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Influence of low oxygen storage on aroma compounds of whole pears and crushed pear flesh
Postharvest Biology and Technology 19 (2000) 279 – 285 www.elsevier.com/locate/postharvbio
Short communication
Influence of low oxygen storage on aroma compounds of whole pears and crushed pear flesh Christian Chervin a,*, Jim Speirs b, Brian Loveys b, Brian D. Patterson c a
CSIRO, Plant Industry, at IHD, Pri6ate Bag 15, South East Mail Centre 3176, Melbourne, Australia b CSIRO, Plant Industry, Horticulture Unit, P.O. Box 350, Glen Osmond, SA 5064, Australia c CSIRO, Plant Industry and Central Coast Campus, Uni6ersity of Newcastle, Brush Rd, Ourimbah, NSW 2258, Australia Received 6 September 1999; accepted 6 March 2000
Abstract Controlled atmosphere storage is known to decrease pome fruit aroma. Here results are presented showing that low O2 storage (3 kPa) for 2 months reduced character impact compounds of ‘Packham’s Triumph’ pears, namely methyl and ethyl decadienoates, during subsequent ripening. This reduction was detected in both whole pears and crushed pear flesh, used here to approximately reproduce mastication. The relative abundance of more than 20 volatile compounds is presented. Analyses after grouping the results by alcohol or acyl moiety suggest that the synthesis of the decadienoate moiety was more depressed than the methyl or ethyl moieties. Other esters were less abundant after low O2 storage, particularly various acetates (butyl, hexyl, heptyl). Their reduction was particularly revealed in crushed flesh which had been incubated at 40°C. Phenyl ethanol and phenylethyl acetate levels were also reduced after low O2 storage. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Low oxygen storage; ‘Packham’s Triumph’ pears
1. Introduction Most studies on aroma changes following controlled atmosphere (CA) storage have been performed with apples, and have been motivated by consumer concern about CA fruit lacking aroma. * Corresponding author. Present address: Ecole Nationale Supe´rieure Agronomique de Toulouse, BP 107, 31326 Castanet, France. E-mail address: [email protected] (C. Chervin).
These studies have shown decreased amounts of esters after prolonged low O2 storage and subsequent ripening under ambient conditions (Patterson et al., 1974; Brackmann et al., 1993; Fellman et al., 1993). It has been suggested that this phenomenon is linked to reduced oxidation of lipids and a consequent lack of precursors to these esters (Williams and Knee, 1977; Brackmann et al., 1993; Fellman et al., 1993). During a study on pear alcohol dehydrogenase (Chervin et al., 1999), we noticed slight decreases
0925-5214/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 5 2 1 4 ( 0 0 ) 0 0 0 9 6 - X
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C. Cher6in et al. / Posthar6est Biology and Technology 19 (2000) 279–285
in aroma of ripe ‘Packham’s Triumph’ pears even after relatively short-term storage under low O2 (2 months at 3 kPa O2) when compared to the air control. The character impact compounds of pears are known to be methyl and ethyl esters of 2,4-decadienoic acids, levels of these compounds steadily increasing over the climacteric phase (Heinz et al., 1965). In this paper, we examine the effects of low O2 storage on the levels of these decadienoate esters in pears, and on a wide range of other pear aroma compounds. Suwanagul and Richardson (1998) have published a comprehensive study of whole pear aroma. Our results complement that study by examining the effects of CA storage on the relative abundances of volatile compounds evolving from whole fruit and homogenised accessory tissue (pear flesh). This is designed as a tentative reproduction of the volatiles emitted during mastication.
2. Material and methods
2.1. Plant material Pears (Pyrus communis L., cv Packham’s Triumph) were picked from a local orchard at the commercial ripeness stage (firmness around 65 N) and transferred to cold storage on the day of harvest under conditions described previously (Chervin et al., 1999). Briefly, pears were stored for 2 months at 0°C, under air (treatment called ‘Air’), or under hypoxia at 3 kPa O2 (treatment called ‘Hyp’), or under air and transferred to 3 kPa O2 for the last 3 days before removal from cold storage (treatment called ‘Air+Hyp’). After removal from cold storage, pears were transferred to 20°C under air (day 0) and allowed to ripen.
