Model Reactions on Generation of Thermal Aroma Compounds - ACS

Chapter 14

Model Reactions on Generation of Thermal Aroma Compounds

Downloaded by FUDAN UNIV on March 24, 2017 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0409.ch014

W.

Baltes, J . Kunert-Kirchhoff, and G. Reese

Institut für Lebensmittelchemie der Technischen Universität Berlin, Strasse des 17. Juni 135, D-1000 Berlin 12, Federal Republic of Germany

Thermal aromas result from the Maillard reaction. By heating carbohydrates with amino acids degradation is accelerated yielding reactive compounds which, by new reactions with amino acids, are converted to heterocyclic products. Results of model investigations of glucose or its degradation compounds with the amino acids serine and phenylalanine are discussed. It is demonstrated that a great many flavor compounds are formed in both model systems. On the other hand, phenylalanine formed by aldol condensations some special aroma products. Furthermore, the generation of thermal aroma compounds depend on the pH, the sugar/amino acid ratio and the temperature.

The o r g a n i z a t i o n of a s p e c i a l symposium d e a l i n g with thermal aromas show the p a r t i c u l a r importance which they have r e c e i v e d i n past y e a r s . One reason may be the i n t e r e s t of f l a v o r companies producing thermal aromas, which are f r e q u e n t l y used i n convenience food prod u c t s . Another reason i s t h e i r complex composition which c h a l l e n g e s s c i e n t i s t s t o f i n d out s u i t a b l e ways f o r t h e i r a n a l y s i s . We have c a r r i e d out some model r e a c t i o n s on the f o r m a t i o n of thermal aromas i n order t o t e s t the c o n d i t i o n s f o r the a n a l y s i s of such aromas and to study the mechanisms of t h e i r f o r m a t i o n and t h e i r dependence on c o n c e n t r a t i o n and temperature. L a s t but not l e a s t we were i n t e r e s t e d t o get an overview about the compounds which can be formed by g e n e r a t i o n of thermal aromas. The aroma p r e c u r s o r s have been s e l e c t e d by t a k i n g i n t o account the s i g n i f i c a n t r o l e of the Mai Hard r e a c t i o n . Indeed, most aroma compounds of t h i s type are formed by the r e a c t i o n of amino a c i d s with sugars or t h e i r d e g r a d a t i o n p r o d u c t s . So we have o b t a i n e d r o a s t beef, r o a s t mutton and heated v e g e t a b l e aromas a f t e r having t r e a t e d a mixture of amino a c i d s and g l u c o s e at d i f f e r e n t temperatures and f o r varied times. NOTE

: Dedicated to Kurt Heyns on his 80th birthday. 0097-6156/89/0409-0143$06.00/0 ο 1989 American Chemical Society Parliment et al.; Thermal Generation of Aromas ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


