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CHAPTER 22

The Flavor of Plums

E. GOMEZ-PLAZA1 and C. LEDBETTER2

University of Murcia2USDA

THE IMPORTANCE OF PLUM GROWING WORLDWIDE

For fresh and dry consumption, or for use as a flavoring, beverage, or condiment toaccompany other foods, plums are widely grown throughout the temperate regionsof the world. Of the pome and stone fruit crops, plums are second only to apples interms of orchard area, with over 2.1 million hectares planted worldwide (Table 22.1).The diversity of habitats where plum species grow naturally have probably assistedin the breeding and development of new varieties adapted to specific localities andenvironmental conditions. Hence, plums are now cultivated throughout the temper-ate zones of both the northern and southern hemispheres.

Worldwide plum production has increased dramatically during the last decade,with a majority of the increase coming from new Asian orchards. Specifically, Chinais the current world's leader in plum production and is responsible for nearly halfof the world's harvested tonnage. The total harvested area of Asian orchards hasincreased approximately 60% since 1996, while maintaining the same yield effi-ciency (Table 22.2). Harvested ton per hectare has similarly remained constant inAfrican orchards during the same time period, while increasing harvested area byapproximately 20%. European plum orchard area has decreased significantlysince 1996; however, the level of plum production per hectare has risen sharply(5.51MT/ha in 1996 vs. 7.85MT/ha in 2006). Romania and Serbia rank second andthird in world plum production with 2006 harvests of 598,753 and 556,227 MT,respectively. Throughout the world, there are 13 nations producing over 100,000 MTof plums annually. The highest overall plum yields per hectare are now achieved inNorth and South American orchards, but harvested orchard area in these regionshave declined during the last decade.

Plums vary widely in their flavor and aroma characteristics, and many plums havebeen developed to be used for specific purposes. Large volumes of plums are usedthroughout Central and Eastern Europe in the production of the popular "slivovitz"

Handbook of Fruit and Vegetable Flavors, Edited by Y. H. HuiCopyright © 2010 John Wiley & Sons, Inc.

415

416 HANDBOOK OF FRUIT AND VEGETABLE FLAVORS

TABLE 22.1. Pome and Stone Fruit Crops,Worldwide Production, 2006 (FAOSTAT Data 2008)

Crop Harvested Area (ha)

Apple 4,786,080Plum 2,154,196Pear 1,669,743Peach 1,448,047Apricot 474,942Sweet cherry 341,436Sour cherry 214,287Quince 63,106

TABLE 22.2. Plum Production and Harvested Orchard Area in the World's GrowingRegions, 1996 and 2006 (FAOSTAT Data 2008)

Production

WorldRegion

AfricaAmericasAsiaEuropeOceania

(MTx

1996

210120133933392

31

1000)

2006

244920

55052727

34

Area(ha

1996

31.6108.0

1037.0615.0

4.6

Harvestedx 1000)

2006

37.895.0

1670.0347.0

3.8

MT/ha

1996

6.6711.133.275.516.87

2006

6.469.703.297.858.76

or plum brandy (Satora and Tuszyriski 2008). Dried plums or "prunes" are producedfrom specific varieties with a high sugar content to ensure consumer palatability.Elevated sorbitol content is also of great importance in prune-type plums as a pre-ventative of excessive caramelization (product darkening) during the drying process(Cinquanta et al. 2002). The fresh plum industry in California orchards has reliedon "Japanese" type plums that ripen from May through the end of September, withdiverse skin and flesh colors. Dozens of varieties are currently grown for bothdomestic and export marketing. Postharvest treatments have been developed toincrease the length of the effective marketing periods by maintaining firmness, juici-ness, and flavor (Mitchell 1987; Salvador et al. 2003; Valero et al. 2002). Both culti-vated and native/wild plums are collected and preserved in a wide variety of forms(chutney, compote, glace, jam, pickling, etc.) for direct consumption or served as anaccompaniment to enhance the culinary experience.

THE IMPORTANT PLUM SPECIES AND THEIR CHARACTERISTICS

Plums are a large and botanically diverse group of stone fruits with over 2000 namedvarieties being selected from over a dozen different Prunus species. Currently, cul-tivated varieties are also diverse in their climactic preferences, as specific varietieshave adapted to various regions throughout the entire temperate fruit-growingzones. Horticultural and botanical characteristics vary greatly between the variousplum species. Numerous botanical characteristics are useful in distinguishingbetween various species, among them fruit color, size, and flavor of the fruit.

