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J Sci FoodAgric 1996,71,329-336

Causal Effectsof Aroma Compounds on RoyalGalaApple Flavours

Harry Y oung,* J anine M Gilbert, ShonaH M urray and RoderickD Ball

T he Horticultural and F ood Research I nstituteof N ew Zealand Ltd, Private Bag92169, A uckland,

New Zealand

(Received 31 July 1995; revised version received 13November 1995; accepted 29 J anuary 1996)

Abstract: Volatile flavour compoundsproduced by Royal Gala apple have beenidentified by GC-MS. Major components were 2-methylbutyl acetate, butylacetate,hexyl acetate,butanol, 2-methylbutanol and hexanol.Odour-port evalu-ation of the components separated by GC indicated that the first four com-poundswere important contributorsto thearoma and flavour. Use of analyticalsensory panels revealed that 2-methylbutyl acetate, butanol and hexyl acetatehad the greatest causal effect on those aroma and flavour attributes consideredimportant for Royal Gala apples.

Key words:apple, sensory panel, flavour components, causal effects.

INTRODZJ CTION

The volatile flavour constituents of apple have been thestudied for over 50 years and have been reviewed byDimick and Hoskin (1983). Although over 200 volatilecompounds have been reported to be in various culti-vars of apple there is li ttle information on the contribu-tion these chemicals have on the sensory perceptionconsumers have of apple flavour. Flath et a1 (1967) rec-ognised the importance of sensory evaluation and theinfluenceof varietal differences. The importance of culti-var was reinforced by an investigation on cider apples(Williams et a1 1980). The flavour volatiles of some ofthe older commercial cultivars such as Delicious,Golden Delicious, Cox’s Orange Pippin and McIntosh

has been the subject of several studies (Flath et al 1967;Sapers et a1 1977; Will iams and Knee 1977; Dirinck et

al 1983).The early studies clearly show that the charac-teristic apple aroma/flavour results from a complexmixture of alcohols, esters, aldehydes and ketones.

The Royal Gala apple (M alus domestica Borkh) is anatural sport of Gala apple which in turn is a cross ofK idds Orange and Golden Delicious cultivars. It is oneof the major commercially grown cultivars in NewZealand and has a strong aroma when ripe. Howeverconsumer feedback has indicated that there is a loss of

* Towhom correspondence should beaddressed.

flavour after storage. L ittle work has been reported onthe volatile flavour constituents of Royal Gala apples.

The objective of this study has been to identify the vola-tile flavour compounds associated with this applevariety and to use an analytical sensory panel to deter-mine the contribution that selected components maketo important sensory attributes.

METHODS

Isolationof flavour volatiles

Royal Gala apples in good condition were purchasedfrom the local supermarket. The apples (2.1 kg) were

peeled, cored and sliced (1-3 mm thick) and placed intoa 3 litre reaction vessel (Quickfit) fitted for vacuumsteam distillation (Y oung et al 1983). No water wasadded and no stirring was done. T he distillation wascarried out over a period of 6 h at 1.7-1.9 kPa, yielding135 ml of aqueous distillate which was then divided into10ml portions and kept frozen (- 20°C) until required.The organic components were recovered from theaqueous distillate (10ml) by passing it through a C I 8solid phase extraction column made by combining thepacking material from two 6 ml Supelclean ENVI-18(Supelco Inc, USA) tubes into a single unit, then recov-ering the adsorbed material by elution with diethyl

329J Sci FoodAyric0022-5142/96/$09.00 1996SCI . Printed in Great Britain



330 H Young et a1

ether (purified by distil lation from LiA lH,) until c

500pl of diethyl ether was collected. A further 300pl ofether was passed through the column and collectedseparately. The residue water which was collected alongwith the first ether layer was removed with a syringeand extracted with the second ether fraction. The twoether fractions were combined and concentrated byslow static distillation (Blomberg and Roeraade 1988) to1OOpl for GC, GC-M S and gas chromatography-olfactometry.

Gas chromatography (GC)

GC was carried out using DBWAX or DB1 fused silicacapillary columns (J and W Scientific, CA, USA), flameionisation detection (210°C) and split/splitless injection(150°C). For analytical work the columns were

30 m x 0-32 mm id and the carrier gas was H, at30 cm s-’. The film thickness of the DBWAX and DB1columns was 0-5 pm and 1 pm, respectively. Thecolumn oven temperature programme was 30”C, hold6 min, 3°C min-‘ to 190°C.

