15 Dietary Fiber from Citrus Wastes: Characterization S. V . T I N G and R. L . R O U S E F F Florida Department of Citrus, University of Florida Agricultural Research and Education Center, Lake Alfred, FL 33850
Nearly 5 million metric tons of citrus peel and pulp waste materials are produced annually in the manufacturing of concentrated citrus juices in Florida. Although these materials are currently processed into cattle feed, they are rich in polysaccharides and can be considered as a source of dietary fiber. Alcohol-insoluble solids (AIS) of 3 different fractions of the processing residues discharged from commercial juice manufacturing equipment were analyzed for pectin, easily hydrolyzable polysaccharides, and cellulose. Two varieties of orange and one of grapefruit that are the predominant citrus fruit used in juice production were included in this study. Between 50-60% of the AIS of a l l samples was removed by a 0.05 M NaOH treatment and about 10-12% by a subsequent acid hydrolysis of the residue. The "cellulose" fraction remaining was nearly completely dissolved in 12 M H SO . Protein and ash comprised about 10% of the AIS and were found mostly in the dilute alkali extract. The neutral sugars were determined by HPLC and GLC of their silylated derivatives of the hydrolysate after removal of acid. Arabinose and galactose were found in the dilute alkali extract. Xylose, arabinose, galactose and glucose were a l l present in the easily hydrolyzable fraction. Glucose was the main sugar component in the fraction dissolved in the 12 M H SO . Carbazol colorimetric method was used to determine the uronic acids in the fractions. 2
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0097-6156/83/0214-0205$06.00/0 © 1983 American Chemical Society
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206
UNCONVENTIONAL SOURCES OF DIETARY FIBER
C i t r u s j u i c e processing i s one of the more Important food i n d u s t r i e s i n the world. Annual production of f r o z e n concentrated orange j u i c e i n the United States and B r a z i l , the two l e a d i n g c i t r u s concentrate producing c o u n t r i e s , u t i l i z e n e a r l y 15 m i l l i o n tons of c i t r u s f r u i t with an enormous q u a n t i t i t y of processing r e s i d u e . Only about 45-50% of g r a p e f r u i t and 50-55% of oranges are recovered as j u i c e ( 1 ) . The r e s i d u e from the f r u i t a f t e r the j u i c e i s removed by mechanical e x t r a c t o r s c o n s i s t e d of the p e e l , the c e n t r a l core and the membrane, and the coarse pulp which i s removed by a screen-type f i n i s h e r . Seeds are included i n the r e s i d u e when seedy v a r i e t i e s of c i t r u s are processed. A d i s t r i b u t i o n of component p a r t s or oranges and g r a p e f r u i t i s shown i n Table I . Table I .
Fruit and variety 'Pineapple orange
Average d i s t r i b u t i o n of component p a r t s of c i t r u s f r u i t s .
Year
1
Peel (Flavedo & albedo)
Membrane, core & seeds (Rag)
Juice sacs (pulp)
Juice
1960- 61 1961- 62
19.9 22.7
17.5 17.7
23.6 21.9
39 .0 37 .7
'Valencia' orange
1959- 60 1960- 61
19.2 20.8
9.8 12.8
19.9 23.7
51 .1 42 .7
'Marsh' grapefruit
1961- 62 1962- 63
27.2 29.2
13.4 14.7
21.5 19.1
37 .9 37 .0
Rouse et a l . (2, 3_, 4 ) . Most of the c i t r u s r e s i d u e s are now manufactured into animal feed. The production of t h i s product i s over a m i l l i o n tons annually ( 1 ) . Although the processing of these r e s i d u e s to such by-products g i v e s a f a i r r e t u r n to the producers, more s o p h i s t i c a t e d products can be obtained from them which are s u i t a b l e as human food. Between 45-75% of the t o t a l s o l i d s i n the c i t r u s peel and membrane has been found to be i n s o l u b l e i n a l c o h o l (Table I I ) , and the main p o r t i o n of t h i s a l c o h o l - i n s o l u b l e m a t e r i a l i s complex carbohydrate (_5) . The a l c o h o l - i n s o l u b l e s o l i d s (AIS) of the coarse pulp i s a l s o l a r g e l y complex carbohydrate, and it a l s o contains p r o t e i n s and inorganic s a l t s Ç6). Of the complex carbohydrates, p e c t i n i s by f a r the most important f r a c t i o n . Rouse et a l . (7) d i v i d e d the p e c t i c substances i n t o water s o l u b l e , oxalate s o l u b l e , and sodium hydroxide s o l u b l e f r a c t i o n s i n t h e i r study. Eaks and S i n c l a i r (8) found a d i f f e r e n c e i n the composition of the h e m i c e l l u l o s e - c e l l u l o s e f r a c t i o n between the mature and immature 'Valencia' orange p e e l AIS.