THE FLAVOR OF PLUMS 417

Based on original geographic distributions of plant habitats, plum germplasm canbe divided into three broad groups of species: (1) European-Asian, (2) NorthAmerican, and (3) Oriental. These groups can be further subdivided, based onvarious pomological characters. Morphological similarities of many plum speciesand interspecific hybridization between species in overlapping habitats have per-plexed both traditional taxonomists and molecular biologists for over a century(Hedrick et al. 1911; Rohrer et al. 2004; Shaw and Small 2005; Waugh 1901). Fromthe numerous plum species found throughout the world's temperate zones, Table22.3 lists only those of economic importance through marketing or use of their fruit.

TABLE 22.3. Grouping of Cultivated Plum Species Based on Geographic Distribution

GeographicGrouping Prunus Species

PomologicalGrouping Representative Varieties

European-Asian

American

Oriental

P. domestica L.

P. domestica var.insititia Bailey

P. cerasifera Ehrh.

P. americana Marsh.

P. nigra Ait.

Prunus hortulanaBailey

Prunus angustifoliaMarsh.

Prunus munsonianaWight and Hedr.

R maritime Marsh.

P. subcordata Benth.

P. salicina Lindl.

P. simonii Carr.

Green Gages

PrunesYellow Eggs

Imperatrices

Lombards

Damsons

Bullaces

MirabellesSt. JuliansMyrobalansCommon wild plums

Wild Goose(Chickasaw) plums

Beach plums

Pacific Coast plums

Japanese plums

Apricot plums

Imperial Gage, Jefferson, ReineClaude

Agen, Hungarian, Italian, SugarGolden Drop, Monroe, Yellow

EggArch Duke, Englebert,

MonarchBradshaw, Compote, Pond,

VictoriaCrittenden, Grand Duke,

ShropshireBlack Bullace, Royal Bullace,

White BullaceDrap d'Or, Late MirabelleSaint JulianLindsayae, Marianna, Nigra

Cherokee, DeSoto, GoldenQueen

Aitkin, Crimson, Oxford

Cumberland, Golden Beauty,Wayland

Caddo Chief, Ogeeche

Jewell, Osage, Pottawattamie,Texas Belle

Bassett's American

Sisson

Blackamber, Friar, Satsuma

Climax, Wickson (hybrids withP. salicina)


European-Asian PlumsThe European or common garden plum, Prunus domestica L., is well-known for itsdiversity of fruit sizes and flavors and is by far the most desirable of the plum speciesin terms of fruit quality. The wide variety of fruit colors and flavors, as well as alengthy fruit ripening season, yields abundant culinary uses. Prunes are producedexclusively from specific European-Asian plum varieties.

Many P. domestica fruits are somewhat oblong in shape, or with an elongatedneck at the point of fruit attachment. Growth habit is usually an upright tree form,and new vegetative shoots are covered with a fine pubescence. A more dwarf andcompact growth habit as well as a more ovate leaf shape distinguish P. domesticavar. insititia from various forms of P. domestica.

Besides strict botanical distinctions between P. domestica and P. domestica var.insititia, each group has been further subdivided into various pomological classes.While some of the representative varieties listed in Table 22.1 are currently in com-mercial production, others have faded from popularity. The pomological classes usedby growers and nurserymen were perhaps more important in a previous time ascompared with plum growers today. Nonetheless, certain classes still have a specialmeaning for producers or in commerce. For canned plums, still a popular productin the United Kingdom, the Green Gage, Yellow Egg, and Lombard plum classesare most recognized and preferred by consumers for their high fruit quality (Luhet al. 1986). The prunes of P. domestica are another specific pomological class withwidespread consumer recognition today. Their high sugar content and rich flavorprofile ensures consumer acceptance after the drying process. Among the driedfruits available to consumers, prunes are second only to raisins in production volume(Anon. 2008).

Unlike the other European-Asian plums, Prunus cerasifera accessions are gener-ally lacking in fruit quality. This species is more commonly known as the cherry orMyrobalan plum and is native to the Caucasus Mountains. The fruits are typicallycherry like in form, of 1-2 cm in diameter, and with watery, soft flesh. The fruits aregenerally somewhat sweet and subacid but with a poor flavor. Fruits from mostaccessions of cerasifera are used for processing into juice, jams, or compotes.

