Citrus Nutrition and Quality - American Chemical Society

6 The Role of Pectin in Citrus Quality and Nutrition ROBERT A. BAKER

Downloaded by UNIV LAVAL on May 4, 2016 | http://pubs.acs.org Publication Date: December 15, 1980 | doi: 10.1021/bk-1980-0143.ch006

U.S. Citrus and Subtropical Products Laboratory, U.S. Department of Agriculture, Science and Education Administration, Southern Region, P.O. Box 1909, Winter Haven, FL 33880

Pectin is one of the major cellular structural components. It exists both in the primary cell wall and in the middle lamella, the intercellular cement between cells. In this capacity it contributes significantly to structural integrity of fruits and vegetables. As a soluble component of juice and an insoluble component of juice particulate material, pectin affects many facets of juice quality. The basic structure of pectin consists of a chain of galacturonic acid units linked viaa(1->4) glycosidic bonds. Although pure galacturonans have been reported (1), most pectin probably contains small quantities of rhamnose as inserts in the main chain (2). Soluble pectins may contain from 150 to 1500 units in the primary chain structure. In addition, most pectins have side chains containing arabinans, galactans, xylose and fucose (3, 4). These are attached by covalent bonding to the free hydroxyl groups of either the rhamnose inserts in the primary chain (5) or the galacturonic acid residues (6) (Figure 1). Recent advances in structural elucidation of pectin have been reviewed by Nelson (7). In most p e c t i n s , many of the carboxyl groups of the g a l a c t u r o n i c a c i d residues are e s t e r i f i e d with methanol. The extent of e s t e r i f i c a t i o n i s expressed as the degree of e s t e r i f i c a t i o n (0-100% D . E . ) or methoxyl content [0-16.32%, the t h e o r e t i c a l upper l i m i t based on the molecular weight of a methylated g a l a c t u r o n i c a c i d u n i t ( 8 ) ] . C i t r u s p e c t i n s that are not d e e s t e r i f i e d by e x t r a c t i v e procedures u s u a l l y have a D . E . i n excess of 50%. When combined, the f a c t o r s of v a r y i n g primary chain l e n g t h , rhamnose i n s e r t s , side c h a i n s , and p a r t i a l methylation y i e l d a family of molecules of almost l i m i t l e s s heterogeneity. Any measurement made on a p e c t i n s o l u t i o n i s at best an average for the molecular species present.

This chapter not subject to U.S. copyright. Published 1980 American Chemical Society Nagy and Attaway; Citrus Nutrition and Quality ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


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Figure 2. "Egg-box model" of calcium pectate formation. Calcium ions (represented by circles) bound by chain-stacked sections of deesterified pectin molecules. Adapted from Ref. 11.

Nagy and Attaway; Citrus Nutrition and Quality ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


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The a t t r i b u t e s of p e c t i n which most d i r e c t l y a f f e c t c i t r u s product q u a l i t y — v i s c o s i t y , g e l l i n g a b i l i t y , and a b i l i t y to p r e c i p i t a t e as p e c t a t e s — a r e p r i m a r i l y determined by chain length (molecular weight) and degree of e s t e r i f i c a t i o n . High molecular weight, high D.E. p e c t i n s form gels when s o l u b l e s o l i d s l e v e l s reach 65% and pH i s i n the range of 2.9-3.4 (9). Low D.E. p e c t i n s w i l l g e l without sugar and at high pH i f s u f f i c i e n t calcium i s present. I f molecular weight i s lowered below a c e r t a i n l e v e l , p e c t i n s l o s e t h e i r a b i l i t y to g e l i n e i t h e r system. P r e c i p i t a t i o n of p e c t i n s as pectates and g e l formation are i n f l u e n c e d not only by D.E. and molecular weight, but a l s o by the d i s t r i b u t i o n of f r e e a c i d and e s t e r i f i e d s i t e s along the molecule (10). Rees (11) and Grant et a l . (12) proposed an "egg-box model" t o e x p l a i n calcium pectate formation (Figure 2 ) . In t h i s model d e e s t e r i f i e d s e c t i o n s of p e c t i n molecules chain stack, c o o p e r a t i v e l y b i n d i n g calcium ions i n the i n t e r v e n i n g cavities. P e c t i n i s o f t e n d i v i d e d i n t o three c a t e g o r i e s on the b a s i s of s o l u b i l i t y (13). Water s o l u b l e p e c t i n , that which can be e x t r a c t e d from t i s s u e w i t h hot water, u s u a l l y has a high D.E. (2). I n s o l u b l e pectates are only s o l u b l e i f c h e l a t i n g agents such as ammonium oxalate or detergents are present. Insoluble pectates are low D.E. p e c t i n s which have formed the i n s o l u b l e s a l t of a d i v a l e n t c a t i o n , such as calcium. The t h i r d category, p r o t o p e c t i n s , are s o l u b i l i z e d only by h e a t i n g with a c i d or alkali. The chemistry of t h i s c l a s s of p e c t i n s i s p o o r l y understood, s i n c e harsh e x t r a c t i o n procedures a l t e r both molecular weight and D.E. The i n s o l u b i l i t y of p r o t o p e c t i n s i n s i t u i s probably a r e s u l t of bonding to h e m i c e l l u l o s e s and mechanical intermeshing with other c e l l w a l l c o n s t i t u e n t s (14). C i t r u s f r u i t s , e s p e c i a l l y c e r t a i n of t h e i r component p a r t s , c o n s t i t u t e one of the r i c h e s t sources of p e c t i n . On a dry weight b a s i s , as much as 30% of orange f r u i t albedo may be p e c t i n ( 8 ) . The rag, comprising the f r u i t core and segment membranes a f t e r j u i c e e x t r a c t i o n , i s a l s o a r i c h source. Since p e c t i n i s a c e l l w a l l component, i t follows that comparatively l i t t l e would be present i n j u i c e expressed from f r u i t . For example, concent r a t i o n s ranging from 0.01 to 0.13% i n orange j u i c e have been reported (15). Much of t h i s would be present as c e l l w a l l fragments and p a r t i c u l a t e m a t e r i a l i n j u i c e pulp and cloud. Even a t these l e v e l s , p e c t i n i n f l u e n c e s j u i c e q u a l i t y , both p o s i t i v e l y and n e g a t i v e l y , by i t s c o n t r i b u t i o n to v i s c o s i t y , g e l a t i o n , and cloud s t a b i l i t y . Quality Viscosity. In f r e s h l y e x t r a c t e d j u i c e , v i s c o s i t y imparted by p e c t i n i s a d e s i r a b l e c h a r a c t e r i s t i c , commonly r e f e r r e d to as body. I f t h i s v i s c o s i t y i s not present, or i s l o s t through d e s t r u c t i o n of the p e c t i n c o l l o i d , j u i c e i s described as t h i n ,



