Food Taste Chemistry - ACS Publications - American Chemical Society

3 Pungency: The Stimuli and Their Evaluation V. S. GOVINDARAJAN

Downloaded by UNIV OF MONTANA on December 14, 2014 | http://pubs.acs.org Publication Date: December 14, 1979 | doi: 10.1021/bk-1979-0115.ch003

Central Food Technological Research Institute, Mysore, India 570013

The quality that distinguishes an appetising meal, which one wants to eat, from a collection of cooked components that satisfy merely one's nutritional requirements, is flavor. Aroma, and the traditionally accepted four tastes, sweet, sour, salt, and bitter, are part of most foods and they are further developed or altered during cooking. Even so, most cooked foods are still considered insipid, and require food additives, such as spices, herbs and pot entiators, to boost the flavour. The important contributions of spices and herbs to sensory qualities, other than aroma, are the new dimensions, pungency and astrigency. Apart from imparting a specific altered aroma, they increase the pre- and post-ingestion al cues which are important in increasing awareness and appreci ation of the food, and thereby lead to increased ingestion (1). Thus, the notion of food flavour - which is commonly understood as aroma and taste - should be expanded to cover the contribution of pungency. P u n g e n c y , an a p p r a i s a l o f t h e t e r m 1

A d e f i n i t i o n o f 'pungency i s r e q u i r e d , i f we a r e t o u n d e r s t a n d t h e p e r c e i v e d i m p r e s s i o n , and a t t e m p t t o e s t i m a t e t h e same. The d i c t i o n a r i e s d e f i n e t h e t e r m a s a s t i n g i n g , i r r i t a t i n g , o r caustic property ( O x f o r d ) and ' k e e n n e s s , s h a r p n e s s , p o i g n a n c y ' (Webster). These a r e e i t h e r too g e n e r a l , o r they a r e r a t h e r u n happy c h o i c e s o f d e s c r i p t o r s w h i c h c o n n o t e some u n d e s i r a b l e c h a r acteristics. The word pungent i s a l s o used t o d e n o t e a c r i d o d o r s , a n d , i n t h e g e n e r a l l i t e r a t u r e , i t i s used a l m o s t a s a synonym o f ' h o t ' and ' i r r i t a n t ' . However, w i t h r e s p e c t t o f o o d , f o r w h i c h t h e t e r m 'pungency' s h o u l d r e a l l y be u s e d , i t assumes a d e s i r a b l e quality. When used a t an o p t i m a l usage l e v e l , t h e s p i c e s have a p a r t from aroma - a ' m o u t h - w a t e r i n g ' q u a l i t y . Together with a r o m a , p e p p e r , g i n g e r and c h i l l i e s p r o v i d e t h e ' p i q u a n t ' s t i m u l u s r e s p o n s e t h a t l e a d s t o a g r e a t e r a c c e p t a n c e and h i g h e r i n t a k e o f food. This d e s i r a b l e nature o f the r e a c t i o n , I b e l i e v e , i d e n t i f i e s 'pungency' a s d i f f e r e n t from t h e o t h e r d e s c r i p t o r s used t o d e f i n e the a t t r i b u t e , such as i r r i t a n t , s t i n g i n g , c a u s t i c , e t c . , a l l o f !

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0-8412-0526-4/79/47-115-053$10.00/0 © 1979 American Chemical Society In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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w h i c h have u n d e s i r a b l e

connotations.


We c o u l d a l s o l o o k a t t h e a t t r i b u t e f r o m t h e s t i m u l u s - s e n s a t i o n angle. N o n c r i e f f (_2), i n h i s t r e a t i s e on t h e C h e m i c a l S e n s e s , has c l a s s i f i e d pungency under t h e common c h e m i c a l s e n s e s , grouping i t along with s t i m u l i e l i c i t i n g burning ( h o t ) , i r r i t a t i o n , l a c h r y m a t o r y or c o u g h - p r o v o k i n g s e n s a t i o n s . I would l i k e to submit t h a t a l l these p r o p e r t i e s are d i f f e r e n t s e n s a t i o n s , t h o u g h some c h e m i c a l s m i g h t e l i c i t more t h a n one o f t h e s e s e n sations. Many e x a m p l e s can be f o u n d w h i c h show t h e s e d i f f e r e n c e s . Ammonia, w h i l e i m p a r t i n g a h i g h l y i r r i t a n t and c a u s t i c s e n s a t i o n c a n n o t be c a l l e d p u n g e n t . The l a c h r y m a t o r y s u b s t a n c e o f o n i o n , t h e p r o p a n o l S — o x i d e , i s not p u n g e n t i n t h e mouth, but p r o d u c e s an i t c h i n g r e a c t i o n i n the nose. S u l p h u r d i o x i d e , a cough-provoking c h e m i c a l , p r o d u c e s no p u n g e n t f e e l i n g i n t h e mouth, but o n l y a c a t c h at the t h r o a t . While a d r i n k i n g c o n c e n t r a t i o n of a l c o h o l c o u l d b r i n g a b o u t t h e s e n s a t i o n o f warmth i n t h e t h r o a t , and even a b u r n i n g s e n s a t i o n a t h i g h e r c o n c e n t r a t i o n s , i t can n e v e r be c a l l e d p u n g e n t . T h e r e a r e o t h e r compounds i n some s p i c e s and h e r b s w h i c h p r o d u c e a t i n g l i n g s e n s a t i o n on t h e t o n g u e , but t h i s a g a i n i s d i s t i n c t f r o m t h e t y p i c a l s e n s a t i o n o f pungency e l i c i t e d by t h e c h a r a c t e r i s t i c components o f s p i c e s , s u c h as c a p s a i c i n , p i p e r i n e and g i n g e r o l . S i n g l e t o n and N o b l e r e c e n t l y described p u n g e n c y as a h o t , p e n e t r a t i n g , b u r n i n g s e n s a t i o n i n t h e mouth w h i c h a t l o w e r l e v e l s may be warm, s p i c y , s h a r p o r h a r s h . This i m p l i e s that a l l these varying s e n s a t i o n s , are merely concentration effects. Such a v i e w may be a c c e p t e d o n l y i n so f a r as t h e t e r m s warm, b u r n i n g , p e n e t r a t i n g and i r r i t a t i n g a r e u n d e r s t o o d i n t h e i r g e n e r a l l y a c c e p t e d m e a n i n g s ; but o r d i n a r y p h e n o l , w h i c h e l i c i t s t h e s e r e s p o n s e s , w i l l a t no c o n c e n t r a t i o n be c a l l e d p u n g e n t . A l s o , t y p i c a l pungent compounds a t a l l l e v e l s , f r o m t h r e s h o l d t o h i g h e r l e v e l s , a r e c l e a r l y i d e n t i f i e d as p u n g e n t . T h e s e s p e c i f i c s p i c e components a r e p e r c e i v e d as p u n g e n t o n l y when t a k e n i n t h e mouth, w h i l e c o n t a c t w i t h o t h e r p a r t s , s u c h as t h e s k i n , i s f e l t as b u r n i n g o r i r r i t a t i o n ; i n t h e nose as w a t e r i n g , and i n t h e eyes as lachrymatory. W h a t e v e r be t h e u l t i m a t e r e a s o n f o r t h e d i f f e r e n t r e s p o n s e s t o t h e same compound a t d i f f e r e n t c e n t r e s o f p e r c e p t i o n , t h e d i s t i n c t r e s p o n s e as pungency when t a k e n i n t h e mouth i s a s s o c i a t e d w i t h s p e c i f i c compounds, b o t h n a t u r a l and s y n t h e t i c , and r e s t r i c t e d t o some s t r u c t u r a l c h a r a c t e r i s t i c s ( s e e l a t e r ) . These p r o p e r t i e s / c h a r a c t e r i s t i c s s h o u l d be c o n s i d e r e d s i g n i f i c a n t enough t o a f f o r d pungency a s t a t u s o f i t s own among t h e d i f f e r e n t s e n sory a t t r i b u t e s of food. The n e x t p o i n t f o r c o n s i d e r a t i o n i s whether pungency i s a p u r e l y g u s t a t o r y m o d a l i t y , o r w h e t h e r i t i s a l s o an o l f a c t o r y modality. T h e r e i s a v a s t and g r o w i n g l i t e r a t u r e on t h e i n t e r r e l a t i o n of the s e n s e s , though the e x t e n t of t h e i r mutual i n f l u e n c e s are

