Sotolon - American Chemical Society

Chapter 5

Sotolon

Downloaded by UNIV OF OKLAHOMA on February 2, 2015 | http://pubs.acs.org Publication Date: February 21, 1989 | doi: 10.1021/bk-1989-0388.ch005

Identification, Formation, and Effect on Flavor Akio Kobayashi Ochanomizu University, Laboratory of Food Chemistry, 2-1-1, Ohtsuka, Bunkyo-ku, Tokyo, Japan Sotolon, 4,5-dimethyl-3-hydroxy-2(5H)-furanone, was isolated and identified as a flavor impact compound from raw cane sugar and has a very low threshold value. The aroma characteristic changes from caramel-like at low concentrations to curry-like aroma at high concentrations. For this reason the compound, which had already been synthesized and also found in its natural form, was not associated with the characteristic brown lump sugar aroma. The syntheses of sotolon homologues and their enantiomers provided some information about the structure-aroma relationship. The formation of sotolon was confirmed in a model system composed of glutamic acid and pyruvate, the latter being estimated as a reaction product of an amino-carbonyl reaction. As a flavor impact compound, sotolon was found in botrytized wine and roasted tobacco. The presence of sotolon in these products is indispensable for a flavor with high sensory qualities. Cane sugar i s one o f the o l d e s t a g r i c u l t u r a l p r o d u c t s known t o man and o r i g i n a t e d i n the t r o p i c s . D e s p i t e the p r i m i t i v e t e c h n o l o g y f o r sugar p r o c e s s i n g t h a t was used i n the e a r l y stage o f i t s d e v e l o p ment, raw cane sugar became e d i b l e and even p a l a t a b l e because o f i t s a c c e p t a b l e f l a v o r . Thus cane sugar h i s t o r y c o n t r a s t s w i t h the development and manufacture o f b e e t sugar produced i n n o r t h e r n Europe s i n c e the 19th c e n t u r y . Brown lump sugar i s p r e p a r e d from c a l c i f i e d cane j u i c e s i m p l y by b o i l i n g down i n t o a s o l i d f o r m , w h i c h i s w i d e l y used as an i n g r e d i e n t o f t r a d i t i o n a l Japanese cakes i n a d d i t i o n t o o t h e r p a r t i a l l y r e f i n e d raw s u g a r s . These a r e n o t o n l y used as s w e e t e n e r s b u t a l s o as f l a v o r i n g i n g r e d i e n t s . The c h a r a c t e r i s t i c c a r a m e l - l i k e , b u r n t - s w e e t aroma o f raw cane sugar i s known t o be formed a t t h e l a s t s t a g e o f the h e a t i n g and c o n d e n s i n g p r o c e s s f o r sugar cane j u i c e , and the f l a v o r i m p a c t compound (FIC) has been e s t i m a t e d t o be a nonenzymatic p r o d u c t formed d u r i n g the b r o w n i n g r e a c t i o n . S i n c e 0097-6156/89/0388-0049$06.00/0 * 1989 American Chemical Society

In Flavor Chemistry; Teranishi, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


50

FLAVOR CHEMISTRY: TRENDS AND DEVELOPMENTS

T a k e i e t a l . Q ) f i r s t r e p o r t e d i n 1936 t h e s e p a r a t i o n and i d e n t i f i c a t i o n o f a v o l a t i l e component i n raw cane sugar and m o l a s s e s , many r e s e a r c h e r s have t r i e d t o i d e n t i f y t h e F I C o f t h e c h a r a c t e r i s t i c s u g a r y aroma i n cane j u i c e o r raw cane sugar p r o d u c t s . F u r f u r a l , h y d r o x y m e t h y l f u r f u r a l , m a l t o l , i s o m a l t o l and 3 - m e t h y l - 2 - h y d r o x y - 2 - c y c l o p e n t e n - l - o n e ( c y c l o t e n e ) have b e e n i d e n t i f i e d as t y p i c a l a m i n o - c a r b o n y l r e a c t i o n p r o d u c t s w i t h a b u r n t sugary aroma; however, t h e i r t h r e s h o l d v a l u e s a r e t o o h i g h t o e x p l a i n t h e s t r o n g c h a r a c t e r i s t i c aroma o f raw cane sugar. Through many r e s e a r c h s t u d i e s i t has become c l e a r t h a t t h i s s u g a r y aroma c o u l d be c o n c e n t r a t e d i n a s p e c i f i c f r a c t i o n , w h i c h i m p l i e s t h a t one o r a few FICs must be p r e s e n t , a l t h o u g h t h e amount o f FICs h a s been too l i t t l e t o i d e n t i f y by t h e u s u a l a n a l y t i c a l method. Separation

