Formation of 2-Acetyl-l-pyrroline and Other Important Flavor...
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Chapter 25
Downloaded by EAST CAROLINA UNIV on November 3, 2016 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0409.ch025
Formation of 2-Acetyl-l-pyrroline and Other Important Flavor Compounds in Wheat Bread Crust Peter Schieberle Deutsche Forschungsanstalt für Lebensmittelchemie, Lichtenbergstrasse 4, D-8046 Garching, Federal Republic of Germany
We showed that 2-acetyl-1-pyrroline is an impact compound in the crust of a "baguette"-type wheat bread. To gain an insight into its formation during baking, the content of the 2-acetyl-1-pyrroline was quantified in model experiments and in breads prepared by different dough formulas (e.g., addition of amino acids or omission of yeast) using a stable isotope dilution assay. The results revealed that heat treatment of a ground yeast/sucrose mixture is an important source of the 2-acetyl-1-pyrroline formed during the baking process. Furthermore, model experiments suggested that the reaction between 1-pyrroline, the Strecker degradation product of proline, and pyruvaldehyde is responsible for the formation of 2-acetyl-1-pyrroline during bread crust formation. In addition, the influence of baker's yeast on the composition of other crust flavor compounds was f o l lowed by an aroma extract dilution analysis.
Shortened f e r m e n t a t i o n o r b a k i n g s t e p s d u r i n g bread making o f t e n r e s u l t i n bread o f poor f l a v o r q u a l i t y . To improve t h e f l a v o r o f bread produced by modern t e c h n o l o g y i t i s n e c e s s a r y t o g a i n i n f o r mation about p r e c u r s o r s and r e a c t i o n r o u t e s l e a d i n g t o i m p o r t a n t bread f l a v o r compounds. I t i s g e n e r a l l y a c c e p t e d (1^) t h a t v o l a t i l e compounds p r e s e n t i n the f l o u r a r e o f minor importance t o t h e aroma o f b r e a d . P r e r e q u i s i t e s f o r f o r m a t i o n o f t h e d e s i r e d c r u s t f l a v o r compounds a r e t h e dough f e r m e n t a t i o n and, e s p e c i a l l y , t h e b a k i n g s t e p s (2, 3). We r e c e n t l y i d e n t i f i e d 2 - a c e t y l - l - p y r r o l i n e (Acp) w i t h a c r a c k e r l i k e odor a s t h e most i n t e n s e f l a v o r compound o f wheat b r e a d c r u s t (4). T r e s s l e t a l . (5) r e p o r t e d t h a t s m a l l amounts o f t h i s compound were formed when model m i x t u r e s c o n t a i n i n g p r o l i n e and monosacchar i d e s were h e a t e d . 0097-6156/89/0409-0268$06.00/0 ο 1989 American Chemical Society Parliment et al.; Thermal Generation of Aromas ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
25. SCHIEBERLE
Formation ofFlavor Compounds in Wheat Bread Crust
To g a i n an i n s i g h t i n t o t h e o r i g i n o f t h e Acp i n wheat b r e a d c r u s t , two d i f f e r e n t e x p e r i m e n t s were c a r r i e d o u t . F i r s t , t h e f o r m a t i o n o f c r u s t o d o r a n t s from y e a s t and from c h e m i c a l l y l e a v e n e d doughs was compared and, s e c o n d l y , model e x p e r i m e n t s on t h e forma t i o n o f t h e Acp were u n d e r t a k e n .
Downloaded by EAST CAROLINA UNIV on November 3, 2016 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0409.ch025
Comparison o f C r u s t F l a v o r Compounds from Y e a s t and C h e m i c a l l y Leavened Wheat F l o u r Doughs A model wheat b r e a d was p r e p a r e d by a s t r a i g h t dough p r o c e d u r e from wheat f l o u r , s a l t and water u s i n g g l u c o n o - d e l t a - l a c t o n e as l e a v e n i n g a g e n t ( S c h i e b e r l e , P., i n p r e p a r a t i o n ) . The b r e a d c r u s t showed an odor n o t e r e m i n i s c e n t o f d a y - o l d b r e a d and, s p e c i f i c a l l y t h e c r a c k e r - l i k e , r o a s t odor n o t e was l a c k i n g . C r u s t v o l a t i l e s were i s o l a t e d i m m e d i a t e l y a f t e r b a k i n g by e x t r a c t i o n w i t h d i c h l o r o m e t h a n e and s u b l i m a t i o n i n vacuo (4)· A p p l i c a t i o n o f aroma e x t r a c t d i l u t i o n a n a l y s i s (6) t o t h e a c i d - f r e e c r u s t ex