In Food Taste Chemistry; Boudreau, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


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not w e l l u n d e r s t o o d . I t i s , h o w e v e r , a common e x p e r i e n c e t h a t o n c e t a k e n i n t o t h e mouth, f o o d s and b e v e r a g e s e l i c i t a t l e a s t d u a l , u s u a l l y t r i p l e , s e n s o r y e f f e c t s due t o t h e i n t e r c o n n e c t i o n o f t h e t h r o a t and n a s a l c a v i t y , and t h e i n n e r v a t i o n o f t h e r e c e p t o r s d i s p o s e d i n r e l a t i o n t o one a n o t h e r . The s e n s e s o f t a s t e and c u t a n eous s e n s i b i l i t i e s i n t h e mouth, and o f o l f a c t i o n i n t h e nose a p p e a r t o a c t s i m u l t a n e o u s l y i n r e s p o n s e t o a s t i m u l u s . However, t h e s e c o u l d be i n d e p e n d e n t l y e x p e r i m e n t e d upon u s i n g v a r i o u s d e v i ces. As pungent compounds c a n be f e l t i n t h e mouth a t l o w t h r e s h o l d l e v e l s , and n o t by s m e l l a t t h o s e l e v e l s , pungency may be c l a s s i f i e d along with the t r a d i t i o n a l t a s t e m o d a l i t i e s . A d d i t i o n a l l y , as w i t h o t h e r t a s t e s , t h e r e i s l i t t l e o r no v a r i a t i o n o f q u a l i t y , b u t o n l y v a r i a t i o n i n i n t e n s i t y , and an o p t i m a l c o n c e n t r a t i o n for acceptance. The a c c e p t e d d e f i n i t i o n o f t h e t e r m t a s t e , 'one o f t h e s e n s e s , t h e r e c e p t o r s f o r w h i c h a r e l o c a t e d i n t h e mouth and a r e a c t i v a t e d by a l a r g e v a r i e t y o f d i f f e r e n t compounds ( Λ ) , w i l l a c c o m o d a t e t h e q u a l i t y o f pungency a s w e l l , Though most i n v e s t i g a t o r s now l i m i t g u s t a t o r y q u a l i t i e s t o f o u r , n a m e l y , s w e e t , s o u r , s a l i n e , and b i t t e r , A y u r v e d a i n I n d i a , and a l s o t h e s c i e n c e s o f o t h e r c u l t u r e s , r e c o g n i s e d s i x k i n d s of t a s t e ; t h e above-mentioned f o u r a s w e l l a s pungent and a s t r i n g e n t . 1

I would l i k e t o make a p a s s i n g r e f e r e n c e t o t h e d i s t i n c t n e s s o f t h e c h e m o r e c e p t o r s . The r e c e p t o r f o r t h e f o u r b a s i c t a s t e s i s now r e c o g n i s e d t o be a n e u r a l c o m p l e x o f s p e c i a l i z e d , e l a b o r a t e l y o r g a n i s e d c e l l s , d i s t r i b u t e d on t h e t o n g u e i n a l o c a l i z e d f a s h i o n . The r e c e p t o r s f o r o t h e r c h e m i c a l s a r e r e p o r t e d t o be t h e s o - c a l l e d f r e e n e r v e e n d i n g s , and a r e f o u n d t h r o u g h o u t t h e o r a l c a v i t y . Re c e p t o r s l o c a t e d on o t h e r p a r t s o f t h e body r e s p o n d i n g t o t o u c h , t e m p e r a t u r e , p r e s s u r e , a r e a l l r e p o r t e d t o have d i s t i n c t i v e n e r v e e n d i n g s , w h i l e f o r i r r i t a t i o n and p a i n no r e c o g n i s a b l e r e c e p t o r b o d i e s have been i d e n t i f i e d (J5). w o u l d f u r t h e r work w i t h t h e more a d v a n c e d e l e c t r o n m i c r o s c o p e s y i e l d f u r t h e r i n f o r m a t i o n and show up d i f f e r e n c e s between t h e f r e e n e r v e ends i n t h e mouth and t h o s e o u t s i d e t h e o r a l c a v i t y ? R e c e n t work i n n e u r o p h y s i o l o g y and e l e c t r o n microscopy has d i s c o v e r e d a c o l d r e c e p t o r f o r former ' f r e e nerve endings ( 5 a ) . Would t h e f r e e n e r v e e n d i n g s o f r e c o g n i s e d t a s t e r e c e p t o r s r e s p o n d t o t h e a p p l i c a t i o n o f s p e c i f i c compounds e l i c i t i n g t h e pungency r e s p o n s e ? The e x t e n s i v e work w i t h c a p s a i c i n by J a n s c o - G a b o r and c o l l e a g u e s ( 5 b ) h a s shown p r o n o u n c e d d e s e w s i t i z a t i o n a g a i n s t a l l k i n d s o f c h e m i c a l s t i m u l i ; i t i s most l i k e l y that receptors belonging t o the slowest conducting C f i b r e s are involved i n i t s effect. A l t e r n a t i v e l y , i s the difference i n the r e s p o n s e t o t h e s p e c i f i c pungency s t i m u l i , w i t h i n t h e mouth and o u t s i d e , r e l a t e d t o t h e l o c a l i s a t i o n of the neural information a t the higher centres of transmission? 1

