Instrumental Analysis of Volatiles in Food Products - ACS Publications

5 Instrumental Analysis of Volatiles in Food Products

Downloaded by CORNELL UNIV on September 7, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0075.ch005

HAROLD P. DUPUY, ΜΟΝΑ L. BROWN, MICHAEL G. LEGENDRE, JAMES I. WADSWORTH, and ERIC T. RAYNER Southern Regional Research Center, Agricultural Research Service, U. S. Department of Agriculture, New Orleans, LA 70179 In recent years, considerable effort has been expended to examine the volatiles and trace components that characterize and contribute to food flavors. Some early attempts to measure food volatile components by gas chromatographic methods consisted of analyzing headspace vapors to detect vegetable and fruit aromas (1) and volatiles of various food products (2). These methods, however, require special preparation of the sample and subsequent transfer of a vapor aliquot to the gas chromatograph. Extraction and distillation techniques have been proposed to pro vide quantities of volatiles sufficient for instrumental detec tion and analysis (3,4,5,6,). These methods are complex, tedious, time-consuming operations that may also produce artifacts. More recently, a direct gas chromatographic procedure was reported for the examination of volatiles in salad oils and peanut butters (7,8). The method does not require prior enrichment of volatiles and is rapid and efficient. Analysis of flavor-scored soybean oils by this direct chromatographic method, or with suitable modifica tions (9,10,11), has shown that their flavor quality can be measured by instrumental means. Our work provides additional evidence of the applicability of this rapid, unconventional gas chromatographic technique for analyzing flavor quality of vege table oils, and projects its utility for other raw and processed food products. M a t e r i a l s and Methods M a t e r i a l s . Tenax GG^, 60-80 mesh (a thermostable polymer, 2,6-diphenyl-p-phenylene o x i d e ) , and Poly MPE Cpoly-m-phenoxylene) were obtained from A p p l i e d Science L a b o r a t o r i e s , State

1/ Names o f companies o r commercial products are given s o l e l y f o r ~~ the purpose o f p r o v i d i n g s p e c i f i c informatipn; t h e i r mention does not imply recommendation or endorsement by the U. S. Department o f A g r i c u l t u r e over others not mentioned.

0-8412-0418-7/78/47-075-060$05.00/0 This chapter not subject to U.S. copyright. Published 1978 American Chemical Society Supran; Lipids as a Source of Flavor ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