Gaschromatography-olfactometry (GCO)

GCO analysis (odour-port assessment) was carried outusing the methodology based on the CHA RM pro-cedure described by Cunningham et al (1986). The GCcolumn used was a 0.53 mm id DBWAX column (30 m)

and carrier gas was H, at 30 cm s-’. Detector (flameionisation) and injector temperatures were 210 and150”C, respectively. Column oven temperature pro-gramme was 30”C, hold 4 min, 5°C min-’ to 195°C.

The outlet of the GC column was connected to anoutlet splitter with 95% to the sniffing port and 5% tothe detector. M ake up gas (N,) at 9 ml min-’ wasadded immediately before the spli tter. The eluate wasassessed by three assessors familiar with Royal Galaaroma. Each assessor assessed the sample for ‘RoyalGala apple’, ‘red apple’ and ‘unknown’ at 1 x ,3x , 15 x

and 15 x dilution in separate runs. Assessors indicatedstart and end of detection of a given peak withoutregard to intensity.

Gas chromatography-massspectrometry (GC-M S)

GC-M S was carried out on a VG-70SE spectrometer,operated in the electron impact mode at 70 eV, fittedwith a directly coupled GC. Gas chromatographic con-ditions were similar to that for GC. The carrier gas wasreplaced with He.

Preparation and sensory evaluationof samples

A total of 256 samples were prepared for sensory testingby adding flavour components to 30 ml aliquots of abase solution. The base solution was prepared from anapple juice concentrate (obtained from ENZA Pro-cessors, Hastings, New Zealand) diluted to 12”Brix andpassed down an Amberlite XAD-2 column to removeresidual aroma material. Each of the four compounds(Table 1) was added to the base at one of the four pre-

determined levels, according to a full factorial design(based on four compounds and four levels of eachchemical). Levels were chosen in a pilot study such thatlevel 2 was the lowest level at which the compoundcould be reliably detected while level 4 was the limit atwhich it was considered to be slightly more intense thanof a freshly cut Royal Gala apple.

Stock solutions of the volatile flavour compoundswere prepared so that 50pl of the solution when addedto 30 ml of the base gave the final level required (Table1).Each solution was prepared by dissolving the com-pound in ethanol (7.5 ml) and made up to 15 ml withreverse osmosis (RO) purified water.

Sixteen trained sensory panellists were asked to assessaroma (by sniffing) and flavour (by tasting) of foursamples over four sessions for intensity of the followingattributes: ‘overall aroma’, ‘red apple aroma’, ‘sweetaroma’, ‘acid aroma’, ‘overall flavour’, ‘red appleflavour’, ‘characteristic apple flavour’, ‘sweet flavour’and ‘acid flavour’. Intensities were recorded on 150 mmline scales with descriptive anchors at each end.

Statistical analysis

For each attribute an accumulated analysis of variance

table was generated from a regression model. This

TABLE 1

Concentrations(mgper 30 ml) of volatile flavour compounds added to thebase solution

Level 1 Level 2 Level 3 Level 4

Butyl acetate 0 0.045 0.096 0.180

Hexyl acetate 0 0.021 0.048 0.066S-2-Methylbutylacetate 0 0.035 0.096 0.186

Butanol 0 0.333 0.651 0.987



Causal effectsof aroma compounds on apple flavour 331

included panellist effects and up to third order inter-actions. This was followed by a REML analysis withrandom panellist effects. F statistics from the accumu-lated analysis of variance tabjle were used as a guide towhich interactions to consider.

RESULTS

The compounds found in thle volatile flavour distillatewere identified by comparison of mass spectra withlibrary spectra and are shown in Table 2. Not listed inTable 2 is hexanal which was only present in traceamount. Hexanal was maskeld on the tail of the intensebutyl acetate peak and was only detected by MS usingthe single ion chromatogram technique. The identifica-tion of l -methoxy- 4- ( 2- propenyl ) - benzeneestragole)with its spice-like or aniseed aroma (Williams et a1

1977) is interesting since these descriptors have not beenused in the sensory characterisation of Royal Gala. Theconfiguration of the 2-methylbutyl acetate has been ten-tatively assigned the S form by analogy with resultsreported in the literature for the related 2-methylbutanoate esters found in Granny Smith apple(Mosandl et a1 1991).GCO analysis suggested four important contributors

to the flavour of Royal Gala apples. These were 2-methylbutyl acetate, butyl acetate, hexyl acetate andbutanol.