North American PlumsBreeding of the North American plum species began in the mid-1800s in an effortto expand plum production into those areas where P. domestica was not well adapted.The wide diversity of native North American plums has led to hundreds of varietiesand interspecies hybrids. Perhaps the most frequently used North American plumwas Prunus americana, owing to its general cold hardiness and late bloom, as com-pared with domesticated plums (Hedrick et al. 1911). Ranging from Eastern Montanaand Wyoming in the west to Rhode Island in the east, and from the Florida pan-handle through the Dakotas to the Canadian border in the north, this species iscertainly the most dominant in terms of its geographic range and density in NorthAmerica. In its native state, P. americana produces somewhat low-topped thornythickets that flourish along riverbanks. Prolific fruit set is very typical, and fruitthinning can provide a dramatic increase in fruit size. Fruits are predominantlyyellow or reddish yellow, typically clingstone in nature and with a characteristically


astringent skin. Fruit quality is said to be generally good, when the astringent skinis removed from the flesh. Subjective evaluations of plum flavor and aroma early inthe 20th century determined that P. americana plums generally surpassed bothJapanese and European plums in flavor "bouquet" (Waugh 1901).

The Wild Goose plums are native to more southerly North America as comparedwith P. americana and Prunus nigra. Numerous varieties were created and namedfrom this group, and its importance in North American plum breeding is secondonly to that of P. americana. Fruit skin is typically much lighter in color and muchthinner as compared with P. americana.

P. nigra is of much lesser horticultural importance as compared with P. americana.Cold hardiness is the single most important character obtained from this species.

The Beach Plum, Prunus maritima, so named for its native seaside distributionon the upper east coast of the United States, has not been a major contributor toplum breeding in North America. Its specific geographic distribution and small fruitsize (1.0-1.5 cm) have provided little reason for its pursuit as a progenitor in plumbreeding programs. "Bassett's American" is said to be the single variety selectedfrom this species.

North America's westernmost plum species, subcordata, was a very importantfood source for settlers of California and Oregon during western migrations in themid-1800s. Typically a small tree or bush, the species can rise to erect specimens of7-10 m in height. The fruits are comparatively large in size for native plums, andskin color is predominantly bright red. Of all the North American native plumspecies, Prunus subcordata most resembles European plums in fruit quality charac-teristics. Small-scale commercial entrepreneurs harvest this native plum in NorthernCalifornia and Southern Oregon to produce wine, juice, and jam for boutiquemarkets.

Oriental PlumsThrough numerous hybridizations, breeding programs in the early 20th centurycombined the qualities of both Oriental plum species, Prunus salicina and Prunussimonii, with those of P. subcordata and P. americana. The diploid nature of theOriental plums made interhybridization possible with the North American plums,whereas European-Asian plums could not be used due to their hexaploid nature.Burbank's variety "Santa Rosa" is still commercially popular today and combinesthe firmness, size, and color of Oriental plums with the aromatic notes from P.americana (Howard 1945). Since Luther Burbank's early efforts in developing plumculture in California, hundreds of varieties have been bred and grown successfully.The vast majority of currently grown Oriental varieties are undoubtedly multi-species hybrids that have stabilized genetically and can be further interhybridizedin the development of newer varieties. Fruit size and diversity in fruit color arebeneficial characteristics attributed to P. salicina. Fruit firmness and distinct flavorare characteristics associated with P. simonii's contribution to currently grownOriental plums.

The Oriental plum species generally exhibit a more spreading growth habit ascompared with trees of European-Asian origin. Compared with the elongated fruitforms of R domestica, Oriental plums have a more rounded fruit shape. P. simoniiis characterized by a flattened pistil end of its fruit. Both P. salicina and P. simonii


have large and juicy fruits as compared with many of the European-Asian plumsand the vast majority of North American plums.

THE FLAVOR OF PLUMS

Stone fruit quality is a complex combination of sensory attributes and proportionsthat provide value in terms of human consumption. While each consumer judgesfruit quality to different standards, the general perception of quality relates to fruitsthat look good, are firm fleshed, and offer ample flavor and nutritive value (Crisosto1994).