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C o l l e g e , Pa. T e f l o n O-rings were purchased from A l l t e k A s s o c i a t e s , A r l i n g t o n Heights, 111·; sandwich-type s i l i c o n e septurns from Hamilton Company, Reno, Nev., and Pyrex g l a s s wool from Corning Glass Works, Corning, Ν. Y. (The O-rings, septums, and g l a s s wool were c o n d i t i o n e d a t 200 C f o r 16 h r p r i o r to use.) Inlet l i n e r s , 10 X 84 mm, were cut from b o r o s i l i c a t e g l a s s tubing. The soybean o i l samples, provided by the AOCS F l a v o r and Nomenclature Committee (AOCS-FNC), were experimental o i l s s p e c i a l l y t r e a t e d to provide a wide range of f l a v o r v a r i a n c e . The o i l s were f l a v o r scored by 12 t a s t e panels from i n d u s t r y , academia, and government l a b o r a t o r i e s on a 1 to 10 s c a l e . The number of sensory judges i n an i n d i v i d u a l panel v a r i e d from 3 to 24, and i n a l l , t o t a l e d 156 panelists. Gas Chromatography CGC). A Tracor-MT-220 gas chromatograph w i t h d u a l independent hydrogen flame d e t e c t o r s was used i n con j u n c t i o n w i t h a Westronics MT22 recorder and a Hewlett-Packard I n t e g r a t o r , Model 3380 A. The columns were s t a i n l e s s s t e e l U-tubes, 1/8 i n . OD, 10 f t l o n g , packed w i t h Tenax GC that had been coated w i t h 8% Poly MPE. Operating c o n d i t i o n s were as f o l l o w s : Nitrogen c a r r i e r gas, 60 ml/min i n e^ch column; hydrogen, 60 ml/min to each flame; a i r , 1.2 f t /hr ( f u e l and scavenger gas f o r both flames). I n l e t temperature was 170 C. Detector was a t 250 C. Column oven was maintained a t 30 C during the i n i t i a l 40 min h o l d p e r i o d . A f t e r removal of the i n l e t l i n e r , the column was heated to 100 C w i t h i n 5 min, then programmed 3 C/min f o r 30 min. The f i n a l h o l d was a t 190 C f o r 30 min u n t i l the column was c l e a r . A T e f l o n 0 - r i n g was p o s i t i o n e d a t the bottom of the i n l e t of the GC to provide a leak proof s e a l . Elec trometer a t t e n u a t i o n was 10 X 4. Mass Spectrometry (MS). A Hewlett Packard (quadrapole) mass spectrometer, Model 5930 A, was i n t e r f a c e d w i t h a Tracor Model 222 GC. The i o n i z a t i o n p o t e n t i a l was 70 eV, and the scan range was from 21 to 350 amu. Scanning and data p r o c e s s i n g were accomplished w i t h an INCOS 2000 mass spectrometer data system. Sample P r e p a r a t i o n and A n a l y s i s f o r GC. A 3-3/8 i n . l e n g t h of 3/8 i n . OD b o r o s i l i c a t e g l a s s tubing was packed w i t h v o l a t i l e f r e e g l a s s wool, loose enough to permit d i f f u s i o n of o i l through out the packing, y e t t i g h t enough to prevent seepage of the sample from the l i n e r onto the GC column. Clearance of 1/4 i n . was allowed a t the bottom of the l i n e r and 1/2 i n . a t the top. The septum nut, septum, and r e t a i n e r nut of the GC were removed, and the l i n e r c o n t a i n i n g the sample was i n s e r t e d i n the i n l e t of the GC on top of the T e f l o n 0 - r i n g . When the r e t a i n e r nut was tightened above the upper r i m o f the l i n e r , a s e a l was formed between the base of the i n l e t and the lower rim. On c l o s i n g the i n l e t system w i t h the septum and septum nut, the c a r r i e r gas was f o r c e d to flow upward and down through the sample. This assembly has been d e s c r i b e d p r e v i o u s l y (9).

Supran; Lipids as a Source of Flavor ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

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LIPIDS AS A SOURCE O F FLAVOR