The main effects of the four selected components onthe sensory attributes studied are shown in Fig 1.

Graphs with solid lines indicate that at the levels usedthe compound has a statistically significant effect(Tukey's LSD, P <0.05) on that sensory attribute.Several first order interactions were also apparent (Figs2-5).

TABLE 2Compounds identified in the volatile flavour extract from Royal Gala

apple"

Components RI KI gb I D

EthanolPropyl acetate2-Methylpropyl acetatePropanoln-Biutyl acetate2-Methylpropanol

2-Methylbutyl acetaten-Biutyl propanoaten-Butanoln-Pentyl acetate2-Methylbut-3-enyl acetate2-Methylbutanol3-Methylbut-3-enyl acetate3-Methylbut-3-en013-Methylbut-2-enyl acetaten-Pentanoln-Hexyl acetateE-Hex-3-enyl acetate

946979

1017104910771oo

112511451156177

199214234251254

257277312

Z-Hex-3-enyl acetate 1320Hex-4-enyl acetate 1329

E-Hex-2-enyl acetate 1337n-Uexanol 1360Z-Hex-3-en01 1390E-Hex-2-en01 1413n-Hexyl-2-methylbutanoate 1430n-Heptanol 1462Camphor 1529n-Octanol 1565n-C)ct-2-enol 16251-M ethoxy-4-(2-propenyl)-benzene 1682

1.510.320.072.72

13.060.59

2.590.04

38.790.250.012.960.01

0.01

0.010.234.980.01

0.01

0.29

1.065.820.710.010.010.030.020.110.020.44

MS, RIMS, RIMSM S, RIM S, RIMS, RI

MS, RIMS, RIMS, RIM S, RI

MSMS, RIM S

MS, RI

MS, RIMS, RIMS, RI

MS, RIMS, RIMS

MS, RIM S, RIM S, RIM S, RI

M S, RIMS, RIMS, RIM S, RI

MS, RIMS, RI

Identification by Retention Index (RI) and/or Library Mass Spectra

Sem-quantitatively data assumng 100% recovery.(MS).



332 H Young et a1

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Fig 1. Mean intensities (measured on 150 mm line scales) for the sensory attributes at various levels of added compounds. Y -axis=mean intensities measured on a 150mm scale, X-axis = levels of added compounds. For concentrations of the added

chemicals refer to Table 1. Solid lines represent significant effects. All intensities are plotted on the same relative scale.



Causal effects of aroma compounds on apple fl avour

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Fig4. The effectsof ahexyl acetatebutan01 nteraction on ‘red apple aroma’,L SD (Tukey,P =0.05)=18.4.

‘Overall aroma’ and ‘overall flavour’ increased withincreasing levels of 2-methylbutyl acetate and butanol(Fig l a and le). In the absence of butanol and increas-ing levels of butyl acetate, ‘overall aroma’ showed adownward, although non-significant, trend (Fig 2). Incontrast, when butanol was added butyl acetate hadlittle effect on ‘overall aroma’. A hexyl acetatebutan01interaction was detected but no obvious pattern wasdiscernible (data not shown).

‘Sweet aroma’ increased with increasing levels of 2-methylbutyl acetate and butanol (Fig lc) while ‘sweet

flavour’ increased with hexyl acetate and 2-methylbutylacetate (Fig lg). The presence of butyl acetate decreasedthe intensity of ‘sweet aroma’. A hexyl acetate/butanoland a 2-methylbutyl acetate/butanol interaction wasfound for ‘sweet flavour’. With both interactions theeffects of increasing acetate were greatest in the absenceof butanol (F ig 3a and 3b).

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‘Acid aroma’ and ‘acid flavour’ were not significantlyaffected by any of the added volatile chemicals. Nointeractions that affected ‘sweet aroma’ or ‘acid flavour’were detected.

‘Red apple aroma’ and ‘red apple flavour’ increasedwith increasing levels of 2-methylbutyl acetate, butanoland hexyl acetate (Fig l b and I f). The addition of butylacetate to level 2 decreased the intensity of ‘red applearoma’, although further addition did not continue thetrend. A hexyl acetate/butanol interaction was found for‘red apple aroma’. I n the absence of butanol this attrib-

ute increased with increasing levels of hexyl acetate,whereas in the absence of hexyl acetate the attributeincreased with increasing butanol level (Fig 4).

