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1 Nutrients and Nutrition of Citrus Fruits
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S. V. TING Florida Department of Citrus, University of Florida, Institute of Food and Agricultural Sciences, Agricultural Research and Education Center, P.O. Box 1088, Lake Alfred, FL 33850
A l l foods for humans must provide some life-sustaining elements. These elements are the nutrients that supply energy after being metabolized in the body and those that are essential for the body to carry on this metabolism. Other qualifications for a food are i t s psychological and social effects. Many foods are eaten as a habit, custom, or t r a d i t i o n ; but all foods must possess acceptable physical attributes, i.e., color and texture, and desirable taste and p a l a t a b i l i t y . The acceptance of fruits as a staple i n human diet has only been practiced since the past century because of their perishability as fresh produce. In most instances, they are used as desserts because most of them are sweet in taste and because of their high economic values. With the advent of canning and other preservation industries and with the better knowledge of n u t r i t i o n , the use of fruits as staple foods has become more prevalent, especially in developed countries. The use o f c i t r u s f r u i t , e s p e c i a l l y oranges, d r a m a t i c a l l y increased i n the U.S. a f t e r World War II ( 1 ) because o f the i n t r o d u c t i o n o f f r o z e n concentrated orange j u i c e (FCOJ) t o the market. C i t r u s f r u i t s , being s u b t r o p i c a l products, d i d not enjoy the p o p u l a r i t y o f other f r u i t s , e . g . a p p l e s , because the l o c a l i t y of production were u s u a l l y not near the world population centers and because o f p e r i s h a b i l i t y o f c i t r u s f r u i t s during storage. T h e i r s u s c e p t i b i l i t y to p h y s i o l o g i c a l d i s o r d e r s and t o storage d i s e a s e s , e s p e c i a l l y molds and r o t s , made the cost a d e t e r r i n g f a c t o r t o consumers. These shortcomings were overcome w i t h the development o f FCOJ, which i s a t t r a c t i v e i n c o l o r , possesses f u l l f r e s h orange f l a v o r and g r e a t l y reduces the cost o f t r a n s p o r t a t i o n with n e a r l y no storage l o s s . Research i n the area o f storage disease c o n t r o l and i n t r a n s p o r t a t i o n o f f r e s h f r u i t have a l s o l e d t o increased consumption i n many developed c o u n t r i e s . C i t r u s f r u i t s and t h e i r products a r e important sources o f vitamin C i n the American d i e t , and are becoming i n c r e a s i n g l y more important t o other developed and developing c o u n t r i e s . Consumer awareness o f the h e a l t h f u l aspects o f c i t r u s , together 0-8412-0595-7/80/47-143-003$05.00/0 © 1980 A m e r i c a n Chemical Society
Nagy and Attaway; Citrus Nutrition and Quality ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
CITRUS NUTRITION AND QUALITY
4
w i t h i t s appealing c o l o r and d e l i g h t f u l aroma and t a s t e , makes c i t r u s products the most popular o f the processed f r u i t products. The improved technology of c i t r u s f r u i t p r o d u c t i o n , p r o c e s s i n g , storage and t r a n s p o r t a t i o n has placed the product w i t h i n economic reach of more people than ever. The c o s t of a s e r v i n g (6 f l o z , 177 ml) of orange j u i c e , g r a p e f r u i t j u i c e , and other j u i c e s were 9 . 7 , 9.0 and 10.7 c e n t s , r e s p e c t i v e l y , as reported i n September 1979 (2:). Table I shows the gallonage and expenditures f o r c i t r u s f r u i t j u i c e s and f r u i t - f l a v o r e d d r i n k s , i n c l u d i n g orange f l a v o r e d , i n the U.S. during September.
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Table I. Consumption of F r u i t J u i c e s and Other F r u i t Beverages i n the United States and T h e i r Total Values-September, 1979 M i l l i o n s of gallons
M i l l i o n s of dollars
Orange j u i c e Grapefruit juice A l l other f r u i t j u i c e s
47.0 6.2 26.1
103.7 12.5 70.0
Orange f l a v o r d r i n k s A l l other f l a v o r d r i n k s
11.6 18.4
16.2 26.2
Source:
{2)
The H i s t o r i c a l Role of C i t r u s F r u i t i n the Human D i e t C i t r u s i s g e n e r a l l y regarded as one of the most important sources of a s c o r b i c a c i d . The r e l a t i o n between c i t r u s f r u i t and a n t i s c o r b u t i c a c t i v i t y a c t u a l l y was f i r s t reported by a Hungarian p h y s i c i a n , Kramer, i n 1732 (3J. The dreadful disease of scurvy was found to be completely prevented w i t h the i n g e s t i o n of green vegetable or pulp of c i t r u s f r u i t . The use of lime and orange i n the d i e t of seamen i n the Royal B r i t i s h A d m i r a l i t y was the o r i g i n of the name "Limey" f o r B r i t i s h s a i l o r s . Harden and Z i l v a (4) found a concentrated f r a c t i o n from the lemon f r u i t t h a t possessed strong reducing p r o p e r t i e s . L a t e r Szent Gyorgyi (5) i s o l a t e d the a n t i s c o r b u t i c substance i n c r y s t a l l i n e form from pepper and c i t r u s fruit. Many vegetables and f r u i t s , other than c i t r u s , c o n t a i n a s c o r b i c a c i d . I t was estimated t h a t c i t r u s f r u i t s and tomatoes p r o vided only 18% of the t o t a l v i t a m i n C i n t a k e i n the American d i e t during the decade of 1910. These two f r u i t s s u p p l i e d 41% of v i t a m i n C i n 1956-58 ( 6 ) . Today orange j u i c e alone provides n e a r l y 60% of the U.S. Recommended D a i l y Allowance (U.S. RDA) of v i t a m i n C i n the American d i e t ( 7 ) . C i t r u s f r u i t s and t h e i r products are now recognized as an important food i n the human d i e t , not only because of t h e i r v i t a m i n C c o n t e n t s , but a l s o because of t h e i r other food a t t r i butes, such as t h e i r pleasant aroma, appealing c o l o r , and p l e a s ant t a s t e of a p p r o p r i a t e r a t i o s of sweetness and t a r t n e s s
Nagy and Attaway; Citrus Nutrition and Quality ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
1.
