Pectin Internal Gel Strength: Theory ... - American Chemical Society

8 Pectin Internal Gel Strength: Theory, Measurement, and Methodology Philip G. Crandall and Louise Wicker

Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: June 5, 1986 | doi: 10.1021/bk-1986-0310.ch008

Institute of Food and Agricultural Sciences, Citrus Research and Education Center, University of Florida, Lake Alfred, FL 33880

The single largest use of pectin is in the manufacture of jelly. About 80 to 90% of the 6 to 7 million kg of commercial pectin is used to make jelly and similar products. The consumer's acceptance, organoleptic properties, and quality of a jelly are largely dependent on the internal gel strength formed by pectin. Most of this pectin is high methoxyl and its gel structure is primarily stabilized by hydrogen bonding and hydropholic interactions. The strength of the gel is affected by the pH of the product, the pectin's methoxyl content, and molecular weight. This review discusses the above factors and the instrumentation used to measure the internal strength of high methoxyl pectin gels. These are categorized into instruments which are nondestructive of the gel and measures its elastic properties or destructive type instruments which exceed the gel's strength. Examples and advantages of each type of instrument are presented. It is hoped this review will be helpful to jelly and pectin manufacturers who are currently evaluating instruments to measure internal gel strength. P e c t i n i s a group d e s i g n a t i o n f o r c o l l o i d a l p o l y g a l a c t u r o n i c a c i d s , the c h e m i s t r y o f w h i c h has been d i s c u s s e d i n p r e v i o u s c h a p t e r s . Most p l a n t s c o n t a i n p e c t i n i n the i n t e r c e l l u l a r l a y e r between the primary c e l l w a l l s o f a d j o i n i n g c e l l s . S i x t o seven m i l l i o n kg o f p u r i f i e d p e c t i n are produced a n n u a l l y , more than h a l f o f which i s e x t r a c t e d from c i t r u s p e e l Of t h i s amount, 80 t o 90% i s used i n the manufacture o f j e l l i e s , jams, and s i m i l a r p r o d u c t s (2^). I n t h i s c h a p t e r , j e l l y w i l l be used t o denote the product formed from p e c t i n , s u g a r , and a c i d under s p e c i f i c c o n d i t i o n s . G e l w i l l mean a s i m i l a r p h y s i c a l s t a t e but not the commercial product ( 3 ) . In m a n u f a c t u r i n g f r u i t j e l l i e s , p e c t i n i s u s u a l l y added t o augment the p e c t i n n a t u r a l l y o c c u r r i n g i n the f r u i t j u i c e i n o r d e r to a c h i e v e the d e s i r e d f i r m n e s s o r c o n s i s t e n c y o f the j e l l y . Jelly 0097-6156/ 86/ 0310-O088S06.00/ 0 © 1986 American Chemical Society

In Chemistry and Function of Pectins; Fishman, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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i s made from not l e s s than 45 p a r t s by weight o f f r u i t j u i c e w i t h 55 p a r t s sugar f o r a t o t a l of not l e s s t h a n 65% s o l u b l e s o l i d s . Jams and marmalades a r e s i m i l a r t o j e l l i e s but u t i l i z e the whole f r u i t , f r u i t p i e c e s , or p i e c e s o f c i t r u s p e e l (4, _5). A U.S. Grade A j e l l y must not s c o r e l e s s t h a n 90 p o i n t s on a 100 p o i n t s c a l e where c o n s i s t e n c y , c o l o r , and f l a v o r o f the g e l r e p r e s e n t 40, 20, and 40 points, respectively. A good c o n s i s t e n c y means "the j e l l y has a t e n d e r t o s l i g h t l y f i r m t e x t u r e and r e t a i n s a compact shape w i t h o u t e x c e s s i v e s y n e r e s i s (weeping)" (4_). The l e v e l s of p e c t i n use range from 0.1 t o 1.0% i n the f i n a l j e l l y or jam and a r e c r i t i c a l to i t s s u c c e s s ( 6 ) . There are s e v e r a l good r e v i e w a r t i c l e s which c o v e r o t h e r uses o f p e c t i n and c o n t a i n f o r m u l a s and p r a c t i c a l i n f o r m a t i o n (_5, 7_ 8-18). Commercial p e c t i n s a r e g e n e r a l l y r e c o g n i z e d as s a f e (GRAS) food i n g r e d i e n t s under FDA r e g u l a t i o n s (19) and are c l a s s i f i e d i n t o two main c a t e g o r i e s a c c o r d i n g t o t h e i r degree o f m e t h y l a t i o n (DM)· Low methoxyl (LM) p e c t i n s have from 25 t o 50% DM and w i l l form a g e l i n the p r e s e n c e o f d i v a l e n t c a t i o n s , s u c h as c a l c i u m . The second main type o f p e c t i n i s the h i g h m e t h o x y l (HM) p e c t i n w i t h DM v a l u e s r a n g i n g from 50 t o 80% (7_> 2 0 ) . HM p e c t i n s w i l l form g e l s i n the p r e s e n c e of sugar and a c i d and a r e f u r t h e r d i v i d e d i n t o broad c a t e g o r i e s o f r a p i d s e t p e c t i n s , 75 to 72% DM, which g e l i n 20 t o 70 s e c ; medium s e t p e c t i n s , 71 t o 68% DM, which g e l i n 100 t o 150 s e c ; and slow s e t p e c t i n s , 66 t o 62% DM, which g e l i n 180 t o 250 s e c (Table I) ( 8 ) .


y

T a b l e I . C a t e g o r i e s of H i g h M e t h o x y l P e c t i n ( C i t r u s ) w i t h Approximate Degree of M e t h y l a t i o n , S e t t i n g Times, and Temperatures

Degree o f methylation Ultra rapid Rapid s e t Medium s e t Slow s e t Reference

>75 75 71 66 -

Setting time (sec)

— 72 68 62

20 100 180 -

Setting temperature (°C)

— 70 150 250

97 92 83 -

95 87 72

(8)

