Volatile Compounds of Rehydrated French

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Food Chemistry 54 (1995) I-7Copyright 0 1995 Elsevier Science Limited

Printed in Great Britain. All rights reserved0308-81461951S9.50

Volatile compounds of rehydrated Frenchbeans, bell peppers and leeks. Part II.Gas chromatography/sniffing port analysis andsensory evaluation

Saskia M. van Ruth, Jacques P. Roozen, Jan L. Cozijnsen & Maarten A. Posthund Wageningen Agricultural University, Department of Food Science, PO Box 8129; Wageningen Agricultural University,Department of Organic Chemistry, PO Box 8026, NL-6700 EC Wageningen, The Netherlands

(Received 17 October 1994; revised version received and accepted 12 January 1995)

The flavours of rehydrated diced French beans, red bell peppers and leeks werecharacterised by gas chromatography/sniffing port analysis (GCYSP) of volatilecompounds released in a mouth model system, and by descriptive sensory analysis.Volatile compounds were identified by combined gas chromatography/massspectrometry. In French beans, bell peppers and leeks, respectively, 10, 16 and 22compounds possessed detectable odours. A common odour profile was shown inthe three vegetables. It comprised each of the odour active compounds presentin French beans: i.e. 2-methylpropanal (chocolate), 2/3_methylbutanal (choco-late), 2,3-butanedione (caramel, fatty), hexanal (grassy, bell pepper), 2-methyl-2-butenal (chemical), octanal (sweet, sickly/musty, grassy, rancid), 1 octen-3-one(mushroom), dimethyl trisulphide (rotten, metal), I-octen-3-01 (fatty, sickly/musty, mushroom) and one unknown compound (chemical, rotten, rancid). Thethree vegetables differed markedly in GCYSP patterns and in scores for sensoryattributes. Use of nose-clips diminished the scores for attributes in sensory analysis.In principal component analysis, correlation of rehydrated vegetables with sensoryattributes and volatile compounds showed considerable contribution of volatilecompounds to the flavour of rehydrated vegetables.

INTRODUCTION

Foods have highly complex chemical compositions,containing both volatile and non-volatile substances.Their sensory responses may be significantly affected bytheir relative concentrations and the temperature of theproduct as consumed (Heath, 1981). Human taste-budsare capable of differentiating only four stimuli. As thenose is capable of discerning hundreds of differentodours, it is not surprising that a major part of flavourresearch has dealt with analysis of volatile compounds(Hoff et al., 1978).

An instrumental approach to characterise flavourcan be regarded as a two-phase arrangement. The firstphase involves representative isolation of volatile com-pounds, because gas chromatographic (GC) patternsare largely influenced by isolation procedures. Releaserates of volatile compounds from a product dependupon partition coefficients of the compound, molecularinteractions and ambient temperatures (Legger &Roozen, 1994). The second phase involves selectionof volatile compounds relevant to the flavour. This

implicates correct determination of the relevant flavourcompounds from the whole range of volatiles present ina particular food product (Dirinck & De Winne, 1994).An interesting approach is sniffing the gas chromato-graphic effluent (GC/sniffing port analysis; GC/SP), inorder to associate odour activity with the eluting com-pounds. Flame ionisation, flame photometric, electroncapture, mass spectrometric, fourier transform, infraredand thermal conductivity detectors are not as sensitiveas the human nose for many odorants (Acree &Barnard, 1994). The interest in determining the individ-ual contribution of volatile compounds present in foods,has led to a new generation of GCYSP techniques,which can be classified into four categories:

(i) Dilution analysis methods for producing titre orpotency values based on stepwise dilution to thresh-old, e.g. CHARM (Acree et al., 1984) and aromaextraction dilution analysis (Ullrich & Grosch, 1987).

(ii) Response interval methods for recording timeduration of perceived odours, and the number ofassessors with odour perception, it also estimatesa titre or potency (Linssen et al., 1993).