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watery, and l a c k i n g a j u i c e - l i k e mouth f e e l . Fresh, s i n g l e s t r e n g t h , or r e c o n s t i t u t e d j u i c e s should, t h e r e f o r e , c o n t a i n some s o l u b l e p e c t i n f o r optimum q u a l i t y . However, when j u i c e i s processed i n t o concentrate, excessive pectin-induced v i s c o s i t y can be d e t r i m e n t a l . I n c r e a s i n g energy c o s t s f o r frozen storage and t r a n s p o r t of concentrate make p r o d u c t i o n of very high B r i x concentrates economically attractive. Concomitant high v i s c o s i t y i s a t e c h n o l o g i c a l problem which must be d e a l t with i f these products are to be accepted. As t o t a l s o l u b l e s o l i d s approaches 65%, v i s c o s i t y increases rapidly. Ingram (16) noted that t h i s l e v e l c o i n c i d e s with the sugar l e v e l r e q u i r e d f o r s u g a r - a c i d - p e c t i n g e l s . Such v i s c o u s preparations cannot be poured or r e c o n s t i t u t e d to single-strength juice easily. To obtain flowable high s o l i d s concentrates, s o l u b l e p e c t i n l e v e l s must be reduced e i t h e r before or during c o n c e n t r a t i o n . T h i s may be accomplished by t r e a t i n g j u i c e before c o n c e n t r a t i o n with a commercially a v a i l a b l e polygalacturonase (PG) p r e p a r a t i o n (17). PG h y d r o l y t i c a l l y cleaves primary p e c t i n chains between adjacent n o n e s t e r i f i e d g a l a c t u r o n i c a c i d r e s i d u e s , reducing v i s c o s i t y r a p i d l y at 30°. Subsequent p a s t e u r i z a t i o n terminates added PG and n a t i v e p e c t i n e s t e r a s e (PE) a c t i v i t i e s , s t a b i l i z i n g j u i c e cloud. An a l t e r n a t i v e approach i s to t r e a t concentrate to reduce viscosity. Berk (18), u s i n g u l t r a s o n i c r a d i a t i o n , succeeded i n lowering v i s c o s i t y of 60° B r i x concentrate to 25% of i t s i n i t i a l level. V i s c o s i t y of 70° B r i x concentrate prepared from t h i s m a t e r i a l was only 50% higher than that of untreated 60° B r i x concentrate. V i s c o s i t y of 70° B r i x concentrate prepared without u l t r a s o n i c treatment was n e a r l y 8 times that of 60° B r i x concentrate. I r r a d i a t i o n was more e f f e c t i v e at an intermediate stage of concentration (e.g., 60° B r i x ) than a f t e r c o n c e n t r a t i o n to 70°Brix. V i s c o s i t y poses a s i m i l a r problem i n the production of pulp wash concentrate. Pulp wash c o n s i s t s of j u i c e s o l i d s obtained by countercurrent washing of pulp a f t e r i t s s e p a r a t i o n from juice. On a °Brix b a s i s , pulp wash l i q u i d s are higher i n p e c t i n than j u i c e from which the pulp has been screened (19). Concentration of pulp wash above 40°Brix i s at times hampered by excessive p e c t i n l e v e l s (20). To c o n t r o l v i s c o s i t y processors may be forced to reduce f i n i s h e r pressure to minimize p e c t i n e x t r a c t i o n , thereby c u r t a i l i n g y i e l d . A more e f f e c t i v e s o l u t i o n i s to t r e a t pulp wash with pectinases to reduce p e c t i n l e v e l s (21). I f pectinases are incorporated i n t o the wash water, t h i s method has the advantage of i n c r e a s i n g t o t a l s o l i d s y i e l d by reducing j u i c e r e t e n t i o n i n the pulp. Gelation. P e c t i n s can form two types of gels : 1) sugara c i d - p e c t i n g e l s , with a 65% sugar l e v e l , low pH, and h i g h l y e s t e r i f i e d p e c t i n , and 2) pectate g e l s , r e q u i r i n g only low



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e s t e r p e c t i n and d i v a l e n t c a t i o n s . The former would be seen only i n very high B r i x concentrates. G e l a t i o n seen i n commercial concentrate (42-45° Brix) i s of the l a t t e r type, and v a r i e s from s o f t curds to f i r m g e l s . This i s a v i s u a l q u a l i t y d e f e c t , but i n severe cases f i r m gels impair r e c o n s t i t u t i o n of concentrate. G e l a t i o n i s accompanied or preceded by c l a r i f i c a t i o n , s i n c e both phenomena i n v o l v e p r e c i p i t a t i o n of low D.E. p e c t i n s with calcium. G e l a t i o n became a s i g n i f i c a n t problem with the i n t r o d u c t i o n of frozen orange j u i c e concentrate, which i n i t i a l l y was o f t e n packed without p a s t e u r i z a t i o n . T h i s product, when not f r o z e n , r a p i d l y developed low e s t e r p e c t i n s by v i r t u e of the PE i t contained. Much of the research generated i n response to t h i s problem has been reviewed by J o s l y n and P i l n i k (22). Briefly, i t has been found that g e l a t i o n i s the r e s u l t of p e c t i n d e e s t e r i f i c a t i o n (23), and increases as j u i c e pulp (24) , PE (25) , and e x t r a c t i o n pressures (24) i n c r e a s e . In concentrate undergoing g e l a t i o n , water s o l u b l e p e c t i n s tend to decrease, w h i l e pectates increase (22). Gel formation i s best prevented by p a s t e u r i z a t i o n at both s u f f i c i e n t temperature and h o l d i n g time to i n a c t i v a t e most PE, coupled with storage of concentrate near -18°. I n t r o d u c t i o n of high temperature evaporative systems has l a r g e l y eliminated the problem of g e l a t i o n . Gel formation by p e c t i n s can be a p o s i t i v e q u a l i t y f a c t o r i n some c i t r u s products, f o r example, g e l l e d c i t r u s salads (26) . These salads c o n s i s t of c i t r u s f r u i t s e c t i o n s suspended i n a g e l of low e s t e r p e c t i n , carrageenan, and l o c u s t bean gum. S i m i l a r p e c t i n mixtures have a l s o been suggested as a s e a l e r f o r g r a p e f r u i t halves (27). Ready-to-eat g r a p e f r u i t h a l v e s may be s a t i s f a c t o r i l y stored 48 hours i f c h i l l e d . Prolonged storage r e s u l t s i n f l a v o r d e t e r i o r a t i o n , d r y i n g and f u n g a l growth. A s t i f f c i t r u s f l a v o r e d p e c t i n g e l a p p l i e d to the cut surface sealed the f r u i t , maintaining f l a v o r and preventing shrinkage or fungal i n v a s i o n f o r up to 19 days. T h i s would be of p a r t i c u l a r value to i n s t i t u t i o n a l consumers. J u i c e Cloud. Mechanical e x t r a c t i o n of c i t r u s f r u i t s y i e l d s a t u r b i d suspension of w a l l fragments and c e l l u l a r o r g a n e l l e s i n a serum composed p r i m a r i l y of c e l l vacuolar fluids. In most c i t r u s j u i c e products, such a suspension of fragments and o r g a n e l l e s i s a d e s i r a b l e component, s i n c e i t provides most of the c h a r a c t e r i s t i c c o l o r and f l a v o r (28). Essence and p e e l o i l s suspended i n j u i c e c o n t r i b u t e d e s i r a b l e c i t r u s notes to f l a v o r , and these o i l s are r a p i d l y adsorbed by j u i c e p a r t i c u l a t e m a t e r i a l s h o r t l y a f t e r e x t r a c t i o n (29). I f steps are not taken to s t a b i l i z e cloud, most c i t r u s j u i c e s w i l l c l a r i f y when allowed to stand. Clarification occurs when n a t i v e PE lowers the e s t e r content of j u i c e s o l u b l e p e c t i n u n t i l i t becomes s u s c e p t i b l e to p r e c i p i t a t i o n as i n s o l u b l e pectates (23). I f p e c t i n l e v e l s are high enough, as i n concentrates, these pectates may form a g e l .