T h i s r e v i e w w i l l d i s c u s s the needs o f the j e l l y m a n u f a c t u r e r to measure g e l s t r e n g t h , the t h e o r y o f g e l f o r m a t i o n , f a c t o r s a f f e c t i n g g e l s t r e n g t h , and i n s t r u m e n t a t i o n used t o measure g e l s t r e n g t h . J e l l y M a n u f a c t u r e r ' s Needs. D u r i n g the f i r s t h a l f o f t h i s c e n t u r y , t h e r e was no u n i f o r m method to measure the a b i l i t y o f p e c t i n t o form a g e l . C o n s e q u e n t l y , j e l l y m a n u f a c t u r e r s had t o c o n s t a n t l y a d j u s t the amount o f p e c t i n used per b a t c h o f j e l l y w i t h l i t t l e a s s u r a n c e t h a t the appearance and t e x t u r e o f the f i n i s h e d p r o d u c t would be o f a h i g h q u a l i t y . A f t e r the a d o p t i o n of the I n s t i t u t e o f Food T e c h n o l o g i s t ' s (IFT-SAG) method i n 1959, a l l p e c t i n s were s t a n d a r d i z e d on t h e i r a b i l i t y t o form a g e l . T h i s method has been used by p e c t i n and j e l l y m a n u f a c t u r e r s f o r more t h a n 25 y e a r s . The d e t a i l s are d i s c u s s e d l a t e r i n t h i s chapter. A l t h o u g h the IFT-SAG method has been the p e c t i n g r a d i n g s t a n d a r d , i t has some


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d e f i c i e n c i e s i n measuring i n t e r n a l g e l s t r e n g t h . So, m a n u f a c t u r e r s a r e l o o k i n g f o r a s u p p l e m e n t a l method f o r p e c t i n e v a l u a t i o n which would c o r r e l a t e w e l l w i t h s p r e a d a b i l i t y , be i n c l o s e agreement w i t h commercial p r a c t i c e , and g i v e r e p r o d u c i b l e r e s u l t s . The equipment s h o u l d be d u r a b l e , o p e r a t o r i n d e p e n d e n t , and i n e x p e n s i v e . Ideally, the p e c t i n s h o u l d be added t o t h e j e l l y t e s t b a t c h , a s i n commercial o p e r a t i o n s , so i t w i l l be i n c o n t a c t w i t h t h e a c i d and sugar d u r i n g the l a s t phase o f c o o k i n g . I f t h e t e s t were r e l a t i v e l y r a p i d , i t c o u l d p r o v i d e r e a l time feedback f o r c o n t r o l o f t h e next b a t c h o f jelly. F i n a l l y , t h e t e s t method s h o u l d r e l a t e t o t h e consumer's p e r c e p t i o n o f t e x t u r e , be i n a c c o r d a n c e w i t h t h e t h e o r y o f g e l f o r m a t i o n , and be based on fundamental t e s t p r i n c i p l e s . Theory o f G e l F o r m a t i o n . I t i s important t o understand the f o r c e s t h a t s t a b i l i z e a p e c t i n g e l s t r u c t u r e and t h e mechanisms o f p e c t i n g e l a t i o n b e f o r e examining t h e i n s t r u m e n t s used t o measure i n t e r n a l gel strength. P e c t i n was d e f i n e d by K e r t e s z (3) as polymers o f c o l l o i d a l polygalacturonic acid containing a s i g n i f i c a n t proportion o f m e t h y l e s t e r groups and h a v i n g c a p a b i l i t i e s o f f o r m i n g g e l s w i t h sugar and a c i d . P e c t i n i c a c i d s a r e composed p r i m a r i l y o f α 1,4 linked D-galacturonic acid u n i t s . In p l a n t s , p e c t i n i s thought o f as a m i x t u r e o f s t r u c t u r a l , c a r b o h y d r a t e m o l e c u l e s w i t h a g e n e r a l i z e d r a t h e r than s p e c i f i c c o m p o s i t i o n . P e c t i n s have a l o c a l i z e d d i s t r i b u t i o n o f c o v a l e n t l y attached n e u t r a l sugars forming h a i r y and smooth r e g i o n s ( 2 1 ) . Rees (22) p r o p o s e d f o u r l e v e l s o f s t r u c t u r e f o r p o l y s a c c h a r i d e s like pectin. P r i m a r y s t r u c t u r e r e f e r s t o t h e n a t u r e and mode o f l i n k a g e o f t h e component g a l a c t u r o n i c a c i d s . In p e c t i n , p o l y - D - g a l a c t u r o n a t e forms a r e g u l a r , b u c k l e d , t w o - f o l d c o n f o r m a t i o n (23) e x c e p t where a k i n k i s formed by t h e i n s e r t i o n o f L-rhamnose o r another sugar. Secondary s t r u c t u r e r e f e r s t o the p y r a n o s e r i n g shapes. N e l s o n ( 2 0 ) reviewed t h e e v i d e n c e t h a t t h e 4 - C - l c o n f o r m a t i o n i s f a v o r e d due t o m i n i m i z a t i o n o f t h e s t e r i c r e p u l s i o n between a x i a l s u b s t i t u e n t s . The r e s t r i c t i v e r o t a t i o n a l a n g l e s (anomeric c a r b o n one t o pendent oxygen) and (carbon four t o pendent oxygen) d e t e r m i n e t h e o v e r a l l c o n f o r m a t i o n o f t h e polysaccharide chain. T e r t i a r y s t r u c t u r e o f p e c t i n i s the s p e c i f i c , r i g i d r o d - l i k e geometry which i s f a v o r e d by n o n c o v a l e n t i n t e r a c t i o n s , r i g i d s e c o n d a r y s t r u c t u r e , e f f i c i e n t p a c k i n g , and i s i n h i b i t e d by l o s s o f c o n f o r m a t i o n a l energy, h y d r a t i o n , i n t e r m o l e c u l a r e l e c t r o s t a t i c r e p u l s i o n , and s t r u c t u r a l irregularities. T e r t i a r y s t r u c t u r e s may remain i n t a c t even a f t e r h y d r a t i o n i n s o l u t i o n o r i n a g e l network. Quaternary s t r u c t u r e i s the i n t e r a c t i o n o f s p e c i f i c , r i g i d u n i t s o f t e r t i a r y s t r u c t u r e t o form a h i g h e r l e v e l o f o r g a n i z a t i o n ( 2 2 ) . S t u d i e s on t h e f o r c e s t h a t s t a b i l i z e t h e p e c t i n g e l network have c o n t r i b u t e d t o o u r u n d e r s t a n d i n g o f t h e mechanism o f g e l a t i o n . Owens and Maclay ( 2 4 ) r e p o r t e d t h a t hydrogen bonding between h y d r o x y l s o f p e c t i n , water, and sugar was promoted by a d e c r e a s e i n pH, due t o a d e c r e a s e i n i n t e r m o l e c u l a r , e l e c t r o s t a t i c r e p u l s i o n . In a d d i t i o n , weak v a n d e r Waals f o r c e s between m e t h y l e s t e r groups c o n t r i b u t e t o t h e s t a b i l i z a t i o n o f t h e g e l network ( 2 4 ) . McCready and Owens (25) and M o r r i s e t a l . (26) proposed t h a t e s t e r groups make a p o s i t i v e c o n t r i b u t i o n t o i n t e r c h a i n a s s o c i a t i o n . M o r r i s e t a l . (26) o b s e r v e d a d e c r e a s e i n g e l s t r e n g t h o f 72% e s t e r p e c t i n