t r a c t l e d t o t h e d e t e c t i o n o f 31 o d o r a n t s . A f t e r s e p a r a t i o n and e n r i c h m e n t , t h e s e compounds were i d e n t i f i e d by comparison o f t h e M S / Ε Ι , MS/CI and r e t e n t i o n d a t a on two columns o f d i f f e r e n t p o l a r i t y t o r e f e r e n c e compounds. Aroma q u a l i t y was a l s o a s s e s s e d . The r e s u l t s o f t h e i d e n t i f i c a t i o n e x p e r i m e n t s ( T a b l e I) r e v e a l e d t h a t 2 ( E ) - n o n e n a l (No. 1 ) , f o l l o w e d by 2(Ε),4(Ε)-decadienal (No. 2) and 3-methyl b u t a n a l (No. 3) showed t h e h i g h e s t F D - f a c t o r s i n t h e c r u s t o f t h e chemically leavened bread. A d d i t i o n a l l y l-octen-3-one, 2(Ζ)-nonenal, 2 ( E ) , 4 ( E ) - n o n a d i e n a l and an unknown compound w i t h a m e t a l l i c odor c o n t r i b u t e d h i g h F D - f a c t o r s t o t h e o v e r a l l f l a v o r (For a d i s c u s s i o n o f F D - f a c t o r s , see C h a p t e r by S c h i e b e r l e and G r o s c h , t h i s b o o k ) . The f l a v o r compounds o f t h e c r u s t from t h e c h e m i c a l l y l e a v e n e d model b r e a d were t h e n compared t o t h o s e r e c e n t l y i d e n t i f i e d (6) i n t h e c r u s t o f a s t a n d a r d wheat b r e a d which was l e a v e n e d by a d d i t i o n o f y e a s t ( T a b l e I ) . One s t r i k i n g d i f f e r e n c e was t h a t Acp (No. 16), which showed t h e h i g h e s t F D - f a c t o r i n t h e y e a s t - l e a v e n e d b r e a d showed a v e r y low F D - f a c t o r i n t h e c h e m i c a l l y l e a v e n e d b r e a d . T h i s i n d i c a t e d , t h a t t h e f l o u r c o n t a i n e d o n l y minor amounts o f t h e p r e c u r s o r ^ ) f o r t h e f o r m a t i o n o f Acp. On t h e o t h e r hand, 2 ( E ) , 4 ( E ) d e c a d i e n a l , 2 ( E ) , 4 ( E ) - n o n a d i e n a l , l - o c t e n - 3 - o n e and 2(Ζ)-nonenal, which a r e u n d o u b t e d l y formed by a h e a t - i n d u c e d o x i d a t i v e d e g r a d a t i o n o f t h e f l o u r l i p i d s , became predominant o d o r a n t s i n t h e c h e m i c a l l y l e a v e n e d compared t o t h e y e a s t - l e a v e n e d b r e a d . Labeling
Experiments
Recent s t u d i e s (8) u s i n g v a r i o u s amino a c i d and sugar model m i x t u r e s showed t h a t Acp i s formed o n l y from p r o l i n e when h e a t e d i n t h e p r e s e n c e o f d i f f e r e n t s u g a r s . To g a i n a more d e t a i l e d i n s i g h t i n t o t h e f o r m a t i o n mechanism o f t h e Acp, two l a b e l i n g e x p e r i m e n t s were p e r f o r m e d ( T a b l e I I ) . In t h e f i r s t , p r o l i n e i n which t h e c a r b o n y l group was l a b e l e d w i t h carbon-13 ( l - C - p r o l i n e ) was r e a c t e d w i t h u n l a b e l e d g l u c o s e . In a second e x p e r i m e n t , u n l a b e l e d p r o l i n e was h e a t e d i n t h e p r e s e n c e o f unique carbon-13 l a b e l e d g l u c o s e ( U - C g l u c o s e ) . The c a r b o n i s o t o p e d i s t r i b u t i o n o f t h e Acp formed was de t e r m i n e d by MS/CI o f e x t r a c t s o b t a i n e d by s i m u l t a n e o u s d i s t i l l a t i o n / e x t r a c t i o n o f t h e r e a c t i o n m i x t u r e s suspended i n w a t e r . 1 3
1 3
Parliment et al.; Thermal Generation of Aromas ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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T a b l e I . Comparison o f t h e Important N e u t r a l / B a s i c V o l a t i l e C r u s t F l a v o r Compounds o f a C h e m i c a l l y Leavened Wheat Bread ( C L - W B ) W i t h Those o f a Y e a s t - l e a v e n e d Wheat Bread (YL-WB) a)
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b)
C
FD- f a c t o r CL-WB YL-WB
No. Compound
Odor D e s c r i p t i o n
RI >
1 2 3 4 5 6 7 8 9 10 11 12 13
2(E)-Nonenal 2(E),4(E)-Decadienal 3-Methylbutanal l-0cten-3-one 2(Z)-Nonenal 2(E),4(E)-Nonadienal unknown Diacetyl 2(E)-Octenal Unknown Hexanal 4(Z)-Heptenal lr 5(Ζ)-0ctadien-3-one >
1508 1778 915 1281 1480 1669 n.d. 968 1393 1786 1064 1214 1353
512 256 256 128 128 128 128 64 64 64 32 32 32
256 32 128 32 64 16 113 114 115 116
91.9 6.9 1.2