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2 S. M. van Ruth, J. P. Roozen, J. L. Cozijnsen, M. A. Posthumus(iii) Time-intensity methods for producing subjective

estimates of perceived intensity recorded simulta-neously with the elution of the chromatographicpeak, e.g. OSME (Sanchez et al., 1992).(iv) Posterior intensity methods for producing sub-jective estimates of perceived intensity, which arerecorded after a peak has eluted (Casimir &Whitfield, 1978).

Correlation between sensory and instrumental flavourdata has been attempted by several multivariate statisti-cal techniques, like factor and principal componentanalyses (Noble, 1978). Such techniques have been usedto reduce large numbers of sensory and instrumentalvariables to a smaller number of factors, or to studyunderlying patterns in the variables (Palmer, 1974;Guedes de Pinho et al., 1994; Luning et al., 1994a).

Recently, the authors reported a dynamic headspacemodel system for isolation of volatile compounds fromvegetables under mouth conditions. Using this modelsystem, release of volatiles from rehydrated Frenchbeans, red bell peppers and leeks did not differ signifi-cantly from their-release in the mouth of 12 assessors(Van Ruth et al., 1995). The present study deals withthe relationship between GCKP detected volatile com-pounds released in the reported mouth model systemfrom rehydrated French beans, red bell peppers andleeks, and the sensory properties of these vegetables.This relationship was examined by multivariate statisticaltechniques.

MATERIALS AND METHODSPlant materialCommercially dried French beans and leeks from TheNetherlands and red bell peppers from Hungary weresupplied in pieces (8 mm X 8 mm) by Top Foods BV(Elburg, The Netherlands). The vegetables were packedin glass jars and stored at 4C in the absence of lightuntil sampling. Diced dried vegetables (1.2 g) wererehydrated by adding 10 ml distilled water, followed byheating in a waterbath at 100C for 10 min and coolingdown in a waterbath at 25C for 4 min.Instrumental analysisThe rehydrated vegetables were transferred into adynamic headspace mouth model system with mastica-tion device. Artificial saliva (4 ml) was added and theheadspace was flushed with nitrogen gas (250 mumin)for 12min in order to trap volatiles in Tenax TA asdescribed previously (Van Ruth et al., 1995). A plungermade about four up and down screwing movementsper min to simulate mastication. In GC/SP, desorptionof volatile compounds from Tenax was performed bya thermal desorption (210C 5 min)/cold trap (-120C240C) device (Carlo Erba TDAS 5000, InterscienceBV, Breda, The Netherlands). Gas chromatography was

carried out on a Carlo Erba MEGA 5300 (InterscienceBV, Breda, The Netherlands) equipped with a Supel-cowax 10 capillary column, 0.25 mm i.d. and 60 m longand a flame ionisation detector (FID) at 275C. Atthe end of the capillary column the effluent was split1 : 2 : 2 for FID, sniffing port 1 and sniffing port 2,respectively. Oven temperature programme and sniffingport detection were performed as reported previously(Van Ruth & Roozen, 1994). In preliminary GC/SPexperiments, 10 assessors (aged 20-40) generated flavourdescriptors for rehydrated French beans, bell peppersand leeks, which were clustered after group sessions ofthe panel. Besides other/I do not know, one of thesedescriptors (Table 1) had to be used for each com-pound detected by the assessors at the sniffing port.Tenax tubes without adsorbed volatile compoundswere used as dummy samples for determining signal-to-noise level of the group of assessors.The volatile compounds trapped in Tenax TA wereidentified by combined GC (Varian 3400, Varian, Wal-nut Creek, CA, USA) and mass spectrometry (MS;Finnigan MAT 95, Finnigan MAT, Bremen, Germany)equipped with a thermal desorptiomcold trap device(TCT injector 16200, Chrompack BV, Middelburg, TheNetherlands). Capillary column and oven temperatureprogram were the same as those used in GC/SP analy-ses. Mass spectra were obtained with 70 eV electronimpact ionisation, while the mass spectrometer wascontinuously scanning from m/z 24 to 400 at a scanspeed of 0.7 s/decade (cycle time 1.05 s).Table I. Comparison of the descriptorsof the gas chromatography/sniffing port panel (GC/SP) and the attributes of the analyticalsensory panel for three rehydrated vegetables