144

THERMAL GENERATION OF AROMAS

In t h i s p u b l i c a t i o n we r e p o r t on the r e s u l t s of r e a c t i o n s between g l u c o s e and s e r i n e or p h e n y l a l a n i n e under the c o n d i t i o n s of cooking a soup, r o a s t i n g of c o f f e e or a u t o c l a v i n g process i n water s o l u t i o n at 120°, 150° and 180° C. The experiments have been completed by a d d i t i o n a l r e a c t i o n of x y l o s e , f r u c t o s e and some c h a r a c t e r i s t i c sugar d e g r a d a t i o n products l i k e c y c l o t e n e , Furaneol and d i a c e t y l and by thermal decomposition of Amador i rearrangement p r o d u c t s . I t i s w e l l knwon t h a t sugars can r e a c t with s u i t a b l e amino compounds very e a s i l y . In the course of these r e a c t i o n s sugars are mostly decomposed and brown melanoidins are formed. By-products of these melanoidins are many v o l a t i l e compounds of c h a r a c t e r i s t i c aroma p r o p e r t i e s . They are a l s o r e s p o n s i b l e f o r the w e l l known aromas of heated food l i k e meat, c o f f e e and bread. T h i s complex accumulation of r e a c t i o n s i s d e s i g n a t e d as M a i l l a r d r e a c t i o n ( 1 ) . In the case of aldosugars the r e a c t i o n s t a r t s by cond e n s a t i o n of the sugar with amino compounds y i e l d i n g a N - g l y c o s i d e which can form the c o r r e s p o n d i n g d e r i v a t i v e of a ketosugar (1-deoxy1-amino-ketose,2) v i a Amadori rearrangement. T h i s r e a c t i o n i s dependent on the pH. On the one hand, Amadori rearrangement i s c a t a l y z e d by protons, but on the other s i d e N - g l y c o s i d e f o r m a t i o n w i l l not take p l a c e when the r e a c t i o n medium i s too a c i d i c . So we have mostly c a r r i e d out our experiments a t pH values of about 5,5 - 6,2. The mechanism of t h i s arrangement i s a s c r i b e d t o e n d i o l s t r u c t u r e s . The r e v e r s e rearrangement of a N-ketoside forming the c o r r e s p o n d i n g aldose d e r i v a t i v e (2-deoxy-2-aminoaldose) i s a l s o known: It i s the Heyns rearrangement which had been d i s c o v e r d 30 years ago. Some "glucose-amino a c i d s " , products of the rearrangement, have been i s o l a t e d from swine l i v e r (3) . Reactions of f r u c t o s e with amino a c i d s have a l s o been observed a f t e r h e a t i n g of f r u i t s ( 4 ) . On one hand the browning of a f r u c t o s e c o n t a i n i n g r e a c t i o n medium occurs s l o w l e r i n comparison t o g l u c o s e ( 5 ) . The i n t e r m e d i a t e s of f r u c t o s e d e g r a d a t i o n a f t e r r e a c t i o n with amino compounds have not been i n v e s t i g a t e d up t o now. On the o t h e r hand i n 1967 J.E. Hodge (1) has g i v e n a d e s c r i p t i o n of d e g r a d a t i o n pathways of g l u c o s e a f t e r r e a c t i o n with amino compounds. He had r e c o g n i z e d t h a t e n d i o l s as i n t e r m e d i a t e s of an Amad o r i rearrangement are u n s t a b l e compounds making the e l i m i n a t i o n of s u b s t i t u e n t s i n the a l l y l - p o s i t i o n more easy. This can be a hydroxy1 as w e l l as an amino group. Today we d i s t i n g u i s h between 2 d e g r a d a t i o n pathways. The f i r s t one c o n t a i n s the 3-deoxyosone as an i n t e r m e d i a t e . 3-Deoxyglucosone had been i s o l a t e d from brown f r u i t pulps and from soy sauce. T h i s d e g r a d a t i o n w i l l form 5 - h y d r o x i m e t h y l - 2 - f u r f u r a l as an end product . T h i s compound i s not o n l y a c h a r a c t e r i s t i c degrad a t i o n product of hexoses i n weak a c i d i c s o l u t i o n s , moreover i t i s a c o n s t i t u e n t of every thermal aroma. In c o f f e e aroma hydroximethyl f u r f u r a l i s one of the most important compounds ( 6 ) . In a d d i t i o n t o the "3-deoxyhexoson-pathway" Hodge p o s t u l a t e d a 1-deoxyosone as an i n t e r m e d i a t e of another d e g r a d a t i o n pathway i n 1967 ( 1 ) . T h i s compound c o n t a i n s carbonyl-and hydroxygroups s i d e by s i d e . I t has been r e c e n t l y i d e n t i f i e d by Ledl ( 7 ) . As F i g u r e 1 demonstrates t h i s d e g r a d a t i o n pathway i s r e s p o n s i b l e f o r the f o r m a t i o n of a g r e a t many of r e a c t i o n products from the carbohydrate molecule. It can a l s o be r e c o g n i z e d t h a t t h e r e are numerous p o s s i b i l i t i e s of k e t o - e n o l - t a u t o m e r i z a t i o n s . The speed of these r e a c t i o n steps depends

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

14.

BALTES ET

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145

Model Reactions on Thermal Aroma Compounds

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C-OH Ç=0 CH

H-C-OH

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H-C-OH

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H-C-OH

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2

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2

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2

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H-C-OH

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H-C-OH

CH OH

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CH -C0-C0-CH

3

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H0-CH -C0-C0-CH

3

3

2

CH3-COOH

CH OH

u.a.

2

2

D

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HO^X/OH

HO, .

-

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Η

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F i g u r e 1: Suggested r e a c t i o n routes of carbohydrate degradation v i a the 1-deoxyosone