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g e l s i n the presence o f 8M u r e a and proposed t h a t the g e l network i s s t a b i l i z e d by n o n c o v a l e n t f o r c e s analogous t o f o r c e s s t a b i l i z i n g tertiary structure i n proteins. Doesburg (27) d i s c u s s e d the phenomenon of g e l a t i o n and d e f i n e d g e l s as two-phase systems w i t h a d i s c o n t i n u o u s phase o f s o l i d m a t e r i a l which r e s t r i c t s a f i n e l y d i s p e r s e d o r c o n t i n u o u s aqueous phase. N e l s o n (20) proposed a g e l model i n which the p e c t i n m o l e c u l e s were random r i b b o n s i n the s o l state,- p r o g r e s s i n g t o l e s s random r i b b o n s o f lower water a c t i v i t y i n the second s t a t e , and were a g e l i n the t h i r d s t a t e where most of the p e c t i n c h a i n s were i n v o l v e d i n some t y p e o f c h a i n s t a c k i n g . In a HM p e c t i n g e l , a c i d , s u g a r , w a t e r , and p e c t i n i n t e r a c t t o form a g e l . At lower pH v a l u e s near pH 3, e l e c t r o s t a t i c r e p u l s i o n i s m i n i m i z e d due t o the g r e a t e r number o f n o n i o n i z e d c a r b o x y l groups which enhance the p r o b a b i l i t y of n o n c o v a l e n t a t t r a c t i o n among the m e t h o x y l , a l c o h o l , and c a r b o x y l groups ( 2 4 ) . The f u n c t i o n o f sugar has been l e s s c l e a r . I t has been suggested t h a t the sugar a c t s as a d e h y d r a t i n g agent (28) and promotes hydrogen bonding between p e c t i n , w a t e r , and sugar ( 2 9 ) . Rees (30) suggested t h a t sugar c o n t r o l l e d the water a c t i v i t y . F u r t h e r p r o g r e s s towards u n d e r s t a n d i n g the i n t e r r e l a t i o n s h i p of s u g a r , a c i d , w a t e r , and HM p e c t i n has been made by O a k e n f u l l and S c o t t ( 3 1 ) . They proposed t h a t h y d r o p h o b i c i n t e r a c t i o n s a r e e s s e n t i a l t o g e l f o r m a t i o n i n HM p e c t i n . However, t h e c o n t r i b u t i o n of hydrogen bonding t o the s t a n d a r d f r e e energy o f f o r m a t i o n i s n e a r l y t w i c e t h a t o f h y d r o p h o b i c i n t e r a c t i o n s and hydrogen bonding a l o n e was not s u f f i c i e n t t o overcome the e n t r o p y b a r r i e r t o g e l a t i o n from the l o s s o f d i s o r d e r ( 3 1 ) . Even though s u c r o s e i n c r e a s e d h y d r o p h o b i c i n t e r a c t i o n s by 67% ( 3 2 ) , o t h e r p o l y o l s , such as s o r b i t o l were more e f f i c i e n t a t s t a b i l i z i n g h y d r o p h o b i c i n t e r a c t i o n s than s u c r o s e ( 3 2 ) . They f u r t h e r r e p o r t e d t h a t the f o r m a t i o n o f j u n c t i o n zones from 18 t o 250 g a l a c t u r o n i c a c i d u n i t s i n l e n g t h , between two p e c t i n m o l e c u l e s , s t a b i l i z e d the p e c t i n g e l network ( 3 2 , 33). The l e n g t h o f the j u n c t i o n zone depends on the h y d r o p h o b i c i n t e r a c t i o n s , w h i c h depend on the c o n c e n t r a t i o n of sugar o r p o l y o l and on the DM v a l u e o f p e c t i n ( 3 1 ) . C o n s i d e r a t i o n o f the g e l f o r m a t i o n f o r Ui p e c t i n s has been p u r p o s e l y o m i t t e d because of space l i m i t a t i o n s . R e f e r e n c e s h e l p f u l i n u n d e r s t a n d i n g the f o r m a t i o n o f LM g e l s a r e 2> 23, 27, 34, 35. Factors A f f e c t i n g I n t e r n a l Gel Strength S e v e r a l o f the f a c t o r s t h a t a f f e c t g e l s t r e n g t h w i l l be d i s c u s s e d such as pH, m o l e c u l a r w e i g h t , type o f p e c t i n , and c o n d i t i o n s used t o t e s t the g e l . The m a j o r i t y o f p e c t i n i s used i n the manufacture o f j e l l i e s and jams, so t h i s r e v i e w w i l l f o c u s on HM p e c t i n s . Jelly grade i s d e f i n e d as the number of grams o f sugar w i t h which one gram of p e c t i n w i l l form a 65% s o l u b l e s o l i d s g e l of s p e c i f i e d s t r e n g t h under s u i t a b l e a c i d c o n d i t i o n s ( 3 ) . O l l i v e r (36) p o i n t e d out two o m i s s i o n s i n t h i s d e f i n i t i o n . F i r s t , no r e f e r e n c e i s made t o a s t a n d a r d g e l s t r e n g t h f o r the " s e t " of the j e l l y . Second, from the d e f i n i t i o n , the amount o f sugar i s the v a r i a b l e ; however, i n p r a c t i c e , the sugar c o n c e n t r a t i o n i s h e l d c o n s t a n t a t 65% s o l u b l e s o l i d s ; t h e r e f o r e , i t i s the amount o f p e c t i n t h a t i s v a r i e d .