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At lower l e v e l s , they s e t t l e out as a f l o e , o c c l u d i n g cloud p a r t i c u l a t e s and removing them from suspension (28). Estimates o f the D.E. necessary f o r such p r e c i p i t a t i o n have been made, ranging from 38% f o r a 25% cloud l o s s (23) to 27% i n j u i c e which was 85% c l a r i f i e d (30). However, the c r i t i c a l e s t e r l e v e l at which p e c t i n p r e c i p i t a t e s as pectate cannot be e s t a b l i s h e d from assays on the D.E. of t o t a l j u i c e p e c t i n . P e c t i n s e x h i b i t a n a t u r a l heterogeneity with respect to e s t e r content (Figure 3a), and when t h i s i s coupled with nonrandom blockwise removal of methoxyl groups by PE (10), a wide range of e s t e r contents may be expected (Figure 3b). A closer approximation of the c r i t i c a l D.E. a s s o c i a t e d with c l a r i f i c a t i o n can be made by examining p e c t i n s f r a c t i o n a t e d according to e s t e r content. In such a study, Baker (31) found that p e c t i n f r a c t i o n s with D.E. of 14% or l e s s were able to c l a r i f y j u i c e , whereas those with D.E. of 21% or more were not. This indicates that the c r i t i c a l D.E. f o r p r e c i p i t a t i o n of p e c t i n s as pectates l i e s between 14 and 21%. Many e a r l y researchers of c i t r u s j u i c e cloud presumed that a s o l u b l e p e c t i n matrix supported and s t a b i l i z e d cloud; d e s t r u c t i o n of t h i s matrix was t h e r e f o r e seen as a c o n t r i b u t i n g cause of c l a r i f i c a t i o n (32). More recent work i n v o l v i n g aqueous suspensions of cloud showed p e c t i n i s not necessary f o r cloud s t a b i l i t y (33, 34). Orange j u i c e cloud i s q u i t e s t a b l e suspended i n water, and can only be made to c l a r i f y i f p e c t i n , c a l c i u m , and s u f f i c i e n t KC1 to s o l u b i l i z e particulate-bound PE are added (33). Even though p e c t i n i s not necessary f o r cloud support, s o l u b l e high e s t e r p e c t i n may delay c l a r i f i c a t i o n by "substrate f l o o d i n g " , that i s , by d i l u t i n g PE a c t i v i t y . Rouse et a l . (35), studying frozen orange j u i c e concentrate, found that packs with high p e c t i n l e v e l were more cloud s t a b l e than packs with low p e c t i n l e v e l at corresponding PE l e v e l s . P e c t i n may a l s o r e t a r d c l a r i f i c a t i o n by i n h i b i t i n g coacervation of cloud p a r t i c u l a t e s and p r e c i p i t a t i n g pectates (36). Although the a d d i t i o n of p e c t i n may r e t a r d c l a r i f i c a t i o n , t h i s a c t i o n does not represent a p r a c t i c a l approach to s t a b i l i z i n g cloud. To achieve cloud s t a b i l i t y , d e e s t e r i f i c a t i o n of s o l u b l e p e c t i n to low e s t e r p e c t i n and p r e c i p i t a t i o n of the l a t t e r as pectates must be prevented (Figure 4). T h i s sequence may be d i s r u p t e d by enzymic d e s t r u c t i o n of e i t h e r p e c t i n or low e s t e r p e c t i n , by b l o c k i n g the p r e c i p i t a t i o n of p e c t a t e s , or by i n a c t i v a t i o n of PE. Commercially, i n a c t i v a t i o n of PE by heat i s the cloud s t a b i l i z a t i o n procedure of choice, since heat i s a l s o needed f o r p a s t e u r i z a t i o n . In a c i d j u i c e s , s p o i l a g e organisms are more e a s i l y i n a c t i v a t e d than i s PE, so cloud s t a b i l i z a t i o n r e q u i r e s higher temperatures than p a s t e u r i z a t i o n (37). Thermal i n a c t i v a t i o n of PE i s not l i n e a r l y r e l a t e d to s t a b i l i t y . Guyer et a l . (38) found most PE i n orange j u i c e could be destroyed without an appreciable e f f e c t on cloud s t a b i l i t y ; a f u r t h e r small decrease i n PE


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Figure 3. Degrees of esterification of fractions from (a) citrus pectin separated on DEAE-cellulose (adapted from Ref. 6) and (b) PE-treated citrus pectin sepa rated on DEAE-cellulose


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INSOLUBLE PECTATES Figure 4. Possible pathways of pectin degradation in juice



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a c t i v i t y r e s u l t e d i n s u b s t a n t i a l cloud s t a b i l i t y . T h i s nonlinear response was explained when Versteeg (39) found that orange j u i c e contains m u l t i p l e forms of PE. One of these, a heat s t a b l e h i g h molecular-weight isozyme, accounted f o r about 10% of t o t a l PE a c t i v i t y . Presumably i t i s t h i s form which c o n t r i b u t e s to cloud l o s s i n incompletely s t a b i l i z e d j u i c e . Enzymic depolymerization of j u i c e s o l u b l e p e c t i n before i t can be d e e s t e r i f i e d by PE a l s o s t a b i l i z e s cloud (30). T h i s has been accomplished with p e c t i n lyase (PL), an enzyme which degrades e s t e r i f i e d p e c t i n to o l i g o g a l a c t u r o n i c a c i d e s t e r s . Since PL cannot attack d e e s t e r i f i e d p e c t i n , cloud s t a b i l i z a t i o n of PE a c t i v e j u i c e r e q u i r e s r e l a t i v e l y l a r g e q u a n t i t i e s of PL to compete with PE f o r e s t e r i f i e d p e c t i n . Cloud l o s s may a l s o be prevented by enzymic d e s t r u c t i o n of low e s t e r p e c t i n s p r i o r to t h e i r p r e c i p i t a t i o n as p e c t a t e s . Commercial fungal PG preparations (40, 41) or PG derived from yeast (42) have been used s u c c e s s f u l l y to s t a b i l i z e cloud of unpasteurized PE a c t i v e j u i c e . As only the low e s t e r p e c t i n s need be destroyed to prevent cloud l o s s , considerably l e s s PG i s r e q u i r e d than PL (30). Cloud s t a b i l i z a t i o n with pectinases permits production of cloud s t a b l e j u i c e s with lower p a s t e u r i z a t i o n temperatures (40). For low e s t e r p e c t i n to p r e c i p i t a t e as pectate and c l a r i f y j u i c e , d i v a l e n t c a t i o n s such as Ca must be a v a i l a b l e to p a r t i c i p a t e i n the coacervaj^ion (36). The use of hexametaphosphates to sequester Ca ions has been suggested as a means to block pectate p r e c i p i t a t i o n and s t a b i l i z e cloud (43). C l a r i f i e d J u i c e . Some c i t r u s j u i c e s , notably lime and lemon, are i n demand as c l a r i f i e d products. Natural c l a r i f i c a t i o n , i n combination with f i l t r a t i o n , i s o f t e n used to achieve a s p a r k l i n g c l e a r serum. However, n a t i v e PE a c t i o n i s slowed by the high a c i d i t y of these j u i c e s , and may not give s a t i s f a c t o r y cloud removal. In t h i s instance, modified p e c t i n s can serve to enhance j u i c e q u a l i t y by removing cloud. Low e s t e r p e c t i n s obtained by treatment of c i t r u s p e c t i n with e i t h e r PE or NaOH c l a r i f y c i t r u s j u i c e s over a wide pH range (44). E s t e r content f o r optimum c l a r i f i c a t i o n depends both upon j u i c e pH and method of d e e s t e r i f i c a t i o n . Conversion of a l l e s t e r i f i e d carboxyl groups i n the p e c t i n chain to f r e e a c i d s y i e l d s p o l y g a l a c t u r o n i c a c i d , which i s l a r g e l y water i n s o l u b l e . I f t h i s a c i d i c polymer i s brought i n t o s o l u t i o n by p a r t i a l n e u t r a l i z a t i o n with base, i t can a l s o f u n c t i o n as a c l a r i f y i n g agent f o r c i t r u s j u i c e s (45). Cloud r e d u c t i o n with p o l y g a l a c t u r o n i c a c i d preparations i s a l s o i n f l u e n c e d by j u i c e pH, but i n t h i s instance polymer molecular weight determines pH optimum.


