Isolation of Thermally Generated Aromas - American Chemical Society

Chapter 5 Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on April 29, 2016 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0409.ch005

Isolation of Thermally Generated Aromas Sara J. Risch and Gary A. Reineccius Department of Food Science and Nutrition, University of Minnesota, St. Paul, MN 55108

The analysis of thermally generated aromas by gas chromatography or gas chromatography/mass spectrometry generally requires that the volatile or aroma compounds be isolated from a food or model system matrix. Many of the techniques that have been used for the isolation of flavors in general are applicable for the isolation of thermally generated aromas. These include solvent extraction, steam distillation, simultaneous steam distillationsolvent extraction, liquid-liquid extraction (dialysis) and both static and dynamic headspace methods. An overview of these methods will be presented with an emphasis on new developments and new applications. Some new developments include a microwave desorption system for dynamic headspace concentration and supercritical fluid extraction. These newer methods are reviewed and the challenges that remain in quantitatively and qualitatively isolating aromas are discussed.

Numerous r e v i e w s have appeared i n the l i t e r a t u r e d e a l i n g w i t h the i s o l a t i o n o f v o l a t i l e compounds from foods ( 1 - 6 ) . T h e r e f o r e , i t appears o f g r e a t e r v a l u e to p r e s e n t some o f the more r e c e n t developments i n t h i s a r e a r a t h e r than r e p e a t what i s c u r r e n t l y a l r e a d y reviewed. 0097-6156/89/0409-0042$06.00/0 © 1989 American Chemical Society Parliment et al.; Thermal Generation of Aromas ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

5.

RISCH AND REINECCIUS

Isolation of Thermally Generated Aromas

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on April 29, 2016 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0409.ch005

Dynamic Headspace Techniques The most p o p u l a r methods f o r f l a v o r i s o l a t i o n are dynamic headspace c o n c e n t r a t i o n and d i s t i l l a t i o n / e x t r a c t i o n ( 1 ) . The dynamic headspace method s t a r t e d w i t h s i m p l y p a s s i n g an i n e r t gas through the sample, c o l l e c t i n g s t r i p p e d o r g a n i c v o l a t i l e c o n s t i t u e n t s and water i n a c o l d t r a p ( 7 ) . T h i s technique g e n e r a l l y r e q u i r e d e x t r a c t i n g any trapped water w i t h an o r g a n i c s o l v e n t such as d i e t h y l e t h e r or d i c h l o r o m e t h a n e . I n an e f f o r t to s i m p l i f y t h i s t e c h n i q u e , i n v e s t i g a t o r s chose to pass the purge v o l a t i l e s through a t r a p which was packed w i t h a m a t e r i a l such as Tenax ( 8 , 9 ) . Tenax has a weak a f f i n i t y f o r water but r e a d i l y binds organic v o l a t i l e s . Thus Tenax c o u l d be loaded w i t h v o l a t i l e s and then t h e r m a l l y desorbed to r e c o v e r the trapped aroma compounds. Since t h i s e a r l y development, numerous i n n o v a t i o n s i n methodology have o c c u r r e d . I t i s of i n t e r e s t t h a t today the loaded Tenax t r a p s are most o f t e n desorbed o f v o l a t i l e s through s o l v e n t e x t r a c t i o n r a t h e r than thermal d e s o r p t i o n (10). I n the p a s t , the loaded t r a p was p l a c e d i n the c a r r i e r f l o w path of the gas chromatograph (GC) and then the t r a p was heated to desorb the volatiles. The desorbed v o l a t i l e s would be sent e n t i r e l y i n t o the GC f o r a n a l y s i s . One can r e a d i l y see d i s a d v a n t a g e s i n the f a c t t h a t the t r a p can o n l y s l o w l y be heated ( i . e . , desorbed) and thus the chromatographic i n j e c t i o n i s too slow to p e r m i t good chromatography of the h i g h l y v o l a t i l e s u b s t a n c e s . The s o l u t i o n was to c o l d t r a p the desorbed v o l a t i l e s thereby g i v i n g e x c e l l e n t chromatography and s e n s i t i v i t y . The disadvantage was, of c o u r s e , t i m e . A second disadvantage i s t h a t the e n t i r e sample i s sent to the GC, i . e . , there i s no o p p o r t u n i t y f o r a second a n a l y s i s of this isolate. S o l v e n t d e s o r p t i o n o f the loaded t r a p s tends to overcome these two l i m i t a t i o n s p l u s p e r m i t s the use of l a r g e r t r a p s , thereby g i v i n g g r e a t e r q u a n t i t i e s o f sample f o r repeated analyses. Simultaneous D i s t i l l a t i o n / A d s o r p t i o n R e c e n t l y , r e s e a r c h e r s have p u b l i s h e d work i n v e s t i g a t i n g the d i f f e r e n t ways to l o a d the t r a p , d i f f e r e n t types of t r a p p i n g , and t e c h n i q u e s to unload the t r a p s . In terms o f l o a d i n g the t r a p , Sugisawa e t a l . (IJ^) have r e p o r t e d on a technique i n v o l v i n g simultaneous d i s t i l l a t i o n and a d s o r p t i o n ( F i g u r e 1 ) . In t h i s t e c h n i q u e , the sample was p l a c e d i n a three necked f l a s k , heated to 60-70°C and then steam was i n t r o d u c e d to the sample. The d i s t i l l a t e was passed through the t r a p p i n g m a t e r i a l (10 mg c h a r c o a l ) and i t s f l o w c o n t r o l l e d by the s t o p c o c k . This d i s t i l l a t i o n was c o n t i n u e d f o r 30 min and the t r a p was d r i e d by p a s s i n g c l e a n n i t r o g e n gas through i t . The loaded t r a p m a t e r i a l was e x t r a c t e d w i t h a s m a l l volume of s o l v e n t ( c a . 0.01 ml of d i c h l o r o m e t h a n e o r carbon d i s u l f i d e ) f o r q u a l i t a t i v e work w h i l e q u a n t i t a t i v e work i n v o l v e d f i v e e x t r a c t i o n s w i t h a l a r g e r volume of s o l v e n t ( c a . 1.0 to 2.0 ml each t i m e ) . R e c o v e r i e s o f model compounds from aqueous s o l u t i o n a t 10 ppm ranged from 23-100%.