V o l a t i l e s were r a p i d l y e l u t e d from the sample as the c a r r i e r gas swept through the heated l i n e r and were adsorbed on the top p o r t i o n o f the column, which was maintained a t 30 C during the i n i t i a l h o l d p e r i o d o f 40 min. The l i n e r c o n t a i n i n g the spent sample was removed from the i n l e t , the i n t e g r a t o r and programmer were turned on immediately, and the temperature was r a i s e d t o 100 C i n 5 min. Temperature programming was then begun. When complete, the temperature was maintained on f i n a l h o l d t o e l u t e and r e s o l v e the v o l a t i l e s adsorbed on the column. The oven was then cooled to 30 C i n p r e p a r a t i o n f o r the next sample. Sample P r e p a r a t i o n and A n a l y s i s f o r MS. A s i l i c o n e membrane separator was used t o i n t e r f a c e the gas chromâtograph w i t h the mass spectrometer. In a t y p i c a l a n a l y s i s , c o n d i t i o n s were the same as those d e s c r i b e d f o r GC, except that helium was the c a r r i e r gas. V o l a t i l e s t h a t a r e r e s o l v e d by GC temperature programming permeate the membrane and enter the mass spectrometer, where the s p e c i f i c peaks are i d e n t i f i e d . Results and D i s c u s s i o n The f o u r experimental soybean o i l s that had been s p e c i a l l y t r e a t e d and f l a v o r - s c o r e d by the AOCS-FNC had the f o l l o w i n g r a t i n g s : 4.0, 5.5, 6.8 and 8.0. These o i l s were examined by d i r e c t GC and combined GC/MS, and the p r o f i l e s o f v o l a t i l e s obtained f o r three o f them Cthe low, medium, and h i g h scored o i l s ) are shown i n F i g u r e 1. Chromatogram 1, which represents a h i g h q u a l i t y o i l w i t h a f l a v o r score o f 8, r e v e a l s few v o l a t i l e components, i n low c o n c e n t r a t i o n s . Pentane, the most prominent peak, i s o f moderate i n t e n s i t y . Two other peaks, hexanal and trans-2, trans-4-decadienal, which have been shown t o be i n d i c a t o r s o f f l a v o r q u a l i t y i n soybean o i l s (9)» a r e o f low i n t e n s i t y . This chromatogram i s t y p i c a l o f that f o r a h i g h q u a l i t y , h i g h f l a v o r scored soybean o i l obtained by the d i r e c t GC method. Chromatogram 2, which was obtained from the soybean o i l w i t h a lower f l a v o r score o f 5.5, r e f l e c t s a marked i n c r e a s e i n the number and i n t e n s i t y o f v o l a t i l e components. I n p a r t i c u l a r , the hexanal response has doubled, the trans-2,trans-4-decadienal has i n c r e a s e d f o u r f o l d , and other components are present i n s i g n i f i c a n t l y l a r g e r q u a n t i t i e s . Chromatogram 3, which represents the poorest q u a l i t y o i l w i t h a f l a v o r score o f 4, shows a dramatic i n c r e a s e i n a l l v o l a t i l e components, and c e r t a i n f l a v o r - r e l a t e d I n d i c a t o r s as pentanal, hexanal, trans-2-heptenal, trans-2, trans-4-heptadienal, trans-2,cis-4-decadienal» and trans-2,trans-4-decadienal a r e conspicuously h i g h . The comparison o f chromatograms 2 and 3 i s e s p e c i a l l y i m p o r t a n t — I t r e v e a l s the s e n s i t i v i t y o f the d i r e c t GC method o f a n a l y s i s over a f l a v o r score range o f only 1.5 u n i t s . I t i s n o t uncommon f o r f l a v o r estimates by i n d i v i d u a l t a s t e r s t o vary as much as three u n i t s f o r a given o i l , whereas the d i r e c t GC method c o n s i s t e n t l y d e t e c t s s u b t l e , y e t r e l i a b l e , d i f f e r e n c e s i n f l a v o r q u a l i t y over a much narrower range (9) ·


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Figure 1. Profiles of vohtihs obtained for three flavor-scored soybean oils. (A) pentane; (B) pentanal; (C) hexanal; (D) trans-2-heptenal; (E) tmns-2,trans-4-heptadienal; (F) trsins-2,cis-4-decadienal; (G) trans-2,trans-4-decadienal


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The c o r r e l a t i o n c o e f f i c i e n t s between instrumental data and f l a v o r scores f o r seven major v o l a t i l e components detected i n the f l a v o r - s c o r e d o i l s a r e shown i n Table I . A l l o f the c o r r e l a t i o n c o e f f i c i e n t s a r e s t a t i s t i c a l l y s i g n i f i c a n t a t confidence l e v e l s o f e i t h e r 95% o r 99%· These r e s u l t s agree w i t h those obtained p r e v i o u s l y w i t h soybean o i l s that had been p r o g r e s s i v e l y degraded by l i g h t exposure (J9). The standard e r r o r s o f estimate f o r the r e g r e s s i o n s o f f l a v o r score on the v a r i o u s peaks ranged from 0.11 to 0.57. The average standard e r r o r s o f the mean f o r the 12 t a s t e panels i n v o l v e d i n t h i s study ranged from 0.15 to Table I Regression A n a l y s i s o f Soybean O i l F l a v o r Scores w i t h Log o f V o l a t i l e Components Volatiles Standard F - Value Correlation error coefficient Pentane Pentanal Hexanal t-2-Heptenal t,t-2,4-Hep t a d i e n a l t , c-2,4-Decadienal t , t - 2,4-Decadienal Total Volatiles