and I d e n t i f i c a t i o n

As t h e c o n t e n t o f t h e aroma s u b s t a n c e was e s t i m a t e d t o be e x t r e m e l y low, we used cane m o l a s s e s as t h e s t a r t i n g m a t e r i a l f o r o u r study, as t h e sugary aroma was a l r e a d y condensed i n i t and t h e m a t e r i a l c o u l d be s u p p l i e d i n b u l k by t h e m a n u f a c t u r e r . One t o n o f cane m o l a s s e s was f i r s t e x t r a c t e d w i t h acetone, and a f t e r e v a p o r a t i n g t h e s o l v e n t , t h e low m o l e c u l a r w e i g h t o r g a n i c m a t e r i a l s were e x t r a c t e d c o n t i n u o u s l y w i t h e t h e r . The e x t r a c t was then d i v i d e d i n t o b a s i c , a c i d i c , weakly a c i d i c and n e u t r a l f r a c t i o n s ; by an o r g a n o l e p t i c e v a l u a t i o n t h e sugary aroma appeared s t r o n g l y i n b o t h t h e w e a k l y a c i d i c and n e u t r a l f r a c t i o n s . As t h e y i e l d o f t h e n e u t r a l f r a c t i o n was much h i g h e r than t h a t o f t h e o t h e r f r a c t i o n s , t h e combined weakly a c i d i c and n e u t r a l f r a c t i o n s were f u r t h e r f r a c t i o n a t e d by s i l i c a g e l column chromatography. The aroma was c o n c e n t r a t e d i n f r a c t i o n 11, w h i c h was then s e p a r a t e d by packed-column GC w i t h peak s n i f f i n g ("nasal a p p r a i s a l " ) . A t a r e t e n t i o n t i m e o f about 60 min, the s t r o n g sugary aroma was noted, a l t h o u g h many peaks o v e r l a p p e d i n t h i s zone. I n t h e next step, t h e e f f l u e n t between t 55 and 65 m i n was t r a p p e d r e p e a t e d l y and a n a l y s e d by h i g h - r e s o l u t i o n gas chromatography (HRGC) combined w i t h mass s p e c t r o m e t r y (MS). T h i s s e p a r a t i o n scheme i s summarized i n F i g u r e 1, (2,3) and t h e r e s o l u t i o n o f t h e GC t r a p p i n g f r a c t i o n by HRGC i s shown i n F i g u r e 2. R

By GC-MS a n a l y s i s , peaks 36, 37 and 39 were e s t i m a t e d t o be 3-hydroxy-4,5-dimethyl-2(5H)-furanone, a c e t a t e o f hydroxymethylf u r f u r a l and 4 - p e n t y l - 2 - p e n t e n o l i d e , r e s p e c t i v e l y . A t t h i s stage, the sample was t o o s m a l l t o a p p l y o t h e r a n a l y t i c a l methods; t h e r e f o r e , we t r i e d t o s y n t h e s i z e a l l t h e p o s s i b l e compounds u s i n g the s y n t h e t i c approaches d e s c r i b e d i n t h e f o l l o w i n g s e c t i o n . None o f t h e t h r e e s y n t h e t i c p r o d u c t s showed t h e c h a r a c t e r i s t i c sugary aroma t h a t we had r e c o g n i z e d i n each s e p a r a t e d f r a c t i o n ; however, t h e y i e l d o f f r a c t i o n 11-GC TRAP from m o l a s s e s was c a l c u l a t e d t o be c a . 1 ppm, and t h e c o n c e n t r a t i o n o f F I C i n m o l a s s e s was e s t i m a t e d t o be i n t h e o r d e r o f ppm o r ppb f r o m i t s peak a r e a i n t h e whole gas chromatogram. By d i l u t i n g these s y n t h e t i c p r o d u c t s i n water t o t h e c o n c e n t r a t i o n o f 1.0 ppm, 3-hydroxy-4,5-dimethyl-2(5H)-furanone