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

THERMAL GENERATION OF AROMAS


44

Figure 1. Apparatus for simultaneous distillation/adsorption. (Reprinted with permission from ref. 11. Copyright 1984 de Gruyter.)


5.



Sugisawa e t a l . ( L I ) demonstrated t h a t t h i s technique c o u l d a l s o be used w i t h Tenax as the a d s o r b e n t . In this s i t u a t i o n , they chose t o t h e r m a l l y desorb the Tenax i n t o the GC c a r r i e r gas f l o w . While no q u a n t i t a t i v e data were p r o v i d e d u s i n g Tenax, the a u t h o r s were q u i t e o p t i m i s t i c about t h i s methodology.


C o l d Trapping

of Volatiles

Badings and DeJong (12) have chosen a d i f f e r e n t approach to t r a p purged v o l a t i l e s . I n t h i s method, s t r i p p e d aroma compounds a r e passed through a very s m a l l d i a m e t e r c o l d t r a p . I n o r d e r t o m i n i m i z e t r a p blockage by f r o z e n w a t e r , an i n i t i a l c o l d condenser was put i n - l i n e . C o l d t r a p p i n g e f f i c i e n c y and thermal d e s o r p t i o n were o p t i m i z e d ( 1 3 ) . U s i n g t h i s t e c h n i q u e , q u i t e good sens i t i v i t y and p r e c i s i o n have been demonstrated. A major advantage o f t h i s technique i s the use o f c o l d t r a p p i n g f o r sample i s o l a tion. T h i s methodology o f f e r s the g r e a t e s t o p p o r t u n i t y f o r the c o l l e c t i o n o f a l l v o l a t i l e s (a -100°C c o l d t r a p i s q u i t e e f f e c t i v e ) , c o n t r i b u t e s v i r t u a l l y no a r t i f a c t s , and w i l l r e a d i l y l i b e r a t e the trapped v o l a t i l e s by h e a t i n g . The primary l i m i t a t i o n t o t h i s method i s t h a t t r a p p i n g time i s r e s t r i c t e d due t o w a t e r f r e e z i n g i n the c o l d t r a p . Sample p u r g i n g may be l i m i t e d to o n l y 10-20 min which then may l i m i t s e n s i t i v i t y . Microwave D e s o r p t i o n