TING
Nutrition
of Citrus
5
Fruits
and because of the awareness by the p u b l i c of t h e i r n u t r i t i v e values and the other n u t r i e n t s they c o n t a i n . Macronutrients i n C i t r u s The energy-supplying n u t r i e n t s are g e n e r a l l y carbohydrates, p r o t e i n and f a t . While c i t r u s products provide l i t t l e p r o t e i n and f a t , t h e i r c o n t r i b u t i o n of carbohydrate i s an e s s e n t i a l p a r t of the n u t r i t i v e value of c i t r u s . The proximate composition of several kinds of c i t r u s f r u i t s (8) are shown i n Table II. Because
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Table II.
Proximate Composition of C i t r u s F r u i t s (g/100 g)
Fruits
Moisture
Protein
Orange Whole f r u i t Juice
86.4 88.3
.7 .6
.2 .2
12.0 10.5
.5 .1
.7 .4
Grapefruit Whole f r u i t Juice Segment
88.9 90.2 91.3
.5 .5 .6
.1 .1 .1
10.1 9.0 7.6
.2 — .2
.4 .2 .4
Tangerine Whole f r u i t Juice
87.0 88,9
.8 J>
.2 .2
11.6 10.1
.5 .1
.4 .3
Source:
Fat
Carbohydrate Sol. Insol.
Ash
(8)
the compositions these values can main p a r t of the hydrate and most amino a c i d s .
vary g r e a t l y due to f r u i t m a t u r i t y and v a r i e t y , f l u c t u a t e c o n s i d e r a b l y from a c t u a l samples. The c a l o r i c values s u p p l i e d by c i t r u s i s from c a r b o of the p r o t e i n value i s a c t u a l l y from f r e e
Carbohydrates Simple sugars. The main p o r t i o n of carbohydrates i n c i t r u s f r u i t are the three simple sugars: sucrose, glucose and f r u c t o s e (9). Together they represent about 80% of the t o t a l s o l u b l e s o l i d s of orange j u i c e (jLO), and the r a t i o s of sucrose: glucose: f r u c t o s e are g e n e r a l l y about 2:1:1 (11_). In over-mature e a r l y and mid-season F l o r i d a oranges, and i n t a n g e r i n e s , the r a t i o s of sucrose to reducing sugars have been found to increase but not i n the l a t e season F l o r i d a oranges (12). In g r a p e f r u i t , the sucrose to non-reducing sugar r a t i o s are l e s s than 1. Most of the f r e e sugars i n lemon and lime j u i c e s are reducing sugars (Table I I I ) and the main s o l u b l e s o l i d i n these f r u i t j u i c e s i s c i t r i c a c i d . In an a c i d i c medium such as c i t r u s j u i c e s , sucrose can be e a s i l y hydrolyzed; t h i s f a c t may account f o r the low sucrose values sometimes found i n canned j u i c e s s u b j e c t to long term storage.
Nagy and Attaway; Citrus Nutrition and Quality ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
CITRUS NUTRITION AND QUALITY
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Table I I I .
Average Sugar Composition of C i t r u s J u i c e s (g/100 g) Total Total Sugars Reducing Sucrose Glucose Fructose
Fruit Orange Grapefruit Tangerine Lemon Lime
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Source:
2.03 1.66 1.13 1.40
2.48 1.75 1.54 1.35
4.51 3.41 2.67 2.75 3.48
4.81 2.56 6.53 0.41 0
9.32 5.97 9.20 3.16 3.48
(12)
The main sugars i n the peel of c i t r u s f r u i t s are a l s o sucr o s e , glucose and f r u c t o s e , although a t r a c e of f r e e xylose was reported (13J. The r e l a t i v e amounts of these sugars are shown i n Table IV. These f r e e sugars are a l s o the major i n g r e d i e n t s of c a t t l e feed manufactured from c i t r u s peel and processing r e s i d u e , e s p e c i a l l y when c i t r u s molasses i s blended i n t o the feed (14). Table IV.