2.2. Sampling procedures Fruit were sampled on days 11 – 14 to be assessed for aroma and grouped for analysis (representing a total of four replicates per treatment, one per day). To limit variation over sampling days, care was taken to choose fruit reaching a similar state of maturity, fruit which were turning from pale green to yellow (visual assessment). All
fruit had softened below the penetrometer lower limit (9.8 N), but screening for similar firmness was not possible as it would have wounded the fruit, changing the balance of aroma measured on whole fruit. Each day, one sample of three whole fruit was weighed, and enclosed in a 1-litre glass jar and incubated for 1 h during which headspace volatiles were absorbed onto a SPME fiber (65 mm, Carbowax-divinylbenzene, Supelco, Bellefonte, PA) (Song et al., 1997). After incubation, the SPME fiber was removed and the absorbed sample was analysed by gas chromatography (see below). The pears were removed from the glass jar and peeled and chopped into small quarters. An average of 3.3 g of each pear (totalling 10 g for the three pears) was homogenised for 30 s in a Polytron homogeniser after addition of an internal standard of uniformly-labelled deuterated hexanol (1 ml at a concentration of 80 nmol ml − 1). After 3 min incubation at room temperature, to simulate the development of volatiles in masticated tissue, further enzyme activity was inhibited by the addition of 3.3 g of CaCl2 and homogenization of the mixture for an additional 10 s (Buttery et al., 1987; Speirs et al., 1998). Then 5.5 g of this slurry was transferred into a 10-ml headspace vial sealed with a silicon/Teflon septum, and the vial was incubated at 40°C for 30 min. A SPME fiber (as above) was inserted into the vial and volatiles in the headspace of the vial were absorbed onto the fiber during a further 30-min incubation period at 40°C.
2.3. GC analysis Absorbed samples were analysed on a gas chromatograph (series 6890, Hewlett Packard) fitted with a capillary DB-wax column (30 m× 0.25 mm i.d.× 0.25 mm, J & W Scientific, Folsom, CA) and individual peaks were identified by reference to authentic spectra derived from a NIST mass spectra database (Speirs et al., 1998).
2.4. Data analysis Peak areas were measured by integration and were normalized against the internal deuterated hexanol standard for homogenised samples. As an
C. Cher6in et al. / Posthar6est Biology and Technology 19 (2000) 279–285
indication, the overall mean area of deuterated hexanol peaks was 5836203 which corresponds to 34 nmol present in the slurry placed in the incubation vial. In the ‘whole pear’ experiment (without deuterated hexanol), the areas were normalised against a mean sample of 806 g, which was the overall mean weight of the three fruit in each experiment. ANOVAs with the storage atmosphere (‘Air’, ‘Hyp’ and ‘Air+Hyp’) as the factor were run separately for each compound or by grouping them by alcohol (e.g. all compounds with an ethyl moiety) or acyl (e.g. all decadienoates) using SigmaStat (v 2.0, SPSS Inc., Chicago).