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PECTINS

Pectin. The h e t e r o g e n e i t y o f p e c t i n i s d i f f i c u l t t o c o n t r o l d u r i n g m a n u f a c t u r e , not w e l l c h a r a c t e r i z e d , and i n f l u e n c e s g e l s t r e n g t h i n commercial j e l l y b a t c h e s . Pectin i s a naturally occurring p o l y s a c c h a r i d e which i s m a i n l y e x t r a c t e d from c i t r u s p e e l and a p p l e pomace. R e s e a r c h e r s have r e p o r t e d on d i f f e r e n c e s among the y i e l d and j e l l y grade o f p e c t i n s from v a r i o u s c i t r u s c u l t i v a r s (37, 38), between f r e s h and d r y c i t r u s p e e l ( 3 9 ) , and i n p e c t i n e x t r a c t i o n c o n d i t i o n s (40, 4 1 ) . F u r t h e r m o r e , t h e r e a r e d i f f e r e n c e s among a p p l e s t a r t i n g m a t e r i a l s (27, 42) and d i f f e r e n c e s between the r e s u l t a n t p e c t i n s made from c i t r u s and a p p l e ( 4 3 ) . In a d d i t i o n t o n a t u r a l v a r i a t i o n i n c o m m e r c i a l l y p r e p a r e d p e c t i n s , the DM o f p e c t i n i n f l u e n c e s i n t e r n a l g e l s t r e n g t h . The s e n s i t i v i t y o f p e c t i n t o d i v a l e n t c a t i o n s and the mechanism o f g e l f o r m a t i o n i s i n f l u e n c e d by the DM v a l u e . Furthermore, j e l l y grade can be i n c r e a s e d by d e c r e a s i n g the DM v a l u e by s a p o n i f i c a t i o n . Doesburg and G r e v e r s (44) showed t h a t the j e l l y grade was 25% h i g h e r i n a 53% DM p e c t i n than i n a 75% DM p e c t i n made from the same s t a r t i n g m a t e r i a l . W i l e s and Smit (45) have a p a t e n t to p r e p a r e a low DM p e c t i n w i t h a h i g h m o l e c u l a r weight (MW) which produces a g e l w i t h an e x t r e m e l y h i g h r u p t u r e s t r e n g t h . A d d i t i o n a l v a r i a b i l i t y e x i s t s because DM v a l u e s a r e heterogeneous among p e c t i n s . Walter and Sherman (46) found t h a t a t a s p e c i f i c DM v a l u e , the p h y s i c a l p r o p e r t i e s o f a HM p e c t i n a r e i n f l u e n c e d by the d i s p e r s i o n and l o c a t i o n o f f r e e c a r b o x y l s . Another HM p e c t i n w i t h a s i m i l a r DM, but d i f f e r e n t d i s t r i b u t i o n o f unmethoxylated c a r b o x y l s , p o s s e s s e d different physical characteristics. The DM i n t e r a c t s w i t h g e l pH t o d e t e r m i n e the maximum g e l s t r e n g t h measurement. In the t y p i c a l c u r v e shown i n F i g u r e 1, the optimum f i r m n e s s o c c u r s a t h i g h e r pH v a l u e s f o r h i g h e r DM p e c t i n s ·

M o l e c u l a r Weight. Swenson e t a l . (48) determined t h a t i n s t r u m e n t s used t o measure the b r e a k i n g s t r e n g t h o f p e c t i n g e l s a s s i g n a h i g h e r grade to h i g h l y p o l y m e r i z e d , h i g h e r MW p e c t i n s than do i n s t r u m e n t s measuring e l a s t i c i t y . C h r i s t e n s e n (49) s u p p o r t e d t h i s by showing t h a t the r e l a t i o n s h i p between b r e a k i n g s t r e n g t h and sag was not c o n s t a n t but depended on the MW o f the p e c t i n . P e c t i n s whose m o l e c u l a r w e i g h t s were near 135,000 were graded e q u a l l y by t h e s e two methods. When the MW was g r e a t e r than 135,000, the d e s t r u c t i v e measurements graded the p e c t i n s h i g h e r . M i t c h e l l and B l a n s h a r d (50) wrote p e c t i n g e l s have e i t h e r s h o r t and v e r y s t i f f o r l o n g e r and more f l e x i b l e c h a i n s . The e l a s t i c i t y modulus i s i n f l u e n c e d p r i m a r i l y by the s h o r t s t i f f c h a i n s and i s independent o f p e c t i n MW above a minimum. B r e a k i n g s t r e n g t h i s i n f l u e n c e d p r i m a r i l y by the l o n g e r , more f l e x i b l e c h a i n s which a r e l a r g e l y MW dependent and remain c r o s s l i n k e d a f t e r the s h o r t s t i f f c h a i n s have r u p t u r e d . M i t c h e l l (51, 52) a l s o found measurements o f the apparent e l a s t i c modulus a t s h o r t times ( l i k e the R i d g e l i m e t e r r e a d i n g ) t o be independent o f MW, above a c e r t a i n l i m i t i n g v a l u e . Thus, a p e c t i n w i t h a h i g h R i d g e l i m e t e r grade would be expected to have a g r e a t e r number of s h o r t s t i f f c h a i n s and a h i g h e r c r o s s l i n k d e n s i t y . This p r o p e r t y may be c o n t r o l l e d to a c e r t a i n e x t e n t by p e c t i n m a n u f a c t u r e r s and s h o u l d be s e l e c t e d f o r by p e c t i n u s e r s f o r products needing a h i g h e l a s t i c i t y c h a r a c t e r i s t i c .



8. CRANDALL AND WICKER

93


DEGREE OF METHYLATION

I

I

I

I

I

I

10

3.2

3.4

3.6

3.8

40

GEL pH F i g u r e 1. R e l a t i o n s h i p among g e l s t r e n g t h , degree o f m e t h y l a t i o n and g e l pH ( E h r l i c h , 7 ) .


94


J e l l y pH. Another f a c t o r which i n f l u e n c e s t h e i n t e r n a l g e l s t r e n g t h i s the j e l l y pH. I n the l a b o r a t o r y , O l l i v e r (53) found pH d i f f e r e n c e s between 2.82 and 3.12 c o u l d cause a v a r i a t i o n i n j e l l y grade o f about 30%. The i n t e r a c t i o n s between j e l l y pH and g e l s t r e n g t h i s shown i n F i g u r e 1. A c i d s a r e used t o c o n t r o l the pH w i t h i n the g e l l i n g range. The optimium pH i s about 3.1 and 3.4 f o r slow s e t HM p e c t i n s and r a p i d s e t p e c t i n s , r e s p e c t i v e l y ( 4 7 ) .


J e l l y Test C o n d i t i o n s . Other f a c t o r s a f f e c t i n g the i n t e r n a l g e l s t r e n g t h a r e t h e r a t e and d u r a t i o n o f b o i l i n g a t e s t j e l l y , s t a g e o f a c i d a d d i t i o n , r a t e o f c o o l i n g , time o f a g i n g , t y p e o f sugar used ( 5 3 , 5 4 ) , and the use o f b u f f e r s a l t s o r s y n t h e t i c f r u i t j u i c e s r a t h e r than water ( 4 3 ) . III.

