J Sci Food Agric 1992, 58, 519-522

Identification of E-Hex-3-enal as an Important Contributor to the Off-flavour Aroma in Kiwifruit Juice Harry Young,* Conrad 0 Perera and Vivienne J Paterson DSIR Fruit and Trees, Mt Albert Research Centre, Private Bag, Auckland, New Zealand

(Received 2 August 1991 ; revised version received 8 October 1991 ; accepted 12 November 1991)

Abstract: The volatile components of the juice of kiwifruit (Actinidia deliciosa (A Chev) Liang et Ferguson var deliciosa cv Hayward) have been isolated by low temperature vacuum distillation followed by ether extraction of the aqueous distillate and by headspace sampling using Chromosorb 105 adsorption. Of the 73 compounds identified, E-hex-3-enal has been shown to be a major contributor to the ‘old cut grass’ and ‘hay’ aroma notes found in kiwifruit juice. Several free carboxylic acids have also been identified as components of the volatiles.

Key words : Kiwifruit, off-flavour, processing, juice, E-hex-3-ena1, sensory.

INTRODUCTION

The flavour volatiles of fresh kiwifruit (Actinidiu deliciosu (A Chev) Liang et Ferguson var deliciosa cv Hayward) grown in several countries have been studied by several groups. Cossa ef a1 (1988) investigated Italian-grown kiwifruit, and Bartley and Schwede (1989) investigated Australian-grown fruit. Young and Paterson (1990) reviewed the compounds found in earlier studies. Little has been published on the flavour volatiles of processed kiwifruit. Pfannhauser (1988) attributed the change in flavour between fresh kiwifruit and frozen kiwifruit puree to the presence of high levels of terpenoid acetates.

Kiwifruit juice, and other processed kiwifruit pro- ducts, have strong unpleasant aroma notes, some of which have been described as ‘hay’, ‘old cut grass’ and ‘cooked gooseberry’ (McMath K pers comm). Kiwifruit juice is frequently used as a minor component in blended fruit juices because of inherent problems with flavour, colour and aroma. With the increase in production of kiwifruit in New Zealand and other parts of the world, there is a growing demand for technology for manu- facturing high quality processed products.

We report here the results of our investigation of the possible causes of the unpleasant aroma in partially processed kiwifruit juice.

* To whom correspondence should be addressed.

EXPERIMENTAL

Two lines of fruit were separately investigated: fruit stored at 1°C for 5-6 months (1988 season); fruit stored at 1°C for less than 1 month (1989 season)

Preparation of kiwifruit juice

Preparation of the juice was based on the method described by Heatherbell et a1 (1990). Fruits, harvested at 6.5” Brix, were removed from storage and ripened by keeping at ambient temperature for 3-7 days. The fruits (average firmness 6.9 N, penetrometer with a 0.8 mm head) were washed and macerated in a food processor without peeling. Filter aid ( 2 YO Celite) was added to the pulp and allowed to stand approx-I0 min before the juice was expressed in a laboratory cloth and rack press. Pectinase (200 mg kg-l, Rohapect D5L, Rohm Enzymes, Germany) was added to the juice which was then kept overnight at 40°C. The mixture was filtered through Celite filter aid and the filtrate was used for analysis of flavour volatiles.

Extraction of the flavour volatiles

Two methods were used for recovery of flavour volatiles for each of the two juice preparations.

519 J Sci Food Agric 0022-5142/92/$05.00 0 1992 SCI. Printed in Great Britain

5 20 H Young, C 0 Pereru, V J Puterson

(1) Vacuum distillation was used for extraction of the bulk sample of the juice for use in ‘sniff analysis’ (monitoring of the GC effluent by nose) and for GC-MS identification. In a typical experiment the juice (800 ml) was distilled at 1 7-20°C/ 1.73 kPa to give 300 ml of distillate using the method described by Young et a1 (1983). The aqueous distillate was extracted in batches of 35 ml with purified diethyl ether ( 5 x 0.5 ml). The ether solution was reduced to between 0.5 and 1 ml using a micro-Dufton fractionating column followed by a slow static distillation technique (Blomberg and Roeraade 1988) to approx-35 pl.

(2) Headspace sampling of 1 g of juice into stainless steel traps filled with 100 mg of Chromosorb 105 (Young and Paterson 1985).