The w i d e s p r e a d u s e o f t h e t e r m s 'pungent o r b i t e component', ' c a p s a i c i n pungency' o r ' s e n s o r y pungency' has t e n d e d t o be c o n fusing. To r e c o g n i s e t h e c a u s e and e f f e c t r e l a t i o n s h i p c l e a r l y , t h e s t i m u l a n t s h o u l d be i d e n t i f i e d o n l y by i t s common o r c h e m i c a l


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name, and t h e s e n s o r y

r e s p o n s e by t h e t e r m p u n g e n c y .


E s t i m a t i o n o f Punqency - P r o b l e m s When pungency i s c o n s i d e r e d as an i m p o r t a n t g u s t a t o r y a t t r i b u t e o f f o o d , an e s t i m a t i o n o f t h i s perceiv/ed pungency i s n e c e s s a r y f o r v a l i d a t i n g any q u a l i t y c o n t r o l p r o c e d u r e o f t h e raw m a t e r i a l , or of the prepared food. Even when t h e c h e m i c a l compounds t h a t a r e r e s p o n s i b l e f o r e l i c i t i n g t h i s s e n s a t i o n a r e known i n each raw m a t e r i a l and i n s t r u m e n t a l methods a r e a v a i l a b l e f o r t h e i r e s t i m a t i o n , t h e s u b j e c t i v e e s t i m a t i o n o f pungency i s n e c e s s a r y t o e s t a b l i s h a c o r r e l a t i o n as a g u i d e t o t h e u s e - l e v e l . S e n s o r y e v a l u a t i o n o f f o o d f o r any p e r c e i v e d f o o d q u a l i t y had i n t h e p a s t been c o n s i d e r e d an a r t and i n g e n e r a l very v a r i a b l e ; but over t h e l a s t two o r t h r e e d e c a d e s , i t s c o n t r i b u t i o n t o t h e s u c c e s s o f p r o c e s s e d f o o d s h a s been r e a l i s e d . As i n t h e d e v e l o p m e n t o f any a n a l y t i c a l method, t h e c a u s e s o f v a r i a t i o n and s o u r c e s o f e r r o r h a v e been s t u d i e d and much o b j e c t i v i t y has been i n t r o d u c e d i n t o t h e p r a c t i c a l methods o f s e n s o r y e v a l u a t i o n (_6_,_7_) · The common u s e o f t h e t e r m ' o b j e c t i v e f o r p h y s i c o - c h e m i c a l measurements and ' s u b j e c t i v e ' f o r e v a l u a t i o n by humans s h o u l d a l s o be d i s c o u r a g e d , s i n c e t h e r e a r e e l e m e n t s o f ' o b j e c t i v i t y ' and ' s u b j e c t i v i t y ' i n both measuring systems. I t i s preferable to specifically state t h e i n s t r u m e n t a l measure and t h e s e n s o r y m o d a l i t y u s e d . Many f o o d l a b o r a t o r i e s a r e p r a c t i s i n g s e n s o r y e v a l u a t i o n by p a n e l s , i n t h e p l a c e o f one o r a few e x p e r t s , i n o r d e r t o a r r i v e a t d e c i s i o n s on t h e raw m a t e r i a l o r f i n i s h e d p r o d u c t q u a l i t y , p r o c e s s c h a n g e , s t o r age l i f e e t c . , (J3) · I t i s h e a r t e n i n g t o s e e t h a t many a s p e c t s o f sensory e v a l u a t i o n of foods a r e r e g u l a r l y d i s c u s s e d a t meetings; and t h a t i n s t i t u t i o n s c o n c e r n e d w i t h s t a n d a r d s and s p e c i f i c a t i o n s a r e , t h r o u g h t h e i r s p e c i a l c o m m i t t e e s , a c t i v e l y (_9) d i s c u s s i n g and c o d i f y i n g d e f i n i t i o n s , d e t a i l s o f methods, g i v i n g g u i d a n c e i n t h e s e l e c t i o n o f methods o f e v a l u a t i o n and a n a l y s i s needed t o answer s p e c i f i c q u e s t i o n s , and e v o l v i n g a u n i f o r m method o f r e p o r t i n g s e n sory evaluation r e s u l t s . 1

