J Sci Food Agric 1994,65, 11 1- 115 Comparative Study of the Aromatic Profiles of Two Different Plum Species : Prunus salicina Lindl and Prunus simonii L Encarna Gomez and Craig A Ledbetter* US Department of Agriculture, Agricultural Research Service, 202 1 South Peach Avenue, Fresno, California 93727-595 1, USA (Received 16 April 1993; revised version 16 November 1993; accepted 5 January 1994) Abstract: Volatile compounds of two plum species (Prunus salicina Lindl and Prunus simonii L) were analyzed by capillary gas chromatography-mass spec- trometry using simultaneous vacuum distillation extraction. Aroma patterns were identified and quantified. A total of 60 compounds were identified, of which 23 were unique to P simonii. The profile of P simonii, as well as a study of the odor unit values of some identified compounds, indicated a much stronger aroma than P salicina. Hexyl acetate, which produces a characteristic apple aroma, was present in P simonii at levels 50-fold higher than in the studied cultivars of P salicina. Key words: Prunus salicina, Prunus simonii, volatile compounds, plum, fruit breeding, aroma. INTRODUCTION tives include enhancing the flavor and aroma character- istics of selections already having large fruit size, high The genus Prunus encompasses numerous tree crops: skin color and firm flesh. almond, apricot, cherry, nectarine, peach and plum. Cultivars and plum species differ in appearance, Within this genus, the plums represent one group aroma and flavor. The sensory properties are very having a wide range of genetic diversity from which important to consumers and can be a determining traits of economic importance can be gleaned. Japanese factor in the development and release of new fruit culti- plums (P salicina Lindl) arrived in the US during the vars. To give plant breeders the information required 1880s (Weinberger 1975). Hybridizations between Japa- for these sensory attributes, it is important to known nese and native American plums at many US locations their chemical nature. have led to numerous cultivars being adapted to a wide Very little has been published on plum volatiles. The range of environmental conditions. first report was carried out by Forrey and Flath (1974) The Agricultural Research Service (ARS) has had an in P salicina var ‘Santa Rosa’. It was determined that active fruit breeding program in Fresno, California, for acetate esters predominated the volatile compounds of the past six decades. From this location, four Japanese plums, but there were also appreciable quantities of the plum cultivars-‘La Roda’, ‘Nubiana’, ‘Queen Ann’ and higher lactones. Based on the study of Williams and ‘RedRoy’-were named and released by the ARS in Ismail (1981), linalool and ethyl butanoate are very 1954. Since these initial cultivar releases, seven other important in the aroma of European plums (P Japanese plums, including ‘Blackamber’ and ‘Friar’, domestica). The studies of Horvat et a1 (1992) with have been made available to stone fruit growers. These hybrids of P salicina and P americana demonstrated two important cultivars account for over 17% of the that, for most of the studied cultivars, (E)-2-hexenal, bearing Japanese plum acreage currently planted in butyl acetate, butyl butanoate and y-dodecalactone California (Tippett et al 1992). Current breeding objec- were the major constituents. Comparative studies of headspace volatiles between Japanese plum, apricot and * To whom correspondence should be addressed. interspecific hybrids between them using intact, tree- 111 J Sci Food Agric 0022-5142/94/%09.000 1994 SCI. Printed in Great Britain 112 E Gbmez, C A Ledbetter ripe fruit samples indicate that aromatic profiles of the temperature of the detector (Hewlett-Packard 5971A studied cultivars ‘Blackamber’ and ‘Friar’ contained far mass selective detector) was 170°C. The spectra were fewer constituents than apricot or plum x apricot recorded at an ionization voltage of 70 eV, with a speed hybrids (Gomez and Ledbetter 1993). of 2.1 scan s-’ over a mass range of m 2-l 20-260. In order to meet the objectives of developing plum Data processing was performed with a Hewlett-Packard cultivars with enhanced quality characteristics, it is 5895 GC ChemStation. necessary to evaluate new germplasm as it becomes Sample components were identified by mass spectrum available and incorporate genotypes having unique or matching with a mass spectral library collection. The enhanced fruit characteristics into the breeding mass spectrum identifications were verified by compari- program. Organoleptic analysis of fruit from a P simonii son of the component’s experimental retention index clone during the 1992 fruit ripening season indicated a with that of authentic reference standards in most of the markedly