3


146


on t h e temperature. In t h e course o f these r e a c t i o n s , water e l i m i n a tions occur and p r e c u r s o r s o f d i c a r b o n y l compounds are formed v i a r e t r o a l d o l r e a c t i o n s . These d i c a r b o n y l s show a high r e a c t i v i t y . A l s o a c e t i c a c i d may a r i s e from a r e a c t i o n o f t h i s t y p e . Most import a n t compounds o f t h i s pathway are p y r u v i c aldehyde, d i a c e t y l , hydrooxyacetone and h y d r o x y d i a c e t y l which can e a s i l y r e a c t with amino a c i d s . The S t r e c k e r d e g r a d a t i o n i s a r e a c t i o n where the amino a c i d i s dec a r b o x y l a t e d and l o s e s i t s amino group. Reaction products are t h e S t r e c k e r aldehyde and - as an i n t e r m e d i a t e - an aminoketone which forms a p y r a z i n e by d i m e r i z a t i o n . T h i s pathway i s c o n s i d e r e d t o be most important f o r t h e o r i g i n o f p y r a z i n e s i n thermal aromas. However, o n l y l i m i t e d knowledge i s a v a i l a b l e about t h e f a t e o f t h e S t r e c k e r aldehydes. As we w i l l demonstrate they are very r e a c t i v e . In a d d i t i o n , F i g u r e 1 demonstrates t h a t some i n t r a m o l e c u l a r c y c l i z a t i o n s a r e p o s s i b l e . So a c e t y l f u r a n e w i l l be p r e s e n t i n every t h e r mal aroma when t h e amount o f carbohydrate i n t h e r e a c t i o n medium was high enough. A l s o m a l t o l , 4-hydroxymaltol, i s o m a l t o l and even Furaneo l (2,5 dimethyl-4-hydroxy-3 [ 2H]-furanone) t h e "ananas-furanone" can be d e t e c t e d i n most cases though t h e i r c o n c e n t r a t i o n s a r e lower than t h e amounts o f a l i p h a t i c d i c a r b o n y l s . B u t t h i s scheme shows t h a t the f o r m a t i o n o f m a l t o l doesn't r e q u i r e t h e s u b s t i t u t i o n s o f t h e 4p o s i t i o n and t h a t t h e f o r m a t i o n o f Furaneol i s n o t a b s o l u t e l y dependent upon a 6-deoxysugar. On t h e c o n t r a r y the sugar molecule can o b v i o u s l y form s u i t a b l e p r e c u r s o r s f o r t h e i r f o r m a t i o n under r e a c t i o n c o n d i t i o n s which a r e s i m i l a r t o the c o n d i t i o n s o f i n d u s t r i a l aroma p r o c e s s i n g . Table 1 shows the most important d e g r a d a t i o n products o f g l u c o s e which have been formed i n an aqueous r e a c t i o n medium c o n t a i n i n g s e r i n e as amino a c i d a f t e r h e a t i n g i n an a u t o c l a v e . These compounds have been q u a n t i f i e d a f t e r GC/MS a n a l y s i s o f t h e v o l a t i l e s by an i n t e g r a t i o n program o f o u r computer. - The compounds c i t e d on t h e l e f t s i d e a r e probably formed v i a t h e 3-deoxyosone pathway (with t h e e x c e p t i o n o f a c e t y l f u r a n and a c e t y l p y r r o l e . The dominant product o f t h i s scheme i s 5 - h y d r o x y m e t h y l - 2 - f u r f u r a l ("HMF") t h e c o n c e n t r a t i o n s of which have been lowered with r a i s i n g temperature. Furandialdehyde i s i t s d e h y d r a t i o n product and 5 - m e t h y l f u r f u r a l probably formed from HMF, t o o . The most important r e a c t i o n product o f t h e 1-deoxyosonpathway i s 5-hydroxy-5,6-dihydromaltol. A t 120° C t h i s compound which we have i d e n t i f i e d i n most o f o u r model r e a c t i o n s , r e p r e s e n t s about 30 % o f a l l v o l a t i l e s i n t h e r e a c t i o n system s e r i n e / g l u c o s e . We are j u s t i n i t i a t i n g s t u d i e s d e a l i n g with f u r t h e r d e g r a d a t i o n and r e a c t i o n pathways o f t h i s compound, which i s very i n s t a b l e and d i s appears a t h i g h e r temperature as w e l l as by treatment on s i l i c a g e l columns. Simultaneously, t h e c o n c e n t r a t i o n s o f c y c l o t e n e (2-hydroxy3-methyl-cyclopenten-2-one) and Furaneol i n c r e a s e . T h i s p o s s i b l e c o r r e l a t i o n l e d t o the assumption t h a t both compounds are degradat i o n products o f t h e d i h y d r o h y d r o x y m a l t o l . T h e r e f o r e i s was suggested t h a t c y c l o t e n e i s formed from t h i s m a l t o l d e r i v a t i v e v i a a hexose reductone and t h e h y d r o x y c y c l o t e n e ( 9 ) . Another f o r m a t i o n pathway of c y c l o t e n e i s suggested t o be a condensation r e a c t i o n o f hydroxyacetone, which can a l s o r e a c t as l a c t i c aldehyde. The f o r m a t i o n o f Furaneol i s proposed t o happen v i a r i n g c o n t r a c t i o n y i e l d i n g a c e t y l formoin, which i s w e l l known t o be a c y c l i z a t i o n product o f 1-deoxyosone. T h i s compound has been i d e n t i f i e d v i a i t s r e a c t i o n products


14.

B A L T E S E T AL.


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American Chemical Society Library 1155 16th St., N.W. Washington, 20036 Parliment et al.; ThermalD.C. Generation of Aromas ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


