Thermal Generation of Aromas - ACS Publications - American

Chapter 27

Downloaded by UNIV OF MINNESOTA on April 16, 2013 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0409.ch027

Formation of Flavor Components in Roasted Coffee R. Tressl Technische Universität Berlin, Seestrasse 13, D-1000 Berlin 65, Federal Republic of Germany

More than 700 c o n s t i t u e n t s have been identified in aroma e x t r a c t s of roasted c o f f e e . H e t e r o c y c l i c aroma components represent the g r e a t e s t amount of the steam v o l a t i l e aroma complex (80 - 85 %) which amounts to 700 900 ppm i n medium roasted Arabica c o f f e e s . The concentration of i n d i v i d u a l components v a r i e s depending on coffee v a r i e t i e s and r o a s t i n g c o n d i t i o n s . T y p i c a l components are formed by thermal degradation of free and bound amino a c i d and chlorogenic a c i d p r e c u r s o r s . Compared to other roasted foods t u f f s , s u l f u r c o n t a i n i n g c o n s t i t u e n t s and phenols are formed i n high amounts and c o n t r i b u t e to d e s i r a b l e coffee f l a v o r or off-flavor. Numerous r e v i e w s on c o f f e e f l a v o r have been p u b l i s h e d o v e r t h e p a s t few y e a r s ( J _ , 2_, 3_) . So f a r , more t h a n 700 components have been c h a r a c t e r i z e d i n r o a s t e d c o f f e e . T h e r e f o r e , a p p r o x i m a t l y 20 % o f t h e 4000 c h e m i c a l s r e p o r t e d i n t h e " L i s t o f V o l a t i l e Compounds i n Food" e d i t e d by TNO (4_) may be consumed by d r i n k i n g c o f f e e . T h i s l i s t o f v o l a t i l e c o n s t i t u e n t s c o n t i n u e s t o grow as d e m o n s t r a t e d by B a l t e s and c o w o r k e r s (_5) Improved a n a l y t i c a l i n s t r u m e n t s and d a t a b a s e s o f t h o u s a n d s o f mass s p e c t r a c o n t r i b u t e d t o the d e t e c t i o n of v o l a t i l e cons t i t u e n t s i n model r e a c t i o n s as w e l l as i n r o a s t e d foods t u f f s . New f o o d c o n s t i t u e n t s a r e more and more d i f f i c u l t t o l o c a t e , i s o l a t e and i d e n t i f y . In a c o n t r i b u t i o n p r e s e n t e d a t t h e f i r s t Weurman Symposium, R i j k e n s and B o e l e n s (6^) e s t i m a t e t h a t f o o d s can c o n t a i n between 5000 t o 10000 c o n s t i t u e n t s . T h i s 0097-6156/89/0409-0285S06.00/0 ο 1989 American Chemical Society In Thermal Generation of Aromas; Parliment, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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286

seems a r e a l i s t i c f o r e c a s t , b u t many o f t h e s e c o n s t i t u e n t s may be p r e s e n t i n v e r y low c o n c e n t r a t i o n s and may have no o r g a n o l e p t i c o r t o x i c e f f e c t . In a r e c e n t r e v i e w on c o f f e e f l a v o r C l a r k e (3^) r e p o r t e d t h a t t h e r e a r e many p u b l i c a t i o n s on q u a l i t a t i v e d a t a , b u t i n f o r m a t i o n on t h e i r a c t u a l q u a n t i t i e s i n c o f f e e i s d i f f i c u l t t o f i n d . Many o f t h e c o f f e e aroma components a r e p r e s e n t i n h i g h c o n c e n t r a t i o n s and t h e s e d a t a were u s e d t o p r e d i c t so c a l l e d c o n s u m a t i o n r a t i o s f o r h e t e r o c y c l i c components {]_, 8^) . A c c o r d i n g t o d a t a p r e s e n t e d by t h e s e a u t h o r s t h e c o n s u m p t i o n o f p y r a z i n e s f r o m c o f f e e i s 380 f o l d h i g h e r compared t o p y r a z i n e s added as f l a v o r i n g s u b s t a n c e s t o a l l o t h e r f o o d s t u f f s . On t h e o t h e r hand, t h e r e a r e numerous c h e m i c a l s d i s c l o s e d i n p a t e n t s w h i c h may be u s e d t o m o d i f y and r e i n f o r c e t h e aroma and f l a v o r o f c o f f e e and b e v e r a g e s . B e t t e r u n d e r s t a n d i n g o f p r e c u r s o r s and r o u t e s by w h i c h i m p o r t a n t and t y p i c a l aroma and f l a v o r components a r e f o r m e d i s e s s e n t i a l t o d e c r e a s e u n d e s i r a b l e , non c o n t r i b u t i n g components and i m p r o v e t h e a c c e p t a n c e and w h o l e s o m e n e s s o f t h i s a t t r a c t i v e beverage. Formation of