Techniques

A f i n a l i n n o v a t i o n i n dynamic headspace methodology i n v o l v e s t r a p p i n g the purged v o l a t i l e s on a c h a r c o a l based t r a p p i n g m a t e r i a l and then d e s o r b i n g the v o l a t i l e s u s i n g microwave h e a t i n g ( F i g u r e 2 ) . Rapid d e s o r p t i o n i s the major advantage o f t h i s technique. V o l a t i l e s a r e t y p i c a l l y desorbed i n 4-5 seconds. T h i s r a p i d d e s o r p t i o n negates the need f o r any c r y o f o c u s i n g o f the desorbed v o l a t i l e s , thereby s a v i n g t i m e , d e c r e a s i n g c o s t , and p e r m i t t i n g o p t i m i z a t i o n o f chromatography w i t h o u t c o n c e r n f o r c r y o f o c u s i n g . The major d i s a d v a n t a g e s o f t h i s technique a r e a c o n c e r n f o r a r t i f a c t f o r m a t i o n on the c h a r c o a l based m a t e r i a l and trap to trap r e p r o d u c i b i l i t y . Static

Headspace

The advent o f automated s t a t i c headspace a n a l y s i s systems has o f f e r e d some new o p p o r t u n i t i e s i n aroma r e s e a r c h v i a t h i s t e c h nique. Wylie ( 1 5 , 16) has p r e s e n t e d some m o d i f i c a t i o n s i n the H e w l e t t Packard commercial system hardware as w e l l as o p e r a t i n g p r o c e d u r e s which have v a s t l y improved the s e n s i t i v i t y o f the automated system. Wylie ( 1 5 , 16) has documented improvements i n s e n s i t i v i t y f o r the a n a l y s i s o f p r i o r i t y p o l l u t a n t s i n water u s i n g the f o l l o w i n g m o d i f i c a t i o n s : 1. Changing from a 1 ml sample loop t o a 3 ml l o o p , ; 2. S a l t i n g out v o l a t i l e s by a d d i n g i n o r g a nic s a l t s . 3. High sample e q u i l i b r a t i o n temperature ( c a . 90°C). W h i l e a r t i f a c t f o r m a t i o n o f t e n l i m i t s sample temperature, t h i s parameter s u b s t a n t i a l l y i n f l u e n c e s s e n s i t i v i t y ; 4. C r y o f o c u s i n g o f v o l a t i l e s , thereby p e r m i t t i n g m u l t i p l e i n j e c t i o n s from the



MICROWAVE

MICROWAVE ENERGY

VALVE 4 yP~

8

NEEDLE VALVE

GC

INJECTOR

INTERFACE

14

VALVE

SORBENT

is

Figure 2. Schematic of a microwave desorption system. (Reprinted with permission from ref. 14. Copyright 1983 Elsevier.)

2450 MHz

GENERATOR

WAVEGUIDE

CAR. GAS


O

o

ON

5.