Average Sugar Composition i n C i t r u s Peel Sucrose Fructose ( Ï dry weight)
Total sugar
F r u i t and variety
Time of harvest
Hamlin oranges Pineapple oranges V a l e n c i a oranges Marsh seedless grapefruit
December January April
10.3 10.8 10.9
16.8 21.2 10.9
11.8 13.9 16.5
38.8 45.8 37.4
December
11.6
12.8
14.3
38.7
Source:
Glucose
(13)
Polysaccharides and Polyuronides. These compounds are found i n the a l c o h o l - i n s o l u b l e f r a c t i o n and c o n s i s t of p e c t i c subs t a n c e s , h e m i c e l l u l o s e , c e l l u l o s e and l i g n i n . The recent i n t e r e s t of d i e t a r y f i b e r i n human n u t r i t i o n has placed s p e c i a l emphasis upon these substances i n foods. Between 45 and 75 percent of the t o t a l s o l i d s i n c i t r u s peel and membrane i s not s o l u b l e i n a l c o hol (13), and most of these a l c o h o l i n s o l u b l e s o l i d s c o n s i s t e d of polysaccharides or polyuronides (15). In orange f r u i t , the peel i s not g e n e r a l l y eaten except i n such s p e c i a l i t y products as candied peel or marmalade. The b i t t e r n e s s i n the peel and the segment membrane of g r a p e f r u i t i s due to n a r i n g i n and l i m o n i n (12) and makes t h a t p o r t i o n of the f r u i t u n p a l a t a b l e . Roe and Bruemmer (16) developed a method to d e b i t t e r the albedo by vacuum i n f u s i o n of t h i s t i s s u e w i t h n a r i n g i n a s e , thus, rendering the e n t i r e f r u i t , except the f l a v e d o , e d i b l e . A d i s t r i b u t i o n of the v a r i o u s component parts of d i f f e r e n t c i t r u s f r u i t i s shown i n Table V (17, 18). Approximately 20 percent of the weight of orange and 30 percent of t h a t of g r a p e f r u i t i s peel ( i n c l u d e s both
Nagy and Attaway; Citrus Nutrition and Quality ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
1.
TING
Nutrition
of
Citrus
7
Fruits
the albedo and f l a v e d o ) , and about 10 percent of the weight of the f r u i t i s segment membrane.
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Table V. D i s t r i b u t i o n of the Components of Oranae and Grapef r u i t (% Fresh Weight) Fruit
Peel
Segment Membrane
Juice vesicles
Seeds
Oranges Pineapple Valencia
19.9 19.2
13.7 9.0
62.6 71.0
3.8 0.8
Grapefruit Seedy
27.2
10.0
59.4
3.4
Source:
(17,
18)
About 30 percent of the polysaccharides i n the peel and pulp may be c l a s s i f i e d as c e l l u l o s e (Table V I ) . No separate c e l l u l o s e f r a c t i o n can be d i s t i n g u i s h e d from the j u i c e p o l y s a c c h a r i d e . Over 50 percent of the t o t a l polysaccharide i n the p e e l , 60 p e r cent or more of t h a t of the pulp and over 90 percent of t h a t of the j u i c e are e x t r a c t e d w i t h the p e c t i c substance, the balance being h e m i c e l l u l o s e . Separate h y d r o l y s i s of these f r a c t i o n s , i n d i c a t e d t h a t some monosaccharides, such as arabinose and g a l a c t o s e , were found i n a l l f r a c t i o n s . Xylose occurred mostly i n the h e m i c e l l u l o s e f r a c t i o n whereas g a l a c t u r o n i c a c i d and glucose were the main monosaccharides i n the p e c t i c substances and the c e l l u l o s e f r a c t i o n s , r e s p e c t i v e l y (Table V I I ) . D i e t a r y F i b e r . The d e f i n i t i o n of d i e t a r y f i b e r i s not very s p e c i f i c and i t g e n e r a l l y includes t h a t group of substances found i n the a l c o h o l - i n s o l u b l e f r a c t i o n s of c i t r u s f r u i t s . The crude f i b e r value as c o n v e n t i o n a l l y reported i s only t h a t p o r t i o n of the d i e t a r y f i b e r c o n s i s t i n g of p a r t i a l l y p u r i f i e d c e l l u l o s e and l i g n i n . Although these compounds are not attacked by the human d i g e s t i v e system as they t r a v e l through the alimentary t r a c t , they are s u b j e c t to p a r t i a l h y d r o l y s i s by the m i c r o f l o r a i n the lower p a r t of the d i g e s t i v e system. The b e n e f i t of d i e t a r y f i b e r has been a t t r i b u t e d to i t s a b i l i t y to decrease the t r a n s i t time of food through the g a s t r o i n t e s t i n a l t r a c t (19). Some f r a c t i o n of the d i e t a r y f i b e r such as p e c t i n has Been a s s o c i a t e d with the property of lowering the c h o l e s t e r o l i n mammals (20), and the methyl content of the polymer has been reported to be c o r r e l a t e d to t h i s c a p a b i l i t y . C i t r u s p e c t i n has a methyl content of 7-10 percent as compared to 6-9 percent f o r apple p e c t i n . P e c t i n from f l e s h y f r u i t s such as strawberry has only 0.2 percent (21). The polysaccharides of the peel and pulp of c i t r u s f r u i t s
Nagy and Attaway; Citrus Nutrition and Quality ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
CITRUS NUTRITION AND QUALITY
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Nagy and Attaway; Citrus Nutrition and Quality ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
Nagy and Attaway; Citrus Nutrition and Quality ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
(13, 17)
+ ++ +++ t --
Juice
Source:
t
+
P e c t i c substances Hemicellulose and Cellulose
Pulp t
t
t
t
Rhamnose
= 50% = trace = not found
+
t ++ +
+ +
P e c t i c substances Hemicellulose Cellulose
Peel
++
t ++ +
+ + +
Fractions
Xylose
Arabinose
P e c t i c substances Hemicellulose Cellulose
Component part + +++ t + +++ ——
+ + t + + + + ++
Glucose
Galactose
+
+++
+++ +
+++ t
t +
t +
Other uronic acids
Polysaccharide
Galacturonic acid
Table VII. R e l a t i v e Amounts of Various Monosaccharides Found i n the Hydrolysate of F r a c t i o r ι of the A l c o h o l - I n s o l u b l e S o l i d s of C i t r u s P e e l , Pulp and J u i c e
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H ι—ι
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10
CITRUS NUTRITION AND QUALITY
provide a good source of d i e t a r y f i b e r . Church and Church (22) reported t h a t an average s i z e orange could supply about 0.8 g of d i e t a r y f i b e r , whereas 236 ml (8 f l oz) s e r v i n g of j u i c e contains only o n e - h a l f t h a t amount.