‘Characteristic apple flavour’ increased with 2-methylbutyl acetate levels while butyl acetate caused aninitial, although not significant decrease (Fig li). Thereis evidence for a 2-methylbutyl acetate/butanol inter-

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Fig5. The effectsof a 2-methylbutyl acetate/butanol) nteractionon ‘characteristic apple flavour’, LSD (Tukey,P =0.05)= 18.6.



Causal effects of aroma compounds on apple J Eavour 335

action (Fig 5). 2-M ethylbutyl acetate had the greatesteffect in the absence of butanol. With butanol present atthe highest level, 2-methylbutyl acetate did not affectthis attribute.

DISCUSSiIONS

Chemical and GCO analyses

The major volatile flavour Components found in RoyalGala apple have all been previously found in apple(Dimick and Hoskin 1983). A lthough butanol was themost abundant compound in the extract prepared byvacuum distil lation, the relative amounts of the com-ponents can vary with different isolation methods(Parliament 1986). For example, we have noticed thatbutyl acetate was the most abundant compound foundwhen headspace sampling was used (results not shown).

Quantitatively the important volatile components inRoyal Gala flavour are butanol, hexanol and the ace-tates. GCO was used to carry out a preliminary deter-mination of the more important contributors to RoyalGala aroma. During sensory profiling, terms thatemerged for describing the positive sensory attributes ofRoyal Gala apple were ‘red apple aroma’, ‘overallaroma’, ‘sweet aroma’, ‘acidic aroma, ‘red apple flavour’,‘acidic flavour’, ‘sweet flavoui;’ and ‘characteristic appleflavour’. ‘Red apple’ as a descriptive term describes aperception of apple aroma and flavour that differen-tiates from other apple flavour perceptions such as‘green apple’ associated with #GrannySmith and ‘woodyapple’ associated with Red Delicious. Through dis-cussions with the panel it was found that the ‘red applearoma’ and ‘red apple flavour’ terms were stronglyassociated with Golden Deliicious apple by panellists,and not with the more traditional red apple varietiessuch as Red Delicious. Indeed ethyl 2-methylbutanoate,which was shown by Flath el: a1 (1967) to be an impor-tant contributor to the flavour of Red Delicious applejuice, was not detected in Royal Gala flavour volatiles.A lthough one of the descriptors included in the GCO

analysis was ‘red apple’, the data did not reveal anyobvious contributors to ‘red a.pple aroma’.

Analytical sensory panel data

The levels of the added compounds approximate thosefound in fresh fruit. It is difficult to precisely matchlevels in the test solutions with those present during theactual consumption of fresh apple. The flavour volatilesappear to be produced over a period of time whentissues have been disrupted eg during the chewingprocess and as noted above the extracted amounts aresensitive to the method and duration of extraction.

As expected the added cornpounds caused the great-est changes with the aroma attributes. Of the four com-

pounds selected for study, butanol and 2-methylbutylacetate had the greatest effects. However similar effectswere echoed in the corresponding flavour attributes,although to a smaller extent. Thus the increasing trendin ‘overall aroma’ caused by 2-methylbutyl acetate andbutanol (c 15 units) (Fig la) is also found in ‘overallflavour’ (c 5 units) (Fig le). A similar trend wasobserved between ‘red apple aroma’ and ‘red appleflavour’ (Fig l b and If).

Butyl acetate, which is quantitatively one of themajor components found in the flavour volatiles ofRoyal Gala apple, did not increase the aroma or flavourattribute intensities with increasing levels. In fact itspresence decreased ‘red apple aroma’ and ‘sweet aroma’intensities. This compound is a reasonably powerfulodorant (Flath et a1 1967; Larson and Poll 1992) andwas one of the compound targeted by GCO analysis asa possible contributor to the aroma. Butyl acetate was

not considered to have apple-like quality when assess-ing Delicious apple essence (Flath et a1 1967) and wasnot one of the components of a synthetic apple odourdeveloped by Durr and Rothlin (1981). It is howeverconsidered as one of the important contributors to theflavour of Cox’s Orange Pippin apple (Williams andKnee 1977). In a study by Knee and Hatfield (1976) theratio of butanol to butyl acetate was c 0.08 and theratio of 2- or 3-methylbutyl acetate to butyl acetate wasc 0.02, in the equilibrium headspace over Cox’s OrangePippin apple discs. A lthough not directly comparabledue to different sampling methods, the butanol :butylacetate and butanol :2-methylbutyl acetate ratios for