I t i s i m p o r t a n t t o c o n s i d e r , a t t h i s j u n c t u r e , i f t h e r e i s any q u a l i t y d i m e n s i o n i n pungency. I n t h e e a r l i e r l i t e r a t u r e , t h e r e a r e some r e f e r e n c e s t o t h e q u a l i t y o f pungency by d e s c r i p t i o n s s u c h as i n s t a n t , v a n i s h i n g , p e r s i s t e n t , e t c . T h e s e o b s e r v a t i o n s on n a t u r a l and s y n t h e t i c compounds w e r e , however, made by d i f f e r e n t p e o p l e , and by p r o c e d u r e s which were n o t s t a n d a r d i s e d . An e x t r e m e example o f t h e i m p o r t a n c e o f c o r r e c t and d e t a i l e d o b s e r v a t i o n i s t h e . . m i s i n t e r p r e t a t i o n o f t h e l a c k o f pungency i n p i p e r i n e c r y s t a l s i s o l a t e d from p e p p e r e x t r a c t s , w h i l e t h e mother l i q u o r had h i g h p u n g e n c y ; t h a t m i s t a k e l e d t o y e a r s o f s e a r c h by many w o r k e r s t o i d e n t i f y t h e n o n - e x i s t e n t isomers o f p i p e r i n e i n pepper ( s e e l a t e r ) . Todd a t al^ have r e c e n t l y o b s e r v e d ( l O ) t h a t c a p s a i c i n , d i h y d r o c a p s a i c i n , and n o r d i h y d r o c a p s a i c i n gave r a p i d p u n g e n t s e n s a t i o n s l o c a t e d i n t h e back o f t h e p a l a t e and t h r o a t , w h i l e h o m o c a p s a i c i n and h o m o d i h y d r o c a p s a i c i n t e n d e d t o p r o d u c e p r o l o n g e d pungent s e n s a t i o n s o f l o w i n t e n s i t y l o c a t e d i n t h e mid-mouth and m i d - p a l a t e


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57

Pungency


regions. They hav/e a l s o f o u n d t h a t t h e s e e f f e c t s a r e p r o b a b l y r e l a t e d t o t h e s o l u b i l i t y o f t h e s e components i n p o l a r s o l v e n t s , h i g h e r f o r t h e f i r s t t h r e e components and l o w e r f o r t h e l a t t e r two homologs w i t h l o n g e r a l k y l s i d e - c h a i n s . I t a p p e a r s t o be c l e a r t h a t , i n common w i t h o t h e r g u s t a t o r y a t t r i b u t e s , t h e r e a r e no q u a l i t y d i f f e r e n c e s i n p u n g e n c y , and t h a t t h e p e r c e i v e d d i f f e r e n c e s a r e e x p l a i n e d by i n t r i n s i c i n t e n s i t y , c o n c e n t r a t i o n , and s o l u b i l i t y o f t h e d i f f e r e n t s t i m u l i r e s p o n s i b l e f o r pungency. As m e n t i o n e d e a r l i e r , i n any f o o d s i t u a t i o n , t h e i n t e r a c t i o n of other sensory m o d a l i t i e s could r e s u l t i n seemingly a p p a r e n t v a r i a t i o n s i n pungency q u a l i t y o f d i f f e r e n t s t i m u l i , b u t such d i f f e r e n c e s a r e not r e a l . We have c a r e f u l l y l o o k e d i n t o t h i s a s p e c t by c o m p a r i s o n o f i s o l a t e d , p u r e , t o t a l p u n g e n t components ( f r e e from aroma) o f t h e s p i c e s , c h i l l i e s , p e p p e r , and g i n g e r . A t c o n c e n t r a t i o n s s l i g h t l y above t h e i r i n d i v i d u a l t h r e s h o l d s , t h e d i f f e r e n t , p u r e s t i m u l i a r e i n d i s t i n g u i s h a b l e f r o m one a n o t h e r even by t r a i n e d p a n e l i s t s . A t h i g h e r l e v e l s , some d i s t i n c t i o n s c o u l d be made; e.g. q u i c k and s t r o n g f o r c a p s a i c i n o i d s , 'low' f o r g i n g e r o l , s l o w f o r s h o g a o l , and ' s l o w and l a s t i n g ' f o r p i p e r i n e . These d i f f e r e n c e s c o u l d , however, be r e l a t e d t o t h e i r known i n t r i n s i c , i n t e n s i t y d i f f e r e n c e s and s o l u b i l i t i e s i n w a t e r . I t would be p r e f e r a b l e t o measure t h e i r s o l u b i l i t y i n s a l i v a , and a l s o t o f i n d o u t whether t h e r e a r e d i f f e r e n c e s i n t h e i r r a t e s o f p e n e t r a t i o n t o t h e receptors. 1

1

I t was e a r l y r e c o g n i s e d t h a t pungency e v a l u a t i o n needs t o be done a s a d i l u t i o n method. S c o v i l l e ( l l ) , a s e a r l y as 1912, determined t h e pungency o f c a p s i c u m by d e t e r m i n i n g t h e g r e a t e s t d i l u t i o n a t w h i c h t h e d e f i n i t e p e r c e p t i o n o f ' b i t e ' ( p u n g e n c y ) would be r e c o g n i s e d ( t h e r e c o g n i t i o n t h r e s h o l d ) , and e x p r e s s e d i t as t h e r e c i p r o c a l of the d i l u t i o n . T h i s f i g u r e h a s s i n c e been c a l l e d t h e S c o v i l l e h e a t u n i t s . Though a p p a r e n t l y s i m p l e , s e v e r a l f a c t o r s i n t e r f e r e i n t h i s s u b j e c t i v e judgement. F o r example, v a r i a t i o n i n a c u i t y o f p a n e l i s t s ; aroma l o w e r i n g t h e t h r e s h o l d s and t h e r e b y b i a s i n g t h e judgement t o h i g h e r v a l u e s t h a n t h e r e a l ; c a r r y - o v e r o f pungency and a d a p t a t i o n e f f e c t b i a s i n g judgement o f s u b s e q u e n t samples t o lower v a l u e s ; other p s y c h o l o g i c a l a n t i c i p a t o r y e r r o r s now r e c o g n i s e d i n s e n s o r y t e s t i n g ; n o n - s t a n d a r d i s a t i o n o f p r o c e d u r e s and p r e p a r a t i o n o f s a m p l e s f o r d i l u t i o n t e s t i n g ; and l a c k o f p u r e r e f e r e n c e compounds had a l l c o n t r i b u t e d t o p o o r r e p r o d u c i b i l i t y and r e s u l t a n t s c e p t i c i s m a b o u t t h e v a l i d i t y o f t h e t e s t . With t h e s t a n d a r d i s a t i o n of t h r e s h o l d t e s t s g e n e r a l l y f o r other t a s t e and aroma s t i m u l i , pungency t e s t i n g has a l s o i m p r o v e d , and we c a n now g e t r e l i a b l e and r e p r o d u c i b l e measurements o f pungency. S t a n d a r d methods f o r e v a l u a t i o n

o f pungency:

The A m e r i c a n S p i c e T r a d e A s s o c i a t i o n (ASTA) ( 1 2 ) , a d o p t e d , i n 1968, an o f f i c i a l method ( 2 1 . 0 ) f o r pungency e v a l u a t i o n f o r c a p s i c u m , w h i c h a p p e a r s t o be an a d a p t a t i o n o f p r o c e d u r e s t h e n i n u s e by some f l a v o u r h o u s e s . T h e s e methods were p r o b a b l y d e v e l o p e d a s a r o u t i n e q u a l i t y c o n t r o l p r o c e d u r e , on s a m p l e s o b t a i n e d u s u a l l y


58


f r o m p a r t i c u l a r s o u r c e s , t o c h e c k whether t h e u s u a l l e v / e l o f p u n gency was b e i n g m a i n t a i n e d ; t h e y depended on s e r i a l d i l u t i o n a r o u n d t h e e x p e c t e d v/alue, and a c c e p t i n g t h e l e v / e l a t w h i c h t h r e e o u t o f f i v e , t r a i n e d , i n - h o u s e p a n e l i s t s f o u n d pungency. The ASTA p r o c e d u r e i m p r o v e d on t h i s by m e n t i o n i n g t h a t t h e t e s t s h o u l d be p e r f o r m e d as a t h r e s h o l d t e s t and t h a t c a r r y - o v e r e f f e c t s s h o u l d be a v o i d e d by p a l a t e c l e a r i n g and a l l o w i n g t i m e between s a m p l e s ; a d i l u t i o n t a b l e t o c o v e r a wide r a n g e o f S c o v i l l e v a l u e s from 100 t o 15 χ 1 0 , e x p e c t e d f o r c h i l l i and i t s o l e o r e s i n s was a l s o p r o vided. However, t h e method gave no r e c o m m e n d a t i o n on c o n c e n t r a t i o n d i f f e r e n c e s i n t h e d i l u t i o n s e r i e s , and r e t a i n e d t h e s i n g l e f i g u r e v a l u e a g r e e d upon by t h r e e o u t o f f i v e j u d g e s . The p r o c e s s i n g and t r e a t m e n t o f d a t a a r e n o t s t a t i s t i c a l l y acceptable, n o r u s e f u l f o r c o r r e l a t i v e work w i t h c h e m i c a l methods o f e s t i m a t i o n s of the s t i m u l i . These a r e p r o b a b l y t h e r e a s o n s f o r t h e p o o r r e p r o d u c i b i l i t y o f t h e method r e c e n t l y r e p o r t e d ( 1 0 ) . This pro c e d u r e h a s , h o w e v e r , been a d o p t e d by t h e B r i t i s h and by t h e I n t e r n a t i o n a l Standards Organisation.


5

A l l t h e s e a s p e c t s have been r e c e n t l y s t u d i e d i n two p u b l i c a t i o n s ( 1 0 , 1 3 ) , w h i c h s t a n d a r d i z e d t h e d i l u t i o n t e s t f o r pungency and c l e a r l y e s t a b l i s h e d c o r r e l a t i o n s w i t h t h e e s t i m a t e s o f t o t a l , and even i n d i v i d u a l , c a p s a i c i n o i d s . These p a p e r s a l s o r e v i e w t h e e a r l i e r attempts a t s t a n d a r d i s a t i o n . Two a p p r o a c h e s a r e p o s s i b l e : use o f a f a i r l y homogenous p a n e l t o d e t e r m i n e t h e t h r e s h o l d p u n gency r e s p o n s e due t o t h e s t i m u l i ; o r u s e o f a g e n e r a l p a n e l t o determine the average t h r e s h o l d of the panel f o r t h e s t i m u l i . The s e c o n d v a l u e w i l l have w i d e r a p p l i c a b i l i t y i n u s e s i t u a t i o n s ; b u t t h e f i r s t v a l u e s h o u l d be u s e f u l f o r c o r r e l a t i v e work. The two methods a p p r o a c h one a n o t h e r when p a n e l s a r e s c r e e n e d and t r a i n e d t o a v o i d a l l b i a s f a c t o r s , and when c a r e f u l l y p l a n n e d d i l u t i o n l e v e l s , d e t a i l s o f p a n e l p r o c e d u r e , t r e a t m e n t o f d a t a , and e x p r e s s i o n of r e s u l t s a r e adopted. I n fact', t h e r e s u l t s p u b l i s h e d i n t h e two i n d e p e n d e n t s t u d i e s ( 1 0 , 1 3 ) , show c l o s e v a l u e s , 17+0.9 m i l l i o n f o r n a t u r a l c a p s a i c i n o i d s and 16.1+0.6 m i l l i o n f o r p u r e c a p s a i c i n and d i h y d r o c a p s a i c i n . The method s t a n d a r d i z e d i n o u r l a b o r a t o r y c a n be summarised by t h e f o l l o w i n g s t e p s , each o f w h i c h s i g n i f i c a n t l y i m p r o v e s r e p r o d u c i b i l i t y and p r e c i s i o n . i ) P r e p a r a t i o n o f sample and p r e l i m i n a r y t e s t i n g , when judges a r e f a m i l i a r i s e d w i t h t h e r e c o g n i t i o n o f pun gency b i a s f a c t o r s , t h e t e s t p r o c e d u r e , and t h e e v a l uation card. The d a t a are u s e d t o group t h e j u d g e s i n t o homogenous p a n e l s o f h i g h and l o w s e n s i t i v i t y u s i n g t h e g r o u p mean and i n d i v i d u a l t h r e s h o l d s , and t o f i x t h e a p p r o x i m a t e t h r e s h o l d f o r each p a n e l . ii)

F i n a l e v a l u a t i o n of t h e sample through t h e use of a d i l u t i o n s e r i e s , i m p o r t a n t l y , an a r i t h e m a t i c s e r i e s with s m a l l increments i n c o n c e n t r a t i o n around t h e


3.