148


(10). I t i s suggested t h a t i t forms Furaneol by d e h y d r a t i o n and and hydrogénation of the e x o c y c l i c methylene group. As was a l r e a d y mentioned, we have r e a c t e d our model mixtures under d i f f e r e n t c o n d i t i o n s . Among them, the experiments of cooking and a u t o c l a v i n g a phospate b u f f e r s o l u t i o n of a sugar/amino a c i d mixture have been very s i m i l a r . Only the temperature was d i f f e r e n t . F u r t h e r more, the r o a s t i n g experiments have been c a r r i e d out by h e a t i n g the sugar/amino a c i d mixtures on an i n e r t m a t e r i a l i n the presence of 10 % b u f f e r s o l u t i o n . In the l a t t e r we have i d e n t i f i e d a g r e a t many of pseudo-aromatic and aromatic compounds. These are, among o t h e r s , predominantly d i f f e r e n t a l k y l - s u b s t i t u t e d f u r a n s , p y r a z i n e s , p y r r o l s and benzo-aromatic compounds. T h i s behaviour of the r e a c t i o n system t o form compounds which are s t a b i l i z e d by mesomeric energy seems t o be r e a s o n a b l e . When cooking at about 100° C predominantly f u r a n s with oxygen c o n t a i n i n g s u b s t i t u e n t s , furanones, pyranones and a l i p h a t i c c a r b o n y l compounds were formed (11). Moreover the number of v o l a t i l e s i n t h i s case are l e s s than i n the case of r o a s t i n g . By h e a t i n g the mixture i n a b u f f e r s o l u t i o n at g r e a t e r than 100° C in an a u t o c l a v e the q u e s t i o n about the i n f l u e n c e of the r e a c t i o n medium on number and k i n d of v o l a t i l e s should be answered. The answer, indeed, i s very simple: most compounds formed i n the a u t o c l a v e have been i d e n t i c a l t o the v o l a t i l e s a f t e r r o a s t i n g the m i x t u r e . Conseq u e n t l y , the r e a c t i o n temperature i s the most d e c i s i v e f a c t o r f o r the f o r m a t i o n of thermal aroma compounds. Table 2 shows the r e l a t i v e q u a n t i t i e s of component groups i n the v o l a t i l e s a f t e r h e a t i n g mixtures of g l u c o s e or f r u c t o s e with s e r i n e at 120°, 150° and 180° C. I t shows optimal f o r m a t i o n of f u r a n s and pyranones at 120° C, whereas furanones possess a maximum at 150° C. Compounds of the o t h e r groups are formed p r e f e r e n t i a l l y a t 150° C, w h i l e the f o r m a t i o n of p y r a z i n e s proceeds b e t t e r the h i g h e r a r e a c t i o n temperature was chosen. A l s o p y r r o l e s need h i g h e r temperatures f o r t h e i r f o r m a t i o n . So we c o u l d demonstrate t h a t o n l y a c e t y l p y r r o l e and 5-methylpyrrole-2-aldehyde has been formed at 120° C whereas many a d d i t i o n a l p y r r o l e s appear a t 150° C or 180°C r e s p e c t i v e l y ( 1 2 ) . By r e a c t i o n of s e r i n e or p h e n y l a l a n i n e with f r u c t o s e i n s t e a d of g l u c o s e we got i d e n t i c a l r e s u l t s with the e x c e p t i o n of h i g h e r p y r a z i n e c o n c e n t r a t i o n s (more than 50 % r e l a t i v e l y h i g h e r ! ) . Main products have been 2,5 and 2 , 6 - d i m e t h y l p y r a z i n e which may be formed v i a the r e a c t i o n of two C g - d i c a r b o n y l s each. In our o p i n i o n these r e s u l t s demonstrate a s p e c i a l ease of f r u c t o s e t o decompose by c h a i n c l e a vage. As another decomposition product of f r u c t o s e we have i d e n t i f i e d f u r f u r y l a l c o h o l a f t e r h e a t i n g a r e a c t i o n mixture a t 120° C. Obv i o u s l y t h i s C -compound has been formed a f t e r cleavage of the c a r bon atom 1 from t h e sugar m o l e c u l e . 5

C a r b o c y c l i c Compounds Ofcourse, the number of components i n c r e a s e with r i s i n g temperature. Examples are c a r b o c y c l i c compounds. Most of them are c y c l o a l k e n o n e s and h y d r o x y c y c l o a l k e n o n e s . One reason f o r t h i s f a c t i s t h e i r i s o m e r i z a t i o n . As we c o u l d demonstrate (12) c y c l o t e n e forms at 180° C 3 i s o m e r i c compounds and a d d i t i o n a l l y 3 methylcyclopentanones or-pentenones v i a e l i m i n a t i o n of one molecule of water ( F i g u r e 2 ) . Hydroxycyclopentenones and-hexenonesare w e l l known t o be important aroma compounds i n caramel f l a v o u r s (13,14).



45 6-7 33 1 1 5 1 7

c

120 C 31 12 2 13 14 7-8 3-4 12-13

150°C 55 6 23 1 7 2 1 5

3-4 2 12 48 7 9 9

-

120°C 29 13 1-2 16 22 5-6 1-2 8

150°C

12 65 3 4 9

-

2 1

180°C

Serine-Fructose %

180°C

Serine-Glucose %

of S e r i n e w i t h Glucose o r F r u c t o s e i n an aqueous system

R e l a t i v e Q u a n t i t i e s (%) o f Component Groups a f t e r H e a t i n g

Furans Furanones Pyranones C a r b o c y c l i c Cpds. Pyrazines Pyrroles Pyridines A l i p h a t i c Cpds.