same sample v i a l o r p o o l i n g o f i n j e c t i o n s from d i f f e r e n t sample v i a l s . C r y o f o c u s i n g must be e f f i c i e n t , however, o r the most v o l a t i l e substances may break through the t r a p and g i v e m u l t i p l e peaks and 5. u s i n g s p l i t l e s s i n j e c t i o n and c r y o f o c u s i n g t o improve chromatography. W y l i e (16) demonstrated t h a t s t a t i c headspace t e c h n i q u e s m o d i f i e d as he suggested w i l l approach the s e n s i t i v i t y o f the more t e d i o u s and l e s s r e p r o d u c i b l e method o f dynamic headspace s a m p l i n g . While the sample c o m p o s i t i o n may p r e v e n t the use o f some o f the m o d i f i c a t i o n s suggested by W y l i e (15, 16), o t h e r m o d i f i c a t i o n s can be used t o advantage. Simultaneous D i s t i l l a t i o n / E x t r a c t i o n As was mentioned e a r l i e r , d i s t i l l a t i o n and subsequent s o l v e n t e x t r a c t i o n remains p o p u l a r i n the aroma r e s e a r c h a r e a Q ) . I n t h i s method f o r aroma a n a l y s i s , the L i k e n s - N i c k e r s o n a p p a r a t u s has been a s t a n d a r d f o r o v e r 20 years (17, 18). The primary l i m i t a t i o n o f the L i k e n s - N i c k e r s o n d i s t i l l a t i o n / e x t r a c t i o n p r o cedure has been i t s o p e r a t i o n a t reduced p r e s s u r e . It is d e s i r a b l e t o operate the system under vacuum i n o r d e r t o reduce the sample b o i l i n g p o i n t t o m i n i m i z e the f o r m a t i o n o f t h e r m a l l y induced a r t i f a c t s . The f a c t t h a t the s o l v e n t s i d e o f the d i s t i l l a t i o n - e x t r a c t i o n a p p a r a t u s i s a l s o under vacuum makes i t d i f f i c u l t t o r e t a i n the s o l v e n t i n the a p p a r a t u s . Even m o d i f i c a t i o n s o f the a p p a r a t u s t o i n c l u d e a d r y i c e / a c e t o n e condenser f o l l o w e d by a l i q u i d n i t r o g e n t r a p do n o t p e r m i t easy o p e r a t i o n under vacuum. Problems a r i s e i n t h a t the s o l v e n t o r aqueous v a p o r s reach the c r y o g e n i c t r a p s , thereby e v e n t u a l l y b l o c k i n g the e x i t o f the condenser. The need t o m i n i m i z e exposure o f the sample t o heat has r e s u l t e d i n the more f r e q u e n t use o f two s t e p procedures. Very o f t e n , the sample i s s i m p l y p l a c e d i n a f l a s h e v a p o r a t o r , a c e r t a i n volume o f d i s t i l l a t e c o l l e c t e d and the d i s t i l l a t e i s solvent extracted v i a e i t h e r separatory funnel o r a c o n t i n u o u s e x t r a c t o r . I n t h i s manner, the d i s t i l l a t i o n process and s o l v e n t c h o i c e are n o t c o n f l i c t i n g p r o c e s s e s . Dialysis The a v a i l a b i l i t y o f and improvement i n membranes has r e k i n d l e d some i n t e r e s t i n d i a l y s i s i n aroma r e s e a r c h . B e n k l e r and R e i n e c c i u s (19, 20) i n i t i a l l y p u b l i s h e d s t u d i e s on the use o f N a f i o n (Dupont) membranes f o r the s e p a r a t i o n o f f a t from f l a v o r i s o l a t e s . T h i s would p e r m i t s o l v e n t e x t r a c t i o n t o be used i n the i s o l a t i o n o f aroma compounds from f a t c o n t a i n i n g f o o d s . Chang and R e i n e c c i u s (21) l a t e r used a c o n t i n u o u s t u b u l a r c o u n t e r c u r r e n t f l o w system t o a c c o m p l i s h t h i s fat/aroma s e p a r a t i o n more efficiently. These membranes can be o b t a i n e d c o m m e r c i a l l y and have been improved i n terms o f membrane t h i c k n e s s and p u r i t y . W h i l e the aroma i s o l a t e o b t a i n e d u s i n g t h i s membrane may n o t p e r f e c t l y reproduce the aroma b e i n g s t u d i e d , t h i s i s an a l t e r n a t e t e c h n i q u e f o r aroma i s o l a t i o n .

American Chemical Society Library 1155 15th St., N.W. Washington, D.C.Generation 20036 of Aromas Parliment et al.; Thermal ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