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I n t a c t F r u i t and S e c t i o n s . A number o f s t u d i e s h a v e b e e n made o f p e c t i c s u b s t a n c e s i n c i t r u s f r u i t d u r i n g g r o w t h and maturation. T h e s e h a v e b e e n r e v i e w e d b y K e f f o r d and C h a n d l e r (46) a n d b y Rouse ( 1 5 ) . In g e n e r a l , once f r u i t has r e a c h e d f u l l s i z e , few n o t a b l e c h a n g e s a r e s e e n d u r i n g m a t u r a t i o n i n t h e b a l a n c e o f w a t e r - , o x a l a t e - , o r NaOH- s o l u b l e f r a c t i o n s o f p e c t i n , or i n i t s ester content. T h i s may i n p a r t be due t o the fact that c i t r u s f r u i t lacks a true c l i m a c t e r i c r i p e n i n g w i t h i t s attendant p h y s i o l o g i c a l changes. F r e e z e damage t o f r u i t a f f e c t s i t s p e c t i n c o n t e n t and d i s t r i b u t i o n among w a t e r - , o x a l a t e - , and N a O H - s o l u b l e f r a c t i o n s , b u t the e x t e n t o f the c h a n g e s d e p e n d s l a r g e l y on f r u i t v a r i e t y ( 1 5 ) . Valencia o r a n g e s , f o r example, a p p a r e n t l y are a b l e to p a r t i a l l y r e c o v e r i n t h e i n t e r i m b e t w e e n w i n t e r f r e e z e and l a t e S p r i n g h a r v e s t . Canned c i t r u s s e c t i o n s c o n s i s t o f e x c i s e d f r u i t segments, n o r m a l l y w i t h segment membrane r e m o v e d , p a c k e d i n w a t e r o r syrup. F i r m n e s s and f r e e d o m f r o m b r o k e n s e c t i o n s a r e p r i m a r y determinants of q u a l i t y i n t h i s p r o d u c t . Section firmness i s d e p e n d e n t upon t h e i r c o n t e n t o f o x a l a t e - and N a O H - s o l u b l e p e c t i n (15). Low e s t e r p e c t i n a d d e d t o g r a p e f r u i t s e c t i o n s i n water d i d not improve firmness or reduce s e c t i o n breakage, but d i d improve d r a i n e d weight (47). However, c a l c i u m s a l t s (as c a l c i u m l a c t a t e ) added to s y r u p c o v e r i n g g r a p e f r u i t sections s i g n i f i c a n t l y i n c r e a s e d the p e r c e n t a g e o f f i r m s e c t i o n s (48). T h i s w o u l d be a n a l a g o u s t o t h e w e l l known c a l c i u m f i r m i n g o f canned tomatoes. Nutrition I n t e r e s t i n p e c t i n from a n u t r i t i o n a l s t a n d p o i n t has i n c r e a s e d w i t h new e v i d e n c e o f i t s i n f l u e n c e on s e v e r a l physiological processes. Long acknowledged as an e f f e c t i v e a n t i d o t e t o d i a r r h e a , p e c t i n h a s now b e e n f o u n d t o be o f p o s s i b l e b e n e f i t i n c o n t r o l o f c h o l e s t e r o l l e v e l s and i n management o f d i a b e t e s . As one o f t h e r i c h e s t p o t e n t i a l s o u r c e s o f p e c t i n , c i t r u s f r u i t s c o u l d e n j o y enhanced n u t r i t i o n a l s t a t u s from these f i n d i n g s . Metabolism of p e c t i n . P e c t i n h a s o n l y r e c e n t l y come, t o be c o n s i d e r e d a p a r t o f t h e d i e t a r y f i b e r c o m p l e x . Previously i t was e x c l u d e d b e c a u s e 1) i t i s n o t f i b r o u s ( e x c e p t a t t h e m o l e c u l a r l e v e l ) , 2) i t e s c a p e s d e t e c t i o n i n s t a n d a r d f i b e r t e s t s o w i n g t o i t s s o l u b i l i t y , and 3) i t u s u a l l y d o e s n o t survive i n t e s t i n a l passage. In a reassessment of which d i e t a r y components s h o u l d be c o n s i d e r e d f i b e r , T r o w e l l (49) proposed that d i e t a r y f i b e r i n c l u d e those c o n s t i t u e n t s of food r e s i s t a n t t o h y d r o l y s i s by man's a l i m e n t a r y enzymes. Spiller ( 5 0 , 51) s u g g e s t e d t h a t c o n f u s i o n s u r r o u n d i n g t h e t e r m " f i b e r " be a v o i d e d b y u s i n g t h e t e r m " p l a n t i x " t o d e n o t e t h o s e p l a n t m a t e r i a l s o f p o l y m e r i c n a t u r e n o t a t t a c k e d b y human d i g e s t i v e enzymes.


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R e g a r d l e s s o f terminology, p e c t i n q u a l i f i e s as f i b e r under these d e f i n i t i o n s . K e r t e s z (52) showed t h a t human s a l i v a , g a s t r i c j u i c e s , t r y p s i n , and p e p s i n were i n e f f e c t i v e i n hydrolyzing pectin. Werch and I v y (53) d e m o n s t r a t e d t h a t p e c t i n p a s s e s r e l a t i v e l y u n c h a n g e d t h r o u g h t h e s t o m a c h and i l e u m . Y e t l i t t l e i f a n y p e c t i n c a n be r e c o v e r e d i n t h e s t o o l , due t o r a p i d d i g e s t i o n b y b a c t e r i a l enzymes p r e s e n t i n t h e c o l o n . Breakdown p r o d u c t s i n c l u d e g a l a c t u r o n i c a c i d , v o l a t i l e a c i d s s u c h a s f o r m i c and a c e t i c , and f i n a l l y , c a r b o n d i o x i d e and w a t e r . A l t h o u g h g a l a c t u r o n i c a c i d i s n o t a b s o r b e d b y t h e human i l e u m o r c o l o n , i t d o e s n o t a p p e a r i n t h e f e c e s (54). A p p a r e n t l y this a c i d i s f u r t h e r degraded t o a c e t i c and f o r m i c a c i d s , w h i c h have b e e n s e e n t o i n c r e a s e i n f e c a l e x c r e t i o n o f s u b j e c t s f e d 30 g o f pectin daily. D e s p i t e the r a t h e r complete d i g e s t i o n o f p e c t i n i n the c o l o n , t h e r e i s some q u e s t i o n a s t o t h e e n e r g y d e r i v e d t h e r e f r o m . V i o l a e t a l . (55) , s t u d y i n g w e a n l i n g r a t s , r e p o r t e d p e c t i n s ' c o n t r i b u t i o n t o d i g e s t i b l e e n e r g y was n e g a t i v e . T h i s was s u p p o r t e d b y Hove and K i n g (56) who u s e d a r a t g r o w t h b i o a s s a y t o d e m o n s t r a t e p e c t i n p r o v i d e d no m e a s u r a b l e e n e r g y t o y o u n g rats. I n c o n t r a s t , C a m p b e l l and P a l m e r (57) c i t e two s t u d i e s (one on r a t s , t h e o t h e r on humans) w h i c h showed a n e t d i g e s t i b l e energy f o r p e c t i n . To t h e e x t e n t t h a t v o l a t i l e a c i d s r e s u l t i n g from c o l o n i c p e c t i n d e g r a d a t i o n a r e absorbed, p e c t i n would contribute to caloric intake. Whether t h i s r e s u l t s i n a n e t e n e r g y g a i n d e p e n d s on o t h e r p e c t i n - i n d u c e d e f f e c t s . In r a t s , i n c r e a s i n g d i e t a r y p e c t i n l e v e l s decrease d i g e s t i v e e f f i c i e n c y , p r o t e i n a b s o r p t i o n (56), a n d l i p i d a b s o r p t i o n (55). P o s i t i v e o r n e g a t i v e d i g e s t i b l e e n e r g y v a l u e s p r o b a b l y d e p e n d on t h e degree to which p e c t i n i n t e r f e r e s with absorption o f other d i e t a r y components. Antidiarrheal Effect. Use o f p e c t i n ( i n t h e f o r m o f s c r a p e d a p p l e s ) h a s b e e n a c k n o w l e d g e d a s a remedy f o r d i a r r h e a f o r o v e r 200 y e a r s . I t s e f f i c a c y i s a t t e s t e d t o by c o m m e r c i a l a n t i d i a r r h e a l p r e p a r a t i o n s b a s e d on p e c t i n . Despite t h i s l o n g h i s t o r y o f u s e , v e r y l i t t l e i s known o f t h e m e c h a n i s m by w h i c h p e c t i n p r o m o t e s a r e t u r n t o n o r m a l b o w e l f u n c t i o n . I t has been s u g g e s t e d t h a t p e c t i n c o u n t e r a c t s d i a r r h e a by a b s o r p t i o n and r e m o v a l o f t o x i n s , o r b y e n l a r g e m e n t o f s t o o l v o l u m e a n d sweeping o f t h e c o l o n w i t h a formed s t o o l (9). I n a s t a b l e d i g e s t i v e s y s t e m p e c t i n c o u l d n o t a b s o r b and remove t o x i n s , a s i t i s l a r g e l y degraded i n t r a n s i t . P e c t i n i s a l s o much l e s s e f f e c t i v e than o t h e r f i b e r s i n i n c r e a s i n g s t o o l b u l k : even a t 36 g/day i n t a k e , Cummings e t a l . (58) f o u n d s t o o l w e i g h t i n c r e a s e d o n l y 33%. However, d u r i n g i n t e s t i n a l u p s e t , n o r m a l c o l o n i c r e t e n t i o n times a r e g r e a t l y reduced. As a r e s u l t , p e c t i n i s n o t d i g e s t e d and c a n be r e c o v e r e d i n t h e s t o o l ( 9 ) . Therefore i t c o u l d c o n c e i v a b l y f u n c t i o n t o some e x t e n t i n t h e manner d e s c r i b e d above.


