account f o r the s p e c i f i c b i n d i n g o f b i l e s a l t and o t h e r o r g a n i c s u b s t a n c e s and the r e p o r t e d n e g a t i v e e f f e c t s u p o n m i n e r a l a v a i l a b i l i t y (4^)· A less r e c o g n i z e d f a c t o r i s t h a t o f the gut m i c r o f l o r a which i f stimulated may incorporate ammonia, carbon skeletons and m i n e r a l s i n t o the c e l l u l a r r e s i d u e (5, 6). This incorporation into c e l l u l a r mass would provide an a l t e r n a t i v e u n a v a i l a b l e sink f o r these substances. The purpose of t h i s i n v e s t i g a t i o n i s to present data on the v a r i e t y o f d i e t a r y f i b e r sources and c h a r a c t e r i z e them r e l a t i v e to the p h y s i c o - c h e m i c a l c h a r a c t e r i s t i c s s u p p o r t i n g m e t a l i o n binding, h y d r a t i o n and f e r m e n t a b i l i t y , Fiber
Sources
The f o l l o w i n g d i e t a r y f i b e r sources were s t u d i e d : P s y l l i u m gum,propol (a glucomannan gum), commercial p e c t i n , solka f l o e (a wood c e l l u l o s e ) , cabbage, soybean h u l l s , a l f a l f a , M a i l l a r d p r o d u c t and s t a n d a r d AACC wheat bran. The P s y l l i u m and p r o p o l gum were donated by the S e a r l e Company, C h i c a g o , IL; o t h e r sources were purchased with the exception of the M a i l l a r d product which was p r e p a r e d a c c o r d i n g to the p r o c e d u r e o f O l s o n e t a l , (7) from s u c r o s e and g l y c i n e . The i n s o l u b l e f i b e r s from wheat bran, a l f a l f a , and soybean h u l l s were p r e p a r e d by a m y l a s e d i g e s t i o n and n e u t r a l d e t e r g e n t e x t r a c t i o n . The cabbage was converted to a coarse powder by a t h r e e f o l d e x t r a c t i o n with 95% e t h a n o l and p a s s i n g the a l c o h o l s l u r r y through a meat g r i n d e r . This treatment r e s u l t e d i n a 30-fold c o n c e n t r a t i o n of the d i e t a r y f i b e r over that i n raw cabbage.
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Water H o l d i n g C a p a c i t y by F i 1 t r a t i o n . The samples were added to c r u c i b l e s that had been weighed at room temperature and hot (100° C). D e i o n i z e d d i s t i l l e d H 0 was added to the sample with backpressure f o r 2 hrs, then f i l t e r e d under f u l l vacuum f o r 1 minute; f i n a l l y the c r u c i b l e was wiped to remove excess water and weighed. The hydrated sample was d r i e d at 100° C overnight and weighed. Grams o f water per d r y gram o f sample was determined. 2
Water H o l d i n g C a p a c i t y by Osmot i c Suet i o n . Water was removed from h y d r a t e d samples by c r e a t i n g an o s m o t i c s u c t i o n using a m o d i f i c a t i o n of the Robertson and Eastwood method (8). D i a l y s i s t u b i n g (Mwt c u t o f f = 2000) was c u t i n t o 10 cm l e n g t h s and one end t i e d . These bags were soaked i n a 0,1% w/v sodium azide s o l u t i o n overnight. Sample was placed i n the bags and 5 ml of a z i d e s o l u t i o n was added. The open end was t i e d and the f i l l e d bag r e s t e d i n a 150 ml b e a k e r w i t h i n a d e s s i c a t o r a t a r e l a t i v e humidity of 100% f o r 24 h r s , f o r hydration. Then 50 ml of an a z i d e s o l u t i o n (0,1%) w i t h 8.75 g o f PEG (3350 M o l e c u l a r weight) was placed i n the beaker. This c o n c e n t r a t i o n of PEG w i l l approximate a s u c t i o n pressure of 270 osmol across the d i a l y s i s membrane a f t e r the sample has l o s t i t ' s n o n a s s o c i a t e d water. R e a d i n g s o f the o s m o l a r i t y were made a t 6, 12, 24 and 48 h r s to determine e q u i l i b r i u m . A f t e r 24 or 48 h r s , the tube was c u t open, A subsample o f the hydrated m a t e r i a l was removed and weighed, A dry weight of the f i b e r subsample was obtained a f t e r overnight drying at 100° C, Grams of water per gram of sample was then c a l c u l a t e d . Measurement of C a t i o n Exchange Capacity, Cation exchange c a p a c i t y (CEC) was measured based