48

T H E R M A L GENERATION OF AROMAS


S o l i d Phase E x t r a c t i o n S o l i d phase e x t r a c t i o n t e c h n i q u e s have been a p p l i e d both i n the i s o l a t i o n and c l e a n - u p o f f l a v o r e x t r a c t s . As an example, U h l i g et a l . (2_2) e x t r a c t e d c e l e r y w i t h d i c h l o r o m e thane and then passed the d i c h l o r o m e t h a n e e x t r a c t over a L C - S i S u p e l c l e a n c a r t r i d g e . The c a r t r i d g e was f l u s h e d w i t h hexane f o l l o w e d by d i c h l o r o m e t h a n e p l u s 0.5% methanol to desorb the c e l e r y aroma components (phthaiides). B i t t e u r and R o s s e t (23) have a p p l i e d s o l i d phase e x t r a c t i o n f o r the r e c o v e r y of b l a c k c u r r a n t aroma compounds from waste w a t e r . They passed s o l u t i o n s o f known compounds i n water through three d i f f e r e n t e x t r a c t i o n columns, e l u t e d each w i t h e i t h e r e t h a n o l o r d i c h l o r o m e t h a n e and then a n a l y z e d the e l u a n t to determine e x t r a c t i o n e f f i c i e n c y . While t h i s study was aimed a t the r e c o v e r y of aroma compounds on a commercial b a s i s , t h e i r r e s u l t s demonstrated t h a t r e v e r s e phase polymers can e f f e c t i v e l y be employed i n the r e c o v e r y o f aroma compounds from d i l u t e aqueous sys terns. Heymann, e t a l (24) have used s o l i d phase e x t r a c t i o n t e c h n i q u e s f o r the i s o l a t i o n of p y r a z i n e s from w i n e s . In t h i s a n a l y s i s , i t was n e c e s s a r y to f i r s t d i s t i l l the p y r a z i n e s from the wine and then pass the d i s t i l l a t e through a Sep-Pak C-18 c a r t r i d g e (Waters A s s o c . ) . To speed the a n a l y s i s , the d i s t i l l a t e was f o r c e d through the c a r t r i d g e a t a r a t e o f 30 ml/min. P y r a z i n e s were e l u t e d from the Sep-Pak w i t h 2 x 1 m l . a l i q u o t s of methanol. R e c o v e r i e s o f wine s p i k e d w i t h m e t h o x y p y r a z i n e s i n the ppb ranged from 10 t o 63%. W h i l e r e c o v e r i e s appear low, they were r e p r o d u c i b l e and q u i t e s u i t a b l e to address the problem b e i n g investigated. S u p e r c r i t i c a l CO2 E x t r a c t i o n S o l v e n t e x t r a c t i o n has a l s o remained a p o p u l a r t e c h n i q u e f o r f l a vor i s o l a t i o n . The most r e c e n t developments i n t h i s a r e a have i n v o l v e d the use o f s u p e r c r i t i c a l CO2 i n b o t h f l a v o r e x t r a c t i o n and a n a l y s i s ( 2 5 - 2 7 ) . S u p e r c r i t i c a l CO2 i s a p a r t i c u l a r l y good c h o i c e i n aroma s t u d i e s s i n c e i t has an e x t r e m e l y low b o i l i n g p o i n t and l e a v e s no o f f - o d o r r e s i d u e to i n t e r f e r e i n e i t h e r a n a l y t i c a l work o r sens o r y e v a l u a t i o n . The f a c t t h a t the s o l v e n t s t r e n g t h o f a s u p e r c r i t i c a l f l u i d depends on d e n s i t y i s an a d d i t i o n a l f a c t o r which may be u s e f u l . One can v a r y s o l v e n t p r o p e r t i e s by changing d e n s i t y , thereby o b t a i n i n g an e f f e c t i v e e x t r a c t i o n of a broad range o f aroma compounds. In r e t r o s p e c t , t h e r e are no t o t a l l y new t e c h n i q u e s f o r the i s o l a t i o n of t h e r m a l l y g e n e r a t e d aroma compounds. The d e v e l o p ments we have seen i n r e c e n t y e a r s have been m o d i f i c a t i o n s o f t e c h n i q u e s which have e x i s t e d f o r s e v e r a l y e a r s . As i n the p a s t , each method has i t s own unique s t r e n g t h s and weaknesses. The c h o i c e o f method i s determined by the food p r o d u c t to be anal y z e d , the v o l a t i l e s o f i n t e r e s t and the a n a l y t i c a l methods to be applied.


5. RISCH AND REINECCIUS



Literature Cited 1.

Reineccius, G.A. In Flavor Chemistry of Lipid Foods; Min, D.B., Croteau, R. Eds. American Oil Chemists Society, 1989; (In Press).