Class

Table 2:


150

T H E R M A L GENERATION OF AROMAS


-OH

NH

9 -

-

9 -

0

-

9 ~

0

7 NH

*

-

^

0

/

^ O H

2

^γ^·0Η 0

0

1

\

Amino Acid

I

0H

^f^ NH I

\ H

Sf^° N £H

3

F i g u r e 2: Reaction of c y c l o t e n e with amino a c i d s



14.

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151

Pyrazines The N - h e t e r o c y c l i c compounds which have a l r e a d y been mentioned are formed by r e a c t i o n of ammonia o r amino a c i d s w i t h v a r i o u s c a r b o n y l compounds from the sugar decomposition. Most important i n t h i s f i e l d i s the S t r e c k e r d e g r a d a t i o n of amino a c i d s v i a r e a c t i o n with c t - d i carbonyl compounds. In t h i s regard every amino a c i d with primary amino group w i l l probably form compounds of t h i s t y p e . M o n o c y c l i c p y r a z i n e s are mostly s u b s t i t u t e d by methyl groups demonstrating the r o l e of d i a c e t y l , p y r u v i c aldehyde and c o r r e s p o n d i n g decomposition products of the sugars. Other s u b s t i t u e n t s of monocyclic p y r a z i n e s are e t h y l - , p r o p y l - , v i n y l - a l l y l - and propenyl groups. Most impor t a n t examples are 2,5-, 2 , 6 - d i m e t h y l p y r a z i n e and 2-ethyl-5-methylp y r a z i n e , whereas a l k e n y l p y r a z i n e s mostly appear o n l y i n t r a c e amounts. B i c y c l i c p y r a z i n e s l i k e f u r a n y l - , f u r f u r y l - , p y r r o l o - and dihydrocyclopentapyrazines respectively tetrahydrochinoxalines mostly r e q u i r e temperatures over 150° C f o r t h e i r f o r m a t i o n , and a l s o monocyclic p y r a z i n e s are formed i n low amounts at 120° C o n l y . V a r i e t y and q u a n t i t y of the p y r a z i n e f r a c t i o n depend on the amino a c i d / s u g a r r a t i o , t o o . We have found 10 p y r a z i n e s a f t e r r o a s t i n g a mixture of an excess of p h e n y l a l a n i n e with g l u c o s e a t 220° C (11). On the other hand we have o b t a i n e d 57 mono- and b i c y c l i c compounds a f t e r h e a t i n g t h i s mixture at 180° C i n the a u t o c l a v e , but i n a mo l a r r a t i o n of 1 : 1. - The hydroxyamino a c i d s s e r i n e and t h r e o n i n e are very important p r o c u r s o r s of p y r a z i n e s because they form aminoc a r b o n y l compounds alone when heated t o temperatures g r e a t e r than 150° C. So we have i d e n t i f i e d more than 120 mono- and b i c y c l i c p y r a z i n e s (15) a f t e r h e a t i n g mixtures of sugars with these amino a c i d s . C y c l o t e n e i s the p r e c u r s o r of d i h y d r o c y c l o p e n t a p y r a z i n e s a f t e r having r e a c t e d with ammonia r e s p e c t i v e l y amino a c i d s t o form c y c l o tene imine. F i g u r e 2 demonstrates not o n l y the i s o m e r i z a t i o n and t r a n s f o r m a t i o n of t h i s compound but a l s o the formula of b i s - d i h y d r o c y c l o p e n t a p y r a z i n e s as s p e c i a l r e a c t i o n p r o d u c t s . They are formed by condensation of c y c l o t e n e - i m i n e and r e p r e s e n t a mixture of 4 d i a s t e r e o m e r i c compounds. These compounds produce i d e n t i c a l mass spec t r a (Shibamoto e t a l , 1 6 ) . We have c a r r i e d out c o r r e s p o n d i n g e x p e r i ments by h e a t i n g Furaneol with p h e n y l a l a n i n e or s e r i n e i n an auto c l a v e at 180 ° C. As F i g u r e 3 demonstrates some monocyclic p y r a z i n e s are formed by r e a c t i o n of the d e g r a d a t i o n products from F u r a n e o l . The presence of f u r o p y r a z i n e s demonstrates t h a t Furaneol can r e a c t as an o c - d i c a r b o n y l compound. F u r o p y r a z i n e s have a l s o been found at t r a c e l e v e l s a f t e r h e a t i n g g l u c o s e with amino a c i d s ( J . KunertK i r c h h o f f and W. B a l t e s , Z e i t s c h r i f t f u r L e b e n s m i t t e l - UntersuchungForschung, i n p r e s s ) . A f t e r h e a t i n g Furaneol alone t o 180° C 17 de g r a d a t i o n products have been i d e n t i f i e d . P y r i d i n e s and P y r r o l e s P y r i d i n e s and p y r r o l e s can be formed i n d i f f e r e n t pathways by M a i l l a r d r e a c t i o n . The f o r m a t i o n of 5 - m e t h y l p y r r o l e aldehyde and 6-met h y l - 3 - p y r i d i n o l e has been observed by Nyhammar e t a l (17) by the r e a c t i o n of i s o t o p e l a b e l l e d 3-deoxyosone with g l y c i n e . The 3-deoxyhexosone r e p r e s e n t s an α - d i c a r b o n y l compound and i n t h i s way the S t r e c k e r d e g r a d a t i o n o c c u r s . Another pathway i s the r e a c t i o n of f u rans with ammonia. Under r o a s t c o n d i t i o n s , we have o b t a i n e d p r i m a r i ly the c o r r e s p o n d i n g p y r r o l e , whereas we found the c o r r e s p o n d i n g py-