on the h i g h a f f i n i t y o f Cu f o r c a r b o x y l i c a c i d groups (9, 10), Samples were weighed i n 50 ml c o a r s e Gooch c r u c i b l e s , p l a c e d i n s i d e a 100 ml b e a k e r and i n c u b a t e d w i t h 50 ml 1 M copper s u l f a t e f o r 2-3 h o u r s . These c r u c i b l e s were removed and washed with d i s t i l l e d , deionized water to remove unbound copper i o n s . Then the samples were washed 3 t i m e s (25 ml/wash) w i t h 0.6 Ν h y d r o c h l o r i c a c i d i n 70% (v/v) p r o p a n o l - 2 . The washes were c o l l e c t e d and b r o u g h t t o a known volume, A l i q u o t s were b r o u g h t to pH 8-9 w i t h 2 Ν ammonium hydroxide, A c o p p e r i n d i c a t o r , c u p r i z o n , was added, a g a i n b r o u g h t to a known volume and the a b s o r b a n c e at 590 nm was measured a f t e r 30 minutes to allow f o r complete c o l o r r e a c t i o n . Copper n i t r a t e standards were prepared ranging from 0 to 8 PPM, Fermentation Technique, A human batch c u l t u r e technique was used to measure in v i t r o f e r m e n t a b i l i t y of the samples (11), The technique was developed to u t i l i z e m i c r o f l o r a present i n human f e c a l m a t t e r . M a i n t e n a n c e o f an a n a e r o b i c e n v i r o n m e n t d u r i n g c o l l e c t i n g , processing and i n o c u l a t i n g was important. The use o f a modified camping t o i l e t allowed c o l l e c t i o n w h i l e maintaining
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anaerobiosis, A r e c e i v i n g medium was used i n the c o l l e c t i o n vessel. The c u l t u r e medium and determination of f e r m e n t a b i l i t y were the same as the rumen in v i t r o procedure (12), Composition of F i b e r s Composition of the d i e t a r y f i b e r sources i s shown i n Table I. The gum s o u r c e s g e n e r a l l y show no i n s o l u b l e f i b e r (as NDF) with the exception of the glucoraannan source which contains about 5% o f a w h i t e f i b r o u s r e s i d u e . Cabbage powder i s i n t e r m e d i a t e w i t h about a t h i r d of i t s d i e t a r y f i b e r i n the form of p e c t i n . This p e c t i n i s unusual i n that i t i s not p r e c i p i t a t e d by q u a t e r n a r y d e t e r g e n t and a l s o posseses s u b s t a n t i a l cold watersolubility i n the absence of c h e l a t i n g agents. This c h a r a c t e r i s t i c has been mentioned by B a i l e y et^ a l , (13), Soybean h u l l s contain s i g n i f i c a n t amounts o f g a l a c t a n and o t h e r p e c t i n r e l a t e d c a r b o h y d r a t e s (14) i n a d d i t i o n to the i n s o l u b l e NDF fraction, Table I,
Composition of Dietary F i b e r Sources Dietary Fiber NDF HC Cell Lignin Nitrogen % DM 1
F i b e r Source
P s y l l i u m , whole seed P s y l l i u m , gum Propol Pectin Wood C e l l u l o s e M a i l l a r d polymer A l f a l f a NDF Cabbage Soy bean h u l l s NDF Wheat bran AACC
87,7 93,5 99,0 100,0 100.0 ND 100,0 73,0 100,0 44,1 2
52,6 ND 4,6 0 100,0 74,2 53,0 44,3 56,0 44,1
23,7 ND 0 0 2,6 0 21,6 7,3 10,5 30,9
25,4 4,1 0 0 94,5 0 23,9 31,1 37,0 9,3
3,5 0,3 0 0 2,8 72,7 7,5 1,0 2,0 3,9
-
-
0 0 0,2 8,6 3,2 2,9 3,2 2.8
Hemicellulose ND, Not Determined
2
Fermentability
and Water Holding Capacity
The d i e t a r y f i b e r i n a l f a l f a and wheat bran i s p r i m a r i l y i n the forms of i n s o l u b l e NDF, and these sources are s i g n i f i c a n t l y l i g n i f i e d and t h e r e f o r e c o n t a i n s u b s t a n t i a l a m o u n t s o f u n f e r m e n t a b l e c a r b o h y d r a t e ( T a b l e I I ) , Soybean h u l I s , cabbage, wood c e l l u l o s e are l e s s l i g n i f i e d and about 90% fermentable. The wood s o u r c e i s not a pure c e l l u l o s e and c o n t a i n s s i g n i f i c a n t amounts of h e m i c e l l u l o s e and l i g n i n . The h e m i c e l l u l o s e f r a c t i o n i s l a r g e enough to o b t a i n a d i g e s t i o n b a l a n c e w i t h human subjects, H e m i c e l l u l o s e i n S o l k a f l o e i s s u b s t a n t i a l l y more fermentable than i t s c e l l u l o s e (6).