8.5 9.3 2.5
16.6 18.0 10.0
83.4 82.0 90.0
Peel Rag Pulp
'Marsh' grapefruit
15.2 10.4 6.4
23.4 18.4 15.8
76.6 81.6 84.2
Peel Rag Pulp
'Valencia' orange
(g/100 g) 13.0 10.2 3.6
Alcohol insoluble s o l i d s (AIS)
22.9 18.8 10.5
77.1 81.2 89.5
1
Peel Rag Pulp
'Pineapple orange
Water
Total solids
6.2 6.8 14.0
5.6 9.7 9.6
5.2 8.1 11.5
Total p r o t e i n as % AIS
Proximate composition of c i t r u s waste m a t e r i a l .
Component
Fruit and variety
Table I I .
2.3 1.8 1.5
2.9 1.8 1.5
2.7 2.0 1.6
Ash as % AIS
208
UNCONVENTIONAL SOURCES OF DIETARY FIBER
The water-soluble p e c t i n s include the more h i g h l y methoxylated p e c t i c a c i d , while the o x a l a t e - s o l u b l e p e c t i c compounds are the calcium pectates of the middle l a m e l l a s of the c e l l w a l l . Sodium hydroxide e a s i l y hydrolyzes both the methoxylated pectates and the calcium p e c t a t e . Thus, e x t r a c t i o n with d i l u t e a l k a l i of the AIS would i n c l u d e a l l these p e c t i c f r a c t i o n s . The h y d r o p h i l i c nature of these carbohydrate c o n s t i t u e n t s could be an important f a c t o r i n c o n s i d e r i n g them as a source of d i e t a r y f i b e r s . Ferguson and Fox (9) studied the p o s s i b i l i t i e s of manufacturing a high f i b e r food product from c i t r u s waste. Belshaw (10) suggested the use of a f l o u r manufactured from the p e e l s of oranges and g r a p e f r u i t i n bread to i n c r e a s e the f i b e r content. Such f l o u r can also incorporate some d e s i r a b l e f l a v o r . Recently, Braddock and Graumlich (11) analyzed the i n s o l u b l e carbohydrates of d i f f e r e n t f r u i t p a r t s of orange and g r a p e f r u i t and discussed the p o s s i b l e u t i l i z a t i o n of c i t r u s p e e l as a source of d i e t a r y fiber. The purpose of t h i s work i s to study the composition of the r e s i d u e s from c i t r u s processing and to devise a method f o r separating and estimating the nature and q u a n t i t i e s of v a r i o u s f r a c t i o n s of the complex carbohydrates from such m a t e r i a l s . Analyzing C i t r u s Residue Processing r e s i d u e s . The processing r e s i d u e s used i n t h i s study were c o l l e c t e d from an FMC Model 091 i n - l i n e j u i c e e x t r a c t o r and from an FMC Model 035 j u i c e f i n i s h e r (FMC Corporation, C i t r u s Machinery D i v i s i o n , Lakeland, F L ) . The j u i c e e x t r a c t o r discharges the peel p a r t i a l l y shredded, while the c e n t r a l core, s e c t i o n membrane and the e n t i r e inner part of the f r u i t are pressed i n the c e n t r a l e x t r a c t i o n tube. A f t e r the j u i c e i s expressed and passed through a p r e f i n i s h e r i n the c e n t r a l e x t r a c t i o n tube, the r e s i d u e discharged i s c o l l e c t i v e l y c a l l e d the r a g . The p r e f i n i s h e d j u i c e then passes through a f i n e screen (.020 in.) i n the f i n i s h e r which removes the pulp. The pressure a p p l i e d to the f i n i s h e r which i s a d j u s t a b l e can determine the amount of l i q u i d r e t a i n e d by the pulp. Two c u l t i v a r s of orange, 'Pineapple' and 'Valencia' and one of g r a p e f r u i t , 'Marsh' were used i n t h i s study as they are the important c u l t i v a r s i n j u i c e manufacturing. Sample p r e p a r a t i o n . Approximately 2 kg of each r e s i d u e component was obtained from each c u l t i v a r . The peel and rag were s e p a r a t e l y passed through a hand-operated g r i n d e r . A f t e r thorough mixing, d u p l i c a t e 100 g a l i q u o t s of each of these components were weighed out and placed i n separate quart Mason j a r s each c o n t a i n i n g about 400 ml of 95% ethanol. The pulp
15.