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A l t e r n a t i v e l y , Werch and I v y (54) s u g g e s t t h e favorable e f f e c t o f p e c t i n may be due t o i n h i b i t i o n o f undesirable o r g a n i s m s by v o l a t i l e a c i d s ( f o r m i c and a c e t i c ) d e r i v e d f r o m p e c t i n breakdown. Such a c i d s promote a r e t u r n t o n o r m a l bowel pH, and a r e b a c t e r i o s t a t i c a g a i n s t many o r g a n i s m s (57). Werch and I v y a l s o h y p o t h e s i z e d t h a t o f f e n d i n g o r g a n i s m s may be s t a r v e d o u t o r o v e r g r o w n by d e s i r a b l e o r g a n i s m s w h i c h use p e c t i n as a s o u r c e o f f o o d . Much more work i s n e e d e d t o e s t a b l i s h the mechanism g o v e r n i n g the a n t i d i a r r h e a l e f f e c t o f p e c t i n . Hypocholesterolemic E f f e c t . No e f f e c t o f p e c t i n has received more a t t e n t i o n i n r e c e n t y e a r s t h a n i t s a b i l i t y t o r e d u c e s e r u m c h o l e s t e r o l l e v e l s . A l t o g e t h e r t h e r e have been a dozen r e p o r t s r e l a t i n g d i e t a r y s u p p l e m e n t a t i o n w i t h p e c t i n t o serum c h o l e s t e r o l l e v e l s i n human s u b j e c t s ( s e e 59). The f i r s t o f t h e s e r e p o r t e d a s t u d y o f m i d d l e - a g e d men whose n o r m a l d i e t s were augmented w i t h a d a i l y r a t i o n o f b i s c u i t s c o n t a i n i n g 15 g o f e i t h e r c e l l u l o s e o r p e c t i n (60). C e l l u l o s e d i d n o t l o w e r serum c h o l e s t e r o l ; c i t r u s p e c t i n d i d , t o a l e v e l a b o u t 5% b e l o w c o n t r o l levels. P e c t i n has a l s o b e e n a d m i n i s t e r e d as p i l l s (61), in w a t e r (62), o r a s a g e l w i t h added f r u i t (63). The c h o l e s t e r o l - l o w e r i n g e f f e c t o f p e c t i n was g r e a t e s t when a d m i n i s t e r e d as a g e l (59). L e v e l s h a v e v a r i e d f r o m 2 g/day (61) t o 40-50 g/day (64). P a l m e r and D i x o n (61) reported s e r u m c h o l e s t e r o l l e v e l s o f human s u b j e c t s w i t h n o r m a l c h o l e s t e r o l l e v e l s were s i g n i f i c a n t l y l o w e r e d by p e c t i n i n t a k e o f 6-10 g/day, b u t n o t 2-4 g/day. Pectin i s particularly e f f i c a c i o u s i n l o w e r i n g serum c h o l e s t e r o l l e v e l s o f h y p e r cholesterolemic p a t i e n t s (64). D e l b a r r e e t a l . (65) were u n a b l e t o r e d u c e s e r u m c h o l e s t e r o l l e v e l s s i g n i f i c a n t l y w i t h 6 g/day, and c o n c l u d e d p e c t i n d i d n o t l o w e r serum c h o l e s t e r o l . However, some r e d u c t i o n was o b s e r v e d . The 6 g/day l e v e l u s e d by t h e s e a u t h o r s was t h e l o w e s t l e v e l p r o v i d i n g s i g n i f i c a n t c h o l e s t e r o l r e d u c t i o n o b s e r v e d by P a l m e r and D i x o n . The c o n c l u s i o n s of D e l b a r r e e t a l . (65) h a v e b e e n r e f u t e d by Kay e t a l . (66). I n s t u d y i n g r e s u l t s f r o m b o t h c h i c k e n s and man, Fisher et a l . (67) c o n c l u d e d t h a t p e c t i n has a h y p o c h o l e s t e r o l e m i c e f f e c t o n l y when f e d w i t h d i e t a r y c h o l e s t e r o l . On c h o l e s t e r o l - f r e e d i e t s , p l a s m a c h o l e s t e r o l i s n o t a f f e c t e d by d i e t a r y p e c t i n . S u b j e c t s f e d p e c t i n w i t h a c h o l e s t e r o l - c o n t a i n i n g d i e t had p l a s m a c h o l e s t e r o l l e v e l s t h a t were l o w e r r e l a t i v e t o t h o s e o f subjects on t h e c h o l e s t e r o l c o n t r o l d i e t , b u t n o t r e l a t i v e t o t h o s e o f s u b j e c t s on a c h o l e s t e r o l - f r e e c o n t r o l d i e t . T h e r e a r e s e v e r a l ways p e c t i n c o u l d r e d u c e s e r u m c h o l e s t e r o l . I n s t u d i e s w i t h human s u b j e c t s , f e c a l e x c r e t i o n o f b i l e a c i d s , f a t t y a c i d s , and t o t a l s t e r o i d s i n c r e a s e d when s u b j e c t s w e r e f e d 15-40 g/day o f p e c t i n (58, 63, 64). Since p e c t i n u s u a l l y lowers s e r u m c h o l e s t e r o l o n l y when c h o l e s t e r o l i s p r e s e n t i n t h e d i e t , i t seems t h a t p e c t i n m i g h t a c t by r e d u c i n g c h o l e s t e r o l absorption. S e v e r a l groups have found t h a t i n r a t s d i e t a r y



