Nutrients and Nutrition of Citrus Fruits - ACS Symposium Series (ACS

1 Nutrients and Nutrition of Citrus Fruits

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

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.


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


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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


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.



6

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


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


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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.


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



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(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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10


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.


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


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


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 -


1.

TING

Nutrition

of

Citrus

13

Fruits

Table IX. Average Vitamin C Content of Freshly Harvested F r u i t s and Vegetables


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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§ 601-

2 ο

Nutrients and Nutrition of Citrus Fruits - ACS Symposium Series (ACS

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