3. Results and discussion Low O2 storage for 2 months under 3 kPa O2 (treatment ‘Hyp’) was sufficient to decrease significantly (P B0.05) the levels of ethyl decadienoate emitted by whole pears (Fig. 1) and by crushed flesh (Fig. 2) relative to the ‘Air’ and ‘Air +Hyp’. The levels of ethyl decadienoate did not differ significantly between the ‘Air’ and ‘Air +Hyp’ tissues. The levels of methyl decadienoate were also lower in ‘Hyp’ samples than with ‘Air’ and ‘Air+Hyp’, but the differences were not significant. As these compounds have been reported to be character impact compounds (Heinz et al., 1965), this may explain why we noticed a decrease in aroma intensity in ‘Hyp’ samples. However a proper sensory analysis would be needed to confirm this trend. When the area results were grouped according to the alcohol or acyl precursors, neither methyl nor ethyl groups differed significantly among the three treatments (P \0.05), whereas decadienoates were significantly lower in ‘Hyp’ samples than in ‘Air’ or ‘Air+Hyp’ (PB 0.05). This suggests that the observed difference was due to lower synthesis of the decadienoate moiety. One potential source of the decadienoate moiety could be a lipoxygenase showing both dioxygenase and fatty acid lyase activities, degrading linoleic acid into trans-2, cis4-decadienal (Andrianarison and Beneytout, 1992). This step, which requires O2, could be reduced by low O2 storage. The next step to the
281
ester (presumably via the acylCoA derivative) is also oxidative. The in vitro alcohol dehydrogenase (ADH) activity was twofold higher in ‘Hyp’ and ‘Air+ Hyp’ samples than in ‘Air’ samples, as reported previously from day 2 to day 12 of the ripening period at 20°C (Chervin et al., 1999). Thus the differences in ethyl decadienoate levels observed between ‘Hyp’ and ‘Air+ Hyp’ samples cannot be ascribed to a difference in ADH activity during the early days of the ripening period. This confirms that it is more likely that the synthesis of the decadienoate moiety was depressed. In the crushed pear experiment (Fig. 2), the ‘Hyp’ treatment resulted in significant reduction in the levels of two additional esters, heptyl acetate (PB 0.05) and phenyl-ethyl acetate (P B 0.05), both naturally of low abundance. Levels of two of the more abundant esters, butyl acetate and hexyl acetate, were also reduced by ‘Hyp’ treatment, but not significantly in this experiment. This suggests that metabolism leading to acetate esters, such as the activity of acetyl CoA alcohol transferase, could be depressed by low O2, as previously reported in apples (Fellman et al., 1993). It is noticeable that phenyl-ethyl acetate and phenyl-ethyl alcohol levels were decreased (PB 0.05) by low O2 storage (Fig. 2). The likely precursor here is phenylalanine through its transaminated product phenylpyruvate. This requires reduction to phenylethanol, not oxidation, and more work is necessary to elucidate this point. From Table 1 it is evident that esters other than acetates are located mainly in the pear skin, as they were not detected in the crushed flesh which lacked the skin. Low O2 storage significantly reduced levels of two terpenes, a-farnesene (E,Z) and undecatetraene (Fig. 1). The levels of a-farnesene (E,E) and copaene were also reduced after low O2 storage but this was not significant. The fact that low levels of a-farnesene (E,E) were found in pear flesh (when its recognised location is the skin) suggests cross-contamination of flesh samples during cutting and peeling.
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C. Cher6in et al. / Posthar6est Biology and Technology 19 (2000) 279–285
In conclusion, we emphasise that, although 3 kPa O2 is not very low (commercial storage is often around 2 kPa O2 and below), and that 2 months
is not a very long storage period, the low O2 storage conditions used here were sufficient to induce significant decreases in pear aroma compounds.
Fig. 1. Volatile compounds emitted by whole ‘Packham’s Triumph’ pears following storage at 0°C, under various atmospheres, plus an average of 12.5 days of ripening at 20°C. In the compound names ‘Me’ replaces methyl, ‘Et’ is for ethyl, and ‘decadienoa’ is for decadienoate. The figure is split in two parts, the lower showing greater details of small peaks after removal of the four largest peak groups. Error bars show S.E., n= 4, *ANOVA with PB 0.05.
C. Cher6in et al. / Posthar6est Biology and Technology 19 (2000) 279–285
283
Fig. 2. Volatile compounds emitted by crushed flesh of ‘Packham’s Triumph’ pears following storage at 0°C, under various atmospheres, plus an average of 12.5 days of ripening at 20°C. In the compound names ‘Me’ replaces methyl, ‘Et’ is for ethyl, ‘Phe’ is for phenyl and ‘decadienoa’ is for decadienoate. The figure is split in two parts, the lower showing greater detail of small peaks after removal of the four largest peak groups. Error bars show S.E., n = 4, *ANOVA with P B0.05.