Instrumentation

S e v e r a l good r e v i e w s have been w r i t t e n on i n s t r u m e n t s used t o measure p e c t i n ' s g e l s t r e n g t h , i n c l u d i n g O l l i v e r ( 3 6 ) , C h r i s t e n s e n ( 4 9 ) , Beltman and P i l n i k ( 5 5 ) , M i t c h e l l (51_, 5 2 ) , and Sherman ( 5 6 ) . Both M i t c h e l l (51) and Sherman (56) have c o n c i s e T a b l e s w h i c h l i s t about 20 i n s t r u m e n t s and g i v e t y p e s o f measurements w h i c h c a n be made. T a b l e I I g i v e s examples o f i n s t r u m e n t s used t o e v a l u a t e p e c t i n g e l s which w i l l be covered i n t h i s r e v i e w . I d e a l l y , i n s t r u m e n t a t i o n t o measure t h e i n t e r n a l g e l s t r e n g t h o f a p e c t i n g e l s h o u l d emulate t h e p e r c e p t i o n o f g e l s t r u c t u r e by the consumer. T h i s p e r c e p t i o n o f g e l s t r u c t u r e by the consumer i s an i n t e r a c t i o n o f s i g h t , t a s t e , and t e x t u r e . V i s u a l o b s e r v a t i o n s a r e combined w i t h the s e n s a t i o n o f t o u c h when a j e l l y i s c u t from a j a r and s p r e a d . I n the mouth, the g e l i s compressed, f r a c t u r e d , and t h e r u p t u r e d p a r t s s l i d e past each o t h e r ( 5 6 ) . Most p o l y s a c c h a r i d e g e l s are composed o f d i f f e r e n t t e x t u r a l p r o f i l e s . T h e r e f o r e , when a s e r i e s o f g e l s i s r a n k e d , the o r d e r c a n v a r y g r e a t l y depending on the weight the consumer p l a c e s on each i n d i v i d u a l a t t r i b u t e ( 5 7 ) . A s e n s o r y e v a l u a t i o n r a t i n g s c a l e has been developed f o r s e v e r a l g e l t e x t u r e a t t r i b u t e s ( 5 8 ) . Models have a l s o been developed t o r e l a t e s p r e a d a b i l i t y as b e i n g the i n v e r s e o f t h e f o r c e ( s h e a r s t r e s s ) e x e r t e d on the s u r f a c e o f a k n i f e ( 5 9 ) . G i v e n t h a t consumers use a wide range o f e v a l u a t i o n methods t o determine t h e t e x t u r e o f a g e l , i t i s e x t r e m e l y d i f f i c u l t t o r e l a t e a l l o f the p e r c e i v e d t e x t u r a l c h a r a c t e r i s t i c s t o a s i n g l e instrument. Sherman (56) and M i t c h e l l (51_) have p o i n t e d o u t t h a t d e s p i t e t h e v a s t range o f t e x t u r a l - i n s t r u m e n t s , s a t i s f a c t o r y i n s t r u m e n t s are n o t yet a v a i l a b l e t o measure a l l t e x t u r a l p r o p e r t i e s . The e l a s t i c modulus i s d e f i n e d as the r a t i o o f s t r e s s t o s t r a i n . Stress i s the f o r c e p e r u n i t a r e a p r o d u c i n g the change i n shape and s t r a i n i s t h e change i n shape o r change i n l e n g h p e r u n i t l e n g t h o f t h e sample (3). I f the s t r a i n (displacement) i s s m a l l , i t i s e s s e n t i a l l y l i n e a r t o s t r e s s ( a p p l i e d f o r c e ) and the g e l w i l l resume i t s o r i g i n a l shape a f t e r t h e f o r c e i s removed. The e l a s t i c l i m i t o f t h e g e l i s t h e l a r g e s t amount o f d e f o r m a t i o n t h a t an e l a s t i c body c a n e x p e r i e n c e and s t i l l r e g a i n i t s o r i g i n a l shape a f t e r removing t h e force. Creep c o m p l i a n c e i s s i m i l a r t o e l a s t i c i t y except a c o n s t a n t f o r c e ( s t r e s s ) i s a p p l i e d t o the g e l and the d e f o r m a t i o n ( s t r a i n ) i s f o l l o w e d over t i m e . A l t e r n a t i v e l y , s t r e s s r e l a x a t i o n c a n be used


CRANDALL AND WICKER

Table

II.

Examples o f i n s t r u m e n t s Stress applied to g e l


Type o f instrument I.

Non-destructive

A.

IFT-SAG

B.

Wageningen

sag

95


used t o examine p e c t i n g e l s Parameter measured

Price range

force of gravity

percent sag

inexpensive

force of gravity

amount o f sag m a g n i f i e d 10 f o l d

inexpensive

inexpensive

C.

F.I.R.A.

torquing blade immersed i n g e l

amount o f water t o make predetermined deflection

D.

Concentric cylinders

torquing corrugated cylinders

constant inexpensive stress/strain to c r e e p compliance expensive stress relaxation

E.

Parallel plates

sliding corrugated plates

strain c r e e p compliance

inexpensive to expensive

F.

Dynamic

oscillator

change i n velocity of waves

inexpensive to expensive

II. Destructive A.

Finger

p r e s s u r e between thumb and forefinger

relative force t o break g e l

inexpensive

B.

Tarr-Baker

syringe piston p r e s s i n g on gel's surface

relative amount o f heavy l i q u i d to rupture g e l

inexpensive

C.

LuersLochmuller

pulling figure from a g e l

weight t o rupture g e l

inexpensive

D.

HerbstreithPektinometer

pulling figure from a g e l

force to rupture g e l

moderately expensive

f i x t u r e s used i n either compression o r t e n s i o n mode

force peak h e i g h t

expensive

E.