TING AND ROUSEFF
Dietary Fiber from Citrus Wastes
209
samples were treated i n the same manner but without p r i o r g r i n d i n g . These alcohol-preserved samples were used i n the p r e p a r a t i o n of AIS. T o t a l s o l i d s was determined by d r y i n g the samples i n a vacuum oven at 75 C f o r 16 hours followed by 2 hours a t 100 C i n a r e g u l a r f o r c e d - d r a f t drying oven. P r e p a r a t i o n of a l c o h o l - i n s o l u b l e s o l i d s . The sample preserved i n a l c o h o l was placed i n a Waring blender and macerated a t high speed f o r 10 minutes and f i l t e r e d through a Whatman No. 1 f i l t e r paper i n a Buchner funnel under vacuum from a water a s p i r a t o r to remove the a l c o h o l s o l u b l e s which were mostly sugars and organic a c i d s . The r e s i d u e on the f i l t e r was returned to the Waring blender with another 400 ml of 80% ethanol. I t was again ground f o r 10 minutes and f i l t e r e d . T h i s operation was repeated, and the r e s i d u e f i n a l l y extracted with 200 ml of acetone. The white r e s i d u e was a i r d r i e d and f i n a l l y d r i e d i n an oven a t 75 C overnight. The weight was considered as percent AIS. Each sample was ground i n a Wiley m i l l to pass a 40 mesh screen and stored i n t i g h t l y stoppered b o t t l e s u n t i l analyzed. T o t a l n i t r o g e n of the AIS was determined by a semi-micro K j e l d a h l procedure (6) on a 0.50 g sample and the p r o t e i n content c a l c u l a t e d u s i n g the f a c t o r 6.25. The ash content was determined on a 0.5 g sample of the AIS i g n i t e d i n a m u f f l e furnace a t 550 C f o r 16 hours. E x t r a c t i o n of p e c t i c substance f r a c t i o n . A one-half gram of the AIS was placed i n a 100 ml volumetric f l a s k , moistened with about 1 ml of 95% ethanol and made to volume with 0.05 NaOH. The contents were allowed to stand a t room temperature f o r 2 hours with o c c a s i o n a l shaking. The mixture was poured onto a f i l t e r i n g c l o t h (Mira-Cloth, American Can Co.) supported on a Buchner f u n n e l . F i f t y ml of the f i l t r a t e was t r a n s f e r r e d i n t o a 100 ml volumetric f l a s k and a c i d i f i e d with 2.5 ml of cone, a c e t i c a c i d . The content was made to volume and a l i q u o t s were used i n the a n a l y s i s of p e c t i n and the accompanied monosaccharides. The r e s i d u e was washed with a copious amount of water followed by 95% ethanol. I t was r i n s e d from the f i l t e r c l o t h with a stream of acetone i n t o a beaker and the mixture f i l t e r e d through a tared s i n t e r e d g l a s s f u n n e l . A f t e r a i r - d r i e d , i t was d r i e d i n an oven a t 75 C and weighed. The d i f f e r e n c e i n weight was considered as that removed by the d i l u t e a l k a l i . A flow diagram d e s c r i b i n g the f r a c t i o n a t i o n , h y d r o l y s i s and subsequent a n a l y s i s of the f r a c t i o n s a r e shown i n Figure 1. Uronic a c i d a n a l y s i s . Carbazol c o l o r i m e t r i c method (7) was used f o r the determination of t o t a l p e c t i n s i n the d i l u t e a l k a l i e x t r a c t of the a l c o h o l - i n s o l u b l e s o l i d s . The s o l u t i o n has to be d i l u t e d i n order f o r the c o n c e n t r a t i o n to f a l l w i t h i n the range of determination by t h i s method.
210
UNCONVENTIONAL SOURCES OF DIETARY FIBER
CITRUS
WASTE
100g 1 ETHANOL
ALCOHOL SOLUBLES
ALCOHOL NSOLUBLES 0 .5 JNNoOH 2 HRS AMBIENT TEMP.
#
FRACTION
PECTIC
RESIDUE 0.5 100
e
0.5 M
MlH S0 2
4
C FOR I HR ACID REMOVED BoCO,
72 % HoSO