24.0* 41.2* 35.1* 743.7** 108.7** 164.9** 105.1** 157.1**

0.961 0.977 0.973 0.999 0.991 0.994 0.991 0.994

a t the 95% confidence

0.57 0.44 0.48 0.11 0.28 0.23 0.28 0.23

*Significant level. * * S i g n i f i c a n t a t the 99% confidence l e v e l . 0.62. Thus the d i r e c t GC procedure f o r e s t i m a t i n g the f l a v o r o f soybean o i l i s a t l e a s t as good, s t a t i s t i c a l l y , as a t y p i c a l t a s t e panel. Table I I compares t a s t e panel f l a v o r scores w i t h those p r e d i c t e d by d i r e c t GC and combined GC/MS a n a l y s i s . Use o f the trans-2-heptenal peak area, the trans-2,trans-4-decadienal peak area, o r the t o t a l v o l a t i l e s area i n the r e g r e s s i o n equation i s equally s a t i s f a c t o r y i n predicting actual f l a v o r scores. Table I I Comparison o f O i l F l a v o r Scores Obtained by Taste Panels and D i r e c t GC A n a l y s i s Taste P r e d i c t e d scores based on instrumental data* panel scores trans-2trans-2,trans-4Total heptenal peak decadienal peak volatiles 4.0 5.5 6.8 8.0

4.0 5.5 6.7 7.9

3.9 5.8 6.6 7.8

4.0 5.6 6.4 8.0

* P r e d i c t e d from r e g r e s s i o n equation:

a+b Clog x ) .



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Although this paper demonstrates the use of direct GC and combined GC/MS analysis in determining flavor quality for soy bean oils, the method has extensive potential for application to other food products. It has proven effective for studying flavor changes associated with the stability of peanut butter (8,12), Neutral volatile components of mayonnaise have been detected, identified, and their flavor relationship examined (13)» Volatile compounds present in various rice products, whole corn, and breakfast cereals were studied for their contribution to quality and flavor (14). Correlating volatile components of raw peanuts with flavor scores of the roasted product also appears to be quite attractive to plant geneticists for use in developing new peanut varieties (15). Using the direct GC or combined GC/MS method of analysis with appropriate conditions should make i t possible to obtain a profile of volatiles for most raw and processed food products, and to utilize such data for assessing flavor characteristics. Recent experiments with bacon samples have demonstrated the versa t i l i t y of the method. Figure 2 shows the profile of volatiles obtained from commercial brands of bacon purchased from a local supermarket. In comparing the chromatograms for brands A, B, and C, no attempt is made to flavor-relate the products. Rather, they are presented merely to indicate the extent to which different peak components can be eluted and resolved by direct GC, and identified by combined GC/MS analysis. Brand C contains more volatile peaks of greater intensity than do brands A or B. Such compounds as acetic acid, 2-methyl-butanal, hexanal, and methoxyphenol are not pronounced in brands A and B, but are quite prominent in brand C. Similarly, an unidentified peak, eluting at approximately 32 min, is present in relatively high concentration in brand Β and is less prominent in brands A and C. These observations indicate the scope and potential of direct GC and combined GC/MS analysis in detecting and identifying food volatiles. When peaks responsible for flavor are established by taste panel procedures, their presence and intensity, or their absence in similar test products, can be determined with confidence. Preliminary experiments at The Center have indicated that nitrosamines can also be determined by combined GC/MS analysis. Further refinement of these i n i t i a l experiments should make possible the simultaneous analysis of bacon or other meat samples for both flavor quality and nitrosamines. It is likely that efforts to increase the sensitivity and resolution of the direct GC and combined GC/MS methods of analysis will greatly enhance their utility in the broad area of flavor chemistry. ABSTRACT A simple, rapid and direct gas chromatographic technique elutes and resolves the volatile components from vegetable oils and relates them to flavor quality. The sample is placed in a