Instron

References

(51, 56)


96

CHEMISTRY AND

FUNCTION OF PECTINS

where a c o n s t a n t s t r a i n i s a p p l i e d and the s t r e s s r e q u i r e d t o m a i n t a i n a c o n s t a n t s t r a i n o v e r time i s measured. A d d i t i o n a l background i s found i n r e p o r t s by M i t c h e l l (52, 60). Nondestructive vs. Destructive Tests. The p e c t i n grade depends somewhat on the e v a l u a t i o n method (6J_). Gel strength i n s t r u m e n t a t i o n may be n o n d e s t r u c t i v e which measures the e l a s t i c p r o p e r t i e s of a g e l , o r d e s t r u c t i v e which measures the i n e l a s t i c o r b r e a k i n g s t r e n g t h o f a g e l . A major p e c t i n p r o d u c e r u n s u c c e s s f u l l y t r i e d s e v e r a l i n s t r u m e n t s and methods f o r more than 20 y e a r s t o measure b o t h e l a s t i c i t y and r u p t u r e s t r e n g t h w i t h a s i n g l e t e s t


(62). N o n d e s t r u c t i v e t e s t s have c e r t a i n advantages i n c l u d i n g the a b i l i t y t o measure the e l a s t i c i t y of the g e l on p r o d u c t s such as j e l l i e s c o n t a i n i n g p a r t i c u l a t e s . N o n d e s t r u c t i v e t e s t s can be used t o b l e n d raw p e c t i n s t o a c o n s i s t e n t j e l l y g r a d e . A l s o , some r e p o r t s have found n o n d e s t r u c t i v e i n s t r u m e n t s t o be l e s s e x p e n s i v e and more r e l i a b l e (63). M i t c h e l l (60) r e p o r t e d t h a t l a r g e d e f o r m a t i o n s o u t s i d e the l i n e a r r e g i o n of a s t r e s s v s . s t r a i n c u r v e a r e more d i f f i c u l t to i n t e r p r e t and more d i f f i c u l t t o measure t h a n s m a l l d e f o r m a t i o n s because r u p t u r e o c c u r s a t a d e f e c t i n the g e l and l a r g e d e f o r m a t i o n s are not as r e p r o d u c i b l e . C o n v e r s e l y , d e s t r u c t i v e t e s t s have advantages o v e r n o n d e s t r u c t i v e t e s t s because t h e y a r e c l o s e r to a consumer's p e r c e p t i o n of s p r e a d i n g a j e l l y . The f o r c e r e q u i r e d t o r u p t u r e a g e l c o r r e l a t e d b e s t w i t h g e l s t r e n g t h a s s e s s e d i n the mouth (57). K u i p e r (64) r e p o r t e d t h a t n o n d e s t r u c t i v e measurements c o r r e l a t e d w e l l w i t h the R i d g e l i m e t e r r e a d i n g , whereas d e s t r u c t i v e measurements c o r r e l a t e d b e t t e r with sensory analyses. Nondestructive Tests IFT-SAG. In the IFT-SAG method, a s t a n d a r d i z e d amount o f sugar i s cooked w i t h a t e s t amount o f p e c t i n . The m i x t u r e i s poured i n t o a 7.94 cm h e i g h t j e l l y g l a s s which c o n t a i n s an e x c e s s o f a c i d and a l l o w e d t o g e l f o r 18 t o 24 h o u r s . The j e l l y i s demolded and the amount of sag under the f o r c e o f g r a v i t y i s measured w i t h a s p e c i a l micrometer c a l l e d a R i d g e l i m e t e r . The

advantages o f the IFT-SAG method have been p r e s e n t e d The method e s t a b l i s h e s t e s t c o n d i t i o n s f o r r e p r o d u c i b l y measuring j e l l y f i r m n e s s . At a pH near 2.2, m i n i m a l e f f e c t o f pH on g e l s t r e n g t h i s o b s e r v e d (36). The e f f e c t s of temperature and a g i n g are a l s o n e g l i g i b l e a t t h i s pH (36, 5θ, 62). The IFT-SAG method uses s i m p l e and i n e x p e n s i v e l a b equipment. It i s p r e c i s e , r e p r o d u c i b l e , and s u b j e c t t o m i n i m a l o p e r a t o r e r r o r . T h i s method i s the s t a n d a r d on which comparisons o f p r i c e o f p e c t i n s and p r e d i c t i o n s of the j e l l y i n g c a p a b i l i t i e s have been made. Thus, raw p e c t i n can be c u t t o a 150 grade o r a m i x t u r e of p e c t i n s of known j e l l y g r a d e s can be c a l c u l a t e d .

(62-65).

The IFT-SAG method d i d not win unanimous s u p p o r t a t i t s i n c e p t i o n because t e s t c o n d i t i o n s do not s t i m u l a t e how p e c t i n s are used i n commercial p r a c t i c e . The t e s t pH o f 2.2 i s t e n times more a c i d t h a n the commercial j e l l y pH of 3.2 and i s w e l l below the maximum g e l s t r e n g t h pH (_7, 36, 53, 62)· The t e s t i s based on a water j e l l y which does not account f o r the n a t u r a l l y o c c u r r i n g b u f f e r s and s a l t s i n f r u i t j u i c e . The low g r a v i t y c o m p r e s s i o n r a t e


8.

CRANDALL AND

WICKER


97

of the IFT-SAG t e s t does not a d e q u a t e l y p r e d i c t the g e l performance under a c t u a l use or o t h e r h i g h compression r a t e s (56)* The t e s t r e q u i r e s 18 t o 24 h o u r s so i t i s not u s e f u l i n r e a l time m o d i f i c a t i o n during j e l l y manufacture. F i n a l l y , t h i s method o f e v a l u a t i o n u n d e r e s t i m a t e s the j e l l y grade of h i g h m o l e c u l a r w e i g h t , h i g h l y polymerized p e c t i n (49).