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p e c t i n does i n ^ e d reduce c h o l e s t e r o l absorption. Using cholesterol-4C, L e v e i l l e and Sauberlich (68) and Kodama et a l . (69) showed c h o l e s t e r o l absorption was impaired by d i e t a r y p e c t i n . K e l l e y and T s a i (70), who a l s o reported a decreased c h o l e s t e r o l absorption i n p e c t i n fed r a t s , concluded that p e c t i n acted by i n t e r f e r i n g with c h o l e s t e r o l absorption and by i n c r e a s i n g c h o l e s t e r o l turnover. However, L e v e i l l e and Sauberlich found r a t l i v e r and serum c h o l e s t e r o l l e v e l s were lowered even when p e c t i n and c h o l e s t e r o l were fed on a l t e r n a t e days. From t h i s the authors concluded that although p e c t i n d i d impair c h o l e s t e r o l absorption, i t p r i m a r i l y a f f e c t e d c h o l e s t e r o l l e v e l s by i n h i b i t i n g b i l e a c i d absorption. This hypothesis appears to be favored (71, 72). B i l e a c i d reabsorption may be impaired by b i n d i n g with p e c t i n during t r a n s i t of the ileum. Selvendran (73) observed a greater adsorption of sodium cholate by vegetable f i b e r s c o n t a i n i n g p e c t i n than by f i b e r s containing l i g n i n . Nagyvary and Bradbury (74) have proposed a model f o r b i l e s a l t b i n d i n g by p e c t i n s or a l g i n a t e s complexed with A1-H-+ i o n s . In support of t h e i r hypothesis, the authors demonstrated strong hypocholesterolemic a c t i v i t y f o r a pectin-aluminum complex. Fuyc^a (75) has t e s t e d and extended t h i s hypothesis, showing that Fe complexes of n a t u r a l i n s o l u b l e p e c t i n a c e o u j ^ f i b e r ( c i t r u s albedo) are even more e f f e c t i v e than A l complexes i n b i n d i n g f a t t y a c i d s . Using an e q u i l i b r i u m d i a l y s i s technique, Baig and Cerda (76) were unable to demonstrate b i n d i n g of b i l e s a l t s to s o l u b l e c i t r u s p e c t i n . This agrees with the f i n d i n g s of Furda (75), who a l s o found no lj>indiny of fatj^y acids to i n s o l u b l e p e c t i n complexed with Η , Ca , or Mg . The e f f e c t i v e n e s s of p e c t i n i n b i n d i n g b i l e s a l t s may w e l l depend on the extent of i t s complexing with t r i v a l e n t cations. Impairment of b i l e a c i d absorption and consequent l o s s of these a c i d s v i a e x c r e t i o n presumably causes an increase i n h e p a t i c conversion of c h o l e s t e r o l to b i l e a c i d s . This conversion lowers serum c h o l e s t e r o l , p a r t i c u l a r l y when serum contains high l e v e l s of c h o l e s t e r o l derived from d i e t a r y i n t a k e . However, when fed with a c h o l e s t e r o l - f r e e d i e t , 10% p e c t i n supplementation stimulated a 3 - f o l d increase i n c h o l e s t e r o l b i o s y n t h e s i s (77). Biosynthesis of phospholipids and t r i g l y c e r i d e s a l s o increased s i g n i f i c a n t l y ; hence, i t was suggested that these increases occurred i n response to diminished f a t absorption occasioned by p e c t i n i n t a k e . T h i s compensatory b i o s y n t h e s i s of c h o l e s t e r o l and l i p i d s may account f o r p e c t i n ' s i n a b i l i t y ( i n most cases) to lower serum c h o l e s t e r o l l e v e l s i n animals fed c h o l e s t e r o l - f r e e d i e t s . Regrettably, most studies r e l a t i n g d i e t a r y p e c t i n to hypocholesterolemia include no data on molecular weight or D.E. of the p e c t i n consumed. These omissions occur d e s p i t e a report as e a r l y as 1962 that high-molecular-weight, high D.E. p e c t i n s were more e f f e c t i v e i n reducing r a t l i v e r c h o l e s t e r o l l e v e l s


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(78). P e c t i n with 62% D.E. l a r g e l y counteracted the increase i n l i v e r c h o l e s t e r o l l e v e l s brought about by d i e t a r y c h o l e s t e r o l . Low e s t e r p e c t i n (30% D.E.), p o l y g a l a c t u r o n i c a c i d (PGA), and 50% e s t e r i f i e d PGA were a l l without e f f e c t . C o n f l i c t i n g reports on the e f f i c a c y of p e c t i n as a hypocholesterolemic agent may be due to v a r i a t i o n s i n these parameters. For example, c i t r u s p e c t i n was found to be more e f f e c t i v e than tomato p e c t i n i n lowering serum and hepatic c h o l e s t e r o l l e v e l s of r a t s (79). D.E. values f o r the c i t r u s and tomato p e c t i n s were 56 and 40%, r e s p e c t i v e l y . Thus, d i f f e r e n c e s i n D.E. l e v e l s rather than p e c t i n source could have accounted f o r the superior e f f e c t i v e ness of c i t r u s p e c t i n . Mokady (80) and Judd et a l . (81) confirmed that serum c h o l e s t e r o l reduction i s most pronounced w i t h high D.E., high-molecular-weight p e c t i n (Table I ) . Table I. E f f e c t of P e c t i n on Rats Fed Free D i e t (54).

a

Cholesterol-

R e l a t i v e values (% of c o n t r o l ) Fecal Blood Fecal cholesterol lipids sterols C o n t r o l (no pectin) Low MW p e c t i n Low e s t e r p e c t i n High MW p e c t i n

100 91 86 76

100 458 390 735

100 278 222 372

Recently the means by which p e c t i n lowers c h o l e s t e r o l l e v e l s and even the v a l i d i t y of t h i s e f f e c t have been questioned. Upon f i n d i n g no b i l e s a l t b i n d i n g c a p a c i t y f o r s o l u b l e p e c t i n , Baig and Cerda (76) proposed that p e c t i n lowered serum c h o l e s t e r o l l e v e l s by forming i n s o l u b l e complexes with the serum low d e n s i t y l i p o p r o t e i n s (LDL) which transport c i r c u l a t i n g c h o l e s t e r o l . Complexing of LDL by c i t r u s p e c t i n was observed i n v i t r o , but the way i n which p e c t i n or some component thereof enters the blood stream to e f f e c t such binding i n vivo has not been determined. P f e f f e r et a l . (82) have found that b i l e s a l t b i n d i n g a c t i v i t y of commercial c i t r u s pectins was l o s t i f these products were d i s s o l v e d , f i l t e r e d , c e n t r i f u g e d , and r e p r e c i p i t a t e d before t e s t i n g . Binding a c t i v i t y was concent r a t e d i n the residue p e l l e t from c e n t r i f u g a t i o n , which was found to be f i n e diatomaceous e a r t h . T h i s contaminant was probably introduced during f i l t r a t i o n steps i n processing and p u r i f i c a t i o n of p e c t i n . The authors concluded that any hypocholesterolemic e f f e c t of commercial p e c t i n was due s o l e l y to i t s diatomaceous earth contamination.