P h e n o l s by

Degradation

of Chlorogenic

Acids

Compared t o o t h e r r o a s t e d f o o d s t u f f s , g r e e n c o f f e e b e a n s c o n t a i n h i g h amounts o f c h l o r o g e n i c a c i d s w h i c h a r e degraded d u r i n g r o a s t i n g . C o f f e a A r a b i c a possesses lower c o n c e n t r a t i o n s o f 5 - c a f f e o y l - and 5 - f e r u l o y l q u i n i c a c i d s t h a n C o f f e a R o b u s t a as shown i n F i g u r e 1. D u r i n g a medium r o a s t 60 t o 77 % o f t h e s e p r e c u r s o r s , w h i c h amount 30 t o 40 g/kg a r e d e g r a d e d and t h e c o r r e s p o n d i n g p h e n o l s a r e f o r m e d i n t h e h i g h ppm r a n g e . O n l y 2 t o 5 % o f t h e d e g r a d e d p r e c u r s o r s a r e d e t e c t a b l e as p h e n o l s and t h e r e s t a r e t r a n s f o r m e d i n t o so c a l l e d h i g h m o l e c u l a r huminic a c i d s ( £ ) . The d e g r a d a t i o n o f c h l o r o g e n i c - , c a f f e i c - , f e r u l i c and q u i n i c a c i d s was s t u d i e d i n model e x p e r i m e n t s and t h e p h e n o l s were c h a r a c t e r i z e d and q u a n t i f i e d i n r o a s t e d c o f f e e (9^, J_0, J M _ ) . The p h e n o l s d e r i v e d f r o m c a f f e i c a c i d p o s s e s s e m e t i c e f f e c t s and t h e i r c o n c e n t r a t i o n s i n r o a s t e d c o f f e e change a c c o r d i n g t o v a r i e t y , r o a s t i n g l e v e l and t r e a t m e n t o f g r e e n c o f f e e b e a n s . In d a r k r o a s t e d b l e n d s , c a t e c h o l and g u a i a c o l i n c r e a s e s i g n i f i c a n t l y , and t h e c o r r e s p o n d i n g 4 - v i n y l d e r i v a t i v e s d e c r e a s e . G u a i a c o l and 4 - v i n y l g u a i a c o l p o s s e s s low t h r e s h o l d s and a r e c o n t r i b u t i n g f l a v o r c o m p o n e n t s . C h l o r o g e n i c a c i d p r e c u r s o r s and t h e i r c o r r e s p o n d i n g p h e n o l s a r e a l s o i n f l u e n c e d by d e c a f f e i n a t i o n p r o c e s s e s ( F i g u r e 2 ) . During d e c a f f e i n a t i o n (I = e t h y l a c e t a t e , I I = superc r i t i c a l C 0 ) 5 - c a f f e o y l - and 5 - f e r u l o y l q u i n i c a c i d s d e c r e a s e s i g n i f i c a n t l y and a r e t r a n s f o r m e d i n t o 4 - v i n y l c a t e c h o l and 4 - v i n y l g u a i a c o l , r e s p e c t i v e l y . The i n c r e a s e o f 4- and 5 - c a f f e o y l q u i n i c a c i d s c a n n o t be e x p l a i n e d by t r a n s e s t e r f i c a t i o n . Obviously, these p r e c u r s o r s are 2

In Thermal Generation of Aromas; Parliment, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