Royal Gala apple were 3.0 and 0.20, respectively. Usingthe 10 ml total headspace and the average weight of anapple disc as 0.15g (K nee and Hatfield 1976) theamount of butyl acetate in Cox’s Orange Pippin wouldbe 12.5 pg g-’, compared with 13.1 pg g-’ found forRoyal Gala, indicating that the two cultivars may bespecifically different in butanol and 2-methylbutylacetate production. This information would suggest thatthe perceived aroma and flavour of Cox’s OrangePippin apple would be quite distinct from that of RoyalGala apple. Because of the butanol/butyl acetate inter-action and butanol is one of the more abundant com-ponents of the flavour volatiles in Royal Gala apple,

butyl acetate is not expected to contribute significantlyto the sensory attributes.

Overall, increasing levels of hexyl acetate, 2-methylbutyl acetate and butanol resulted in increasingintensities of ‘red apple aroma’ and ‘red apple flavour’.These two attributes have been considered by panellistsas important sensory properties of Royal Gala apple.Hexyl acetate showed a downward trend between level2 and level 3. Since it is observed in the aroma andflavour attributes it is unlikely that this is a spuriousresult. In the absence of butanol , ‘red apple aroma’increased with increasing levels of hexyl acetate (75-101units) while in the absence of hexyl acetate ‘red apple



336 H Young et a1

aroma’ increased with increasing levels of butanol.

Becauseof this hexyl acetatebutan01 interaction and as

butanol and hexyl acetate are both found in the flavour

volatiles of Royal Gala apple, 2-methylbutyl acetate is

probably the most effective contributor to ‘red apple

aroma’. I t is noteworthy that this compound is the only

one which made a signif icant contribution to ‘character-

istic apple flavour’. T he alkyl moiety of 2-methylbutyl

acetate is branched while the other two acetates used in

this study are n-alkyl esters. 2-M ethylbutyl acetate is

the major component of ripe Bisbee Delicious apple

aroma and has been suggested as an indicator of

physiological maturity (M attheis et a1 1991).

T he increase in intensity of ‘sweet aroma’ and ‘sweet

flavour’ with increasing levels of 2-methylbutyl acetate

is consistent with the fact that aroma compounds can

affect sweet flavour perception in fruit. Narain and

T homas (1990) report that ethyl butanoate affected the

sweet flavour in peaches, ethyl acetate has been weaklycorrelated with sweetness in apple (Watada et a1 1981),

Binder and F lath (1989) found that methyl benzoate

contributed to the sweet aroma of guava. Frank et a1

(1989) found that strawberry odour enhanced the sweet-

ness of sucrose solutions. T he enhancing of ‘sweet

aroma’ and ‘sweet flavour’ by butanol is interesting as

alcohols do not appear to have been previously report-ed as having causal effects on sweetness perception.

Because of the hexyl acetatebutan01 and 2-methylbutylacetatebutan01 interactions found for ‘sweet flavour’

the contribution of butanol to ‘ sweet flavour’ intensityof Royal Gala apples is probably very small.

T he lack of effects by the four compounds on ‘acidaroma’ and ‘acid flavour’ attributes is not totally unex-pected since esters and alcohols have not been pre-viously associated with acid flavour notes. I t is also

possiblethat ‘acid flavour’ is strongly influenced by the

sugars and non-volatile acids.

CONCLUSIONS

T he compound that had the most effect on the sensory

attributes which are particularly important for Royal

Gala apple is 2-methylbutyl acetate. This compound

affected eight of the nine attributes. Butanol, the most

abundant component found in the volatile flavour

extract, also has a strong effect on the aroma and

flavour attributes. T he association of 2-methylbutyl

acetate, hexyl acetate and butanol with the ‘red apple’attributes confirm that ‘red apple’ as defined for Royal

Gala is quite different from that of Delicious apple. Thesuppressive effect of butyl acetate on perceived inten-

sities of the sensory attributes highlight the importance

of extending odour-port results by use of thorough

sensory testing.

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Dirinck P, De Pooter H, Wil laert G, Schamp N 1983 Applica-tion of a dynamic headspace procedure in fruit flavouranalysis. In: Analysis of Volatiles: Proceedings of an Inter-national Workshop, ed Schreier P. de Gruyter, Berlin,Germany, pp 381-400.

Durr P, Rothlin M 1981 Development of a synthetic applejuice odour. Lebensm Wiss Technol 14 313-314.

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