KOBAYASHI

Sotolon: Identification, Formation, & Flavor Effect

Cane M o l a s s e s

1000 kg

Acetone (1400g), s t i r o v e r n i g h t , D i s t i l l o f f acetone Oleoresin

s e p a r a t e upper

layer

31.6 kg

C o n t i n u o u s e t h e r e x t r a c t i o n f o r 40hr D i s t i l l o f f ether Ether Extract

752 g

S e p a r a t e b a s i c and s t r o n g l y a c i d i c Weakly A c i d i c and N e u t r a l S i l i c a g e l column F r a c t i o n 11

Fractions

fractions 90.2 g

chromatography

17.3 g

Preparative

gas chromatography

Ψ F r . 11-GC TRAP (3.3% o f whole peak a r e a ) HRGC-MS Figure

1. Scheme f o r E x t r a c t i o n and F r a c t i o n a t i o n




52


5.

KOBAYASHI

53 Sotolon: Identification, Formation, & Flavor Effect


showed a s t r o n g b u r n t sugary aroma r e m i n i s c e n t o f those i n t h e s e p a r a t e d f r a c t i o n s f r o m molasses, a l l t h e g a s - c h r o m a t o g r a p h i c and s p e c t r o m e t r i c data f o r the s y n t h e s i z e d product c o i n c i d i n g w i t h those f o r t h e n a t u r a l one. T h e r e f o r e , i t became c l e a r t h a t a h i g h concen t r a t i o n o f t h e s y n t h e t i c p r o d u c t produced a n o t h e r e f f e c t on t h e o l f a c t o r y organ. In t h e next s t e p t o c o n f i r m t h e c o n t r i b u t i o n t o t h e sugary aroma i n m o l a s s e s , a l l t h e compounds i d e n t i f i e d i n f r a c t i o n 11-GC TRAP were r e c o n s t r u c t e d w i t h t h e c o n c e n t r a t i o n s a p p e a r i n g on gas chromatogram Β i n F i g . 2, as i s shown i n T a b l e 1,(A) w i t h o u t and (B) w i t h s o t o l o n . An o r g a n o l e p t i c o m i s s i o n t e s t on t h i s m i x t u r e c o n f i r m e d 3hydroxy-4, 5-dimethyl-2(5H)-furanone t o be t h e most i m p o r t a n t FIC f o r a raw cane sugar aroma, and t h e o t h e r main components i n T a b l e 1, i.e., v a n i l l i n , m a l t o l , 4 - p e n t y l b u t a n o l i d e , 4 - v i n y l p h e n o l and 2,6dimethoxy phenol, seemed t o improve t h e o v e r a l l sugary aroma. From t h e s e r e s u l t s , we(4) gave the t r i v i a l name " s o t o l o n " t o 2-hydroxy4,5-dimethyl-2(5H)-furanone, which i s b u i l t up f r o m "soto" (raw sugar i n Japanese) and " o l o n " ( e n o l l a c t o n e ) as a main FIC i n raw c a n e sugar.