Acknowledgements We thank the Australian Apple and Pear
Growers Association and the Horticultural Research and Development Corporation for research grant AP 9520, partly used for this study.
284
C. Cher6in et al. / Posthar6est Biology and Technology 19 (2000) 279–285
Table 1 Identification relevance of volatile compounds of ‘Packham’s Triumph’ pearsa Compound name
Retention time (min)
Ion match (%)
Referenced before
From whole pears and ground pear flesh Butyl acetate Pentyl acetate Hexyl acetate 1-Hexanol Heptyl acetate 1-Octanol a-Farnesene (E,E) Methyl-2,4-trans-cis decadienoate Ethyl-2,4-trans-cis decadienoate
6.05 7.77 9.29 10.43 10.62 12.76 14.68 15.14 15.55
83 83 78 74 83 90 83 98 99
ab ab ab ab ab ab ab ab ab
From whole pears mainly Butyl butanoate Ethyl hexanoate Methyl octanoate Butyl hexanoate Hexyl butanoate Ethyl octanoate Octyl acetate Copaene Methyl decanoate Hexyl hexanoate Methyl decenoate Ethyl decanoate a-Farnesene (E,Z) Undecatetraene
8.48 8.72 10.82 11.11 11.13 11.36 11.83 12.05 13.13 13.29 13.45 13.57 14.41 16.02
90 91 91 91 83 94 86 98 95 91 91 96 81 45
ab ab ab ab ab ab ab c ab ab ab ab a d
From ground pear flesh mainly Hexanal 2-Hexenal (E) 6-Methyl-5-hepten-2-one Nonanal 2-Octenal (E) Acetic acid 6-Methyl-5-hepten-2-ol Phenyl-ethyl acetate Phenyl-ethyl alcohol Dodecanol Nonanoic acid
6.17 8.5 10.18 10.86 11.33 11.58 11.77 15.35 16.22 16.62 18.28
86 90 94 90 72 91 80 90 95 95 95
a ab a d c b d a b c d
a The compounds appearing mainly in one category were absent from the other or had too small an area to be identified. The ‘ion match’ figure gives an idea of the certainty of the identification. When marked ‘a’ the compounds have been previously reported in pears according to Suwanagul and Richardson (1998), or ‘b’ according to Maarse and Visscher (1989). If they were not, then reports were searched for in CAB abstracts (1989–1999) and are marked ‘c’ when found in relation to those compounds in Pyrus fruit or ‘d’ in other plants.
References Andrianarison, R.H., Beneytout, J.L., 1992. Appearance of new lipoxygenase isozymes during seedling germination of Vicia sati6a. Plant Sci. 87, 143–150. Brackmann, A., Streif, J., Bangerth, F., 1993. Relationship
between a reduced aroma production and lipid metabolism of apples after long-term controlled-atmosphere storage. J. Am. Soc. Hort. Sci. 118, 243 – 247. Buttery, R., Teranishi, R., Ling, L.C., 1987. Fresh tomato aroma volatiles: A quantitative study. J. Agric. Food Chem. 35, 540 – 544.
C. Cher6in et al. / Posthar6est Biology and Technology 19 (2000) 279–285 Chervin, C., Speirs, J., Truett, J.K., 1999. Expression of alcohol dehydrogenase in ripening pears and associated alcohol production. J. Am. Soc. Hort. Sci. 124, 71–75. Fellman, J.K., Mattinson, D.S., Bostick, B.C., Mattheis, J.P., Patterson, M.E., 1993. Ester biosynthesis in ‘Rome’ apples subjected to low-oxygen atmospheres. Postharvest Biol. Technol. 3, 201 – 214. Heinz, D.E., Creveling, R.K., Jennings, W.G., 1965. Direct determination of aroma compounds as an index of pear maturity. J. Food Sci. 30, 641–643. Maarse, H., Visscher, C.A., 1989. Pear (Pyrus communis L.). In: Maarse, H., Visscher, C.A. (Eds.), Volatile Compounds in Food: Qualitative and Quantitative Data, vol. I. Products 1-66. TNO-CIVO Food Analysis Institute, Zeist, The Netherlands, pp. 172 –175. Patterson, B.D., Hatfield, G.S., Knee, M., 1974. Residual effects of controlled atmosphere storage on the production
.