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Organic A c i d s . The most predominate s o l u b l e c o n s t i t u e n t s of c i t r u s j u i c e , f o l l o w i n g the sugars, are the organic acids and t h e i r s a l t s . They represent about 10 percent of the t o t a l s o l u b l e s o l i d s i n c i t r u s j u i c e s . The proper r a t i o s of sugar and the acids and t h e i r b u f f e r s give the c i t r u s j u i c e s t h e i r d e l i g h t f u l t a s t e . The organic acids of c i t r u s f r u i t i n c l u d e a group of c a r b o x y l i c acids (23) w i t h d i f f e r e n t a c i d s predominant i n various component parts of the f r u i t . C i t r i c a c i d i s the main a c i d i n the j u i c e , r e p r e s e n t i n g from 80 percent of the t o t a l a c i d i t y i n j u i c e from ripened oranges, about 90 percent of t h a t of g r a p e f r u i t and n e a r l y a l l of t h a t of lemon. The pH of the j u i c e g r e a t l y a f f e c t s the sourness of the product (24) and i s , i n t u r n , a f f e c t e d by the c a t i o n s , e s p e c i a l l y potassium. The major a c i d s i n c i t r u s peel are m a l i c , o x a l i c (25), malonic (26), and qui n i c (27). Organic acids are metabolized i n the body and should be considered as a source of energy. In the case of the s a l t s of these a c i d s , the organic p o r t i o n i s metabolized l e a v i n g the f r e e c a t i o n s to be combined with other anions. Thus, c i t r u s j u i c e i s c l a s s e d as an a l k a l i n e food (28). Protein The amount of p r o t e i n i n c i t r u s f r u i t i s r e l a t i v e l y low (Table I I ) , and the j u i c e and peel have about the same amount (29). Much of the value t h a t i s considered as p r o t e i n i s e i t h e r f r e e amino a c i d s or non-protein c o n s t i t u e n t s which c o n t a i n n i t r o g e n . The t o t a l n i t r o g e n of orange j u i c e s was found to increase w i t h the m a t u r i t y of the f r u i t and ranged between .068 to .120 g per 100 ml (30). The a c t u a l p r o t e i n values obtained by Clements (31) were about 20 percent of the acetone powder. Nearly 30 percent of the a l c o h o l - i n s o l u b l e s o l i d s of j u i c e and about 20 percent of t h a t of v e s i c u l a r pulp were found to be p r o t e i n as determined by the K j e l d a h l procedure (32). These values are the actual p r o t e i n that was p r e c i p i t a t e d by a l c o h o l and are only a f r a c t i o n of the t o t a l p r o t e i n values u s u a l l y reported f o r orange j u i c e ( 8 ) . The main source of p r o t e i n s i n c i t r u s j u i c e i s probably i n the form of enzymes and the p l a s t i d s . At l e a s t 47 d i f f e r e n t enzymes have been reported to occur i n c i t r u s f r u i t s (33). C i t r u s f r u i t s a l s o contain several phenolic amines (34), some of which such as synepherine, may have p h y s i o l o g i c a l importance (35). Among the v a r i o u s f r e e amino acids reported i n c i t r u s j u i c e s (32), a r g i n i n e i s the only semi-indispensable amino a c i d t h a t occurs i n moderate amounts. The m a j o r i t y of amino acids i n c i t r u s are considered to be nonessential according to the c l a s s i f i c a t i o n
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by Block and B o i l i n g (36). At most, the c o n t r i b u t i o n of f r e e amino acids i n c i t r u s j u i c e s to human n u t r i t i o n i s minimal. Lipids
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From a d i e t a r y s t a n d p o i n t , the c o n t r i b u t i o n of c i t r u s l i p i d s i s i n s i g n i f i c a n t ; and only between .06 and .09 percent has been found i n oranges (37). They a r e , however, of importance because of t h e i r e f f e c t s on the development of o f f - f l a v o r s (12), thus lowering the p a l a t a b i l i t y of these products. The near absence of l i p i d s i n c i t r u s makes i t a d e s i r a b l e food f o r those on a limited fat diet. Micronutrients in Citrus The term micro i s used purely to i n d i c a t e the p h y s i c a l quan t i t i e s r e q u i r e d i n n u t r i t i o n r a t h e r than f o r t h e i r importance. These n u t r i e n t s represent the v i t a m i n s , minerals and other sub stances t h a t have d i e t a r y s i g n i f i c a n c e . F a t - S o l u b l e Vitamins i n C i t r u s F r u i t Among the several vitamins i n t h i s c l a s s i f i c a t i o n , only v i t a m i n A i s present i n a p p r e c i a b l e q u a n t i t y as c a r o t e n o i d p r o v i t a m i n A i n c i t r u s (38). No v i t a m i n D has ever been reported i n c i t r u s nor any p l a n t v i t a m i n D p r e c u r s o r s , such as e r g o s t e r o l . Several of the s t e r o l s present i n c i t r u s f r u i t s are reported (39, 40, 4 1 ) , but they are not r e l a t e d to v i t a m i n D. Vitamin Ε. The amount of vitamin Ε i n c i t r u s i s n u t r i t i o n a l l y i n s i g n i f i c a n t . Braddock (42) reported only 0.1 mg i n 100 ml of orange j u i c e . The U.S. RDA f o r t h i s v i t a m i n i s 30 mg. Newhall and Ting (43) found as much as 1 mg i n 100 grams of f l a v e d o on a f r e s h weight b a s i s . Its a n t i o x i d a n t property plays an impor t a n t r o l e i n the keeping q u a l i t y of c i t r u s o i l s . Vitamin A. The v i t a m i n A of c i t r u s f r u i t i s e n t i r e l y i n the form of provitamin A c a r o t e n o i d s . The carotenes (both the alpha and beta form) and the cryptoxanthins i n c i t r u s are considered as the main p r e c u r s o r s . The carotenes are only a minor component of the t o t a l carotenoids of oranges ranging from about 5 to 10 p e r cent (44-46). In Dancy tangerine and V a l e n c i a oranges, c r y p t o xanthin i s the main v i t a m i n A precursor (47, 48). High perform ance l i q u i d chromatography (HPLC) has been usëcT to separate cryptoxanthin from other oxygenated carotenoids (413, 50). Only the b e t a - c r y p t o x a n t h i n has provitamin a c t i v i t y . P r i o r use of open column chromatography (51) could not separate the d i f f e r e n t monooxygenated c a r o t e n o i d s , thus g i v i n g higher v a l u e s . Using the HPLC method, Stewart (52) analyzed the carotenoids of several v a r i e t i e s of oranges and mandarins, and found t h a t b e t a - c r y p t o x a n t h i n i s the