GOviNDARAjAN

Pungency

59

a p p r o x i m a t e t h r e s h o l d , m a i n t a i n i n g one ' j n d ' w h i c h has been n o r m a l l y f o u n d t o be o n e - t e n t h o f t h e t h r e s h o l d c o n c e n t r a t i o n - and o b t a i n i n g 15-20 j u d g e m e n t s w i t h a minimum o f 5 p a n e l i s t s and o f 3 to 4 repeat t e s t s . iii)

Judgements a r e decoded i n t e r m s o f S c o v i l l e u n i t s (SU = r e c i p r o c a l o f d i l u t i o n s ) and t h e mean v a l u e i s e x p r e s s e d as a r a n g e , i n SU + cr.


i v ) The p a n e l i s d e f i n e d by i t s a v e r a g e t h r e s h o l d i n SU + 6-

2

?

8

9

CH == C H - ( C H ) 2

*

2

8

undecenamide

- R e l a t i v e t o n a t u r a l c a p s a i c i n a s 100. D a t a f r o m

(10,24)

W i t h t h i s a s b a c k g r o u n d , we c o u l d d e c i d e on methods o f e s t i m a t i o n o f c a p s a i c i n o i d s i n c a p s i c u m by i n s t r u m e n t a l methods. Many m e t h o d s , u t i l i s i n g p a r t i t i o n , c o l u m n , p a p e r , and t h i n - l a y e r c h r o m a t o g r a p h i c s e p a r a t i o n s , combined w i t h c o l o r i m e t r y and gas c h r o m a t o g r a p h y , have been s u g g e s t e d f o r e s t i m a t i o n o f t^he c a p s a i c i n oids (26). A l l t h e s e e a r l i e r methods g i v e e s t i m a t e s o f t h e t o t a l c a p s a i c i n o i d s ; b u t t h e r e c e n t g a s c h r o m a t o g r a p h i c method a f t e r s i l y l a t i o n i s c a p a b l e o f s e p a r a t i n g and e s t i m a t i n g t h e i n d i v i d u a l n a t u r a l c a p s a i c i n o i d s and some o f t h e s y n t h e t i c s ( 1 0 ) . For r o u t i n e q u a l i t y e v a l u a t i o n , i t i s c o n s i d e r e d s u f f i c i e n t t o d e t e r m i n e a c c u r a t e l y t h e t o t a l c a p s a i c i n o i d s ; t h e m i n o r compon-^ e n t s g e n e r a l l y amount t o a b o u t 1 0 $ , and have 50 t o 60$ o f t h e p u n gency r e s p o n s e o f t h e m a j o r c a p s a i c i n o i d s . For t h i s purpose S a l z a r (26) compared a number o f t h e e a r l i e r methods, and f o u n d t h e p a p e r c h r o m a t o g r a p h i c method (27) t o be s i m p l e , r a p i d , r e p r o d u c i b l e and a c c u r a t e . The method i s a p p l i c a b l e d i r e c t l y t o t h e o l e o r e s i n , and i s b a s e d on t h e f a c t t h a t , o f t h e t h r e e major compone n t s o f a t o t a l e x t r a c t , t h e c a p s i c u m c o l o r would be a d s o r b e d by t h e p a p e r and a l o n g w i t h t h e f a t would be moved l i t t l e by t h e b u f f e r e d methanol s o l v e n t , while t h e c a p s a i c i n o i d s a r e c l e a r l y s e p a r a t e d , moving t o a h i g h R f p o s i t i o n ( F i g u r e l ) . The a c c u r a c y o f t h e d e t e r m i n a t i o n was a l s o i m p r o v e d by t h e u s e o f t h e more s e n s i t i v e and s p e c i f i c G i b b s r e a g e n t f o r t h e c o l o r i m e t r y . 1

However, capsaicinoids there a r e high when t h e r e i s

t h e v a l u e s by any o f t h e s e methods m e a s u r i n g t o t a l would g i v e a l o w c o r r e l a t i o n w i t h pungency^ when p r o p o r t i o n s o f t h e C8 and C i o c a p s a i c i n o i d s , o r g r o s s a d u l t e r a t i o n w i t h s y n t h e t i c s . The d e t e c t i o n


64



o f a d u l t e r a t i o n by s y n t h e t i c s has been a t t e m p t e d s i n c e 1955, but a l l t h e methods were r a t h e r c o m p l e x , and s o , u n s a t i s f a c t o r y f o r r a p i d and r o u t i n e s c r e e n i n g . R e c e n t l y , Todd _et al_ (28) have d e v e l oped e l e g a n t , r a p i d , t h i n - l a y e r c h r o m a t o g r a p h i c methods f o r s e p a r a t i o n o f c a p s a i c i n o i d s and s y n t h e t i c s by u t i l i z i n g t h e d i f f e r e n c e i n s o l u b i l i t y between the d i f f e r e n t c a p s a i c i n o i d s as t h e c h a i n l e n g t h i n c r e a s e d , and by s w i t c h i n g t o p a r t i t i o n c h r o m a t o g r a p h y i n the r e v e r s e d phase, with s i l v e r — i o n complexing, to separate the s a t u r a t e d and u n s a t u r a t e d compounds. A s e p a r a t i o n o f t h e f i v e s y n t h e t i c s from t h e n a t u r a l c a p s a i c i n o i d s as a l s o t h e f i v e n a t u r a l c a p s a i c i n o i d s i n t o t h r e e s p o t s was a c h i e v e d i n t h e f i r s t d i r e c t i o n . T a b l e IV.

C a p s a i c i n o i d s and r e l a t e d s y n t h e t i c compounds - T h i n l a y e r c h r o m a t o g r a p h y , R values P

Reversed Phase

Polyamide Methanol ( A g ) / water/acetic acid +

Compounds

Methanol/water 60:40 ( v / v ) As

Vanillyl

octanamide

Vanillyl namide

such

0.70

Brominated

0.70

Methanol (Ag )/ water/acet i c acid 52:40:8 ( v / v ) +

—

52:40:8

—

pelargo0.64

0.64

Capsaicin

0.52

0.65

0.39

0.64

Nordihydrocapsaicin

0.52

0.52

0.25

0.42

Vanillyl

capramide

0.44

0.44

—

—

0.32

Dihydrocapsaicin

0.41

0.41

0.17

Homocapsaicin

0.41

0.63

0.30

0.57

V a n i l l y l undecenamide

0.36

0.25

Homodihydrocapsaicin

0.31

0.31

0.10

0.26

V a n i l l y l undecanamide

0.20

0.20

—

—

Data from

(28).