Descriptors(GC/SP panel)Bell pepperBurnedCaramelChemicalChocolateCitrusCooked vegetablesCucumberDetergentFattyFrench beanFruityGarlicGrassyHerbalLeekMetalMushroomOnionRancidRottenSickly/mustySourSpicySweet

Attributes(sensory panel)Bell pepperBitterBurnedChemicalCitrusCooked vegetables

French bean

GrassyLeekMushroomOnionRottenSharpSourSpicySweet


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Volati le compounds of French beans, bell peppers and leeks. Part I ISensory evaluationA panel of 21 judges (aged 20-55) was selected andtrained for quantitative descriptive analysis (QDA). Acomputer interactive interviewing system for compos-ing questionnaires was used to gather survey informa-tion (Ci2 system; Sawtooth Software Inc., Ketchum,ID, USA). Samples of each of the diced dried vegeta-bles (1.8 g) were rehydrated by adding 15 ml water andheating in a waterbath at 100C for 10 min. They werecooled down and stored in a waterbath at 50C untilthey were served to the panel and assessed for flavourevaluation. Flavour attributes were generated duringtraining sessions, which resulted in a list of 16 attributes(Table 1). Sensory attributes of each of the rehydrateddiced vegetables were evaluated with and without useof nose-clips (Jaeger Nederland BV, Breda, The Nether-lands) by scoring perceived intensities on a 70 mmvisual analogue scale on a portable computer screen.Statistical evaluationSensory data were subjected to Students t-tests todetermine significant differences between the three vege-tables. The SPSS/PC+ program (SPSS Inc., Chicago, IL,USA) was used for principal component analysis(PCA) of the sensory data as well as for correlation ofinstrumental and sensory data. A significance level ofP < 0.05 was used throughout the study.

RESULTS AND DISCUSSIONVolatile compounds of rehydrated diced French beans,bell peppers and leeks were isolated in the mouth modelsystem and analysed by GC/SP. Figure 1 represents thesniffing chromatograms of each of the three vegetables.The volatile compounds were identified by GC/MS andtheir retention times, and characterised by their FIDpeak areas and the odours described by the assessors ofthe sniffing panel (Table 2). GC sniffing of dummysamples showed that detection of an odour at thesniffing port by three or less out of 10 assessors can beconsidered as noise. GC/SP revealed that 10, 16 and22 volatile compounds isolated from French beans, bellpeppers and leeks, respectively, possessed detectableodours. Although differences in GC/SP patterns acrossthe vegetables were obvious, a common GC/SP profilewas shown in the three vegetables. It incorporates allodour active compounds isolated from French beans:2-methylpropanal (chocolate), 2/3-methylbutanal (choco-late), 2,3-butanedione (caramel, fatty), hexanal (grassy,bell pepper), 2-methyl-2-butenal (chemical), unknown(chemical, rotten, rancid), octanal (sweet, sickly/musty,grassy, rancid), 1-octen-3-one (mushroom), dimethyltrisulphide (rotten, metal) and 1-octen-3-01 (fatty,sickly/musty, mushroom). The presence of 2-methyl-propanal and 2/3-methylbutanal is typical for driedvegetables. These volatile compounds can be formed byStrecker degradation of valine and (iso)leucine during

10

gi81

a

20 30Qeention time /vinI(4

23II

0 10 20 30 40Qetention time lrr~ n

5(

(c)Fig. 1. Sniffing chromatograms of volatile compounds ofrehydrated diced (a) French beans, (b) bell peppers, and (c)leeks. Numbers in the chromatograms refer to compounds inTable 2.