152

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GENERATION

OF

AROMAS

r i d i n o l e s a f t e r r e a c t i o n i n a n a q u e o u s s y s t e m a t 130° - 160° C. (G. R e e s e a n d W. B a l t e s , Z t s c h r . L e b e n s m . U n t e r s . F o r s c h . , i n p r e s s ) . This r e a c t i o n r e q u i r e s a carbonyl f u n c t i o n i n the p o s i t i o n 2 o f the f u r a n d e r i v a t i v e . By r i n g e n l a r g e m e n t t h e c a r b o n y l g r o u p i s c o n v e r t e d t o a hydroxy1 group i n the β-position, whereas the a l i p h a t i c r e s i d u e (from a ketone!) remains i n the oc-position o f the p y r i d i n e ring. Whereas "normal" α - a m i n o a c i d s g i v e r i s e t o the f o r m a t i o n o f about 25 d i f f e r e n t p y r r o l s u n d e r r o a s t c o n d i t i o n s , s e r i n e a n d t h r e o n i n e a r e e v e n more a c t i v e . We h a v e o b t a i n e d more t h a n 6 0 d i f f e r e n t p y r r o l e s i n c l u d i n g f u r f u r y l - and f u r a n y l p y r r o l e s and d i h y d r o p y r r o l i z i n e s a f t e r r o a s t i n g in the presence o f sugars. This v a r i e t y o f pyrroles can be o b t a i n e d by the r e a c t i o n o f p r o l i n e o r h y d r o x y p r o l i n e , which a l r e a d y each c o n t a i n s a p y r r o l e r i n g . To our s u r p r i s e , h i s t i d i n e f o r m s a r e l a t i v e l y h i g h number o f p y r r o l e s b y r o a s t i n g w i t h s u g a r s . By r o a s t i n g h y d r o x y a m i n o a c i d s we h a v e a l s o o b t a i n e d m o r e t h a n 4 0 d i f f e r e n t p y r i d i n e s , whereas the r e a c t i o n o f p h e n y l a l a n i n e y i e l d e d o n l y a b o u t 20 d e r i v a t i v e s . M o s t p r o d u c t s p o s s e s s e d a c e t y l - o r h y d r o x y l g r o u p s i n a d d i t i o n t o m e t h y l - a n d e t h y l r a d i c a l s , when t h e m i x t u r e was h e a t e d i n an a u t o c l a v e . During our experiments o f heating phenylalanine with glucose i n an a q u e o u s s o l u t i o n a t 180° C we o b t a i n e d a compound p o s s e s s i n g an uncommon s t r u c t u r e w h i c h i s r e p r e s e n t e d i n F i g u r e 4. I t i s a n O L - N hydroxymethylpyrrolyl-propionic l a c t o n e . Two compounds o f t h i s t y p e h a v e b e e n i s o l a t e d b y D i c k e r s o n e t a l (18) f r o m f l u e c u r e d t o b a c c o . The a u t h o r s d e s c r i b e d t h e i r f l a v o r a s s p i c y a n d p e p p e r y . A f t e r r o a s t i n g d i f f e r e n t a m i n o a c i d s w i t h g l u c o s e a t 2 0 0 ° C ( 1 8 - 2 0 ) some compounds o f t h i s t y p e h a v e b e e n i s o l a t e d . We r e a c t e d HMF w i t h p h e n y l a l a n i n e i n a n a u t o c l a v e a t 180° C. U n d e r t h e s e c o n d i t i o n s t h e p y r r o l o l a c t o n e d i d not a r i s e . That l e d us t o the c o n c l u s i o n t h a t t h e p r e c u r s o r o f t h i s compound m i g h t b e t h e 3 - d e o x y o s o n e o c c u r i n g , w h i c h a f t e r r e a c t i o n o f t h e c a r b o n atom 2 w i t h t h e amino a c i d s h o u l d y i e l d the s u b s t i t u t e d ketimine. Oxazoles and Oxazolines In o u r e x p e r i m e n t s o x a z o l e s a n d o x a z o l i n e s w e r e f o u n d o n l y r a r e l y . A f t e r r o a s t i n g s e r i n e a n d t h r e o n i n e we h a v e f o u n d t r a c e s o f 20 d i f f e r e n t o x a z o l e s which were m o s t l y s u b s t i t u t e d by methyl groups. N i n e o f t h e s e o x a z o l e s we f o u n d i n c o f f e e aroma f o r t h e f i r s t t i m e ( 2 1 , 2 2 ) . A b o u t 30 o x a z o l e s h a v e b e e n d e s c r i b e d t o b e p r e s e n t i n t h e r m a l aromas o f c o f f e e , c o c o a and meat. I t h a s been s u g g e s t e d t h a t hydroxyamino a c i d s are t h e i r p r e c u r s o r s . In our experiments with se r i n e a n d t h r e o n i n e o n l y t r a c e s o f t h e s e compounds h a v e b e e n f o r m e d , h e n c e we a r e n o t i n a g r e e m e n t w i t h t h i s m e c h a n i s m . On t h e c o n t r a r y , we a s s u m e a r e a c t i o n p a t h w a y c o n s i s t e n t w i t h t h e s u g g e s t i o n o f V i t z thum (23) o r R i z z i ( 2 4 ) . T h e l a t t e r o b t a i n e d s u b s t i t u t e d o x a z o l i n e s v i a t h e S t r e c k e r d e g r a d a t i o n o f amino a c i d s where a c y c l i z a t i o n o c c u r r e d a f t e r d e c a r b o x y l a t i o n . On t h e o t h e r h a n d , V i t z t h u m e t a l have assumed an a c e t y l a t i o n o f t h e i n t e r m e d i a t e amino enol formed by t h e S t r e c k e r d e g r a d a t i o n . I n e a r l i e r e x p e r i m e n t s we h a v e i d e n t i f i e d d i - a n d t r i m e t h y l o x a z o l e s a f t e r r e a c t i o n o f d i a c e t y l w i t h am monia o r amino a c i d s ( 2 5 ) .