In contrast to apricots and other types of Prunus, less research has been accom-plished on the aroma and flavor of plums. As previously stated, cultivars and plumspecies differ in appearance, aroma and flavor. The sensory properties are veryimportant to consumers and can be a determining factor in the development andrelease of new fruit cultivars. The first reported study of plum volatiles was carriedout by Forrey and Flath (1974) with P. salicina. After this first study, some of themost significant works on flavor and aroma of plums can be found in the studies ofIsmail and others (1980, 1981) and Etievant and others (1986) in P. domestica,Gomez and Ledbetter (1993) and Gomez and others (1993) in P. salicina, Gomezand Ledbetter (1994) in P. simonii, and Horvat and others (1992) in P. salicina x P.americana.

MAIN COMPOUNDS IDENTIFIED IN THE DIFFERENT PLUM SPECIES

A list of the identified compounds in the different species is shown in Table 22.4.The volatile compounds identified in plums include carbonyl compounds, esters,hydrocarbons, lactones, and other miscellaneous compounds. When compared withthe aromatic profile of other Prunus such as apricot, the results indicated that plums

TABLE 22.4. Compounds Identified in the Different Plum Species

R R P.Constituent salicina simonii domestica

KetonesButanone x3-Octanone xIsophorone x xAcetophenone x xoc-Ionone xGeranylacetone x x(3-Ionone x x3-Hydroxy-2-butanone x xp-Damascenone1-Phenyl acetone xFurfuryl methyl ketone x

P. salicina xP. americana Reference

991,21,2,821,2

x 1,2,34, 7, 8, 9

x 368


TABLE 22.4. Continued

Constituent

AlcoholsMethanolEthanolPropanol2-Methyl-3-buten-l-ol2-Methyl-propanolButanol3-Methyl-l-butanol2-Methyl-l-butanolButane-2,3-diolPentanolPenten-2-ol4-Methylpentanol(Z)-3-Hexen-l-ol(£)-2-Hexen-l-olHexanol

2-EthylhexanolHeptanolOctanolLinalool

Nonanola-TerpineolGeraniol4-TerpineolNerolNerolidolBenzyl alcohol2-PhenylethanolEugenolMethyl eugenol

AldehydesAcetaldehyde2-Methylpropanal2-Methylbutanal3-Methylbutanal2-Methyl-2-pentenalHexanal

(£)-2-Hexenal

Heptanal2-Heptenal(E,£)-2,4-HeptadienalPhenylacetaldehydeOctanalNonanal

P.salicina

XXXXX

XX

XX

XX

X

XXXX

X

XXXX

X

XX

X

XXXX

X

p.simonii

XXXX

XX

X

X

X

X

XX

X

X

p.domestica

XXXXXX

X

XXX

X

X

XX

XXX

X

X

XXX

P. salicina xR americana Reference

44444,77,8,94,9484,842

x 1,2,3,41,2,4

x 1, 2, 3, 4,7, 9, 10

1,2,64,91,2,4,9

x 1, 2, 3, 4,6,7

2,91,2,429124,84,9

x 3x 3

49994

x 1,2,3,7,8, 9, 10

x 1, 2, 3, 9,10

1,2,81,21,21, 2, 9, 108

x 1,2,3,5,6, 7, 8,10



P.Constituent salicina

Decanal x(3-Cyclocitral(E,Z)-2,4-Decadienal(E,E)-2,4-DecadienalBenzaldehyde x

FurfuralMethylfurfural

EstersEthyl acetate xPropyl acetate x2-Methyl-l-propyl acetate xButyl acetate x3-Methylbutyl acetate xPentyl acetate x(Z)-3-Hexenyl acetate xHexyl acetate x

(E)-2-Hexenyl acetate xHeptyl acetate xOctyl acetate xNonyl acetateDecyl acetateDodecyl acetateBenzyl acetate xEthyl phenyl acetate xGeranyl acetateBornyl acetate xButyl propanoatePentyl propanoateHexyl propanoate3-Hexenyl propanoatePentyl 2-methylpropanoateMethyl butanoateEthyl butanoate xPropyl butanoateButyl butanoate x2-Methylpropyl butanoateMethylbutyl butanoatePentyl butanoateHexyl butanoate(E)-2-Hexenyl butanoate xHeptyl butanoate xOctyl butanoateEthyl 2-methylbutanoateEthyl 3-methylbutanoateButyl 2-methylbutanoateHexyl 2-methylbutanoateButyl pentanoate