more intense flavor when compared with cases, as indicated in Table 1. commercial cultivars of Japanese plums. Personnel evaluating this clone reported a strong apple flavor. This clone is productive and fruit size averages 80-90 g. RESULTS AND DISCUSSION The flesh is firm, orange-colored and crisp. This study reports on the volatile constituents present Table 1 presents the volatile constituents identified and in P simonii and a comparison with the profiles quantified in P simonii and two cultivars of P salicina obtained from two P salicina Lindl cultivars. (‘Blackamber’ and ‘Friar’). The quantification of these compounds has to be considered approximate because the response factor of all compounds has not been cal- EXPERIMENTAL culated and all responses are considered the same as the internal standard, 3-nonanone. Clonal accessions of ‘Blackamber’ and ‘Friar’ Japanese In both species a total of 12 alcohols, 6 hydrocar- plums and P simonii were cultivated at the ARS Horti- bons, 10 aldehydes, 7 ketones, 21 esters and 4 lactones cultural Crops Research Laboratory in Fresno, Califor- have been identified. Aldehydes and alcohols of six- nia. P simonii (PI 91527) was obtained from the carbon atoms were identified in both species. These National Clonal Germplasm Repository in Davis, Cali- compounds were hexanal, 2-hexenal, hexanol, 2-hexenol fornia. Trees were grown in a flood-irrigated orchard, and 3-hexenol. The presence of these compounds was and each clonal accession was grown on ‘Nemaguard’ due to the lipoxygenase activity, action initialized by the rootstock. disruption of the fruit tissues when it was blended Fresh, tree-ripened plums were collected and imme- (Frankel 1982). These C, compounds are important to diately stored at - 18°C until the analysis. All analyses plum aroma (Ismail et a1 1981) and contribute to the were performed within one month of harvest. The vola- green note of the fruit (Guichard et a1 1990). tile constituents were isolated using simultaneous Both P salicina cultivars had very similar profiles that vacuum distillation extraction. were quite different from P simonii. In each of the vola- The fruit was cut in half and the stones were removed tile constituent classes, specific compounds were identi- and discarded. The skin and pulp (500 g) were blended fied and quantified in P simonii that were not present in with 1.5 liters of HPLC grade water for 30 s. 3- either ‘Blackamber’ or ‘Friar.’ Of the seven identified Nonanone was used as an internal standard. The ketones, a-ionone appeared only in the profile of P mixture was added to a 5-liter round-bottomed flask. simonii. Among the alcohols, octanol, nonanol and gera- A modified Likens-Nickerson distillation extraction niol only appeared in P simonii. The number of hydro- head was used (Schultz et a1 1977; Takeoka et a1 1988). carbons present in P simonii was smaller than in P The fruit slurry was subjected to vacuum distillation salicina; however, esters were both qualitatively and extraction (60 mmHg) for 3 h with 60 ml of hexane. quantitatively higher in P simonii. Twelve of the 21 Residual water was removed by freezing the sample identified esters only appeared in P simonii. Also, the at -40°C. The hexane extract was then concentrated concentration of lactones, important for the fruity with a Vigreux column under reduced pressure aroma (Guichard et al 1990), was greater in P simonii. (200 mmHg) to a final volume of 0.3-0.4 ml. Esters are the main compounds responsible for the The concentrated extract was injected into a Hewlett- fruity aroma. Among the esters identified soley in P Packard 5890 gas chromatograph (Hewlett-Packard, simonii, butyl propanoate and 3-methyl- l-butyl acetate Avondale, PA). Separations were performed using a have a strong banana aroma and 2-methylpropyl buta- DB-5 MS column (30 m x 0.25 mm, J & W Scientific notate and octyl acetate have a pleasant fruity aroma. Folsom, CA). The oven temperature was programmed Some components were identified in both species but from 50 to 250°C (4°C min-’). Helium was used as a with great differences in their concentrations from one carrier gas and the average linear velocity was species to the other. These compounds were hexenal, 60 cm s-’. The injector was maintained at 200°C. The hexanal and hexanol, the esters butyl acetate, hexyl Study of the aromatic profiles of two plum species 113 TABLE 1 Concentration of the volatile compounds identified in the two plum species (pg kg-') Constituent Blackamber Friar P simonii Identification' Klb Ketones 2,2,6-Trimethyl-cyclohexanone' - 0.7' MS 1036 Isophorone 1.o 0.8 KI, MS 1059 Acetophenone - - KI, MS 1065 a-Ionone - 4.4 MS 1422 Dihydro-B-ionone 0.4 - KI, MS 1433 Geranylacetone 0.6 5.0 KI, MS 1449 B-Ionone 3.1 18.2 KI, MS 1482 Alcohols