T a b l e 1. The s y n t h e t i c m i x t u r e s i n F r . 11-GC t r a p Peak No. 7 11 14 21 21 22 25 26 36 37 39 43 50 54 60 63 66

Compound

Furfurly alcohol Damascenone Guaiacol 5,6-Εροχν-β-ΐοηοηβ Maltol 2-Acetylpyrrole Phenol 4-Pentylbutanolide Sotolon 5-Acetoxymethylfurfural 4-Ρ enty1-2-penteno1ide 2,6-Dimethoxyphenol Isoeugenol 4-Vinylphenol 5-Hydroxymethylfurfural Vanillin Acetovanillone

o f the i d e n t i f i e d

A Cone. 0.20 0.13 0.72 0.04 0.21 2.53 5.70 3.38

-

0.84 0.17 67.12 0.17 1.90 1.27 14.14 1.48

(%)

compounds

Β Cone.

(%)

0.20 0.12 0.69 0.04 0.20 2.45 5.51 3.27 3.27 0.81 0.16 64.92 0.17 1.84 1.22 13.68 1.42


54

FLAVOR


Synthetic

CHEMISTRY:

TRENDS A N D DEVELOPMENTS

Approaches

The s y n t h e s i s o f s o t o l o n c a n be t r a c e d back t o 1947(5). S u l s e r et^ al.(6) s u b s e q u e n t l y improved t h e s y n t h e t i c method and o b t a i n e d s o t o l o n as a homolog o f 3,4-dialkyl-2-hydroxy-butenolactone. They a l s o i d e n t i f i e d I I i n a v e g e t a b l e - p r o t e i n h y d r o l y s a t e as a f l a v o r i n g compound^), a l t h o u g h t h e y d e s c r i b e d t h e aroma c h a r a c t e r o f I and I I as M a g g i h e r b - l i k e a t a c o n c e n t r a t i o n o f 5-1 ppm, and as a w a l n u t - l i k e h e r b a l aroma a t 0.1 ppm w i t h t h r e s h o l d v a l u e s between 1-0.5 ppb. On t h e o t h e r hand, R o d e l and Hempel(7) d e s c r i b e d t h e aroma c h a r a c t e r o f I as f r u i t y and a l c o h o l i c a t t h e c o n c e n t r a t i o n o f 100 ppm i n water, and as a Maggi h e r b a l and c e l e r y - l i k e aroma a t 50 ppm. We s u g g e s t t h e f o l l o w i n g r e a s o n s f o r t h e r e h a v i n g been no d e s c r i p t i o n o f a sugary aroma f o r s y n t h e t i c I ( s o t o l o n ) : (a) The previous s t u d i e s concerned the f l a v o r substances o f a vegetable p r o t e i n h y d r o l y s a t e , and when s y n t h e t i c p r o d u c t s showed a h e r b a l aroma, t h e y d i d n o t a t t e m p t an o r g a n o l e p t i c t e s t a t l o w e r c o n c e n t rations, (b) These u n s a t u r a t e d l a c t o n e s a r e u n s t a b l e and e a s i l y p o l y m e r i z e d t o a v i s c o u s o i l , a l t h o u g h t h i s change was o f t e n o v e r l o o k e d because o f t h e s t r o n g odor. We, t h e r e f o r e , r e i n v e s t i g a t e d the s y n t h e s i s o f s e v e r a l s o t o l o n homologs by f o l l o w i n g t h e g e n e r a l s y n t h e t i c r o u t e d e s c r i b e d i n F i g u r e 3, and e v a l u a t e d t h e aroma c h a r a c t e r s i n t h e i r pure s t a t e s a t a c o n c e n t r a t i o n c l o s e t o t h e r e s p e c t i v e t h r e s h o l d v a l u e . The s t r u c t u r e and p u r i t y o f d i s t i l l e d s o t o l o n (bp 0.2 mm Hg 84-86°C) and t h e o t h e r homologs were c o n f i r m e d by GC, MS, IR, PMR and CMR. The t h r e s h o l d v a l u e s e v a l u a t e d by an e x p e r i e n c e d p a n e l f r o m Takasago P e r f u m e r y Co. L t d . a r e summarized i n the b o t t o m l i n e o f F i g u r e 3 ( u n p u b l i s h e d d a t a ) . I n an e a r l i e r r e p o r t (4), we a s s i g n e d t h e t h r e s h o l d v a l u e o f s y n t h e t i c s o t o l o n t o be 0.002 ppb, a l t h o u g h t h e new r e s u l t s have reduced t h i s v a l u e t o 10" ppb, and t h o s e o f some l o n g e r a l k y l - s u b s t i t u t e d homologues a r e e v e r l o w e r . S o t o l o n i s u n s t a b l e i n i t s pure s t a t e even i n a r e f r i g e r a t o r , a l t h o u g h i t c a n be p r e s e r v e d as a d i l u t e d s o l u t i o n i n h i g h p o l a r s o l v e n t s such as water, e t h y l e n e g l y c o l o r g l y c e r i n . These r e s u l t s suggest t h a t s o t o l o n i s e a s i l y polymerized t o a higher m o l e c u l a r w e i g h t p r o d u c t and t h u s g r a d u a l l y l o s e s i t s s t r o n g aroma character. 3