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of volatiles compounds by two varieties of apples. J. Sci. Food Agric. 25, 843 – 849. Song, J., Gardner, B.D., Holland, J.F., Beaudry, R.M., 1997. Rapid analysis of volatile flavor compounds in apple fruit using SPME and GC/time-of-flight mass spectrometry. J. Agric. Food Chem. 45, 1801 – 1807. Speirs, J., Lee, E., Holt, K., Kim, Y.-D., Scott, N.S., Loveys, B., Schuch, W., 1998. Genetic manipulation of alcohol dehydrogenase levels in ripening tomato fruit affects the balance of some flavor aldehydes and alcohols. Plant Physiol. 117, 1047 – 1058. Suwanagul, A., Richardson, D.G., 1998. Identification of headspace volatile compounds from different pear (Pyrus communis L.) varieties. Acta Hort. 475, 605 – 623. Williams, A.A., Knee, M., 1977. The flavour of Cox’s Orange Pippin apples and its variation with storage. Ann. Appl. Biol. 87, 127 – 131.
Short communication
Influence of low oxygen storage on aroma compounds of whole pears and crushed pear flesh Christian Chervin a,*, Jim Speirs b, Brian Loveys b, Brian D. Patterson c a
CSIRO, Plant Industry, at IHD, Pri6ate Bag 15, South East Mail Centre 3176, Melbourne, Australia b CSIRO, Plant Industry, Horticulture Unit, P.O. Box 350, Glen Osmond, SA 5064, Australia c CSIRO, Plant Industry and Central Coast Campus, Uni6ersity of Newcastle, Brush Rd, Ourimbah, NSW 2258, Australia Received 6 September 1999; accepted 6 March 2000
Abstract Controlled atmosphere storage is known to decrease pome fruit aroma. Here results are presented showing that low O2 storage (3 kPa) for 2 months reduced character impact compounds of ‘Packham’s Triumph’ pears, namely methyl and ethyl decadienoates, during subsequent ripening. This reduction was detected in both whole pears and crushed pear flesh, used here to approximately reproduce mastication. The relative abundance of more than 20 volatile compounds is presented. Analyses after grouping the results by alcohol or acyl moiety suggest that the synthesis of the decadienoate moiety was more depressed than the methyl or ethyl moieties. Other esters were less abundant after low O2 storage, particularly various acetates (butyl, hexyl, heptyl). Their reduction was particularly revealed in crushed flesh which had been incubated at 40°C. Phenyl ethanol and phenylethyl acetate levels were also reduced after low O2 storage. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Low oxygen storage; ‘Packham’s Triumph’ pears
1. Introduction Most studies on aroma changes following controlled atmosphere (CA) storage have been performed with apples, and have been motivated by consumer concern about CA fruit lacking aroma. * Corresponding author. Present address: Ecole Nationale Supe´rieure Agronomique de Toulouse, BP 107, 31326 Castanet, France. E-mail address: [email protected] (C. Chervin).