P. P.simonii domestica

xxX

X

XX

X

X XX X

XX

X X

XX XX XX

X

X

XX

XX X

X

X

X

XX

XX

X X

X

XX

X XX

XX

X

XX

X

P. salicina xP. americana Reference

6222

x 3, 4, 7, 8,10

8,108,10

4,944

x 1,2,3,4,9x 2,3,7x 3, 4, 9

1,2,4,9x 1,2,3,4,

6,9x 1,2,3,9

2,4,72,4,82

x 324,84,7222,999

99

x 3, 4, 7, 99

x 3,4,92,9797,92,6,96,9999999



Constituent

Hexyl pentanoateMethyl hexanoateEthyl hexanoatePropyl hexanoateMethyl butyl hexanoateButyl hexanoatePentyl hexanoateHexyl hexanoate(Z)-3-Hexenyl hexanoateEthyl heptanoateMethyl octanoateEthyl octanoate

Propyl octanoateButyl octanoateHexyl octanoateOctyl octanoateEthyl nonanoateMethyl decanoateEthyl decanoateEthyl anysateMethyl salicylateMethyl cinnamateEthyl cinnamate

Lactonesy-Hexalactoney-Octalactoney-Nonalactone8-Decalactoney-Decalactone

y-Undecalactoney-Dodecalactone

Hydrocarbons1,4-Dimethyl benzene1,2,3-TrimethylbenzeneTridecaneTetradecanePentadecaneHexadecaneHeptadecane2-HeptadeceneLimonenea-Pinene(3-PineneCarvone

P. P. Rsalicina simonii domestica

XX

X XXX

X XX

XX X

XX

X X

XX

XX

X XX

XXX

XX X

X

X X X

X

X X

X

XXXXXXX

X XXXX

R salicina xR americana

X

X

X

XXXX

XXXXX

X

Reference

794,7,9776,3,723,7,91,2991, 2, 3, 4,

7,933,7,93,93994,7,94879

3,41,2,3,43,73,61,2,3,4,

6,7,821,2,3

229999992,6998

1, Gomez and others (1993); 2, Gomez and Ledbetter (1994); 3, Horvat and others (1992); 4, Forrey andFlath (1974); 5, Ismail and others (1977); 6, Gomez and Ledbetter (1993); 7, Ismail and others (1981); 8,Ismail and others (1980); 9, Etievant and others (1986); 10, Sabarez and others (2000).


presented lower numbers of volatile compounds. In the extensive study of Etievantand others (1986) with P. domestica, 130 compounds were detected, including 62esters, 14 hydrocarbons, 11 aldehyde, 10 alcohols, 8 lactones, and 8 ketones, and inone study with P. salicina and P. simonii (Gomez and Ledbetter 1994), a total of 12alcohols, 6 hydrocarbons, 10 aldehydes, 7 ketones, 21 esters, and 4 lactones wereidentified in both species.

Aldehydes and alcohols of six-carbon atoms were identified in all species. Thesecompounds were hexanal, 2-hexenal, hexanol, 2-hexenol, and 3-hexenol. The pres-ence of these compounds is probably due to lipoxygenase activity, actions initializedby the disruption of the fruit tissues when it was blended (Frankel 1982), since mostof the studies include a homogenization step, except when the headspace aroma isstudied. Accordingly, most of these compounds were not identified in the study ofplum headspace volatile compounds by Gomez and Ledbetter (1993). These C6-compounds seem to be important to plum aroma (Ismail et al. 1981) and contributeto the green note of the fruit (Guichard et al. 1990).

Other carbonyl compounds have been identified in plums. The presence ofnonanal is significant as a characteristic constituent of skin waxes of plums havinga fragrant, woody-like aroma (Ismail et al. 1981; Williams and Ismail 1981). Thecuticular wax layer removed from whole plums had a creamy, fragrant wood-likeodor. These characteristics were considered a significant contribution relative tothe aroma of some plums. Nonanal has been identified in the volatiles isolatedfrom many fruits and vegetables, but it has only been reported to be of significancein the aroma of lemon oil (Ikeda et al. 1962), cauliflower, and broccoli (Butteryet al. 1976), although it has also been isolated from peach leaves (Kemp et al.1971).

Among the ketones, geranylacetone, detected in P. salicina and P. simonii, can beregarded as a norterpenoid arising from isoprenoid degradation (Takeoka et al.1990). Another ketone found in some samples was isophorone, identified previouslyin kiwi flowers (Tatsuka et al. 1990).