RCH C00C H 2

2

C

RCHCOOC H 2

(COOC H ) 2

R R

1

Threshold V a l u e (ppb)

5

H

^0° 2 5

5

5

COCOOC H

2

2

R

s

^OH

R'CHO^

5

I

II

III

IV

V

VI

VII

Me

Me

Me

Me

Me

Et

Et

Me

Et

Pro

Bu

iso-Bu

Me

Et

1X10"

3

1X10"

5

1X10"

5

5X10"

6

1X10"

3

4

2.5X10""

F i g u r e 3. G e n e r a l S y n t h e t i c Route t o 3 , 4 - D i a l k y l - 2 h y d r o x y - b u t e n o l a c t o n e s and t h e i r T h r e s h o l d V a l u e s


2.5X10"

5


5.

KOBAYASHI


A l l t h e compounds produced a b u r n t sugary aroma, w h i c h became more b u r n t and heavy as t h e s u b s t i t u t e d a l k y l c h a i n i n c r e a s e d i n l e n g t h . I t i s i n t e r e s t i n g t h a t t h e e t h y l s u b s t i t u t e d l a c t o n e I I has a 100 t i m e s l o w e r t h r e s h o l d v a l u e t h a n t h a t o f s o t o l o n , and t h a t t h i s compound has been c o n s i d e r e d t o be an FIC i n t h e p r o t e i n h y d r o l y s a t e . T h i s was because i t had been p r e p a r e d f r o m t h r e o n i n e by h e a t i n g w i t h h y d r o c h l o r i c a c i d and subsequent d e h y d r a t i o n , h y d r o l y s i s , c o n d e n s a t i o n ( A l d o l type) and d e c a r b o x y l a t i o n , and i t showed a s t r o n g c u r r y - l i k e o r h e r b a l aroma a t c o n c e n t r a t i o n s h i g h e r t h a n 1 ppm. S o t o l o n has an a s y m m e t r i c c a r b o n i n i t s m o l e c u l e , and t h e r e f o r e , s t e r e o s p e c i f i c syntheses(8) o f s o t o l o n e n a n t i o m e r s would be e f f e c t i v e f o r c o r r e l a t i n g t h e s t e r e o s t r u c t u r e and o l f a c t o r y s e n s a t i o n . I f t h e aroma c h a r a c t e r t u r n e d o u t t o be q u a n t i t a t i v e l y o r q u a l i t a t i v e l y d i f f e r e n t between t h e s e e n a n t i o m e r s , we c o u l d e x p e c t t o e l u c i d a t e whether s o t o l o n i s a n a t u r a l l y o c c u r r i n g compound o r a p r o d u c t formed by c h e m i c a l r e a c t i o n d u r i n g t h e sugar m a n u f a c t u r i n g p r o c e s s . S t a r t i n g from D-and L - t a r t a r i c a c i d s , t h e r e s p e c t i v e 2,3epoxybutenes, (2R,3R) and (2S,3S), were p r e p a r e d by t h e known method. l , 3 - D i t h i a n e - 2 - c a r b o x y l i c a c i d condensed w i t h t h e r e s p e c t i v e e p o x i d e s and r e m o v a l o f t h e t h i o a c e t a l group gave o p t i c a l l y a c t i v e s o t o l o n e s . The [ α ] ^ * v a l u e i n e t h e r f o r ( R ) - s o t o l o n was -6.5° and f o r ( S ) - S o t o l o n was +7.1°. The s y n t h e t i c scheme i s summarized i n F i g u r e 4. 3

L-Tartaric acid F i g u r e 4.