Gas chromatography (GC)

GC was carried out using DBWAX fused silica capillary columns (J and W Scientific, California, USA) and a flame ionisation detector (FID). For analytical work splitless injection (at 150°C) and a 30 m x 0.32 mm ID (0.5 pm film thickness) column was used with H, carrier gas at 30 cm s-’. The column temperature programme was 30°C hold 6 min, 3°C min-’ to 190”C, hold 20 min. For ‘sniff analysis’, performed by the authors, a gas chromatograph fitted with a FID, a 95: 5 outlet splitter (5 parts to FID) and sniffing port was used in conjunction with a 30m xO.5 mm ID column. Make-up gas (10 ml min-’ of N,) was added to the outlet end of the column. The carrier gas was H, at 30 cm s-’ and the temperature programme was 35”C, hold 6 min, 3°C min-’ to 19O”C, hold 20 min. Samples were introduced using a cool on-column injector. For semi-preparative GC, liquid-nitrogen-cooled glass capillary tubing (I mm ID) replaced the sniffing port.

For volatiles collected by adsorption on to Chromo- sorb 105 the samples were heat desorbed directly on to the column (Young and Paterson 1985).

Gas chromatography-mass spectrometry (GC-MS)

GC-MS was carried out on a VG-70SE (VG Analytical, Manchester, UK) magnetic sector mass spectrometer using electron impact ionisation. A capillary column similar to that used for analytical GC was coupled via a restrictor to the MS source. The carrier gas was He, otherwise conditions were essentially the same as that used for analytical GC.

Identification of compounds

Unless noted otherwise, compounds were identified by comparison of their mass spectra with library spectra

and/or those of standard compounds and their retention indices with those of the standard compounds.

RESULTS AND DISCUSSION

The intense ‘hay’ and ‘cooked gooseberry’ aroma was readily detected in the ether extracts of the aqueous distillate collected from the juice of stored (1988) fruit. Major components were ethyl acetate, E-hex-2-enal and E-hex-2-en-1-01 (Table 1).

Juice prepared from the 1989 fruit had a more appealing fruity aroma but the ‘hay’ and ‘cooked gooseberry’ notes were still present. The most obvious differences between the two juice preparations were the high relative levels of esters, predominately ethyl buta- noate, in juice prepared from 1989 fruit. The ratio of ethyl butanoate to E-hex-3-enal was > 180: 1, compared with a ratio of 1 :2.5 in the 1988 fruit. Despite the high level of ethyl butanoate, which has a fruity aroma and low odour threshold (Flath et a1 1967), it was not able to mask the ‘hay’ note. Thus processing the fruit im- mediately after harvest when the aroma volatiles contain high levels of esters is unlikely to solve the off-aroma problem.

Headspace sampling into Chromosorb 105 showed that highly volatile compounds were not lost during ether extraction/concentration or masked by the ether peak. Apart from peaks normally found with blank Chromosorb 105 traps, no peaks were found to be obscured by the solvent peak.

By monitoring the eluate during gas chromatography of the extracts at a ‘sniffing port’ we were able to attribute the ‘hay’ aroma note to E-hex-3-enal. A similar aroma note was detected when the ‘ sniff test’ was carried out on a reference sample of E-hex-3-enal at con- centrations similar to those of the extracts. We were not able to detect the ‘hay’ note in other regions of the chromatogram. In contrast the Z-isomer had a green note.

The ‘cooked gooseberry ’ note was detected just prior to the elution of E-hex-2-en- 1-01. GC-MS detected the presence of hexadienals in this region. However, a test with a similar concentration of hexa-2: 4-dienal standard did not demonstrate the ‘cooked gooseberry ’ character. The levels of compounds involved were too low to give definitive mass spectra. From a careful study of the single-ion chromatograms generated from full-scan MS data for this region of the chromatogram, it was obvious that one or more as yet unidentified compounds co- eluted.