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main provitamin A c i t r u s c a r o t e n o i d . The mandarin-type c i t r u s f r u i t i s a good source o f provitamin A, whereas oranges are s i g n i f i c a n t l y poorer (Table V I I I ) . With g r a p e f r u i t , the white v a r i e t i e s have no v i t a m i n A, but the pink and red v a r i e t i e s have been found to c o n t a i n a p p r e c i a b l e amounts o f beta-carotene (53, 54). Table V I I I .
Vitamin A Content o f C i t r u s J u i c e s % U. S. RDA 177 ml (6 f 1 . oz)
Fruit
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E a r l y and mid-season Late season Tangerines Dancy Honey Robinson Tangélos Orlando Grapefruit White Ruby Source:
1.6-2.7 1.7 19 64 23 4.7 None 30
(44, 52-54)
Wàter-Sôluble Vitamins i n C i t r u s F r u i t Vitamin C. Perhaps the most important c o n t r i b u t i o n of c i t rus f r u i t s to human n u t r i t i o n i s a t t r i b u t e d to t h e i r high ascorbic acid. Although c i t r u s products are not the only source f o r high contents of v i t a m i n C among f r u i t s and v e g e t a b l e s , t h e i r p o p u l a r i t y are l a r g e l y due to t h e i r d e s i r a b l e f l a v o r , t a s t e and c o l o r . In Table IX (55) are l i s t e d some of the more common f r u i t s and vegetables and t h e i r v i t a m i n C content. An average 177 ml (6 f l oz) s e r v i n g o f e i t h e r orange or g r a p e f r u i t j u i c e could provide 100 percent U.S. RDA of v i t a m i n C (60 mg). Tangerine j u i c e , a l though c o n t a i n i n g l e s s v i t a m i n C than other c i t r u s j u i c e s , would provide a s u b s t a n t i a l amount toward the recommended d a i l y a l l o w ance. Nearly 3/4 of a l l v i t a m i n C i n an orange and 5/6 i n a grapef r u i t i s found i n the peel ( 5 6 ) , however, c i t r u s j u i c e s and t h e i r products provide a major p o r t i o n of the v i t a m i n C i n the American d i e t . Considerable v a r i a t i o n s i n v i t a m i n C content can be found i n d i f f e r e n t c i t r u s products due to such f a c t o r s as v a r i e t y s , m a t u r i t y and c u l t u r a l p r a c t i c e s of the f r u i t (57) from which the products o r i g i n a t e and to the processing p r a c t i c e s and storage c o n d i t i o n s of these products before they reach the consumer. The decrease of a s c o r b i c a c i d w i t h f r u i t m a t u r i t y i s i l l u s -
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Table IX. Average Vitamin C Content of Freshly Harvested F r u i t s and Vegetables
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Product Guava Broccoli Green pepper Turnip greens Cabbage Orange Lemon Grapefruit Tangerine Potato Tomato Pineapple Banana Apple Peaeh Source:
mg/100 g 300 120 120 120 60 50 50 40 25 30 25 25 10 10 4
(55)
t r a t e d i n Figure 1. Vitamin C content of the e a r l y and midseason oranges are higher than t h a t of the l a t e season f r u i t . These f a c t s i n d i c a t e t h a t high q u a l i t y does not c o r r e l a t e w i t h higher n u t r i t i o n a l q u a l i t y . The more mature f r u i t and those of the l a t e season v a r i e t y are g e n e r a l l y regarded as of b e t t e r q u a l i t y . S t a b i l i t y of Vitamin C i n C i t r u s F r u i t s and J u i c e s The ease of o x i d a t i o n of reduced a s c o r b i c a c i d i s the b a s i s f o r a simple method of a n a l y s i s by dye t i t r a t i o n (58). Ascorbic a c i d as i t occurs i n c i t r u s j u i c e i s i n the reduced form. When subjected to o x i d a t i o n , a s c o r b i c a c i d changes to the dehydro form. Dehydroascorbic a c i d has nearly the same p h y s i o l o g i c a l a c t i v i t y as the reduced form and i s e a s i l y converted to the l a t t e r . Further o x i d a t i o n of the dehydroascorbic a c i d converts i t to 2 , 3 - d i k e t o gulonic a c i d . This r e a c t i o n i s i r r e v e r s i b l e , and the o x i d i z e d product i s devoid of b i o l o g i c a l a c t i v i t y . These r e a c t i o n s are shown i n Figure 2. Nearly 90 percent or more of the v i t a m i n C found i n c i t r u s j u i c e and c i t r u s products i s i n the reduced form (Table X) (59). Vitamin C i n c i t r u s j u i c e i s remarkably s t a b l e during the short period i t i s g e n e r a l l y kept a f t e r e x t r a c t e d from the f r u i t . F r e s h l y e x t r a c t e d orange and g r a p e f r u i t j u i c e s r e t a i n e d about 98 percent of t h e i r o r i g i n a l v i t a m i n C at 21,1 f o r 3 days. At 4.4 orange and g r a p e f r u i t j u i c e s r e t a i n e d 96 and 99 percent, respect i v e l y , of the o r i g i n a l amount a f t e r one-week's storage (60). Orange j u i c e t h a t has been heated to b o i l i n g f o r 15 min s t i l l r e t a i n e d about 96 percent of the v i t a m i n C (59). Atmospheric oxygen Q
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Ε
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§ 601-
2 ο <
1937-38
4 Οί
1936-37
ο
CÛ
oc o 20h o co < 30 1020 30 9 19 29 8 18 28 10 20 30 9 19 29 9 19 29 8 18 NOV DEC JAN FEB MAR APR MAY JUNE Figure 1.