5

(2S,3S) S t e r e o s p e c i f i c Synthesis

(3R,4S) of Sotolon

(S)-I Enantiomers

Both t h e e n a n t i o m e r s showed t h e same aroma c h a r a c t e r a t t h e same c o n c e n t r a t i o n near t h e t h r e s h o l d v a l u e o f (+) s o t o l o n ; moreover, t h e r e was no d i f f e r e n c e i n t h e i n s e c t a t t r a c t a n c y (house f l y and cockroach) among t h e two e n a n t i o m e r s and t h e racemate o f s o t o l o n . L a t e r , we(2) t r i e d t o a n a l y z e t h e aroma compound i n f r e s h sugar cane j u i c e and c o u l d n o t i d e n t i f y s o t o l o n i n t h e same f r a c t i o n as t h a t s e p a r a t e d f r o m cane m o l a s s e s . These r e s u l t s s u g g e s t t h a t s o t o l o n was p r e s e n t i n a r a c e m i c f o r m p r e p a r e d by m u t u a l i n t e r a c t i o n o f t h e c o n s t i t u e n t s i n sugar cane j u i c e .


56


Formation

of Sotolon

A b u r n t , sugary aroma i s r e p r e s e n t a t i v e o f some cooked foods. A number o f sugary aroma compounds a r e l i s t e d below:


Sotolon F i g u r e 5. Structure

Maltol

Cyclotene

Sugary Aroma Compounds Having

Furaneol a Common

Partial

These have a p a r t i a l common s t r u c t u r e , i . e . e n o l y z e d α-diketone and α-hydrogen t o t h e e n o l i z a b l e c a r b o n y l i s s u b s t i t u t e d by a m e t h y l group. F o r a c h a r a c t e r i s t i c sugary aroma, t h e p r e s e n c e o f e n o l hydrogen was e s s e n t i a l , because t h e a c e t y l e s t e r o r m e t h y l e t h e r o f s o t o l o n showed no c h a r a c t e r i s t i c aroma. The e x t r a o r d i n a r i l y low t h r e s h o l d v a l u e o f s o t o l o n may be due t o i t s c o e x i s t e n c e w i t h h y d r o p h i l i c and h y d r o p h o b i c ( a l k y l - s u b s t i t u t e d l a c t o n e ) p a r t i a l s t r u c t u r e s i n one m o l e c u l e . The l o w e r t h r e s h o l d v a l u e o f t h e e t h y l s u b s t i t u t e d homolog (II) than t h a t o f s o t o l o n may a l s o be e x p l a i n e d from t h e b a l a n c e o f t h e s e o p p o s i t e p h y s i c o c h e m i c a l p r o p e r t i e s . A p a r t from s o t o l o n , t h e o t h e r compounds i n F i g . 5 c a n be e x p l a i n e d as t h e p r o d u c t s o f a M a i l l a r d r e a c t i o n , and t h e i r c a r b o n s k e l e t o n s s i m p l y o r i g i n a t e from t h e a c t i v e Amadori i n t e r m e d i a t e ; i n o t h e r words, t h e y s t i l l p r e s e r v e t h e s t r a i g h t carbon c h a i n s t r u c t u r e o f m o n o s a c c h a r i d e s . I n s p i t e o f b e i n g a s i m p l e Cg l a c t o n e , s o t o l o n has a branched carbon s k e l e t o n , w h i c h i m p l i e s a n o t h e r f o r m a t i o n p r o c e s s i n t h e M a i l l a r d r e a c t i o n . S u l s e r e t al.(6) r e p o r t e d t h a t e t h y l s o t o l o n (II) was p r e p a r e d from t h r e o n i n e w i t h s u l f u r i c a c i d , and t h a t 2 - o x o b u t y r i c a c i d , a d e g r a d a t i o n p r o d u c t o f t h r e o n i n e , was a better starting material to obtain II. This f i n a l reaction i s a C l a i s e n t y p e o f c o n d e n s a t i o n , w h i c h would p r o c e e d more s m o o t h l y under a l k a l i n e c o n d i t i o n s . As we(10) o b t a i n e d I I from 2 - o x o b u t y r i c a c i d (see f i g u r e 6) w i t h a h i g h y i e l d i n t h e p r e s e n c e o f p o t a s s i u m c a r b o n a t e i n e t h a n o l , a mixed c o n d e n s a t i o n o f 2 - o x o b u t y r i c and 2-oxop r o p a n o i c ( p y r u v i c ) a c i d s was attempted under t h e same c o n d i t i o n s , and a m i x t u r e o f s o t o l o n (22% y i e l d ) and I I were o b t a i n e d ; however, t h e