These studies have shown decreased amounts of esters after prolonged low O2 storage and subsequent ripening under ambient conditions (Patterson et al., 1974; Brackmann et al., 1993; Fellman et al., 1993). It has been suggested that this phenomenon is linked to reduced oxidation of lipids and a consequent lack of precursors to these esters (Williams and Knee, 1977; Brackmann et al., 1993; Fellman et al., 1993). During a study on pear alcohol dehydrogenase (Chervin et al., 1999), we noticed slight decreases
0925-5214/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 5 2 1 4 ( 0 0 ) 0 0 0 9 6 - X
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C. Cher6in et al. / Posthar6est Biology and Technology 19 (2000) 279–285
in aroma of ripe ‘Packham’s Triumph’ pears even after relatively short-term storage under low O2 (2 months at 3 kPa O2) when compared to the air control. The character impact compounds of pears are known to be methyl and ethyl esters of 2,4-decadienoic acids, levels of these compounds steadily increasing over the climacteric phase (Heinz et al., 1965). In this paper, we examine the effects of low O2 storage on the levels of these decadienoate esters in pears, and on a wide range of other pear aroma compounds. Suwanagul and Richardson (1998) have published a comprehensive study of whole pear aroma. Our results complement that study by examining the effects of CA storage on the relative abundances of volatile compounds evolving from whole fruit and homogenised accessory tissue (pear flesh). This is designed as a tentative reproduction of the volatiles emitted during mastication.
2. Material and methods
2.1. Plant material Pears (Pyrus communis L., cv Packham’s Triumph) were picked from a local orchard at the commercial ripeness stage (firmness around 65 N) and transferred to cold storage on the day of harvest under conditions described previously (Chervin et al., 1999). Briefly, pears were stored for 2 months at 0°C, under air (treatment called ‘Air’), or under hypoxia at 3 kPa O2 (treatment called ‘Hyp’), or under air and transferred to 3 kPa O2 for the last 3 days before removal from cold storage (treatment called ‘Air+Hyp’). After removal from cold storage, pears were transferred to 20°C under air (day 0) and allowed to ripen.
2.2. Sampling procedures Fruit were sampled on days 11 – 14 to be assessed for aroma and grouped for analysis (representing a total of four replicates per treatment, one per day). To limit variation over sampling days, care was taken to choose fruit reaching a similar state of maturity, fruit which were turning from pale green to yellow (visual assessment). All
fruit had softened below the penetrometer lower limit (9.8 N), but screening for similar firmness was not possible as it would have wounded the fruit, changing the balance of aroma measured on whole fruit. Each day, one sample of three whole fruit was weighed, and enclosed in a 1-litre glass jar and incubated for 1 h during which headspace volatiles were absorbed onto a SPME fiber (65 mm, Carbowax-divinylbenzene, Supelco, Bellefonte, PA) (Song et al., 1997). After incubation, the SPME fiber was removed and the absorbed sample was analysed by gas chromatography (see below). The pears were removed from the glass jar and peeled and chopped into small quarters. An average of 3.3 g of each pear (totalling 10 g for the three pears) was homogenised for 30 s in a Polytron homogeniser after addition of an internal standard of uniformly-labelled deuterated hexanol (1 ml at a concentration of 80 nmol ml − 1). After 3 min incubation at room temperature, to simulate the development of volatiles in masticated tissue, further enzyme activity was inhibited by the addition of 3.3 g of CaCl2 and homogenization of the mixture for an additional 10 s (Buttery et al., 1987; Speirs et al., 1998). Then 5.5 g of this slurry was transferred into a 10-ml headspace vial sealed with a silicon/Teflon septum, and the vial was incubated at 40°C for 30 min. A SPME fiber (as above) was inserted into the vial and volatiles in the headspace of the vial were absorbed onto the fiber during a further 30-min incubation period at 40°C.
2.3. GC analysis Absorbed samples were analysed on a gas chromatograph (series 6890, Hewlett Packard) fitted with a capillary DB-wax column (30 m× 0.25 mm i.d.× 0.25 mm, J & W Scientific, Folsom, CA) and individual peaks were identified by reference to authentic spectra derived from a NIST mass spectra database (Speirs et al., 1998).
2.4. Data analysis Peak areas were measured by integration and were normalized against the internal deuterated hexanol standard for homogenised samples. As an
C. Cher6in et al. / Posthar6est Biology and Technology 19 (2000) 279–285
indication, the overall mean area of deuterated hexanol peaks was 5836203 which corresponds to 34 nmol present in the slurry placed in the incubation vial. In the ‘whole pear’ experiment (without deuterated hexanol), the areas were normalised against a mean sample of 806 g, which was the overall mean weight of the three fruit in each experiment. ANOVAs with the storage atmosphere (‘Air’, ‘Hyp’ and ‘Air+Hyp’) as the factor were run separately for each compound or by grouping them by alcohol (e.g. all compounds with an ethyl moiety) or acyl (e.g. all decadienoates) using SigmaStat (v 2.0, SPSS Inc., Chicago).