Seasonal changes of ascorbic acid in juice of Florida oranges (average values for 2 seasons)
r-CO
I C-OH I C-OH
LOC-H I HO-CH I CH 0H 2
L-ASCORBIC ACID
Figure 2.
C0I CO I CO I H-COI HO-CH I CH 0H 2
DEHYDROASCORBIC ACID
COOH I CO I CO I H-C-OH I HO-CH I C H OH α
KETO-
GULONIC
ACID
Oxidation of 1-ascorbic acid
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Table X. Reduced Ascorbic A c i d and Dehydroascorbic C i t r u s Products
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Product
Source:
Acid i n
Reduced Dehydro (mg/100 g)
Canned orange j u i c e grapefruit juice tangerine j u i c e
43.1 32.0 27.2
0.9 1.8 1.1
FCOJ Orange segments G r a p e f r u i t segments
45.2 36.0 31.5
1.6 1.5 2.3
(59)
i s an e s s e n t i a l element i n the l o s s of vitamin C during long term storage. In sealed c o n t a i n e r s , there i s a r a p i d l o s s of the v i t a min followed by a slow decrease and becoming r e l a t i v e l y constant. Smoot and Nagy (61) found t h a t when stored at high temperature, the l o s s of vitamin C continues even a f t e r the headspace oxygen i s exhausted and that the d i k e t o g u l o n i c a c i d f r a c t i o n increased noticeably. A comparison of the v i t a m i n C content of f r e s h l y squeezed orange j u i c e and r e c o n s t i t u t e d FCOJ showed no marked d i f f e r e n c e i n the a s c o r b i c a c i d content during storage i n a home r e f r i g e r a t o r f o r up to one week (62) (Figure 3 ) . The l o s s of vitamin C i n i n t a c t oranges during t F e i r marketing period are not expected to be more than 10 percent of the o r i g i n a l (63). Thiamine (Vitamin B,). C i t r u s products are a l s o good sources of thiamine. They proviae comparable amount or more of v i t a m i n B. than foods t h a t are known s u p p l i e r s of t h i s n u t r i e n t on the basis of n u t r i e n t density as measured by the Index of N u t r i e n t Q u a l i t y (INQ) (64). L i s t e d i n Table XI are the INQ of some common foods. The vegetables have high INQ since the index i s c a l c u l a t e d on u n i t n u t r i e n t per KCal, but c i t r u s products are higher i n INQ than some other f r u i t s . Wholewheat bread, u s u a l l y c o n s i d ered as a source of vitamin B. has an INQ much lower than t h a t of orange j u i c e . Thiamine i n canned orange j u i c e i s r a t h e r s t a b l e . A l o s s of only 16-17 percent was reported a f t e r the product was stored f o r 18 months at 27° (65). F o l i c Acid ( f o l a t e ) . Chemically, f o l i c a c i d i s a p t e r y l glutamic acid.. The several forms that occur i n nature depend on the numbers of glutamic a c i d u n i t s and methyl groups i n the molec u l e s . Because of i t s usual low c o n c e n t r a t i o n , f o l i c a c i d i s g e n e r a l l y determined i n food m a t e r i a l s by the m i c r o b i o l o g i c a l assay with [Lactobacillus c a s e i and measured t u r b i d i m e t r i c a l l y or t i t r i m e t r i c a l l y . D e f i c i e n c y of t h i s v i t a m i n could r e s u l t i n
macroytic anemia (66). Orange j u i c e contains more f o l a t e than many other f r u i t j u i c e (Table X I I ) ( 6 7 ) .
Nagy and Attaway; Citrus Nutrition and Quality ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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A V E R A G E V A L U E S FOR P E R C E N T A S C O R B I C RETAINED AFTER REFRIGERATION lOOp^.