Esters are the main compounds responsible for the fruity aroma. Amongthe esters identified in plums, butyl propanoate and 3-methyl-l-butyl acetate havea strong banana aroma, and 2-methylpropyl butanoate and octyl acetate have apleasant fruity aroma. These esters, together with hexanal and some lactones,contribute to the plum-like aroma and the characteristic fruity flavor of some plumcultivars.

Among alcohols, some terpenols have been identified. The concentration of oneof the most significant terpenols, linalool, was in general, present at lower concentra-tion than in other Prunus such as apricots (Gomez et al. 1993). This was also truefor the other terpenoids, as we could not detect geranial, or pseudoionone, com-pounds normally appearing in other Prunus species (Tang and Jennings 1968).

Hydrocarbons are more prevalent, quantitatively, than in other Prunus such asthe apricot. This was probably related to the composition of the skin, which is veryrich in waxes. One of the identified compounds, limonene, besides its sensory sig-nificance, has been described as an important odor attractant for some insects(Leskey et al. 2001).

A large number of lactones have been identified. y-Octalactone, decalactone, andy-dodecalactone were the most important quantitatively in plums.


MAJOR AROMA COMPOUNDS IN DIFFERENT PLUM SPECIES

P. domesticaOn the basis of the studies of Williams and Ismail (1981) with P. domestica, linalooland ethyl butanoate are very important compounds in the aroma of Europeanplums. These authors also determined that hexanal, when diluted, has a plum-likearoma. Ismail and others (1981) also considered y-decalactone as well as ethyl non-anoate and benzaldehyde to be associated with the plum aroma of this species.When headspace analysis was employed, all extracts were dominated by hexanoland nonanol, together with 3-methylbutanol and linalool. These authors stated thatthe regions of the chromatogram containing benzaldehyde, linalool, methyl cin-namate, and y-decalactone were associated with fresh plum aroma.

The same authors studied the aromatic profile of P. domestica canned fruits,showing that the headspace aroma of these plums was dominated by carbonyls,benzaldehyde, and linalool, whereas methyl cinnamate, y-octalactone, and y-decalactone were not detected. The absence of these compounds from headspaceanalyses may account for the differences in the aroma of canned plums as comparedwith freshly harvested fruit.

The studies of Etievant and others (1986) showed that hexyl, butyl, and ethylesters were very abundant in the headspace of fresh mirabelle plums (P. domesticass. insitia), whereas they could not detect linalool, a-terpineol, or geraniol. Estersaccounted for 88% of total volatiles, the major being hexyl esters. They stated thatthe absence of linalool, damascenone, and methyl cinnamate (claimed to be maincontributors to P. domestica aroma) might explain the differences in the aroma ofmirabelle plums.

Dried European plums, known as prunes, are consumed in large amounts due,among other things, to their health benefits. During drying, there are a number ofchemical reactions that may influence the aroma of the final product. Sabarez andothers (2000) studied the volatile changes during dehydration of d'Agen prunes. Asin previous studies with fresh European plums, the determined main volatiles wereC6-compounds (especially when the fruit was blended) and nonanal and phenylac-etaldehyde. After drying, C6-compounds disappeared, and the two aldehydes wereretained. Three major new compounds were generated and ^identified in the dryproduct: benzaldehyde (probably due to the degradation of its glucoside precursor,amygdalin, during heating as stated by Williams and Ismail 1981), 2-furancarboxy-aldehyde (from degradation of sugars alone or in contribution with amino acids),and ethyl cinnamate.

P. salicinaThe first study on P. salicina indicated that esters and lactones were the main volatileconstituents in this fruit (Forrey and Flath 1974). In this primary study, it was dem-onstrated that acetate esters predominated in the Santa Rosa variety, but appreciablequantities of the higher y-lactones appeared as well. Many carbonyl compounds andrelated esters were also identified by Gomez and others (1993), with y-decalactoneand y-dodecalactone being the most important lactones found in the study.


The relative contribution of various constituents to the blended plum aroma wasdetermined in the study of Gomez and others (1993) by calculating the number ofodor units (Uo). The odor unit was defined by Guadagni and others (1966) as theconcentration of the compound divided by its odor threshold. This value gives anindication of the significance each volatile contributes to the plum aroma. The com-pounds with the highest odor unit value were hexanal, described, when diluted asa plum-like aroma (Williams and Ismail 1981); nonanal, a characteristic constituentof plum skin waxes, with a fragrant woody-like aroma; (E,E)-2,4-decadienal; andthe lactones y-decalactone and y-dodecalactone, which are described as beingresponsible for the fruity, peach, and coconut background aromas of the fruit(Takeoka et al. 1990).