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Glucose Tolerance. Ingestion of carbohydrates temporarily i n c r e a s e s serum g l u c o s e l e v e l s a n d , i n r e s p o n s e , serum i n s u l i n levels. D i m i n i s h e d i n s u l i n l e v e l s o f d i a b e t i c s p e r m i t an excessive blood glucose r i s e , with u r i n a r y e x c r e t i o n of glucose. I n 1977 J e n k i n s e t a l . (83) r e p o r t e d t h a t b o t h h y p e r g l y c e m i a and i n s u l i n p e a k i n g a f t e r c o n s u m p t i o n o f c a r b o h y d r a t e s were reduced by d i e t a r y p e c t i n . A d d i t i o n o f 10 g o f p e c t i n t o f o o d r e d u c e d b l o o d g l u c o s e m e a s u r e d 15 m i n a f t e r e a t i n g , and d e c r e a s e d t h e i n s u l i n p e a k f o r 30 m i n . I w a s a k i e t a l . (84) a l s o r e p o r t e d on t h e p o s s i b l e use o f p e c t i n i n treatment o f diabetes. These authors observed a s i g n i f i c a n t r e d u c t i o n o f u r i n a r y g l u c o s e e x c r e t i o n when 15 g/day o f p e c t i n was i n c o r p o r a t e d i n the d i e t . C o n s u m p t i o n o f 8 g o f p e c t i n w i t h 30 g o f g l u c o s e i n p e e l e d o r a n g e s l o w e r e d t h e b l o o d g l u c o s e r e s p o n s e when compared t o c o n s u m p t i o n o f an e n e r g e t i c a l l y e q u i v a l e n t q u a n t i t y o f o r a n g e j u i c e c o n t a i n i n g o n l y 0.2 g o f p e c t i n (85). S i m i l a r r e s u l t s w i t h a p p l e s , a p p l e p u r e e , and j u i c e h a v e b e e n d i s c u s s e d b y H e a t o n (86). R e c e n t l y H o l t e t a l . (87) r e p o r t e d t h a t n o r m a l s u b j e c t s f e d 50 g o f g l u c o s e w i t h 14.5 g o f p e c t i n h a d l o w e r b l o o d glucose l e v e l s than c o n t r o l s . The a u t h o r s h y p o t h e s i z e d t h a t t h e d e p r e s s i o n o f g l u c o s e r e s p o n s e was an i n d i c a t i o n o f delayed absorption. To t e s t t h i s h y p o t h e s i s , t h e y f e d s u b j e c t s p e c t i n w i t h acetaminophen. A b s o r p t i o n o f a c e t a m i n o p h e n was a l s o s l o w e d b y p e c t i n , b u t t o t a l a b s o r p t i o n was u n a f f e c t e d . F u r t h e r , d e p r e s s i o n o f the blood glucose response by p e c t i n d i d n o t o c c u r i n a p a t i e n t whose s t o m a c h h a d b e e n removed. T h u s , i t was s u g g e s t e d t h a t p e c t i n may m o d e r a t e t h e g l u c o s e (and h e n c e t h e i n s u l i n ) r e s p o n s e b y d e l a y i n g t h e r a t e a t w h i c h stomach emptying o c c u r s . Detoxication of Metals. Pectins or pectin derivatives have been proposed as a n t i d o t e s f o r heavy m e t a l p o i s o n i n g f o r n e a r l y 200 y e a r s . K e r t e s z (9) h a s r e v i e w e d t h e e a r l y w o r k i n t h i s area. P e c t i n complexes l e a d so s t r o n g l y as t o q u a n t i t a t i v e l y remove i t f r o m s o l u t i o n . A b s o r p t i o n ôf l e a d , a r s e n i c , and s e l e n i u m b y s e v e r a l a n i m a l s p e c i e s h a s b e e n r e d u c e d b y pectin- or apple-containing diets. Use o f p e c t i n as a p r o p h y l a c t i c a g e n t i n l e a d p o i s o n i n g c o n t i n u e s t o be o f i n t e r e s t . B o n d a r e v (88) r e c e n t l y r e p o r t e d an i n c r e a s e i n e x c r e t i o n and a d e c r e a s e i n bone a c c u m u l a t i o n o f l e a d when r a t s f e d 6 mg/day o f l e a d a l s o r e c e i v e d 72-432 mg/day o f l o w e s t e r p e c t i n . PaskinsH u r l b u r t e t a l . (89) a c h i e v e d an 87% d e c r e a s e i n l e a d a b s o r p t i o n by p e c t a t e f e d r a t s . These s t u d i e s bear out the e a r l y o b s e r v a t i o n s o f F e l l e n b u r g ( s e e 9_, p . 572), who c o n c l u d e d t h a t p e c t i n o f d e c r e a s e d e s t e r c o n t e n t would have an enhanced a b i l i t y t o complex m e t a l s . In c o n c l u d i n g t h i s review o f p o t e n t i a l n u t r i t i o n a l a p p l i c a t i o n s o f p e c t i n , one m i g h t a s k : Can l e v e l s o f p e c t i n c o m p a r a b l e t o t h o s e u s e d i n t h e s e s t u d i e s be o b t a i n e d b y a r e a s o n a b l e consumption o f f r u i t o r f r u i t products? The answer


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would appear to be a q u a l i f i e d yes. T r u s w e l l , f o r example, c i t e s two studies a t t r i b u t i n g a hypocholesterolemic e f f e c t to the d a i l y consumption of s e v e r a l apples (59). Kay and S t i t t (85) s l i g h t l y reduced the hyperglycemic response to glucose with whole oranges. However, subjects consumed 624 g of f r u i t , and f r u i t was " t h i n l y peeled", presumably l e a v i n g much o f the albedo f o r consumption. In most c i t r u s f r u i t products as c u r r e n t l y consumed, p e c t i n content would not meet even the minimum l e v e l s found e f f e c t i v e i n n u t r i t i o n a l i n v e s t i g a t i o n s . Those p o r t i o n s of c i t r u s r i c h e s t i n p e c t i n — t h e p e e l , core and segment membranes—are u s u a l l y discarded as i n e d i b l e . J u i c e p e c t i n l e v e l s are f a r too low to allow f o r the necessary intake. J u i c e sacs or pulp o f g r a p e f r u i t as normally eaten contain only 0.3% p e c t i n on a f r e s h weight b a s i s (90). An intake of 6 g of p e c t i n [the minimal dose g i v i n g a s i g n i f i c a n t hypocholesterolemic e f f e c t (61)] would r e q u i r e consumption o f 2 kg/day of g r a p e f r u i t . By comparison, g r a p e f r u i t p e e l i s approximately 3.5% p e c t i n on a f r e s h weight b a s i s (91). Since 40% of a g r a p e f r u i t may be p e e l , a 500 g f r u i t would contain 7 g o f p e c t i n i n the p e e l alone. E d i b i l i t y o f t h i s p e e l i s diminished i n part by presence of the b i t t e r compound n a r i n g i n i n the albedo, and i n part by o i l from glands i n the flavedo. These obstacles to consumption of p e e l have been p a r t i a l l y overcome with a procedure described by Roe and Bruemmer (92, 93). Flavedo of f r u i t i s removed with a mechanical p e e l e r , and albedo i s vacuum infused with a s o l u t i o n c o n t a i n i n g naringinase, sugar, f l a v o r , c o l o r and g e l a t i n . Naringinase reduces b i t t e r n e s s ; sugar, f l a v o r , and c o l o r enhance p a l a t a b i l i t y and a c c e p t a b i l i t y ; and g e l a t i n reduces leakage o f the infused s o l u t i o n . This treatment improves the hedonic r a t i n g of g r a p e f r u i t p e e l from i n e d i b l e to marginally acceptable (93). With f u r t h e r refinement, t h i s technique could perhaps make a v a i l a b l e a new d i e t a r y source of p e c t i n . N u t r i t i o n a l b e n e f i t could thus be obtained from a source now considered as waste.