ACID
FRESH
24
48 H O U R S IN
72 96 REFRIGERATOR
120
Figure 3. Changes in ascorbic acid content of freshly squeezed orange juice and reconstituted orange juice concentrate during storage in refrigerator (62)
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Table XI. Comparison of Index of N u t r i e n t Q u a l i t y (INQ) of the Amino Acids of C i t r u s Products With Those of Various Other Common Foods Foods
Index of n u t r i e n t q u a l i t y (INQ)
M i l k (whole) M i l k (skim) Eggs ( h a r d - b o i l e d ) Beef Grapes (seedless) Grape j u i c e Apple (whole) Apple j u i c e Peaches (raw) Peaches (canned i n water) Banana G r a p e f r u i t segments G r a p e f r u i t j u i c e (canned) G r a p e f r u i t j u i c e (from concentrate) Orange (peeled) Orange j u i c e (canned) Orange j u i c e (from concentrate) Tangerine (peeled) Tangerine j u i c e (canned) B r o c c o l i (cooked) Cabbage (raw) Tomato (raw) Tomato j u i c e White bread Whole wheat bread Source:
1.20 2.12 1.00 .33 1.71 .74 1.00 .33 1.00 .53 1.20 2.00 1.40 2.00 4.00 2.83 3.83 2.50 2.40 4.00 5.00 5.60 5.33 2.94 2.50
(64)
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Table XII.
F o l i c A c i d Content of Some F r u i t J u i c e s
Product Orange j u i c e Mixed f r u i t j u i c e s Grapefruit j u i c e Tangerine j u i c e Grape j u i c e Prune j u i c e Apple j u i c e Tomato j u i c e Downloaded by UNIV OF WOLLONGONG on May 4, 2016 | http://pubs.acs.org Publication Date: December 15, 1980 | doi: 10.1021/bk-1980-0143.ch001
Source:
Average 35 (infant) 8 8 21 0.1 0.3 0.2 9.9
Range (mi crôgràms/100 ml ) 26-40 7-10 3-24 17-25 0.1 0.2-0.3 0.1-0.2 9.7-10
(67)
Vitamin B^. A n a l y s i s of r e c o n s t i t u t e d f r o z e n concentrated orange j u i c e has shown t h a t i t contains an average of 55 meg per 100 ml (68). While t h i s amount i s not considered h i g h , orange j u i c e i s comparable to m i l k i n supplying t h i s n u t r i e n t . Atking ÊÏ. ϋ1· (69) reported t h a t cow's m i l k s u p p l i e s an average of 54 meg of v i t a m i n B per 100 ml and orange j u i c e 56 meg i n equal v o l ume. Canned and f r e s h g r a p e f r u i t j u i c e s contain 8 and 18 meg per 100 m l , r e s p e c t i v e l y (70). Vitamin B5 i s a group of 3 r e l a t e d compounds, namely p y r i d o x i n e , pyridoxamine and p y r i d o x a l . I t i s e s s e n t i a l f o r many of the enzymes i n amino a c i d metabolism. Deter minations of v i t a m i n Bg i n c i t r u s are made by m i c r o b i o l o g i c a l assay. Although the RDA f o r t h i s n u t r i e n t i s 2 mg, the a c t u a l requirement could be as low as 1.25 mg. With high p r o t e i n i n t a k e , the need could be as much as 1.75 mg (71). 6
N i a c i n , R i b o f l a v i n and Pantothenic A c i d . These three v i t a mins are a l l present i n c i t r u s j u i c e s but the amount of each p r e s ent i n orange j u i c e i s only between 2 to 4 percent U.S. RDA i n a 177 ml s e r v i n g (68). In g r a p e f r u i t j u i c e , the amount i s s l i g h t l y l e s s but i s a l s o near the 2 percent U.S. RDA region (72). While these amounts are not l a r g e , they a r e , however, greater i n p r o p o r t i o n to the average c a l o r i c intake as expressed by the INQ (7). Mineral N u t r i e n t s i n C i t r u s Potassium and Sodium. Potassium i s the most abundant mineral of c i t r u s j u i c e s and other c i t r u s products, amounting to 40 p e r cent of the t o t a l ash (73). In c o n t r a s t , c i t r u s f r u i t s are low i n sodium, g e n e r a l l y l e s s than 1 mg/100 ml j u i c e (74). In 100 ml of orange j u i c e , 4 to 6 meq of potassium may be a v a i l a b l e (73). S l i g h t l y l e s s was found i n canned g r a p e f r u i t j u i c e (72). These two elements are the main c a t i o n s of the c e l l . Persons w i t h high blood pressures are u s u a l l y placed on a low sodium d i e t . Although potassium d e f i c i e n c y i n normal a d u l t s i s r a r e ,
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people on d i u r e t i c medicine or on an improper d i e t have been known to need a supplemented i n t a k e of t h i s element. Calcium, Magnesium and Phosphorus. Calcium and magnesium are the two major d i v a l e n t c a t i o n s of c i t r u s f r u i t , but both o c cur i n r e l a t i v e l y low amounts i n the j u i c e s ranging between 6-15 mg/100 ml (74). The c o n t r i b u t i o n of c i t r u s j u i c e s f o r these two mineral n u t r i e n t s would only be 2-3 percent U.S. RDA per a s e r v i n g of 177 ml (6 f l o z ) . However, when c a l c u l a t e d on the basis of c a l o r i c i n t a k e , c i t r u s j u i c e s provide a l l these three n u t r i e n t s (phosphorus, calcium and magnesium) near or above t h e i r c a l o r i c contributions. While phosphorus i s r e l a t e d to the two b i v a l e n t c a t i o n i n human n u t r i t i o n as i n s t r u c t u r a l f o r m a t i o n , i t i s a l s o present i n the blood c e l l s as phosphates and i n p r o t e i n , l i p i d s , carbohydrates and enzymes such as ATP and ADP. The U.S. RDA f o r phosphorus i s one gram, and orange j u i c e and g r a p e f r u i t j u i c e may cont a i n between 15 to 20 mg per 100 m l . Trace Elements as Mineral N u t r i e n t s . Within t h i s group of elements are copper, z i n c , i r o n and manganese. A l l of these mine r a l s are s u p p l i e d during the c u l t i v a t i o n of c i t r u s f r u i t , and are e s s e n t i a l as p l a n t n u t r i e n t s . They are important i n many of the enzymatic r e a c t i o n s i n the metabolic a c t i v i t i e s of the body. These minerals are a l l found to be near, or s l i g h t l y above, the c a l o r i c c o n t r i b u t i o n of the c i t r u s products (7_, 75). Other C o n s t i t u e n t s i n C i t r u s w i t h P o s s i b l e D i e t a r y