14.

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Steps i n thermal aroma f o r m a t i o n


The most important steps of thermal aroma f o r m a t i o n v i a the M a i l l a r d reaction are: 1. The f i r s t s t e p o f the r e a c t i o n i s the condensation of amino a c i d s t o carbon atom 1 of a l d o s e s (or C-2 of ketoses) and the rearrange ment t o the keto ( a l d o ) - s u g a r (Amadori or Heyns-rearrangement). The i n t e r m e d i a t e e n d i o l s t r u c t u r e s g i v e r i s e t o a f a c i l e sugar decomposition by which d i f f e r e n t a l i p h a t i c o r c y c l i c mono- o r d i carbonyl compounds are formed. For Amadori rearrangement products two r e a c t i o n pathways are known: the 1-deoxy or the 3-deoxyosone pathways. 2. cx-Dicarbonyl compounds r e a c t with amino a c i d s t o y i e l d the S t r e c k e r aldehydes and aminoketones which can be converted v i a dimerization to yield pyrazines. 3. Sugars are the main p r e c u r s o r s of α-dicarbonyls. The amino a c i d / sugar r a t i o i s d e c i s i v e f o r the p r o p o r t i o n of p y r a z i n e s , f u r a n s , furanones, pyranones i n the v o l a t i l e f r a c t i o n . 4. Sugars as w e l l as amino a c i d s are decomposed by heat treatment. The f i n a l r e a c t i o n products from sugars are o f t e n i d e n t i c a l with the products formed by the M a i l l a r d r e a c t i o n . 5. Temperature i s most i n f l u e n c i n g f a c t o r i n the composition of thermal aromas. T h i s means t h a t most α-amino acid/sugar-models w i l l b a s i c a l l y form s i m i l a r compounds i n the v o l a t i l e f r a c t i o n at the same temperature. Consequently, the predominant primary sugar d e g r a d a t i o n products con s i s t of f u r a n s , furanones, pyranones, cyclopentenones and c y c l o p e n tanones with or without hydroxylgroups p l u s some a l i p h a t i c c a r b o n y l s as w e l l as aromatic compounds. By r e a c t i o n with ammonia a g r e a t va r i e t y of p y r r o l e s , p y r a z i n e s , p y r i d i n e s , p y r i d i n o l s and o x a z o l e s are formed. In the presence of s u l f u r c o n t a i n i n g amino a c i d s , t h i a z o l e s , thiophenes and compounds with more than one S-atom (e.g. t r i t h i o l a nes, t r i t h a n e s ) are formed. On the o t h e r hand a g r e a t many of s p e c i a l r e a c t i o n products are formed when secondary amino a c i d s are r e a c t e d with sugars i n o r d e r t o produce thermal aromas. Reaction of S t r e c k e r aldehydes The q u e s t i o n of the f a t e of the " S t r e c k e r " aldehydes r e q u i r e s an ans wer. By c o n v e r t i n g the amino a c i d p h e n y l a l a n i n e t o y i e l d aroma com pounds, phenylacetaldehyde i s l i b e r a t e d . Because of i t s phenyl r i n g i t i s a good d e t e c t o r compound. We were a b l e t o e s t a b l i s h some of i t s r e a c t i o n p r o d u c t s . For example, we have i d e n t i f i e d , among o t h e r s , p h e n y l e t h y l p y r a z i n e , p h e n y l f u r a n , p h e n y l e t h y l p y r r o l e and p h e n y l p y r i d i n e . We assume t h a t a l d o l condensations are r e s p o n s i b l e f o r the f o r m a t i o n of t h e s e compounds. F i g u r e 5 i l l u s t r a t e s our assumption.We have i d e n t i f i e d s e v e r a l compounds the s t r u c t u r e s of which make pro bable an a l d o l condensation ( 3 - ( 2 ' - f u r y l ) - 2 - p h e n y l - 2 - p r o p e n a l , phe nyl hydroxyketones) l i k e l y . T h i s assumption i s supported by the i d e n t i f i c a t i o n o f p y r a z i n e s with up t o 5 carbon atom s i d e c h a i n s i n o t h e r experiments.