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Sources
T a b l e l l shows a c o m p a r i s o n between f e r m e n t a b i l i t y v a l u e s u t i l i z i n g rumen and human i n o c u l a on the same s u b s t r a t e s , A comparison of t h i s nature may have inherent i n e q u a l i t i e s due to d i f f e r e n c e s i n inoculum source and c o l l e c t i o n procedures. These p a r a m e t e r s have been e v a l u a t e d by a number o f w o r k e r s (11, 17, 18). The d i f f e r e n c e s i n n u t r i t i o n a l schemes between a n i m a l species have an e f f e c t on m i c r o b i a l populations. Fermentability estimates of a p a r t i c u l a r substrate are b i o l o g i c a l l y acceptable with respect to the inoculum source. Table I I ,
Physicochemical and B i o l o g i c a l P r o p e r t i e s of Dietary F i b e r Sources Fermentability Water Holding Capacity Exchange Osmotic Human suction, meq Cu/ Fecal Rumen Filtration 0,3 osmol 100 g - % - -g H 0/g F i b e r 2
_1 P s y l l i u m gum _1 Propol .1 Pectin Wood C e l l u l o s e 1.4 M a i l l a r d product 2,4 A l f a l f a NDF 5,8 Cabbage 20,7 Soybean Hulls NDF 5,9 Coarse Bran, NDF 3,5
3,2 2,5 5.0 1,0 1.7 1,8 3.0 1,7 1.3
3 O 227 5 37 36 92 18 87
1
_1 _1 90 23 (o) 46 91 9
53
2
ο 2
56 100 98 94 (0). 57 91 89 71 2
2
Not determined—see d i s c u s s i o n . 2 Used whole plant t i s s u e . F e r m e n t a b i l i t y using human f e c a l inoculum ranges from a high of 91% (cabbage) to a low of 0% ( M a i l l a r d product) demonstrating the s e n s i t i v i t y of human m i c r o f l o r a to d i f f e r e n t f i b e r sources, F e r m e n t a b i l i t y of c e l l u l o s e by human f e c a l m i c r o f l o r a (23%) i s s u b s t a n t i a l l y l e s s than rumen m i c r o f l o r a (94%). The m i c r o f l o r a in the human i n t e s t i n e may be more a f f e c t e d by f i b e r composition than rumen m i c r o f l o r a . Work by Bryant (19) and J e r a c i (11) leads to s p e c u l a t i o n that v a r i a t i o n among inoculum sources i n humans on a p a r t i c u l a r s u b s t r a t e c o u l d be g r e a t e r than i n o t h e r s p e c i e s . The water holding c a p a c i t y (WHC) of d i e t a r y f i b e r could have important r e l a t i o n s h i p s to many other f i b e r c h a r a c t e r i s t i c s (15, 16), A new t e c h n i q u e i s u t i l i z e d so t h a t w a t e r s o l u b l e components of d i e t a r y f i b e r can be measured for WHC, This method d i f f e r s s i g n i f i c a n t l y from p r e v i o u s t e c h n i q u e s (8) i n t h a t the d i a l y s i s tubing had a molecular weight c u t o f f of 2000, The WHC measured by f i l t r a t i o n i s c o n s i s t a n t l y higher for the i n s o l u b l e fibers. Wood c e l l u l o s e shows the lowest value by both methods. The t h r e e water s o l u b l e p o l y s a c c h a r i d e s are among the h i g h e s t values measured by osmotic s u c t i o n with p e c t i n holding 5,0 g H 0 2
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UNCONVENTIONAL SOURCES OF DIETARY FIBER
per gram o f f i b e r . Cabbage h o l d s 3,0 g water per gram o f f i b e r . This i s probably due to the l a r g e amount of p e c t i n i n t h i s f i b e r source. The M a i l l a r d p r o d u c t i s u n u s u a l i n t h a t i t has the s m a l l e s t change i n WHC between the methods. The f e r m e n t a b i l i t y o f t h e s e f i b e r s , the m i c r o b i a l mass produced and the p r o p e r t i e s of the m i c r o b i a l products and f i b e r residues may be of greater importance i n the l a r g e i n t e s t i n e than the c h a