27. TRESSL

II

I

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5-CQA

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green

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19

il

53

110

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287

Formation of Flavor Components in Roasted Coffee

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1: C o n t e n t (g/kg) o f 5 - c a f f e o y l - and 5 - f e r u l o y l q u i n i c a c i d s i n c o f f e e A r a b i c a and R o b u s t a and t h e i r c o r r e s p o n d i n g p h e n o l s (ppm) f o r m e d d u r i n g roasting

II

44

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2: C h l o r o g e n i c a c i d p r e c u r s o r s (g/kg) and t h e i r c o r r e s p o n d i n g p h e n o l s (ppm) f o r m e d b y d e c a f f e i n a t i o n process (I e t h y l a c e t a t e , I I super critical , b e f o r e and a f t e r d e c a f f e i n a t i o n , respectively)



288

bound as g l y c o s i d e s . These d a t a d e m o n s t r a t e t h a t dec a f f e i n a t e d c o f f e e s p o s s e s s lower amounts o f p h e n o l s , (when r o a s t e d u n d e r c o m p a r a b l e c o n d i t i o n s ) t h a n c a f f e i n a t e d b l e n d s . S i m i l a r r e s u l t s were p r e s e n t e d by Konig e t a l . (9^) f o r s o c a l l e d s t o m a c h f r i e n d l y c o f f e e s w h e r e c h l o r o g e n i c a c i d p r e c u r s o r s a r e p a r t i a l l y degraded by washing c o f f e e beans under e l e v a t e d temperatures.


Carbohydrates

and Amino

Acid

Precursors

S u c r o s e , r e d u c i n g s u g a r s , f r e e and p e p t i d e amino a c i d s a r e o t h e r i m p o r t a n t p r e c u r s o r s w h i c h d e t e r m i n e aroma and f l a v o r o f r o a s t e d c o f f e e . F e l d m a n e t a l . (j_2) i n v e s t i g a t e d t h e s e p r e c u r s o r s i n g r e e n R o b u s t a s and A r a b i c a s and q u a n t i f i e d t h e i r changes during r o a s t i n g . Arabicas p o s s e s s h i g h e r amounts o f s u c r o s e t h a n R o b u s t a s and l o w e r amounts o f r e d u c i n g s u g a r s . D u r i n g r o a s t i n g s u c r o s e , g l u c o s e , f r u c t o s e a n d g a l a c t o s e d e c r e a s e b e l o w 0.5 % a n d a r a b i n o s e and rhamnose c a n be d e t e c t e d a f t e r r o a s t i n g w h i c h i n d i c a t e s t h a t p o l y s a c c h a r i d e s a r e a l s o degraded ( 1 3 ) . W a l t e r e t a l . (J_4) d e m o n s t r a t e d t h a t g r e e n c o f f e e beans possess approximately 5 t o 10 % o f p e p t i d e a m i n o a c i d s as f r e e amino a c i d s . M e i c h e l b e c k and Zahn (15) s h o w e d t h a t c o f f e e p r o t e i n s c o n t a i n 0.47 - 0.56 % c y s t i n e a n d o n l y 0.02 % c y s t e i n e b y p o l a r o g r a p h i c m e t h o d s . T h e y c o u l d n o t f i n d d i f f e r e n c e s between A r a b i c a s and R o b u s t a s . T r e s s l e t a l . (^_3) s h o w e d d i f f e r e n c e s i n t h e a m o u n t s o f f r e e amino a c i d s o f A r a b i c a s , R o b u s t a s and d e c a f f e i n a t e d b l e n d s . Robustas p o s s e s s h i g h e r amounts o f j-aminob u t y r i c a c i d , S t r e c k e r - a c t i v e , and b a s i c amino a c i d s and lower amounts o f g l u t a m i c a c i d t h a n A r a b i c a . These r e s u l t s were c o n f i r m e d by T r a u t w e i n (J_6) who i n v e s t i g a t e d m o r e t h a n 50 s a m p l e s . T h e r e a r e l i t t l e o r n o s i g n i f i c a n t d i f f e r e n c e s i n t h e p e p t i d e amino a c i d s b e t w e e n t h e two v a r i e t i e s . D u r i n g r o a s t i n g f r e e amino a c i d s r e a c t v i a M a i l l a r d R e a c t i o n and p r o t e i n bound C y s , L y s , A r g , S e r , T h r d e c r e a s e c o n s i d e r a b l y (J_6 ) . S t r o b e l (J_7 ) s h o w e d s i g n i f i c a n t d i f f e r e n c e s i n the concentration of free c y s t e i n e between A r a b i c a s and R o b u s t a s and b l a m e d s u l f u r constituents f o r the less d e s i r a b l e "robusta taste" (rubbery note). Aroma