3. Results and discussion Low O2 storage for 2 months under 3 kPa O2 (treatment ‘Hyp’) was sufficient to decrease significantly (P B0.05) the levels of ethyl decadienoate emitted by whole pears (Fig. 1) and by crushed flesh (Fig. 2) relative to the ‘Air’ and ‘Air +Hyp’. The levels of ethyl decadienoate did not differ significantly between the ‘Air’ and ‘Air +Hyp’ tissues. The levels of methyl decadienoate were also lower in ‘Hyp’ samples than with ‘Air’ and ‘Air+Hyp’, but the differences were not significant. As these compounds have been reported to be character impact compounds (Heinz et al., 1965), this may explain why we noticed a decrease in aroma intensity in ‘Hyp’ samples. However a proper sensory analysis would be needed to confirm this trend. When the area results were grouped according to the alcohol or acyl precursors, neither methyl nor ethyl groups differed significantly among the three treatments (P \0.05), whereas decadienoates were significantly lower in ‘Hyp’ samples than in ‘Air’ or ‘Air+Hyp’ (PB 0.05). This suggests that the observed difference was due to lower synthesis of the decadienoate moiety. One potential source of the decadienoate moiety could be a lipoxygenase showing both dioxygenase and fatty acid lyase activities, degrading linoleic acid into trans-2, cis4-decadienal (Andrianarison and Beneytout, 1992). This step, which requires O2, could be reduced by low O2 storage. The next step to the
281
ester (presumably via the acylCoA derivative) is also oxidative. The in vitro alcohol dehydrogenase (ADH) activity was twofold higher in ‘Hyp’ and ‘Air+ Hyp’ samples than in ‘Air’ samples, as reported previously from day 2 to day 12 of the ripening period at 20°C (Chervin et al., 1999). Thus the differences in ethyl decadienoate levels observed between ‘Hyp’ and ‘Air+ Hyp’ samples cannot be ascribed to a difference in ADH activity during the early days of the ripening period. This confirms that it is more likely that the synthesis of the decadienoate moiety was depressed. In the crushed pear experiment (Fig. 2), the ‘Hyp’ treatment resulted in significant reduction in the levels of two additional esters, heptyl acetate (PB 0.05) and phenyl-ethyl acetate (P B 0.05), both naturally of low abundance. Levels of two of the more abundant esters, butyl acetate and hexyl acetate, were also reduced by ‘Hyp’ treatment, but not significantly in this experiment. This suggests that metabolism leading to acetate esters, such as the activity of acetyl CoA alcohol transferase, could be depressed by low O2, as previously reported in apples (Fellman et al., 1993). It is noticeable that phenyl-ethyl acetate and phenyl-ethyl alcohol levels were decreased (PB 0.05) by low O2 storage (Fig. 2). The likely precursor here is phenylalanine through its transaminated product phenylpyruvate. This requires reduction to phenylethanol, not oxidation, and more work is necessary to elucidate this point. From Table 1 it is evident that esters other than acetates are located mainly in the pear skin, as they were not detected in the crushed flesh which lacked the skin. Low O2 storage significantly reduced levels of two terpenes, a-farnesene (E,Z) and undecatetraene (Fig. 1). The levels of a-farnesene (E,E) and copaene were also reduced after low O2 storage but this was not significant. The fact that low levels of a-farnesene (E,E) were found in pear flesh (when its recognised location is the skin) suggests cross-contamination of flesh samples during cutting and peeling.
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C. Cher6in et al. / Posthar6est Biology and Technology 19 (2000) 279–285
In conclusion, we emphasise that, although 3 kPa O2 is not very low (commercial storage is often around 2 kPa O2 and below), and that 2 months
is not a very long storage period, the low O2 storage conditions used here were sufficient to induce significant decreases in pear aroma compounds.