the drying process. The other components of the com-mon odour profile can be formed mostly in lipid oxida-tion reactions (Tress1 et al., 1981; Whitfield, 1992).Most compounds of the profile possess low thresholdsin water (< 5 mg/kg) (Leffingwell & Leffingwell, 1991).Many of the compounds with detectable odours wereidentified in Part I of the present study (Van Ruth et


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4 S. M. van Ruth, J . P. Roozen, J . L. Cozijnsen, M. A. PosthumusTable 2. Odour active volatile compounds of rehydrated diced French beans, bell peppers and leeks; their retention times and odourdescriptionsNo. Retention Compoundtime(mm) French beans

Odour descriptionBell peppers Leeks

1 6.3 Methanethiol - -2 6.7 Carbon disulphide - -3 7.2 Dimethyl sulphide - -4 7.7 Propanal - -5 8.0 2-Methylpropanal6 8-7 1 Propanethiol7 10.0 2-Propene-I-thiol8 11-l 2/3-Methylbutanal9 12.5 3-(Methylthio)-I-propene

Chocolate Chocolate--Chocolate

-Chocolate-

10 13.9 2, 3-Butanedione Caramel, fatty Caramel11 16.1 I-Penten-3-one - Chemical, metal12 19.8 Hexanal Grassy, bell pepper Grassy, bell pepper13 20-9 2-Methyl-2-butenal Chemical Chemical, metal, leek14 24.9 Unknown15 27.2 2-Heptanone/heptanal -16 31-5 Unknown Chemical, rotten,rancid17 32.6 (Z)- 1 Methyl-propenyldisulphide -

Citrus, fruity, rottenBell pepper, cookedvegetables, cucumberRotten, rancid,sickly/musty, chemical,citrus, leek-18 33.419 34.0 (J)J~n?v$thyl-propen yldisulphide Sweet, sickly/musty,grassy, rancidMushroomRotten, metal

-Citrus, cucumber, leek

20 35.2 I-Octen-3-one21 39.3 Dimethyl trisulphide MushroomRotten, metal22 406 Propenyl propyi disulphide23 41.7 I-Octen-3-0124 42.1 2, 4-Heptadienal25 46.1 2-Isobutyl-3-methoxypyrazine26 466 Unknown27 49.1 Dipropyl trisulphide28 49.6 2, 4-Nonadienal

Fatty, sickly/musty,mushroom----

-Sickly/musty, fatty,rancid, bell pepperFatty, sickly/mustyBell pepper--Cucumber, cookedvegetables, grassy

RottenSweet, chemicalRotten, cookedvegetables, leek, spicyCooked vegetables,grassy, chemicalChocolateOnionGarlicChocolate, spicyOnion, rotten, herbal,chemicalCaramel -GrassyRancid, cookedvegetables, onion--Rancid, rotten,chemical, sickIy/musty,onionChemical, onion, rotten,herbal, burnedRotten, garlic, leekFatty, citrus, sweetMushroomRotten, onion, leek,metalHerbal, garlic, leek, fattyFatty, herbal, leek

--OnionOnion, garlic-

-1-Octen-3-one eluted simultaneously with 3-hepten-2-one in bell peppers.

al., 1995). Some of these compounds contribute to theflavour of the vegetables, despite their very low FIDresponses. 2-Isobutyl-3-methoxypyrazine is present inthe rehydrated bell peppers and has a characteristic bellpepper odour. Several sulphur-containing volatile com-pounds in leeks possess leek odours. However, novolatile compounds identified in rehydrated Frenchbeans possessed a particular French bean flavour. Theodour active compounds 2-methylpropanal and 2/3-methylbutanal, present in the French beans, have beenreported in dried beans previously (Lovegren et al.,1979). Stevens et al. (1967) and Toya et al. (1974)reported that I-octen-3-01 belongs to the most impor-tant flavour compounds of canned snap beans. Severalof the odour active compounds of bell peppers werereported previously (Wu et al., 1986; Luning et al.,19946). The bell pepper odour of 2-isobutyi-3-