Perspective

Aroma and f l a v o r c o m p o n e n t s o f r o a s t e d c o f f e e were i n v e s t i g a t e d b y n u m e r o u s a u t h o r s . I n 1926 R e i c h s t e i n and Staudinger (J_8) c h a r a c t e r i z e d f u r f u r y l m e r c a p t a n (2-furylmethanthiol) as an i m p o r t a n t aroma c o n s t i t u e n t o f r o a s t e d c o f f e e which i s a c h a r a c t e r impact component. A t t h e t u r n o f t h i s c e n t u r y , o n l y 13 c o m p o n e n t s w e r e k n o w n . T h i s n u m b e r i n c r e a s e d t o 60 i n t h e f o l l o w i n g f i f t y y e a r s . From 1965 t o 1975 t h e number o f c o f f e e c o n s t i t u e n t s i n c r e a s e d d r a m a t i c a l l y t o more t h a n 600. S t o l l e t a l . ( 1 9 ) , Goldman e t a l . (20) and V i t z t h u m e t


27.

TRESSL


289

a l . (2J_, 2_2) c o n t r i b u t e d most t o t h i s d e v e l o p m e n t . In the l a s t t e n y e a r s t h e i d e n t i f i c a t i o n o f new c o f f e e c o n s t i t u e n t s p l a t e a u e d a t o v e r 700 ( 2_3, 24^) . M o s t o f t h e c o m pounds were r e p o r t e d w i t h o u t p r e s e n t i n g q u a n t i t a t i v e d a t a , f l a v o r , aroma q u a l i t i e s o r t h r e s h o l d v a l u e s . Therefore, t h e i m p o r t a n c e o f i n d i v i d u a l aroma and f l a v o r compounds among t h e 700 c o n s t i t u e n t s i n r o a s t e d c o f f e e i s s t i l l an open q u e s t i o n .


Furans

and

Reductones

F u r a n s and r e d u c t o n e s a r e m a j o r c o m p o n e n t s i n r o a s t e d c o f f e e as shown i n F i g u r e 3. A r a b i c a s p o s s e s s h i g h e r amounts of f u r a n a l d e h y d e s and F u r a n e o l t h a n R o b u s t a s , w h e n r o a s t e d u n d e r c o m p a r a b l e c o n d i t i o n s . The aldehydes and r e d u c t o n e s a r e S t r e c k e r - a c t i v e c o m p o n e n t s and f u r t h e r transformed i n t o t y p i c a l aroma and f l a v o r compounds as demonstrated i n model experiments. By a p p l y i n g t h e c o n c e p t o f o d o u r v a l u e s (2_5) to t h i s c l a s s o f compounds o n l y F u r a n e o l and M a l t o l c o n t r i b u t e t o c o f f e e f l a v o r . B o t h c o n s t i t u e n t s a r e known as i m p o r t a n t f l a v o r compounds and a r e u s e d as n a t u r e iden t i c a l f l a v o r i n g s i n many f o o d s . F u r a n e o l c o n t r i b u t e s a f r a g r a n t c a r a m e l n o t e t o c o f f e e and i s known as important c o m p o u n d i n p i n a p p l e a n d s t r a w b e r r i e s . The consumption o f F u r a n e o l i n t h e USA i n t h e s e p r o d u c t s i s r e s p e c t i v e l y 4 1 , 8 0 0 t o 2 , 6 5 0 t o 1,100 kg ( p e r y e a r ) compared t o 2,718 kg of s y n t h e s i z e d F u r a n e o l u s e d i n n a t u r e identical f l a v o r i n g . S i m i l a r f i g u r e s w e r e p r e s e n t e d f o r M a l t o l and o t h e r c o f f e e c o m p o u n d s b y S t o f b e r g a n d G r u n d s h o b e r (£) . R o a s t i n g c o n d i t i o n s ( b l e n d I = medium and b l e n d I I = d a r k r o a s t ) i n f l u e n c e t h e s p e c t r u m o f f u r a n s and reduc t o n e s c o n s i d e r a b l y . R e a c t i v e a l d e h y d e s and e n o l o n e s d e c r e a s e and f u r f u r y l a l c o h o l , f u r a n c a r b o x y l i c a c i d and M a l t o l i n c r e a s e s i g n i f i c a n t l y . C o m p o n e n t s 1_ t o _5 a r e f o r m e d v i a 3 - d e o x y - a n d 6^ t o v i a 1 -deoxyosones. In a d d i t i o n , c a r a m e l i z a t i o n may a l s o b e i n v o l v e d . oc - D i c a r b o n y l s ,