F i g u r e 6. P r e p a r a t i o n o f S o t o l o n Homologues by a Mixed Condensation o f 2-0xoalkanoic A c i d



5.

KOBAYASHI


s e l f - c o n d e n s a t i o n p r o d u c t o f 2-oxopropanoic a c i d , d e s m e t h y l s o t o l o n e , was n o t i d e n t i f i e d i n the r e a c t i o n m i x t u r e . In t h e a c t u a l m a n u f a c t u r i n g p r o c e s s f o r sugar, t h e r e e x i s t v a r i o u s M a i l l a r d r e a c t i o n s e r i e s , and p y r u v a t e seemed t o be a d e g r a d a t i o n p r o d u c t o f c a r b o h y d r a t e . On the o t h e r hand, 2 - o x o b u t y r i c a c i d c o u l d be d e r i v e d from g l u t a m i c a c i d w h i c h i s the main f r e e amino a c i d i n sugar cane j u i c e as w e l l as a s p a r t i c acid.(11) An e q u i m o l a r amount o f g l u t a m i c and p y r u v i c a c i d s was d i s s o l v e d i n water, the pH was a d j u s t e d t o 8 w i t h p o t a s s i u m c a r b o n a t e and the m i x t u r e b o i l e d f o r 4 hours. S o t o l o n was i d e n t i f i e d i n the e t h e r e x t r a c t o f the r e a c t i o n m i x t u r e by GC-MS the y i e l d was below 0.1 %. From t h i s d a t a , t h e g l u t a m a t e seemed t o have been o x i d i z e d t o α -keto g l u t a r a t e i n t h e p r e s e n c e o f p y r u v a t e , b e f o r e i t was condensed w i t h t h e p y r u v a t e and subsequent d e c a r b o x y l a t i o n t o y i e l d s o t o l o n . In p r a c t i c e , t h e c h a r a c t e r i s t i c aroma o f brown lump sugar appears a t the l a s t m a n u f a c t u r i n g s t a g e when the h i g h - s u g a r - c o n t e n t l i q u i d (Bx. 73) i s h e a t e d a t 135 °C; t h e r e f o r e , s o t o l o n must be formed a t t h i s s t a g e f o l l o w i n g the complex a m i n o - c a r b o n y l r e a c t i o n j u s t d e s c r i b e d . A s p e c u l a t i v e f o r m a t i o n mechanism f o r s o t o l o n and i t s homologues i s shown i n F i g . 7, i n which an u n s t a b l e k e t o a c i d o x i d i z e s an amino a c i d t o the α-keto a c i d , w h i c h t h e n condenses w i t h a n o t h e r k e t o a c i d t o f o r m the furanone s t r u c t u r e . The l a s t s t a g e o f the r e a c t i o n i s d e c a r b o x y l a t i o n , w h i c h would be dependent on the pH and t e m p e r a t u r e conditions. COOH

CH^COCOOH·

CH

Sotolon - American Chemical Society

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