Fig. 1. Volatile compounds emitted by whole ‘Packham’s Triumph’ pears following storage at 0°C, under various atmospheres, plus an average of 12.5 days of ripening at 20°C. In the compound names ‘Me’ replaces methyl, ‘Et’ is for ethyl, and ‘decadienoa’ is for decadienoate. The figure is split in two parts, the lower showing greater details of small peaks after removal of the four largest peak groups. Error bars show S.E., n= 4, *ANOVA with PB 0.05.
C. Cher6in et al. / Posthar6est Biology and Technology 19 (2000) 279–285
283
Fig. 2. Volatile compounds emitted by crushed flesh of ‘Packham’s Triumph’ pears following storage at 0°C, under various atmospheres, plus an average of 12.5 days of ripening at 20°C. In the compound names ‘Me’ replaces methyl, ‘Et’ is for ethyl, ‘Phe’ is for phenyl and ‘decadienoa’ is for decadienoate. The figure is split in two parts, the lower showing greater detail of small peaks after removal of the four largest peak groups. Error bars show S.E., n = 4, *ANOVA with P B0.05.
Acknowledgements We thank the Australian Apple and Pear
Growers Association and the Horticultural Research and Development Corporation for research grant AP 9520, partly used for this study.
284
C. Cher6in et al. / Posthar6est Biology and Technology 19 (2000) 279–285
Table 1 Identification relevance of volatile compounds of ‘Packham’s Triumph’ pearsa Compound name
Retention time (min)
Ion match (%)
Referenced before
From whole pears and ground pear flesh Butyl acetate Pentyl acetate Hexyl acetate 1-Hexanol Heptyl acetate 1-Octanol a-Farnesene (E,E) Methyl-2,4-trans-cis decadienoate Ethyl-2,4-trans-cis decadienoate
6.05 7.77 9.29 10.43 10.62 12.76 14.68 15.14 15.55
83 83 78 74 83 90 83 98 99
ab ab ab ab ab ab ab ab ab
From whole pears mainly Butyl butanoate Ethyl hexanoate Methyl octanoate Butyl hexanoate Hexyl butanoate Ethyl octanoate Octyl acetate Copaene Methyl decanoate Hexyl hexanoate Methyl decenoate Ethyl decanoate a-Farnesene (E,Z) Undecatetraene
8.48 8.72 10.82 11.11 11.13 11.36 11.83 12.05 13.13 13.29 13.45 13.57 14.41 16.02
90 91 91 91 83 94 86 98 95 91 91 96 81 45
ab ab ab ab ab ab ab c ab ab ab ab a d
From ground pear flesh mainly Hexanal 2-Hexenal (E) 6-Methyl-5-hepten-2-one Nonanal 2-Octenal (E) Acetic acid 6-Methyl-5-hepten-2-ol Phenyl-ethyl acetate Phenyl-ethyl alcohol Dodecanol Nonanoic acid
6.17 8.5 10.18 10.86 11.33 11.58 11.77 15.35 16.22 16.62 18.28
86 90 94 90 72 91 80 90 95 95 95
a ab a d c b d a b c d
a The compounds appearing mainly in one category were absent from the other or had too small an area to be identified. The ‘ion match’ figure gives an idea of the certainty of the identification. When marked ‘a’ the compounds have been previously reported in pears according to Suwanagul and Richardson (1998), or ‘b’ according to Maarse and Visscher (1989). If they were not, then reports were searched for in CAB abstracts (1989–1999) and are marked ‘c’ when found in relation to those compounds in Pyrus fruit or ‘d’ in other plants.
References Andrianarison, R.H., Beneytout, J.L., 1992. Appearance of new lipoxygenase isozymes during seedling germination of Vicia sati6a. Plant Sci. 87, 143–150. Brackmann, A., Streif, J., Bangerth, F., 1993. Relationship
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