Furanones

and

Esters

F i g u r e 4 p r e s e n t s some t y p i c a l α-dicarbonyls,furanones and e s t e r s o f r o a s t e d c o f f e e . D i a c e t y l and 2.3-pentand i o n e add a b u t t e r y t o p n o t e t o c o f f e e and a r e important p r e c u r s o r s f o r c o n t r i b u t i n g p y r a z i n e s . C o m p o n e n t s 4_ a n d _5 were i d e n t i f i e d as p o s s e s s i n g r o a s t y , s u l f u r y n o t e s by s n i f f i n g t e c h n i q u e s . 4_ a n d _5 w e r e i s o l a t e d a n d identified b y MS- a n d H - N M R - s p e c t r o s c o p y (J_3 ) . I n m o d e l e x p e r i m e n t s we c o u l d d e m o n s t r a t e , t h a t 6_ ( a c e t o l ) i s a n important p r e c u r s o r s i n t h e f o r m a t i o n o f t h e s e c o m p o n e n t s . I t was a l s o shown t h a t 5 - m e t h y l - 2 , 3 - h e p t a n e d i o n e i s f o r m e d by aldol condensation f r o m a c e t o l and 2 - m e t h y l b u t a n a l , 4_ b y condensation o f a c e t o l and g l y o x a l , r e s p e c t i v e l y . I t i s o b v i o u s l y an a n a l o g o u s r e a c t i o n t o t h e f o r m a t i o n o f kahweofuran from mercaptoacetone d u r i n g r o a s t i n g of green


290


Robusta

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3: F u r a n s a n d r e d u c t o n e s i n d i f f e r e n t c o f f e e v a r i e t i e s formed by r o a s t i n g ( u n i t s i n ppm)


27.


TRESSL


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4:

Typical α-dicarbonyls, i n roasted c o f f e e (n.d. (units i n ppm)

f u r a n o n e s and e s t e r s = not determined)


292

T H E R M A L GENERATION OF AROMAS

c o f f e e b e a n s . R o a s t i n g l e v e l and r o a s t i n g technology i n f l u e n c e t h e s p e c t r u m o f t h i s c l a s s o f compounds more than the v a r i e t y of the beans.