Wageningen Sag. Doesburg (66) d e v e l o p e d a p r o c e d u r e i n which s u g a r - a c i d g e l s were c a s t i n t o a two p i e c e c y l i n d e r . The g e l was cut i n h a l f and the sag of the g e l was measured by a p i n a t t a c h e d t o a p i v i t o l p o i n t e r and a s c a l e . T h i s arrangement a m p l i f i e d the amount o f sag t e n t i m e s . A good c o r r e l a t i o n between the o r g a n o l e p t i c q u a l i t y o f h i g h sugar g e l s was found w i t h the Wageningen sag measurements· B.A.R.—F.I.R.A. J e l l y T e s t e r . T h i s i n s t r u m e n t ' s measurements a r e based on a fundamental p r i n c i p l e where the e l a s t i c modulus i s measured by the t o r q u e on a b l a d e immersed i n a j e l l y . The t o r q u e i s a p p l i e d by water f l o w i n g i n t o a bucket and a measurement i s made on the amount o f water needed t o produce a 30° t u r n o f the b l a d e — w h i c h i s w i t h i n the e l a s t i c l i m i t s o f the g e l 03, 67). M o d i f i c a t i o n s have been made on t h i s i n s t r u m e n t u s i n g an e l e c t r i c motor connected t o a t o r s i o n w i r e to t u r n the b l a d e . Wires w i t h known t o r s i o n a l moments a r e used as s t a n d a r d s . S t u d i e s have shown a s t r o n g c o r r e l a t i o n between R i d g e l i m e t e r and F.I.R.A. g r a d i n g . Other r e s e a r c h e r s have m o d i f i e d t h i s j e l l y t e s t e r to measure a 15° t u r n ( e l a s t i c i t y ) , f o l l o w e d by measurement of the b r e a k i n g s t r e n g t h o f g e l s and have found good c o r r e l a t i o n t o s e n s o r y e v a l u a t i o n s ( 5 5 ) . Concentric C y l i n d e r Instruments. S a v e r b o r n (68) d e v e l o p e d an i n s t r u m e n t t h a t r e q u i r e s the p e c t i n m i x t u r e be poured between two c o n c e n t r i c , c o r r u g a t e d c y l i n d e r s and a l l o w e d t o s e t . The i n n e r c y l i n d e r i s t w i s t e d by a t o r s i o n w i r e and the e x t e n t o f t o r s i o n caused i n the g e l i s measured. The c o r r u g a t i o n s p r e v e n t s l i p p a g e o f the g e l . K e r t e s z (3) c i t e s t h i s as one o f the f i n e s t i n s t r u m e n t s d e v i s e d f o r j e l l y s t r e n g t h measurements. Other i n s t r u m e n t s which use c o n c e n t r i c c y l i n d e r s i n c l u d e t h o s e d e s c r i b e d i n (69-72) and were r e v i e w e d by M i t c h e l l ( 5 1 ) . These i n s t r u m e n t s can be used f o r fundamental measurements on g e l s l i k e c r e e p c o m p l i a n c e , s t r e s s r e l a x a t i o n , and r i g i d i t y modulus. Parallel Plate. P l a s h c h i n a e t a l . (73) s t u d i e d the c r e e p o f HM p e c t i n g e l s p l a c e d between two c o r r u g a t e d p a r a l l e l p l a t e s . Creep compliance c u r v e s were o b t a i n e d f o r 0.5 t o 2.5% p e c t i n a t temperatures from 25 t o 55°C. R e v e r s i b l e and i r r e v e r s i b l e s t r a i n components were s e p a r a t e d . P e c t i n macromolecules were c h a r a c t e r i z e d as b e i n g v e r y s t i f f and o n l y a s l i g h t d e c r e a s e i n e n t r o p y was r e q u i r e d to form a p e c t i n g e l ( 7 3 ) . M i t c h e l l and B l a n s h a r d (50) used an automated p a r a l l e l p l a t e v i s c o e l a s t o m e t e r t o s t u d y the c r e e p compliance on low methoxyl p e c t i n s . The v a l u e o f t h e s e e x p e r i m e n t s was t h a t a c o n t i n u o u s r e s p o n s e was o b t a i n e d from the g e l r a t h e r than a s i n g l e p o i n t measurement. Dynamic T e s t i n g . G e l s can be c h a r a c t e r i z e d by dynamic t e s t i n which an o s c i l l a t o r y s t r e s s i s a p p l i e d t o the g e l and the phase


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a n g l e of the s t r a i n measured. Most g e l s show a phase d i f f e r e n c e l e s s than 90° out of phase w i t h the imposed s t r e s s . Gross (74) and Gross e t a l . (75) reviewed dynamic t e s t i n g and used i t t o s t u d y the t e x t u r e s of LM p e c t i n g e l s . They determined the dynamic modulus and phase a n g l e s and found t h a t a t 100 HZ, the g e l s t h a t appeared t o be the most f i r m were the weakest a t 200 HZ. However, they found some good c o r r e l a t i o n s between some of the dynamic t e s t s and sensory evaluations. D e s t r u c t i v e Tests F i n g e r T e s t . The f i r s t d e s t r u c t i v e t e s t s on a p e c t i n g e l were performed by s q u e e z i n g a s l i c e o f j e l l y between the thumb and f o r e f i n g e r u n t i l the g e l b r o k e . T h i s was made i n t o a f o r m a l i z e d j e l l y e v a l u a t i o n t e s t c a l l e d the F i n g e r T e s t . When the a n a l y s t s had adequate e x p e r i e n c e , the r e s u l t s were r e p r o d u c i b l e and d i f f e r e n c e s of 5% o r g r e a t e r between the t e s t j e l l y and a ' s t a n d a r d ' j e l l y c o u l d be d e t e c t e d ( 3 ) . A s i d e from the obvious d i f f e r e n c e s among a n a l y s t s , a ' s t a n d a r d ' j e l l y must be prepared f o r each t e s t from a ' s t a n d a r d ' p e c t i n w h i c h i s kept r e f r i g e r a t e d t o m i n i m i z e changes i n the s t a n d a r d . The c h a r a c t e r i s t i c s o f the ' s t a n d a r d ' j e l l y a r e not specified (49). Tarr-Baker. The T a r r - B a k e r (Delaware J e l l y S t r e n g t h T e s t e r ) i s based on the work of T a r r ( 7 6 ) , Baker ( 7 7 ) , and Baker and Woodmansee (78). A f o r c e i s a p p l i e d t o the g e l ' s s u r f a c e by a s y r i n g e p i s t o n powered by compressed a i r . Measurements can be i n f l u e n c e d by a ' s k i n ' on the j e l l y ' s s u r f a c e o r uneven a p p l i c a t i o n of p r e s s u r e ( 3 ) . Swenson e t a l . (48) m o d i f i e d the T a r r - B a k e r apparatus t o produce a b a l a n c e - p l u n g e r type i n s t r u m e n t . A l t h o u g h they found a l i n e a r s t r e s s - s t r a i n r e g i o n w i t h i n the e l a s t i c l i m i t s of the g e l ; they a l s o found e l a s t i c i t y was somewhat dependent on the r a t e o f l o a d i n g . T h e i r i n s t r u m e n t was c a p a b l e of s e v e r a l e l a s t i c and b r e a k i n g t e s t s on one g e l . There was l e s s than a 2% e r r o r between the t r u e and assumed grades u s i n g b r e a k i n g s t r e n g t h . C h r i s t e n s e n (49) determined a r a t i o between a m o d i f i e d 'sag' grade t o the b r e a k i n g s t r e n g t h as measured by the T a r r - B a k e r a p p a r a t u s f o r 22 p e c t i n s o f v a r y i n g MW. The b r e a k i n g / s a g r a t i o ranged from 1.38 t o 0.85, but heat o r enzyme treatment f u r t h e r reduced the r a t i o t o 0.39. The r e l a t i o n s h i p between sag grade and b r e a k i n g s t r e n g t h was r e p o r t e d t o be dependent on the MW of p e c t i n ( 4 9 ) . Doesburg (66) showed lower pH v a l u e s (2.0 to 2.3) caused s h o r t s e t t i n g times and reduced the i n t e r n a l g e l s t r e n g t h compared t o pH 3.1 as measured by the T a r r - B a k e r i n s t r u m e n t . He d i d not observe a good c o r r e l a t i o n o f o r g a n o l e p t i c q u a l i t y t o T a r r - B a k e r measurements. Luers-Lochmuller Pektinometer. The Pektinometer was developed i n Germany (79) and measures the amount o f f o r c e n e c e s s a r y t o p u l l a metal f i g u r e out a f t e r b e i n g c a s t i n s i d e a p e c t i n g e l . A hot j e l l y i s poured i n t o a s p e c i a l c o n t a i n e r w i t h c o r r u g a t e d s i d e s c o n t a i n i n g the m e t a l f i g u r e . The c o r r u g a t e d s i d e s prevent s l i p p a g e of the g e l as the amount of f o r c e n e c e s s a r y t o break the g e l i s measured. S t e i n h a u s e r e t a l . (80) used the L u e r s - P e k t i n o m e t e r i n h i s comparison of f o u r methods f o r d e t e r m i n i n g g e l s t r e n g t h and found the r e p r o d u c i b i l i t y was l i n e a r . Uhlenbrock (81) adopted t h i s i n s t r u m e n t t o determine the b r e a k i n g s t r e n g t h of s u g a r - a c i d g e l s .