Literature Cited 1. Zitco, V.; Bishop, C. T. Can. J. Chem., 1966, 44, 1275. 2. VanBuren, J. P. J. Text. Stud., 1979, 10, 1. 3. Aspinall, G. O. In "The Carbohydrates", Academic Press: New York, 1970; p. 515. 4. Fogarty, W. M.; Ward, D. P. In "Progress in Industrial Microbiology"; Hockenhull, D. J. D., Ed.; Churchill Livingstone: Edinburgh, 1974; p. 59. 5. Albersheim, P. In "Plant Carbohydrate Chemistry"; Pridham, J. Β., Ed.; Academic Press: London, 1974. 6. Pilnik, W.; Voragen, A. G. J. In "The Biochemistry of Fruits and Their Products"; Hulme, A. C., Ed.; Academic Press: New York, 1970; p. 53.



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7. Nelson, D. B. Proc. Int. Soc. Citriculture, 1977, 3, 739. 8. Sinclair, W. B. In "The Orange"; Sinclair, W. B., Ed.; Univ. of Calif.: Riverside, 1961, p. 191. 9. Kertesz, Ζ. I. "The Pectic Substances"; Interscience Publishers, Inc.: New York, 1951. 10. Deuel, H.; Stutz, E. Adv. Enzymol., 1958, 20, 341. 11. Rees, D. A. Chem. Ind., 1972, 1972, 630. 12. Grant, G. T.; Morris, E. R.; Rees, D. Α.; Smith, P. J. C.; Thom, D. FEBS Letters, 1973, 32, 195. 13. Rouse, A. H.; Atkins, C. D. Univ. Fla. Agric. Expt. Stn., 1955, Bull. 570. 14. Joslyn, M. A. Adv. Food Res., 1962, 11, 1. 15. Rouse, A. H. In "Citrus Science and Technology"; Nagy, S.; Shaw, P. E.; Veldhuis, M. Κ., Ed.; Avi Pub. Co.: Westport, CN., 1977; Vol. I, p. 110. 16. Ingram, M. Board for Scientific and Industrial Res., Israel, 1948. 17. Baker, R. Α.; Bruemmer, J. H. Proc. Fla. State Hortic. Soc., 1971, 84, 197. 18. Berk, Z. Food Technol., 1964, 18, 1811. 19. Rouse, A. H.; Atkins, C. D.; Moore, E. L. Proc. Fla. State Hortic. Soc., 1959, 72, 227. 20. Braddock, R. J.; Kesterson, J. W. Proc. Fla. State Hortic. Soc., 1975, 88, 292. 21. Braddock, R. J.; Kesterson, J. W. Proc. Fla. State Hortic. Soc., 1974, 87, 310. 22. Joslyn, Μ. Α.; Pilnik, W. In "The Orange"; Sinclair, W. Β., Ed.; Univ. Calif. Press: Riverside, 1961; p. 373. 23. Rouse, A. H. Proc. Fla. State Hortic. Soc., 1949, 62, 179. 24. Olsen, R. W.; Huggart, R. L.; Asbell, D. M. Food Technol., 1951, 5, 530. 25. Atkins, C. D.; Rouse, A. H.; Huggart, R. L.; Moore, E. L.; Wenzel, F. W. Food Technol., 1953, 7, 62. 26. Moore, E. L.; Rouse, A. H.; Atkins, C. D.; Hill, E. C. Proc. Fla. State Hortic. Soc., 1969, 82, 224. 27. Rouse, A. H.; Moore, E. L.; Atkins, C. D. Proc. Fla. State Hortic. Soc., 1969, 82, 227. 28. Stevens, J. W.; Pritchett, D. E.; Baier, W. E. Food Technol., 1950, 4, 469. 29. Scott, W. C.; Kew, T. J.; Veldhuis, M. K. J. Food Sci., 1965, 30, 833. 30. Krop, J. J. P. Agric. Res. Rep., Wageningen, Netherlands, 1974, No. 830. 31. Baker, R. A. J. Agric. Food Chem., 1979, 27, 1387. 32. Dietz, J. H.; Rouse, A. H. Food Res., 1953, 18, 169. 33. Baker, R. Α.; Bruemmer, J. H. Proc. Fla. State Hortic. Soc., 1969, 82, 215. 34. Primo Yufera, E.; Koen Mosse, J.; Royo Iranzo, J. First Int. Cong. Food Sci. Technol., 1965, 2, 337.


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35. Rouse, A. H.; Atkins, C. D.; Moore, E. L. Food Technol., 1957, 11, 218. 36. Baker, R. Α.; Bruemmer, J. H. Proc. Fla. State Hortic. Soc., 1972, 85, 225. 37. Bissett, O. W.; Veldhuis, M. K.; Rushing, Ν. B. Food Technol., 1953, 7, 258. 38. Guyer, R. B.; Miller, W. M.; Bissett, O. W.; Veldhuis, M. K. Food Technol., 1956, 10, 16. 39. Versteeg, C. Agric Res. Rep., Wageningen, Netherlands, 1979, No. 892. 40. Baker, R. Α.; Bruemmer, J. H. J. Agric. Food Chem., 1972, 20 1169. 41. Baker, R. Α.; Bruemmer, J. H. Patent 3,754,932, 1973. 42. Krop, J. J. P.; Pilnik, W. Lebensm.-Wiss. Technol., 1974, 7, 121. 43. Stevens, J. W.; Pritchett, D. E. Patent 2,599,519, 1950. 44. Baker, R. A. Proc Fla. State Hortic. Soc., 1980, in press. 45. Baker, R. A. J. Food Sci., 1976, 41, 1198. 46. Kefford, J. F.; Chandler, Β. V. "The Chemical Constituents of Citrus Fruit"; Academic Press: New York, 1970. 47. Berenovsky, N.; Mannheim, C. H. Confructa, 1976, 21, 181. 48. Olsen, R. W.; Barron, R. W.; Huggart, R. L.; Wenzel, F. W. Proc. Fla. State Hortic. Soc., 1966, 79, 326. 49. Trowell, H. Lancet, 1974, 1, 503. 50. Spiller, G. A. Am. J. Clin. Nutr., 1978, 31, 5231. 51. Spiller, G. Α.; Fasset-Cornelius, G. Am. J. Clin. Nutr., 1976, 29, 934. 52. Kertesz, Ζ. I. J. Nutr., 1940, 20, 289. 53. Werch, S. C.; Ivy, A. C. Am. J. Dig. Dis., 1941, 8, 101. 54. Werch, S. C.; Ivy, A. C. Am. J. Dis. Child., 1941, 62, 499. 55. Viola, S.; Zimmerman, G.; Mokady, S. Nutr. Rep. Int., 1970, 1, 367. 56. Hove, Ε. L.; King, S. J. Nutr., 1979, 109, 1274. 57. Campbell, L. Α.; Palmer, G. H. In "Topics in Dietary Fiber Research", Spiller, G. Α.; Amen, R. J . , Ed.; Plenum Press: New York, 1978, p. 105. 58. Cummings, J. H.; Southgate, P. A. T.; Branch, W. J.; Wiggins, H. S.; Houston, H.; Jenkins, P. J. Α.; Jivraj, T.; Hill, M. J. Br. J. Nutr., 1979, 41, 477. 59. Truswell, A. S. In "Pietary Fiber: Current Pevelopments of Importance to Health"; Heaton, K. W., Ed.; Technomic Pub. Co., Inc.: Westport, CN, 1979, p. 105. 60. Keys, Α.; Grande, F.; Anderson, J. T. Proc. Soc. Exp. Biol. Med., 1961, 106, 555. 61. Palmer, G. H.; Dixon, D. G. Am. J. Clin. Nutr., 1966, 18, 437. 62. Jenkins, D. J. Α.; Leeds, A. R.; Newton, C.; Cummings, J. H. Lancet, 1975, 1, 1116. 63. Kay, R. M.; Truswell, A. S. Am. J. Clin. Nutr., 1977, 30, 171. 64. Miettenen, Τ. Α.; Tarpila, S. Clin. Chim. Acta., 1977, 79, 471. 9


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