As stated previously, hybridizations between Japanese and Native Americanplums at many U.S. locations have led to numerous cultivars being adapted to awide range of environmental conditions. The studies of Horvat and others (1992)with hybrids of P. salicina and P. americana demonstrated that for most of thestudied cultivars, (E)-2-hexenal, butyl acetate, butyl butanoate, and y-dodecalactonewere the major constituents. The major lactone reported in this study was y-dodecalactone, whereas other lactones were found in trace amounts. The distribu-tion of plum lactones differed from those of peaches and other plums wherey-decalactone and 8-decalactone in peaches had been found in higher concentra-tions than y-dodecalactone (Horvat et al. 1990).

P. simoniiThis plum presented a very interesting aromatic profile, with a high number ofidentified compounds. In a study where its profile was compared with P. salicina(Gomez and Ledbetter 1994), some compounds were only found in P. simonii. Forexample, among ketones, p-ionone appeared only in the profile of P. simonii as wellas the alcohols, octanol, nonanol, and geraniol. The number of esters was bothqualitatively and quantitatively higher in P. simonii. Twelve of the 21 identifiedesters appeared only in P. simonii. Also, the concentration of lactones, important forthe fruity aroma (Guichard et al. 1990), was greater in P. simonii.

Some of the most characteristic components of plum aroma showed a signifi-cantly higher concentration in P. simonii samples. These compounds were hexenal,hexanal, and hexanol; the esters butyl acetate, hexylacetate, and (Z)-S-hexenylacetate; as well as y-decalactone and y-dodecalactone. These esters and lactones,together with hexanal, probably contribute to the plum-like aroma and the charac-teristic fruity flavor of P. simonii.

In the study of Gomez and Ledbetter (1994), it can be seen that the most importantcompounds, as determined by the odor units, are P-ionone (despite its low concentra-tion, this compound has a very low odor threshold) and nonanal, as well as hexylacetate. This ester is also present at a high concentration in apples (Willaert et al.1983) and is responsible for the characteristic apple-like aroma found in this species.

P. simonii has demonstrated itself to be a useful parent in plum breeding pro-grams because of its unique aromatic components and quantitative presence. Of the60 quantified compounds in this study, 23 are only present in P. simonii, showingsignificantly more esters than other plum species; taken collectively, they probablycontribute to the intense fruity aroma of P. simonii.


THE FLAVOR OF PLUMCOTS

The Agricultural Research Service's Horticultural Crops Research Laboratory inFresno, CA, began the development of plum-apricot hybrids in 1989. With thesehybrids, it was expected to provide consumers with new fruit types. The hybrids arecommonly known as "plumcots" and vary widely with regard to skin and flesh color,fruit size and shape, ripening period, and flesh texture and flavor. The main breedinginterest was focused on those types having flesh texture characteristics similar tothat of Japanese plum and aroma/flavor characteristics resembling those of apricot.

The studies of Gomez and Ledbetter (1993,1997) and Gomez and others (1993)demonstrated that almost all of the compounds identified in apricots and plumswere also found in plumcots (Table 22.5). There were also some unique compoundsonly found in one or more of the hybrids, and not in apricot or plum: ethyl benzoate;ethyl salicylate, and p-phellandrene. Ethyl salicylate has been described as having apleasant odor, whereas ethyl benzoate has been identified also in peaches (Horvatet al. 1990). Those esters are described as having a sweet and fruity aroma.

Among plumcots, the compounds having the higher odor units were those char-acteristic of apricot such as lactones, linalool, and (£,E)-2,4-decadienal and thosecharacteristic of plums such as hexyl acetate and nonanal. Besides these compounds,the contribution of the C6-compounds to the plumcot aroma was higher than ineither apricot or plum.

From the results obtained in this study, it appears that the ability to producearomatic volatiles may be paternally transmitted as discrete characteristics. A secondpoint with regard to the structural genes responsible for the transmission of specificaromatic constituents is that compounds that are quantitatively important com-pounds in the parents of plum x apricot hybridizations may also be produced inquantitatively high levels in the progeny. Geranylacetone was identified in all ofthe parents of this study but was quantitatively much higher in one of the hybrids.Similarly, geranylacetone was also identified in all plumcot progeny, but at higherlevels in progeny for which the elevated geranylacetone apricot parent had beenused. A similar case can be made with y-decalactone and y-dodecalactone. Anothercompound, nonanal, contributed significantly to Blackamber (P. salicind) aroma.Those plumcots having Blackamber as a parent had higher odor unit values fornonanal than those plumcots for which another plum (P. salicina cv. Friar) was used.