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The b r e a k i n g s t r e n g t h v a r i e d from 500 t o 600 g and the of v a r i a t i o n was 1 t o 2%.

coefficient

1


Herbstreith-Pektinometer. L u e r s - L o c h m u l l e r s method has been f u r t h e r r e f i n e d i n the H e r b s t r e i t h - P e k t i n o m e t e r which uses a s p e c i a l l y d e s i g n e d c l e a r , c o r r u g a t e d cup and p l a s t i c f i g u r e . One hundred grams of the p e c t i n m i x t u r e from a l a b o r a t o r y o r commercial b a t c h i s poured i n t o the cup w i t h a s e l f c e n t e r i n g f i g u r e . The g e l i s h e l d f o r 2 hours a t 20°C. The cup i s l o c k e d i n p l a c e and a hook i s a t t a c h e d t o the c e n t r a l f i g u r e . An e l e c t r i c motor p u l l s up on the f i g u r e r u p t u r i n g the g e l and the peak f o r c e r e q u i r e d i s measured on a 1 kg s t r a i n gauge. The f i n a l v a l u e i s p r i n t e d out i n Pektinometer u n i t s . I n s t r o n . The I n s t r o n U n i v e r s a l T e s t i n g Machine can be used i n compression and t e n s i o n experiments where the f o r c e on a l o a d c e l l i s measured w h i l e moving the c r o s s head a g i v e n d i s t a n c e o r t i m e . In e v a l u a t i n g g e l s t r u c t u r e , fundamental c h a r a c t e r i s t i c s such as b r i t t l e n e s s , h a r d n e s s , and e l a s t i c i t y can be q u a n t i t a t i v e l y measured and r e l a t e d t o s e n s o r y a t t r i b u t e s such as chewiness and gumminess. Sherman (56) found sample d i m e n s i o n and c r o s s head speed a f f e c t these readings. Gels were not l i n e a r i n t h e i r force-compression behavior. Slow c r o s s head speeds can l e a d t o s t r e s s r e l a x a t i o n , so from low compression t e s t r a t e s , i t was i m p o s s i b l e t o p r e d i c t how a g e l would behave a t h i g h compression r a t e s as i n the mouth. O a k e n f u l l and S c o t t (32) used an I n s t r o n t o measure apparent shear m o d u l i and r u p t u r e s t r e n g t h s . A l t h o u g h no s a t i s f a c t o r y t h e o r y c o u l d be proposed t o r e l a t e r u p t u r e s t r e n g t h and d i s r u p t i o n of i n t e r m o l e c u l a r f o r c e s , they d i d f i n d that hydrophobic i n t e r a c t i o n s play a role i n g e l formation. M i t c h e l l (52) r e v i e w s s e v e r a l i n v e s t i g a t i o n s u s i n g I n s t r o n measurements on s e v e r a l t y p e s o f g e l s . Gels were t e s t e d i n compression and the apparent modulus was c a l c u l a t e d from the i n i t i a l s l o p e of the curve and the r u p t u r e s t r e n g t h was c a l c u l a t e d from the peak f o r c e . Conclusions T h i s r e v i e w p o i n t s out why a p e c t i n g e l ' s i n t e r n a l s t r e n g t h measurements are i m p o r t a n t . Some o f the important q u a l i t i e s f o r t e s t methods and i n s t r u m e n t a t i o n t o measure i n t e r n a l s t r e n g t h a r e given. I t a l s o r e l a t e s the c u r r e n t t h e o r y of the f o r m a t i o n of a HM p e c t i n g e l and the f a c t o r s a f f e c t i n g g e l s t r e n g t h . There a r e two g e n e r a l c a t e g o r i e s of i n s t r u m e n t a t i o n — n o n d e s t r u c t i v e and d e s t r u c t i v e . Examples and advantages o f each c a t e g o r y a r e a l s o given. I n t e r n a l g e l s t r e n g t h measurements a r e c u r r e n t l y under r e v i e w . In the next few y e a r s , new methods and m o d i f i c a t i o n s o f e x i s t i n g t e c h n i q u e s and i n s t r u m e n t a t i o n w i l l be d e v e l o p e d . These t e s t s w i l l more f u l l y c h a r a c t e r i z e the p r a c t i c a l a s p e c t s o f a p e c t i n ' s i n t e r n a l gel strength. Acknowledgments The a u t h o r s w i s h t o acknowledge the a s s i s t a n c e o f K a t h r y n C. i n the p r e p a r a t i o n o f the m a n u s c r i p t . F l o r i d a A g r i c u l t u r a l Experiment S t a t i o n J o u r n a l S e r i e s No. 7184.


Davis


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Pectin Internal Gel Strength: Theory ... - American Chemical Society

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