2.

Parliment, T.H. In Biogeneration of Aroma; Parliment, T . H . , Croteau, R. Eds. ACS Symposium Series No. 317; American Chemical Society, 1986; Washington, D.C. p. 34-52.

3.

Schreier, P. In Chromatographic Studies of Biogenesis of Plant Volatiles; Huthig: New York, 1984; p. 1-32.

4.

Maarse, H . ; Betz, R. In Isolation and Identification of Volatile Compounds in Aroma Research; Akademic-Verlag: Berlin, 1981; p. 1-59.

5.

Teranishi, R.; Flath, R.; Sugisawa, H. In Flavor Research Recent Advances; Marcel Dekker, Inc.: New York, 1981; p. 11-51.

6.

Reineccius, G.A.; Anandaraman, S. In Food Constituents and Food Residues: Their Chromatographic Determination; Lawrence, J . E . Ed. Marcel Dekker, Inc.: New York, 1984; p. 195-293.

7.

Chang, S.S.; Vallese, F.M.; Huang, L . S . ; Hsieh, O.A.L.; Min, D.B.S. J . Agric. Food Chem. 1977, 25, 450-455.

8.

Jennings, W.G. In Flavour '81; Schreier, P. Ed. W. de Gruyter: New York, 1981; p. 233-252.

9.

Charalambous, G. Analysis of Foods and Beverages, Headspace Techniques; Academic Press: New York; 1978.

10.

Olafsdottir, G.; Steinke, J . A . ; Lindsay, R.C. J . Food S c i . , 1985, 50 1431-1436.

11.

Sugisawa, H . ; Chen, C . ; Nabeta, K. In Analysis of Volatiles; Schrier, P. ed. W. de Gruyter: New York, 1984; p. 355-369.

12.

Badings, H.T.; DeJong, C. In Analysis of Volatiles; W. de Gruyter: New York, 1984; p. 401-407.

13.

Badings, H.T.; DeJong, C . ; Dooper, R.P.M. In Proceedings 6th International Symposium on Capillary Chromatography; Sandra, P.; Bertsch, W. Eds. Huethig-Verlag: New York, 1985; p. 666.

14.

Rektorik, J. In Proceedings of the 5th International Symposium on Capillary Chromatography; Rijks, J. Ed. Elsevier Science Publ.: Amsterdam, 1983; p. 396-405.



50

T H E R M A L GENERATION OF AROMAS

15.

Wylie, P.L.

J. Amer. Waterworks 1988, 80, 65-72.

16.

Wylie, P.L.

Chromatographia 1986, 21, 251-258.

17.

Likens, S.T.; Nickerson, G.B. Amer. Soc. Brew. Chem. Proc. 1964, 5-13.

18.

Schultz, T.H.; Flath, R.A.; Mon, T.R.; Eggling, S.B.; Teranishi, R. J. Agric. Food Chem. 1977, 25, 446-449.

19.

Benkler, K.; Reineccius, G.A. J. Food Sci. 1979, 44, 1525-1529.

20.

Benkler, K.; Reineccius, G.A. J. Food Sci. 1980, 45, 1084-1085.

21.

Chang, Y.I.; Reineccius, G.A. J. Agric. Food Chem. 33, 1168-1173.

22.

Uhlig, J.W.; Chang, A.; Jen, J.J. 658-660.

23.

Bitteur, S.; Rosset, R.

24.

Heymann, H.; Nobel, A.C.; Boulton, R.B. J. Agric. Food Chem. 1986, 34, 268-271.

25.

Hawthorne, S.B.; Krieger, M.S.; Miller, 1988, 60, 472-477.

26.

Flament, I.; Chevallier, C.; Keller, U. In Flavor Science and Technology; Martins, M.; Dalen, G.A.; Russwurm, H. Eds. Wiley: New York, 1987; p. 151-164.

27.

Sugiyama, K.; Saito, M. J. Chrom. 1988, 442, 121-131.

J. Food Sci. 1987, 52,

J. Food Sci. 1988, 53, 141-147.

D.J.

Anal. Chem.

RECEIVED February 28, 1989


Isolation of Thermally Generated Aromas - American Chemical Society

Recommend Documents