r a c t e r i s t i c s of the unfermented d i e t a r y f i b e r . Cation Exchange C a p a c i t i e s C a t i o n adsorption appears to be important i n the formation of c a t i o n i c bridges as a mechanism f o r b i l e a c i d , f a t t y acid and m i n e r a l a d s o r p t i o n i n the upper i n t e s t i n e (20), The a f f i n i t y o f copper f o r c a r b o x y l i c a c i d groups has been employed i n p e c t i n p r e c i p i t a t i o n (10, 21) and i n determinations of c a t i o n exchange c a p a c i t y (CEC) o f d i e t a r y f i b e r (9^), Table II contains e s t i m a t e s o f c a t i o n exchange c a p a c i t y w i t h p e c t i n h a v i n g the g r e a t e s t amount of copper adsorption per unit weight. Cabbage, with one t h i r d of i t s d i e t a r y f i b e r value d e r i v i n g from p e c t i n , s i m i l a r i l y has a s u b s t a n t i a l exchange value (92 meq/100 g). The c o a r s e b r a n , a l f a l f a and soybean h u l l s were n e u t r a l d e t e r g e n t f i b e r preparations using the amylase m o d i f i c a t i o n o f R o b e r t s o n and Van Soest (22) and were thus f r e e o f p e c t i n and s t a r c h . Under t h e s e c o n d i t i o n s l i g n i n w i t h b o t h c a r b o x y l and h y d r o x y l groups on the phenylpropane u n i t s has a predominant r o l e i n CEC, D i f f e r e n c e s among d i e t a r y f i b e r s are a l s o d e r i v e d from the h e m i c e l l u l o s e f r a c t i o n i n c l u d i n g the g l u c u r o n i c and g a l a c t u r o n i c acid content. Sugar r e s i d u e s o f the g l u c u r o n i c a c i d become a v a i l a b l e f o r m e t h y l a t i o n , a m i d a t i o n or c a t i o n i c complexes dependent on the degree o f o x i d a t i o n o f the t e r m i n a l h y d r o x y l . T h i s f r a c t i o n appears to be the p r i m a r y s o u r c e o f copper a d s o r p t i o n i n wheat b r a n . A l f a l f a and soybean h u l l s have more moderate exchange v a l u e s o f 36 and 18 meq/100 g r e s p e c t i v e l y . The g l y c i n e M a i l l a r d p r o d u c t exchange v a l u e o f 37 meq/100 g i s primarily a lignin-like effect. Wood c e l l u l o s e and P s y l l i u m are r e l a t i v e l y inert polysaccharides with very l i t t l e copper adsorption, P r o p o l c o u l d not be a c c u r a t e l y measured s i n c e i t forms a g e l . This prevents removal of excess copper. Addition o f y t t e r b i u m w i t h the p r o p a n o l does not cause p r e c i p i t a t i o n . This i s supportive evidence that the exchange would be near zero. Summary D a t a have been p r e s e n t e d e m p h a s i z i n g the v a r i a b i l i t y o f d i e t a r y f i b e r sources. Water h o l d i n g c a p a c i t y estimates d i f f e r e d by technique as w e l l as source. The most important advantage of the osmotic s u c t i o n technique i s that the measurement includes the w a t e r s o l u b l e d i e t a r y f i b e r components. T h i s i s a more r e a l i s t i c model of the d i g e s t i v e t r a c t . The human f e c a l batch iji
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v i t r o fermentation method can be used to study f i b e r d i g e s t i o n . V a r i a t i o n i n degree o f f e r m e n t a t i o n a r e due t o d i f f e r e n c e s i n f i b e r c o m p o s i t i o n and/or i n o c u l u m s o u r c e . C a t i o n exchange c a p a c i t y e s t i m a t e s also emphasize chemical composition d i f f e r e n c e s w i t h l i g n i n and u r o n i c a c i d s b e i n g the p r i m a r y adsorption s i t e s .
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