66

LIPIDS AS A SOURCE

40r

20-

O F FLAVOR

500 mg FRIED BACOM (I) BRAND A


0"

1. ACETIC ACID 2. 2-METHYLPROPANOL 3. 2-METHYLBUTANAL 4. HEXANAL 5. FURFURYL ALCOHOL 6. DIMETHYLPYRAZINE 7. METHOXYPHENOL 8. DIMETHOXYBENZENE 9. DIMETHOXYPHENOL

60h 40h

20h

ι

I ι ι ι ι I 10 20

30

ι

I ι ι ι ι I ι ι ι ι I ι ι ι ι 40 50 60 70

ι ι ι

80

RETENTION TIME. MINUTES

Figure 2.

Profiles of volatiles obtained for three brands of bacon


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gas chromatograph l i n e r and secured in the heated i n j e c t i o n port of the gas chromatograph, without p r i o r enrichment of v o l a t i l e s . Carrier gas, flowing through the heated oil, r a p i d l y and effici ently elutes the v o l a t i l e s , which are adsorbed on the r e l a t i v e l y cool gas chromatographic column and subsequently resolved by temperature programming. The p r o f i l e of v o l a t i l e s obtained is an i n d i c a t i o n of the quality of oil f l a v o r . S p e c i f i c peaks of the chromatogram are i d e n t i f i e d by mass spectrometry. Correla t i o n between taste panel f l a v o r scores f o r the oils and the instrumental data obtained is s i g n i f i c a n t at 99% and 95% c o n f i dence l e v e l s .


Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Buttery, R. G., T e r a n i s h i , R., A n a l . Chem. (1961) 33, 1439. T e r a n i s h i , R., B u t t e r y , R. G., Lundin, R. Ε., A n a l . Chem. (1962) 34, 1033. Smith, D. E., Coffman, J. R., Anal. Chem. (1960) 32, 1733. Chang, S. S., J. Am. Oil Chem. Soc. (1961) 38, 669. Lee, C. H., Swoboda, P. A. T., J. Sci. Food A g r i c . (1962) 13, 148. Selke, Ε., Moser, Η. Α., Rohwedder, W. K., J. Am. Oil Chem. Soc. (1970) 47, 393. Dupuy, H. P., Fore, S. P., G o l d b l a t t , L. Α., J. Am. Oil Chem. Soc. (1973) 50, 340. Fore, S. P., Dupuy, H. P., Wadsworth, J . I., and L. A. G o l d b l a t t , J . Am. Peanut Res. Educ. Assoc. (1973) 5, 59. Dupuy, H. P., Rayner, E. T., Wadsworth, J . I . , Legendre, M. G., J. Am. Oil Chem. Soc. (1977) 54, 445. W i l l i a m s , J. L., and Applewhite, T. H., J. Am. Oil Chem. Soc. (1977) 54, 461. Jackson, H. W., and G i a c h e r i o , D. J., J. Am. Oil Chem. Soc. (1977) 54, 458. Fore, S. P., Dupuy, H. P., and Wadsworth, J . I . , Peanut Sci. (1976) 3 (2) 86. Fore, S. P., Legendre, M. G., and F i s h e r , G. S., J. Am. Oil Chem. Soc. (1977), submitted f o r p u b l i c a t i o n . Legendre, M. G., Dupuy, H. P., Ory, R. L., and McIlrath, J . A g r i c . Food Chem. (1977), submitted f o r p u b l i c a t i o n . Brown, M. L., Wadsworth, J . I . , Dupuy, H. P., and Mozingo, R. W., (1977) Peanut S c i . in p r e s s .

RECEIVED December 22, 1977


Instrumental Analysis of Volatiles in Food Products - ACS Publications

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