(v/v)


3.

GoviNDARAjAN

Pungency

65

B r o m i n a t i o n o f t h e s e p a r a t e d s p o t s by e x p o s u r e t o b r o m i n e v/apor i n a t a n k , and f u r t h e r d e v e l o p m e n t i n t h e s e c o n d d i r e c t i o n a c h i e v e d t h e s e p a r a t i o n o f t h e bunched u n s a t u r a t e d from t h e s a t u r a t e d n a t u r a l components w i t h one m e t h y l e n e g r o u p l e s s . The n a t u r a l , s a t u r a t e d and u n s a t u r a t e d c a p s a i c i n o i d s c o u l d be c l e a r l y s e p a r a t e d i n a s i n g l e d e v e l o p m e n t , and more r a p i d l y by t h e use o f r e v e r s e d p h a s e , o r p o l y a m i d e , p l a t e s , and u s i n g d e v e l o p i n g s o l v e n t s w i t h s i l v e r i o n f o r c o m p l e x i n g . The s e p a r a t i o n p o s s i b i l i t i e s a r e c l e a r f r o m t h e R f v a l u e s f o r t h e d i f f e r e n t s y s t e m s g i v e n i n T a b l e IV. T h e s e methods a r e a p p l i c a b l e t o t h e e x t r a c t s or d i l u t e d o l e o r e s i n s d i r e c t l y , not r e q u i r i n g any p r e - s e p a r a t i o n ; h e n c e , t h e y a r e u s e f u l f o r r a p i d s c r e e n i n g i n the f i e l d f o r d e t e c t i o n of a d u l t e r a t i o n , o r f o r s e l e c t i o n o f i m p r o v e d s t r a i n s i n a g r o n o m i c and g e n e t i c work. The e s t i m a t i o n o f i n d i v i d u a l components and d i f f e r e n t s y n t h e t i c s was e f f e c t i v e l y a c h i e v e d by gas c h r o m a t o g r a p h y a f t e r s i m p l e s i l y l a t i o n (10). F i g u r e 2 shows t h e s e p a r a t i o n o b t a i n e d , c l e a r l y away f r o m o t h e r v o l a t i l e s i n a c a p s i c u m e x t r a c t . The i n d i v i d u a l c a p s a i c i n o i d s were q u a n t i t a t e d i n r e l a t i o n t o v a n i l l y l o c t a n a m i d e as t h e i n t e r n a l s t a n d a r d c h o s e n , b e c a u s e i t e l u t e d j u s t p r i o r t o t h e c a p s a i c i n o i d s and t h e r e s p o n s e f a c t o r s o f t h e c a p s a i c i n o i d s w i t h r e f e r e n c e t o t h i s s t a n d a r d were c l o s e t o u n i t y . The stand a r d i s e d c o n d i t i o n s were as f o l l o w s : S i l y l a t i o n w i t h N , 0 - b i s ( t r i m e t h y l s i l y l ) - t r i f l u o r o a c e t a m i d e i n t e t r a h y d r o f u r a n gave c l e a r r a p i d r e a c t i o n a t room t e m p e r a t u r e . The s i l y l a t e d e x t r a c t s were i n j e c t e d d i r e c t l y on t o a s t a i n l e s s s t e e l column o f 2 m χ 3 mm, f i l l e d w i t h Z% SE-30 on Chromosorb-GHP, (100-120 mesh); t h e c o l u m n t e m p e r a t u r e was programmed f r o m 170° t o 215°C a t 4°/min. and h e l d a t 215°C f o r 10 m i n u t e s . The i n j e c t i o n p o r t t e m p e r a t u r e was i m p o r t a n t f o r r a p i d v o l a t i l i s a t i o n o f a l l t h e components w i t h out d e c o m p o s i t i o n , and was f i x e d a t 200°C. The f l a m e i o n i s a t i o n d e t e c t o r t e m p e r a t u r e was k e p t a t 250°C, and n i t r o g e n f l o w a t 20 m l / m i n . The p e r c e n t a g e s o f t h e i n d i v i d u a l c a p s a i c i n o i d s were c a l c u l a t e d from t h e a r e a s o f t h e p e a k s , t h e r e s p o n s e f a c t o r s , and t h e w e i g h t and a r e a of t h e r e f e r e n c e compound. P u n g e n c i e s of s a m p l e s were c a l c u l a t e d b y m u l t i p l y i n g t h e p e r c e n t a g e s of i n d i v i d u a l c a p s a i c i n o i d s from gas c h r o m a t o g r a p h y w i t h t h e c o r r e s p o n d i n g t h r e s h o l d p u n g e n c y , and t h e t o t a l was e x p r e s s e d i n S c o v i l l e u n i t s . Figure 3 i l l u s t r a t e s the c l o s e r e l a t i o n b e t ween t h e pungency c a l c u l a t e d from t h e gas c h r o m a t o g r a p h i c compo s i t i o n and t h e d i r e c t l y ( s e n s o r i l y ) e s t i m a t e d v a l u e of pungency. A c o r r e l a t i o n c o e f f i c i e n t g r e a t e r t h a n 0.95 was o b t a i n e d f o r p u n gency v a l u e s v a r y i n g from 50 ΓΊ t o 2.0 MH. I t i s , t h e r e f o r e , c l e a r l y p o s s i b l e t o e v a l u a t e by i n s t r u m e n t a l methods t h e q u a n t i t y of s y n t h e t i c s p r e s e n t as a d u l t e r a n t s i n a sample o f c a p s i c u m e x t r a c t or o l e o r e s i n , t o d e t e r m i n e i n d i v i d u a l c a p s a i c i n o i d s or t o t a l c a p s a i c i n o i d s , and t o p r e d i c t t h e pungency of t h e c a p s i c u m p r e p a r a t i o n s .