In the last decade, the world has seen dramatic increases in plum production,with many breeding programs developing new plum varieties adapted to wide-ranging environmental conditions. Varietal development for the last several decadeshas focused primarily on the producer, with very little attention being paidto specific organoleptic characteristics. Currently, there is a renewed focus onconsumer acceptance in plum improvement, with flavor and fruit aroma beingimportant criteria in new variety development. Plums exhibit a great deal of geneticdiversity relative to their natural habitats, as well as both in the kinds and amountsof compounds responsible for characteristic plum flavor. However, specific researchon plum flavor and the responsible aromatic compounds must progress to the levelof flavor research achieved in other fruits and vegetables. Collaborations betweenflavor chemists and plum breeders will certainly assist in the development of newhigh flavor plum varieties for the future. With the wealth of plum germplasmdiversity available to the breeder and modern analytic tools available to the

TABLE 22.5. Aroma Compounds Detected in Plumcots (Gomez et al. 1993)

§GO

Ketones

3-Hexanone2-Hexanone2,2,6-TrimethylcyclohexanoneIsophoroneAcetophenone2-EthylcyclohexanoneDihydro-p-ionone

oc-IononeGeranylacetone2,6-Bis(l,l-dimethylethyl-2,5-

cyclohexadiene-l,4-dionep-IononePseudoionone(E,E) -Pseudoionone6,10,14-Trimethyl-2-

pentadecanone

Alcohols

1 -Methylcyclopentanol(Z)-S-hexen-l-ol(E)-2-hexen-l-olHexanol2-EthylhexanolLinalool2,6-

Dimethylcyclohexanol4-Terpinenola-TerpineolGeraniol

NerolNerolidolFarnesol2,6-Bis(l,l-

dimethy lethy 1) -4-ethylphenol

Aldehydes

Hexanal(E)-2-hexenalHeptanal(£)-2-heptenal(£,£)-2,4-heptadienalPhenylacetaldehydeCitral methyl acetal

Nonanal2-Undecenalp-Cyclocitral

(E,£)-2,4-decadienal(£,Z)-2,4-decadienal

Esters

Butyl acetate3-Methylbutyl acetate(Z)-3-hexenyl acetateHexyl acetate(£)-2-hexenyl acetateEthyl benzoate(Z)-3-hexenyl butanoate

(E)-2-hexenyl butanoateMethyl salicylateEthyl octanoate

Octyl acetateEthyl phenylacetateEthyl salicylateBornyl acetate

Lactones

y-Octalactoney-Nonalactoney-Decalactone8-Decalactoney-Undecalactoney-Dodecalactone

Hydrocarbons

2,3-Dimethyl-2-pentene1 ,4-Dimethylbenzene1,3,5-Cyclooctatriene2,4-Dimethyl-2-decenea-Phellandrenea-Pinene1 ,2,3-Trimethylbenzene

1,3,5-Trimethylbenzene2,2,8-Trimethyldecanem-Cymene

Limonene2,5-Dimethyl-2-undecene2,2,5 ,5-Tetramethylhexane3,8-Dimethylundecane

6,10,14-Trimethyl-5,9,13-pentadecatrien-2-one

6-Methyl-5-hepten-2-one

2-Ethyl-3-hydroxyhexyl-2-methylpropanoate

Geranyl acetate

(Z)-3-hexenyl hexanoate

(£)-2-hexenyl hexanoate

Ethyl decanoate

Methyl-10-methyldodecanoate

Ethyl pentadecanoate

Naphthalene

2-Oxo-l-methyl-3-isopropylpyrazine

l-Methyl-4-(methylthio)benzene

Megastigma-4,6(Z),8(Z)-triene

Megastigma-4,6(£:),8(£:)-triene

l,2,3,4-Tetrahydro-l,l,6-trimethylnaphthalene

Megastigma-4,6(£),8(Z)-triene

Megastigma-4,6(Z),8(£)-triene

2-Ethyl-l,4-dimethylbenzene

Dihydroactinidiolide


flavor chemist, future plum varieties might well possess both the horticultural traitsimportant to the producer as well as a complex and pleasing flavor bouquet desiredby consumers.

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