Pyrroles

and

Pyridines

Many p y r r o l e s h a v e b e e n i d e n t i f i e d i n r o a s t e d c o f f e e . Regarding t h e i r formation i n coffee p y r i d i n e , pyrrole and N - m e t h y l - 2 - f o r m y l p y r r o l e a r e t r i g o n e l l i n d e r i v a t i v e s ; _3 a n d A_ t y p i c a l p y r r o l e s f r o m p r i m a r y a m i n o a c i d s ; a n d 6_ t o £ h y d r o x y p r o l i n e d e r i v e d M a i l l a r d p r o d u c t s (2^6) . P y r i d i n e a n d p y r r o l e ( F i g u r e 5) i n c r e a s e according t o t h e r o a s t i n g l e v e l o f t h e b e a n s and a r e h i g h e r i n Arabicas than i n Robustas under comparable c o n d i t i o n s . 5 t o 8 are c o n t r i b u t i n g aroma components a c c o r d i n g to t h e i r low t h r e s h o l d v a l u e s (21). Pyrazines M o r e t h a n 50 p y r a z i n e s h a v e b e e n i d e n t i f i e d i n r o a s t e d c o f f e e ( F i g u r e 6). They are formed d u r i n g Strecker d e g r a d a t i o n of p r i m a r y amino a c i d s w i t h α-dicarbonyls, c y c l i c e n o l o n e s , f u r a n a l d e h y d e s a n d r a n g e f r o m 180 to 220 ppm i n medium r o a s t e d c o f f e e . They p o s s e s s r o a s t e d n u t t y , g r e e n , c e r e a l and e a r t h y aroma q u a l i t i e s and t h e i r t h r e s h o l d s v a r y i n a w i d e r a n g e . From t h e F l a v o r U n i t d a t a , i t c a n b e s e e n t h a t JJ_ a n d J_2 which are formed from 2 , 3 - p e n t a n e d i o n e , are c o n t r i b u t i n g compounds w h e r e a s t h e m o r e c o n c e n t r a t e d J_ a n d 4 p o s s e s s l e s s s i g n i f i c a n c e . Roasting technology ancT r o a s t i n g l e v e l d e t e r m i n e t h e s p e c t r u m o f p y r a z i n e s by i n c r e a s i n g t h e a m o u n t s o f m e t h y l - a n d e t h y l p y r a z i n e s . The consumption o f p y r a z i n e s f r o m r o a s t e d c o f f e e i s 600 t o 700 fold h i g h e r c o m p a r e d t o a l l t h e s y n t h e s i z e d p y r a z i n e s added as f l a v o r i n g materials to f o o d s t u f f s . The o d o r o f g r e e n c o f f e e i s p r e d o m i n a n t l y d e t e r m i n e d b y 2 - m e t h o x y - 3 - i s o b u t y l - a n d i s o p r o p y l p y r a z i n e s (_28 ) . The c o n c e n t r a t i o n o f 2 - m e t h o x y - 3 - i s o b u t y l p y r a z i n e v a r i e s i n r o a s t e d c o f f e e f r o m 10 t o 50 p p b a n d i s t h e r e f o r e a c o m p o u n d w i t h a h i g h f l a v o r v a l u e (> 1000 F.U.) in r o a s t e d c o f f e e . N i t z e t a l . {29_) d e m o n s t r a t e d t h a t t h e "peasy" o f f - f l a v o r i n c e r t a i n A f r i c a n c o f f e e beens i s c a u s e d b y a 5 t o 20 f o l d i n c r e a s e o f 2 - m e t h o x y - 3 - i s o p r o p y l p y r a z i n e , o b v i o u s l y f o r m e d by f u n g i . Sulfur

Containing

Furans

S u l f u r c o n t a i n i n g c o m p o u n d s p l a y an i m p o r t a n t r o l e i n t h e f l a v o r s p e c t r u m o f r o a s t e d c o f f e e . They amount 5 t o 15 ppm and a r e i n f l u e n c e d by v a r i e t y , r o a s t i n g l e v e l and s t o r a g e c o n d i t i o n s . More t h a n 100 s u l f u r compounds were c h a r a c t e r i z e d i n r o a s t e d c o f f e e ( 2_4 ) . F i g u r e 7 shows s u l f u r c o n t a i n i n g f u r a n s and their r a n g e i n r o a s t e d c o f f e e . I t c a n be s e e n t h a t R o b u s t a s


TRESSL



27.