66



Cp

Wmnmnmnr

1

2

3

4

Figure 1. Tracings of separation of total capsaicinoids of capsicum extracts and reference pure compounds by paper chromatography: Spot (I) red capsicum; Spot (2) natural capsaicinoids pure; Spot (3) green capsicum fresh; and Spot (4) synthetic capsaicin; (C) color; (F) fat, and (Cp), capsaicinoids/capsaicin ((27) and unpublished).

Journal of Food Science

Figure 2. GC pattern of silylated capsicum extract (See text for GC conditions and Tables II and III for abbreviations) (10)


3.

GOviNDARAjAN

67

Pungency

Pepper ( P i p e r nigrum

Linn.)

Pungency i s t h e q u a l i t y w h i c h has made p e p p e r w o r l d famous as t h e K i n g o f S p i c e s ; i t i s g i v e n an even g r e a t e r w e i g h t a g e t h a n a r o ma when d e t e r m i n i n g t h e o v e r a l l q u a l i t y o f p e p p e r .


The c h e m i s t r y o f t h e p u n g e n t compounds of p e p p e r has been u n d e r s t u d y s i n c e t h e i s o l a t i o n o f p i p e r i n e i n 182Q. P i p e r i n e was shown t o be a p i p e r i d i d e o f p i p e r i c a c i d , and had t h e t r a n s , t r a n s configuration. The t h r e e o t h e r p o s s i b l e i s o m e r s were soon p o s t u l a t e d , and t h e y were named i s o p i p e r i n e ( c i s , t r a n s ) , i s o c h a v i c i n e ( t r a n s , c i s ) and c h a v i c i n e ( c i s , c i s ) even b e f o r e t h e y were s y n t h e sized. The a s s i g n m e n t o f t h e i s o m e r i c c o n f i g u r a t i o n s t a r t s f r o m t h e a m i d e end. T h e r e was much c o n t r o v e r s y a b o u t t h e i d e n t i t y of t h e h i g h l y p u n g e n t compound i n t h e mother l i q u o r , a f t e r c r y s t a l l i s a t i o n o f p i p e r i n e from p e p p e r e x t r a c t s . B u c h e i m s c l a i m s o f t h i s compound b e i n g t h e c i s - c i s i s o m e r and b e i n g more pungent t h a n p i p e r i n e were b e l i e v e d f o r a l o n g t i m e , even t h o u g h i t was c l e a r l y shown t h a t p i p e r i n e was not pungent i n t h e c r y s t a l l i n e s t a t e , b u t , was ext r e m e l y pungent as a s o l u t i o n i n a l c o h o l . The e a r l y s t u d i e s a r e r e p o r t e d i n d e t a i l i n a r e c e n t r e v i e w which a l s o g i v e s a comprehensive bibliography (16). 1

Grewe et_ a l _ (29) s y n t h e s i s e d t h e f o u r p o s s i b l e i s o m e r i c a c i d s by s t e r e o s p e c i f i c r o u t e s , as a l s o t h e c o r r e s p o n d i n g p i p e r i d i d e s , and d e t e r m i n e d t h e i r s p e c t r a l and o t h e r p h y s i c a l c h a r a c t e r i s t i c s . T h i s c l e a r e d t h e c o n f u s i o n i n e a r l i e r works on t h e s u p p o s e d i s o l a t i o n o f c h a v i c i n i c a c i d s f r o m p e p p e r . They a l s o d e t e r m i n e d t h e i r pungency and showed t h a t , e x c e p t i n g p i p e r i n e , 5 ( 3 , 4 - d i o x y m e t h y l e n e phenyl)-2-trans,4-trans-pentadienoic a c i d , a l l other isomers are p o o r l y p u n g e n t . They c o n c l u d e d t h a t n e i t h e r c h a v i c i n e nor t h e o t h e r two, c i s - t r a n s or t r a n s - c i s , i s o m e r s e x i s t i n n a t u r a l p e p p e r . They a l s o showed t h a t most of t h e i s o m e r s were u n s t a b l e t o l i g h t and were e a s i l y c o n v e r t e d i n t o t h e t r a n s , t r a n s p i p e r i n e . D e C l e y n and V e r z e l e ( 3 g , 3 1 , i Z ) i s o l a t e d t h e f o u r p o s s i b l e i s o m e r s from p i p e r i n i c a c i d i r r a d i a t e d i n an u l t r a - v i o l e t r e a c t o r , by c o u n t e r - c u r r e n t d i s t r i b u t i o n ; and a l s o t h e p i p e r i d i d e s o b t a i n e d s y n t h e t i c a l l y f r o m t h e t r e a t e d p i p e r i n i c a c i d by h i g h pressure l i q u i d c h r o m a t o g r a p h y ( F i g u r e 4 ) . The s t r u c t u r e s o f t h e i s o m e r s were d e r i v e d m a i n l y from NMR d a t a . T h e i r pungency was r e c o r d e d , but p o s s i b l y not by r i g o r o u s methods ( 3 1 , 3 2 ) . The r e s u l t s c o n f i r m e d t h e o b s e r v a t i o n o f Grewe _e_t SLL (29) t h a t p i p e r i n e i s t h e pungent p r i n c i p l e o f p e p p e r , and t h a t o t h e r i s o m e r s have l i t t l e ' t a s t e . The d a t a on t h e p r o p e r t i e s o f t h e i s o m e r s a r e c o l l e c t e d i n T a b l e V. 1

D e C l e y n and V e r z e l e had o r i g i n a l l y (30) h e l d t h e v i e w t h a t t h e l o s s o f pungency i n p e p p e r on s t o r a g e i s p o s s i b l y due t o l i g h t i n d u c e d i s o m e r i s a t i o n o f a p a r t of the p i p e r i n e t o t h e o t h e r i s o mers w h i c h a r e not p u n g e n t . B u t , i n t h e i r l a t e s t work ( 3 3 ) , t h e y f i n d l i t t l e e v i d e n c e o f t h e p r e s e n c e o f i s o m e r s i n pepper s a m p l e s



68 FOOD TASTE CHEMISTRY


3.

GoviNDARAjAN T a b l e \l.

General

Pungency

69

P i p e r i n e and i s o m e r s and t h e i r

properties

Formula:5 4 3 2 1 /—> n- ^ i-CH=CH-CH=CH-CO-N > VNk

«

Food Taste Chemistry - ACS Publications - American Chemical Society

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