methoxypyrazine is well known (Buttery et al., 1969;Luning ef uf., 19940). Sulphur-containing componentsare the main odour active compounds of rehydratedleeks. Several of these compounds were reported bySchreyen ef al. (1976a,b), i.e. I-propanethiol, methylpropenyl disulphide, propenyl propyl disulphide anddipropyl trisulphide, each of them possessing leek/onion odours.An analytical sensory panei of 21 judges evaluatedthe rehydrated diced French beans, red bell peppersand leeks. Although individual judges responded differ-ently to the three vegetables, the panel was consistentacross replications. Most of the attributes generated bythe sensory panel were similar to the descriptors of theGC/SP panel (Table 1). Apparently, volatile com-pounds contribute to the flavour of French beans, bellpeppers and leeks. Th e results of the sensory evaluation


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Volati le compounds of French beans, bell peppers and leeks. Part I I 5

rotten sweet sour

mushrocm

i \cooked orinnvegetables(b)sweetrotten ~ :our

vegetables(cl

Fig. 2. Spider-web diagrams of scores for sensory attributesof rehydrated diced (a) French beans, (b) bell peppers, and(c) leeks assessed (--) with and (- - - - -) without nose-clips.

of each of the vegetables are presented in spider-webdiagrams (Figs 2(a)-(c)). Use of nose-clips diminishedscores for sensory attributes in general, which is in

bitter

_Frerictibear\ bell

pepper

agreement with packaging migration studies of Linssenet al. (1991). This confirms our suggestion that the con-tribution of volatile compounds to the Bavour of thevegetables is significant. As expected, the scores fortaste attributes, like sweet, sour, sharp and bitter,as well as for the particular vegetable less relevantattributes (indicated by low intensity scores with use ofnose-clips) decreased less than others.

Mean scores for the sensory attributes for each ofthe vegetables and the statistical significances of mutualdifferences (Studentss t-test, P< 0.05) are shown inTable 3. Intensity scores for the bell peppers were simi-lar to those reported previously (Van Ruth & Roozen,1994). French beans scored high in French bean,cooked vegetables, grassy, sweet and spicy at-tributes, the bell peppers in bell pepper, sweet,cooked vegetables, spicy, sour, sharp and bitterand leeks in leek, onion, cooked vegetables, sweet,sharp, spicy and grassy. The three vegetables didnot differ significantly in scores for the attributes spicyand mushroom, however, each of the other attributesrevealed significant mutual differences between the veg-etables. Sensory data were subjected to PCA, the firstfive principal components of which are shown in Table4. PCA shows relationships between attributes: leekand onion loaded high on the first component, indi-cating that these attributes were strongly associatedwith each other. Sweet, sour, citrus and bell pep-per loaded high on the second component. Two associ-ated attributes were determined for each of the otherthree components.

PCA was performed on combined GC/SP (numberof assessors perceiving an odour active compound) andQDA sensory data sets (intensities of sensory attributesperceived without nose-clips) (Fig. 3). The first twoprincipal components explained 75% of the variance inthe data sets. Vegetable sample scores in the PCA mapTable 3. Scores for sensory attributes of rehydrated dicedFrench beans, bell peppersand leeks, assessed without nose-clips__ (mean, n = 21)Code Attribute Frenchbeans Bellpeppers

A Sweet 30.2a 42.6bB Sour 9.la 35.8bC Sharp 11.8a 33.4bD Bitter 12.6a 26.4bE French bean 56.6b 7.2aF Bell pepper 7,la 59,ObG Leek 5.6a 5.9aH Onion 5.5a 8.8aI Cooked vegetables 54,Ob 38.4aJ Grassy 41.1c 18.3aK Spicy 28.7a 36.0aL Citrus 4,Oa 12.9bM Mushroom 19.4a 16.2aN Chemical 7.la 17.5b0 Burned 7.9a 16.8bP Rotten 3:6a 8,lab