A feature of the volatiles in the present investigation is the presence of free acids, previously identified in only one other study of kiwifruit flavour volatiles (Wheeler 1990). This may be due to better analytical methodology rather than the result of the juicing process, as GC

E- Hex-3-enal and of-flavour in kiwifruit juice 521

TABLE 1 List of chemicals identified in volatiles from kiwifruit juice"

K P Components Re1 amt' KI

897 912 924 95 1 970 977 987 972

101 1 1040 1046 I049 I062 1075 1083 1090 I I32 1136 1145 1147 1152 1 I61 1 I75 1 I75 1 I94 I I98 1212 1227 1245 1266 1296 1312 1327 1334 1336 1349 1369

1988 1989

Ethyl acetate Methanol 3-Methylbutanal Ethanol Ethyl Propanoate Ethyl 2-methylpropanoate Pen tanal Methyl butanoate Acetonitrile? Pent- I-en-3-one But-2-enal Ethyl butanoate Ethyl 2-methylbutanoate 2-Methylpent- l-en-3-one Butyl acetate Hexanal Propyl butanoate E-pent-2-enal Ethyl pentanoate E-Hex-3-enal Z-Hex-3-enal Butan- 1-01 Ethyl but-2-enoate Pent- I-en-3-01 Heptanal Methyl hexanoate Z-Hex-2-enal E-Hex-2-enal Ethyl hexanoate Pentan- 1-01 Octanal Ethyl Hex-3-enoate Pent-2-en-1-01 (ms) E-Hept-2-enal Cyclopentanol 6-Methylhept-5-en-2-one Hexan-1-01

19.74 Tr

0.42 1.53

0.13 0.04

0.05

0.29 0.03

Tr

5.08

0.74 0.10

1.02 I 0.03 0.49

37.92 0.19 0.30

0.04

0.56

3.86

10.9 0.64 0.0 1

1.88 0.08 0.18 0.88 0.08 0.13

28.93 0.05 0.0 1 0.02 2.09 0.0 1 0.13 0.08 016 002 0.19

0.46

0.03 0.0 1 0.42

13.80 0.63 0.17 0.02 0.1 1 0.02 0.16 0.27 008 1.38

25.7

1379 1399 1405 1414 1423 1441 1464 1471 1478 1500 1512 1507 1530 1536 1542 1548 1561 1573 1594 1625 1639 1639 1641 1658 1685 1753 1767 1866 1872 1962 1966 1984 2075 220 1 2292 2474

Rrl amt Components -

~

1988 1989

E-Hex-3-en- 1-01 Z-Hex-3-en- 1-01 Hexa-2,4-dienal (ms) E,E-Hexa-2,4-dienal (ms) E-Hex-2-en- 1-01 E-Oct-2-enal Acetic acid 2-Furaldehyde Hepta-2,4-dienal (ms) E,E-Hepta-2,4-dienal (ms) Formic acid 1 -Ethylcyclohexene? 2-Bornanone Benzaldeh yde E-Non-2-enal Ether contaminant 3,7-Dimethyl octa- 1,6-dien-3-01 Octan- 1-01 5-Methyl-2-furaldehyde (ms) Z-Hex-2-enyl butanoate Methyl benzoate Ethyl 2-furoate Butanoic acid E-Dec-2-enal Ethyl benzoate Pentanoic acid Undeca-2-enal Hexanoic acid Undec-5,9-dienoic acid? 2-Ethylhexanoic acid (ms) Heptanoic acid Hex-3-enoic acid Octanoic acid Nonanoic acid Decanoic acid Acid?

0.42 0.27 0.02 0.03

10.06 0.03 0.57 0.02 0.25

0.02

0.20 0.45

6.14 0.05 0.06 0.02

Tr

0.0 1 1 0.06

0.58 0.09

0.90

0.18 o.20 1 4.19 0.32 038 0.04

0.1 I 0.18

0.04 2.89 0.15 0.04 0.01 0.08 0.03

0.12 0.09 0.14 0.0 I

0.3 0.02 0.0 1 0.02

0.07

0.05 0.47

0.02 0.0 1 0.06

0.04

0.02 0.1 1 0.14 0.03 0.19

Isolated by Method 1 KI, Kovat indices

' Area YO (uncorrected) Tr = trace. ms = identified by MS only. ? = Tentative identification.

columns used in our earlier studies on fresh fruit were not adequate for analysing free acids.

Hex-3-enals have been only tentatively identified in New Zealand-grown kiwifruit (Paterson et a1 1991) but have been found in kiwifruit grown in California (Takeoka et a1 1986). E-Hex-3-enal has also been found in other fruit such as guava but its sensory property was not considered (Idstein and Schreier 1985; Binder and Flath 1989).

ACKNOWLEDGEMENT

This research was partially funded by the N Z Kiwifruit Marketing Board.

REFERENCES

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