^

Ro bu s ta

Arabica

^ Blend

26

46

32

190

5.3

7.5

6.5

10

3.8

4.3

I H HO

^N'XHO

V

4.2

3.5

2.5

2.5

10

1.4

1.5

2.8

2

2.5

4.4

ο

Ο

1.5

Ν

ο

Τ Figure

5:

2.9

P y r i d i n e and t y p i c a l p y r r o l e s i n r o a s t e d R o b u s t a and A r a b i c a c o f f e e s ( u n i t s i n ppm)


293

294



Figure

6: R a n g e o f t y p i c a l p y r a z i n e s i d e n t i f i e d i n r o a s t e d c o f f e e ( u n i t s i n ppm)


27. TRESSL



Range

^ 0 ^ C H - SH

Arabica

295

Robusta

500-4000

1 0 0 0 - 2000

2 0 0 0 - 3800

° 0 - 200

150 - 200

100 - 150

200 - 1 0 0 0

250 - 500

500 - 1 0 0 0

20 - 50

30 - 50

200-600

200-400

2

^0^CH -SH

1

2

Ô^CH -S-CH

3

Ô^CH -S-CH

3

2

2

Ô^CH -S-S-CH 2

X N

0^CH -S-S-CH 2

3

^0^CH -S-CH -CH 2

2

2

S-CH Ô^CH -S-S-CH 3

S-S-CH 0 . Figure

7:

20 -

30

4 0 0 - 600

3

^T>CH -S-CH ^

2

10

20

3

f\ 2

1 -

10 -

2

1 0

0^

-

* 1 -

7

0

10

3

1

-

5

S u l f u r c o n t a i n i n g f u r a n s c h a r a c t e r i z e d under d i f f e r e n t r o a s t i n g c o n d i t i o n s ( u n i t s i n ppm)



296


p o s s e s s h i g h e r amounts o f f u r f u r y l m e r c a p t a n , f u r f u r y l m e t h y l s u l f i d e and f u r f u r y l m e t h y l d i s u l f i d e and A r a b i c a s more o f t h e c o r r e s p o n d i n g 5 - m e t h y l d e r i v a t i v e s ( a t s i m i l a r r o a s t i n g l e v e l s ) . Furfurylmercaptan possesses a threshold o f 5 p p t . From 0.1 - 1 ppb f u r f u r y l m e r c a p t a n has an aroma l i k e f r e s h brewed c o f f e e , w h i l e f r o m 5 - 1 0 ppb i t i s p e r c e i v e d with a s u l f u r y , m e r c a p t a n - l i k e note comparable to s t a l e d c o f f e e . 5-Methylfurfurylmercaptan has a t h r e s h o l d o f 50 ppt and d e l i v e r s a s u l f u r y n o t e above 5 ppb. B o t h c o n s t i t u e n t s a r e f o r m e d d u r i n g r o a s t i n g and i n c r e a s e 5 t o 10 f o l d d u r i n g s t o r a g e o f r o a s t e d c o f f e e b e a n s as d e m o n s t r a t e d by S i l w a r e t a l . (_30.) · T h e r e f o r e , b o t h c o n s t i t u e n t s d e t e r m i n e t h e aroma and f l a v o r o f f r e s h r o a s t e d c o f f e e and a r e a l s o r e s p o n s i b l e f o r t h e undesirable aroma o f s t a l e d c o f f e e . The c o n c e n t r a t i o n s o f f u r f u r y l s u l f i d e s and - d i s u l f i d e s w i c h p o s s e s s t o a s t e d , b r e a d - and m e a t - l i k e aromas do n o t change d u r i n g s t o r a g e o f r o a s t e d c o f f e e . S u l f u r c o n t a i n i n g f u r a n s a r e f o r m e d by h e a t i n g furanaldehydes w i t h c y s t e i n e and m e t h i o n i n e , r e s p e c t i v e l y ( 2 4 ) . Thiolanones In r o a s t e d c o f f e e s i m i l a r r e a c t i o n s seem t o be i n v o l v e d f o r m i n g m e t h y l m e r c a p t a n by t h e S t r e c k e r d e g r a d a t i o n o f f r e e m e t h i o n i n e and f o r m i n g H^S f r o m p e p t i d e c y s t e i n e . Figure 8 presents a d d i t i o n a l f l a v o r c o n t r i b u t i n g cons t i t u e n t s of r o a s t e d c o f f e e . 3-Thiolanone 6^ and 2-methyl3-thiolanone T_ were i d e n t i f i e d by S t o l l e t a l . (19) and p a t e n t e d as c o f f e e f l a v o r s . The two t h i o l a n o n e s

Thermal Generation of Aromas - ACS Publications - American

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