Importance
Several c o n s t i t u e n t s of c i t r u s f r u i t s have been reported to have d i e t a r y importance, although there are strong c o n t r o v e r s i e s about them. While these substances do not cause d e f i c i e n c y symptoms when not taken i n s u f f i c i e n t q u a n t i t i e s , t h e i r presence i n food may c o n t r i b u t e to the betterment of h e a l t h . Among them are the b i o f l a v o n o i d s and i n o s i t o l . Bioflavonoids Flavonoids are a group of compounds c o n t a i n i n g a C5-C3-C5 s t r u c t u r e . At l e a s t 50 f l a v o n o i d s have been i s o l a t e d and i d e n t i f i e d from c i t r u s (76), but only two are found i n a p p r e c i a b l e q u a n t i t i e s . Hesperidin i s the major f l a v o n o i d i n oranges whereas n a r i n g i n i s major i n g r a p e f r u i t . Some f l a v o n o i d s have been found to have b i o l o g i c a l a c t i v i t i e s ; the term b i o f l a v o n o i d s i s sometimes used to d e s c r i b e these substances. Szent-Gyorgyi (77) had considered c a l l i n g these substances "Vitamin P". Since these substances are not e s s e n t i a l i n the human d i e t , i n as much as t h e i r d e f i c i e n c y does not cause s p e c i f i c symptoms, the term "Vitamin P" was not accepted by most n u t r i t i o n i s t s . Tests of some of the b i o f l a v o n o i d preparations f a i l e d to show any p h y s i o l o g i c a l
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a c t i v i t i e s . Thus, i t was concluded by many n u t r i t i o n i s t s t h a t the b i o f l a v o n o i d s are not of any n u t r i t i o n a l importance. B i o f l a v o n o i d s as i s o l a t e d from oranges f o r pharmaceutical purposes c o n t a i n mostly h e s p e r i d i n , and when p u r i f i e d , only h e s p e r i d i n may be present. The p h y s i o l o g i c a l a c t i v i t i e s , however, are present i n some of the minor components of the c i t r u s f l a v o n o i d p o o l . Robbins (78) found t h a t the f u l l y methoxylated f l a v o n e s , espec i a l l y n o b i l e t i n and t a n g e r e t i n , and heptamethoxyflavone are e f f e c t i v e i n preventing the adhesion of red blood c e l l s . The decrease i n t h i s red blood c e l l adhesion has been a s s o c i a t e d with a l e s s e n i n g of some heart d i s e a s e s .
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Inositol I n o s i t o l i s a commonly o c c u r r i n g substance i n plants and has been found to be present i n c i t r u s j u i c e i n the amount of about 150-200 mg/100 ml (79). I t has been considered a growth f a c t o r f o r some animals (807 but i t has not been proven e s s e n t i a l i n humans. I t i s doubtful t h a t t h i s substance i s c r i t i c a l i n the human d i e t . Conclusion With i n c r e a s i n g knowledge of n u t r i t i o n and of n u t r i e n t s and food values of c i t r u s f r u i t and w i t h the development of technology of p r o c e s s i n g , packagin, and t r a n s p o r t a t i o n to reduce the cost of the product, c i t r u s f r u i t s and t h e i r products have become s t a p l e food items i n most developed and some developing c o u n t r i e s i n recent y e a r s . The s u p e r i o r i t y of c i t r u s i s not only dependent on the f a c t t h a t i t contains high v i t a m i n C, although i t i s important, but t h a t i t contains several hundred other chemical c o n s t i t u e n t s i n c l u d i n g many other v i t a m i n s , p o l y s a c c h a r i d e s , amino acids and m i n e r a l s . Many of these are e s s e n t i a l i n human n u t r i t i o n . The i n c l u s i o n of c i t r u s i n the d i e t provides sugars as a quick source f o r energy as w e l l as many other substances which may c o n t r i b u t e to a person's w e l l being. The major assets of c i t r u s as food are t h e i r d e s i r a b l e c h a r a c t e r i s t i c aroma, t a s t e and c o l o r and i n the i n t e r r e l a t i o n s h i p of a l l the d i e t a r y n u t r i e n t s i n a pool as i n a l l n a t u r a l foods such as c i t r u s when consumed. As a food i t can be enjoyed by persons of a l l ages and at a l l times. Many c i t r u s c o n s t i t u e n t s e i t h e r already i d e n t i f i e d or to be i s o l a t e d may c o n t r i b u e to the betterment of h e a l t h .
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76. Horowitz, R. M. In "Citrus Science & Technology"; Nagy, S.; Shaw, P.; Veldhuis, M., Eds.; Avi Publishing Co.: Westport, CT, 1977; Vol. 1, Chapter 10. 77. Szent-Gyorgyi, Α.; Rusznyak, I. Nature, (London), 1936, 138, 27. 78. Robbins, R. C. Int. J. Vitam. Nutr. Res., 1977, 47, 373. 79. Wolford, R. M. Sunshine State Agric. Res. Rept., 1958, 3(4), 10-11. 80. Cunha, T. J.; Kirkwood, S.; Phillips, P. H.; Bobstedt, G. Proc. Soc. Exp. Biol. Med., 1943, 54, 236-238. 1980.
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