153


H CL

.0

χχ. • αχ

NH

H 0 2


180° C

Χτοφτ

JOT

2

COOH

X # T &

F i g u r e 3: Reaction of Furaneol with p h e n y l a l a n i n e

N-^CHO

F i g u r e 4: oC-N-Hydroxymethylpyrrolyl-phenylpropionic

lactone

α χ ,

I n

x

I

I

O

o

»

02

H

H

F i g u r e 5: Condensation products of phenylacetaldehyde


14.

B A L T E S ET

AL.


155

We are j u s t beginning t o understand the f o r m a t i o n and composition of thermal aromas, but undoubtedly t h e r e i s s t i l l a l o t of work t o do u n t i l we w i l l be able t o c o n t r o l a l l i n f l u e n c e s . Literature cited 1.


2. 3. 4. 5. 6.

Hodge, J.E. in: The Chemistry and Physiology of Flavors; Schulz, H.W., Day, E.A., Libby, L.M.; AVI Publ.: Westport Coun., 1967; p. 465. Amadori, M.; Atti, R. Acad. Naz. Lincer, Mem. C1. Sci. Fis.Nat. 1931; p 6, 13, 72 Kuhn, R., Weygand. F. Ber. Dtsch. Chem. Ges. 1937, 70, 769. Heyns, K., Paulsen, H., Ann. Chem. Liebigs 1959; 622, 160. Anet, E.F.L.I.; Reynolds, T.M. Austrial. J. Chem. 1957, 10, 182. Schwimmer, S.; Olcott, H.S. J. Am. chem. Soc. 1953, 75, 4855. Baltes, W. in: Septienne Collogue International sur la Chemie des Cafes verts, Torrefies et leur Drives; ASIC: Paris, 1975,

p. 91 7. 8.

Beck, J., Ledl, F., personal communication. Otto, R.; Baltes, W. Ztschr. Lebensm. Unters. Forsch. 1981, 172 286. 9. Helak, B., Thesis, Technische Universität Berlin, 1988. 10. Ledl, F.; Fritsch, G. Zeitschr. Lebensm. Unters. Forsch. 1984, 178, 41. 11. Baltes, W.; Mevissen, L.; Zeitschr. Lebensm. Unters. Forsch. 1988, 187, 209. 12. Baltes, W. in: Frontiers of Flavor, G. Charalambous Ed.; Elsevier: Amsterdam; 1988; p. 575. 13. Pittet, A.O.; Rittersbacher, P.; Muralidinara, R. J. Agric. Food Chem. 1970, 18, 929. 14. Hodge, J.E.; Mills, F.D.; Fischer, B.E. Cer. Scr. Today 1972, 17, 34. 15. Baltes, W.; Bochmann, G. Zeitschr. Lebensm. Unters. Forsch. 1987, 184, 485. 16. Nishimura, O.; Mikara, S.; Shibamoto, T. J. Agric. Food Chem. 1980, 28, 39. 17. Nyhammar, T.; Ohlson, K.; Pernemalm, P.A. ACS Sympo. Ser. 1983, 215, 72. 18. Dickerson, J.P.; Roberts, D.L.; Miller, C.W.; Lloyd, R.A.; Rix, C.E. Tobacco 1976, 20, 71. 19. Galliard, T.; Phillips, D.R.; J. Reynolds Biochim. Biophys. Acta 1976, 441, 181; Galliard, T.; Matthew, J.A.; Wright, A.J. and M.J. Fishwick, J. Sci. Food Agric. 1977, 28, 863. 20. Shigematsu, H.; Kurata, T.; Kato, H.; Fujimaki, A. Agric. Biol. Chem. 1971, 35, 2097, ibid 1972, 36, 1631, Shigematsu, H.; Shibata; S.; Kurata, T.; Kato, H., Fujimaki, M. J. Agric. Food Chem. 1975, 23, 233. 21. Baltes, W.; Bochmann, G. J. Agric. Food Chem. 1987, 35, 340. 22. Baltes, W.; Bochmann, G. Ztschr. Lebensm. Unters. Forsch. 1987, 23. Vitzthum, O.G.; Werkhoff, P. Ztschr. Lebensm. Unters. Forsch. 1974, 156, 300. 24. Rizzi, G.P. J. Org. Chem. 1969, 34, 2002. 25. Piloty, M.; Baltes, W. Ztschr. Lebensm. Unters. Forsch. 1979, 168, 374. Received March 13, 1989


Model Reactions on Generation of Thermal Aroma Compounds - ACS

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