Leeks

39.3b16.6a37.4b28.1b6.3a4.9a60.9b47.5b42.3a30,Ob34.3a4.5a17.5a7.5a19.3bc8.2b

Different following letters within a row indicate significantdifferences (Students t-test, P


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6 S. h4. van Ruth, J. P. Roozen, J. L. Cozijnsen, M. A. PosthumusTable 4. Attributes of the first five principal components con-cerning rehydrated diced French beans, bell peppers and leeks(loadings in parentheses)First25% Second16% Third13% Fourth9% Fifth7%Leek(0.9)Onion(0.9)

Sweet(0.7)Sour(0.7)Citrus(0.7)Bellpepper(0.7)

Grassy Burned Mushroom(0.9) (0.8) (0.7)Cooked Chemical Rottenvegetables (0.7) (0.7)(04)

rpercentage of explained variance.

show that leeks (LEEK) was separated from bell peppers(BELL) and French beans (BEAN) along the first com-ponent. BELL and LEEK were separated along thesecond component. The diagram reveals three sites inwhich vegetable sample, volatile compounds and sen-sory attributes are closely related to eadh other. Thefollowing numbers refer to volatile compounds inTable 2 and capitals refer to sensory attributes in Table3. French bean (E) and cooked vegetables (I) showedhigh negative loadings on both the first and secondcomponent and correlated well with BEAN. BELLcorrelated well with 2-methoxy-3-isobutylpyrazine (25)and bell pepper (F). The compound 2-methoxy-3-isobutylpyrazine was described by the GC/SP panel asbell pepper. Sensory attributes onion (H) and leek(G), as well as several volatile compounds having leek

I] 1225 16F

24N

M

(BEaK]E 13,

r1

J

or onion odours according to GC/SP, have highloadings on the first component and low loadingson the second. These sensory attributes and volatilecompounds, among which are methanethiol (l),3-(methylthio)- 1 propene (9) and propenyl propyl disul-phide (22), correlated with LEEK. Correlation betweenthe rehydrated vegetables, sensory attributes and sev-eral volatile compounds having corresponding odoursaccording to GUSP, showed that volatile compoundscontribute considerably to the flavour of rehydratedvegetables. This is in agreement with studies of Noble(1978), in which a correlation between sensory andinstrumental data of wines was determined and Rothe etal. (1994), who reported relationships between volatilecompounds and blue cheese flavour. Investigations ofTeule and Crouzet (1994) showed a relationshipbetween volatile compounds and formation of cookedaroma in dehydrated apple products. Burdach andDoty (1987) reported that persons who have lost theirsense of smell, frequently perceive this loss as one oftaste, rather than one of smell. This indicates that stim-ulation of the olfactory receptors by foods via theretronasal route is a primary determinant of theirflavour. In GC/SP the volatile compounds are sampledseparately by the assessors in GC elution sequence.Nevertheless the instrumental data of GC/SP ofvolatiles released in a mouth model system with masti-cation device correlated quite well with sensory data.

CONCLUSIONSThe presence of a common odour profile was shown inthe three rehydrated vegetables by GC/SP. This profile

59P

D cA

0

Fig. 3. Scores of vegetable samples (BEAN, French beans; BELL, bell peppers; and LEEK, leeks) and loadings of volatile com-pounds and sensory attributes on the first (horizontal) and second (vertical) principal component axes. Numbers refer to volatilecompounds in Table 2 and capitals to sensory attributes in Table 3.


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Volati le compounds of French beans, bell peppers and leeks. Part I I 7incorporates each of the odoui active compounds ofrehydrated French beans. Correlation between rehy-drated vegetables, sensory attributes and severalvolatile compounds isolated in a mouth model system,showed major contribution of volatile compounds tothe llavour of rehydrated vegetables.

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