Characterization of aroma-active compounds in choch (Lucuma hypoglauca Standley) fruit

ABSTRACT

Lucuma hypoglauca Standley, locally named choch, is apparently native from Southern Mexico, but is also cultivated in Central and South America. The fruit is consumed fresh and it is widely accepted in diverse regional markets. Owing to the great potential of commercialization as an exotic fruit, it is important to analyze the aroma of this fruit. The objective of this present study was to analyze the volatile compounds causing the aroma of choch fruit. The volatile compounds of choch fruit were isolated by simultaneous distillation-solvent extraction (SDE) and analyzed by gas chromatography-flame ionization detector and gas chromatography-mass spectrometry. A total of 30 volatile constituents were detected, which represented 2.31 mg kg⁻¹ of the fruit. The composition of the volatile constituents of the fruit included 12 ketones (27.5% of the total volatile composition), seven terpenes (64.8%), four esters (4.1%), four alcohols (2.2%), two aldehydes (1.1%), and a sulfur compound (0.4%). The major compounds were (E)-β-caryophyllene (56.3% of the total volatile composition), with lesser amounts of 3-hydroxy-2-butanone (6.1%), 2-pentanone (5.6%), and (E)-3-penten-2-one (5.6%). By application of odor activity values (OAVs), six constituents were considered as aroma-active volatiles, of which the most important were (E)-3-penten-2-one, (E)-β-caryophyllene, methional, 3-methylbutanal, 3-heptanone, butanal, and 3-hexanone.

KEYWORDS:

• Aroma-active compounds
• Choch
• Gas chromatography-mass spectrometry
• Lucuma hypoglauca
• Simultaneous distillation-solvent extraction

Introduction

Lucuma hypoglauca Standley, locally named choch, is apparently native from Southern Mexico, but is also cultivated in Central and South America. This species is a medium-sized tree that grows quite slowly. The tree is evergreen, with large, elliptic-oblong, and dark green leaves. The young branches are densely foliated. Senescent leaves become bright red in color and usually remain on the branches for several weeks before abscission occurs, giving a pleasing contrast to the green leaves. The tree has a densely branching habit, forming a compact, rounded crown. The fruit is oval-globose in shape, about 7 cm long and 5 cm broad, with a thick, hard outer shell and three or four large elliptical seeds 4 cm long and 2.5 cm thick, with a hilar scar elliptic-linear 2.5 cm long. The flesh surrounding the seeds is yellow and soft with a sweet fruity flavor resembling that of a sweet potato. The main crop of fruit matures during July and August.^[1] Owing to its organoleptic characteristics, the fruit is consumed while fresh. Despite its wide acceptance in diverse regional markets and its great potential of commercialization as an exotic fruit, there is no information available regarding the aroma of this fruit. Thus, the objective of the present study was to analyze the volatile compounds causing the aroma of choch fruit.

Materials and methods

Chemicals and reagents

Fresh, healthy, and ripe fruits used were harvested at the Botanical Garden of Chapingo Autonomous University (UACH) in Merida, Yucatan, Mexico.

The standards used for identifications were supplied by Aldrich (Steinheim, Germany) and Fluka (Buchs, Switzerland). An n-alkane solution (C₈–C₃₂) from Sigma–Aldrich (St. Louis, MO) was employed to calculate the linear retention index (LRI) of each analyte. Dichloromethane was purchased from Merck (Darmstadt, Germany) and it was previously redistilled and checked for purity.

Isolation of volatile compounds by simultaneous distillation-solvent extraction

Volatile compounds were isolated according to a previous reported method.^[2,3] Two hundred grams of sliced fruits were blended with 500 mL of distilled water; 0.2 mg of methyl nonanoate was added as an internal standard, and the volatile compounds were isolated by means of a simultaneous distillation-solvent extraction (SDE) apparatus using 40 mL of dichloromethane for 1 h. The extract was dried over anhydrous Na₂SO₄ and concentrated to 0.6 mL in a Kuderna-Danish evaporator with a Vigreux column and then to 0.2 mL with a gentle nitrogen stream. The concentrated extract was stored in a glass screw-top vial at −20°C until being analyzed. Three independent extractions were performed and each extract was injected twice into the Gas chromatography-flame ionization detection (GC-FID) and Gas chromatograph-mass spectroscopy (GC-MS).

GC-FID and GC-MS analyses

A Perkin-Elmer Autosystem XL (Shelton, CT, USA), equipped with a 30 m x 0.25 mm x 0.25 μm film thickness RTX-5MS (Restek, EE.UU.) fused-silica capillary column and with a flame ionization detector, was used. The oven temperature was held at 50°C for 2 min and then raised to 280°C at 4°C/min and held for 10 min. Carrier gas (helium) flow rate was 1 mL/min. The injection and detector temperatures were 240°C and 250°C, respectively. The retention times of a series of n-alkanes (C₈-C₃₂) were used to calculate the retention indices for all identified compounds and for the reference standards. The estimated concentrations for all compounds were made by GC peak area comparisons of the SDE extract components with the area of a known quantity of internal standard. The concentrations were expressed as mg/kg methyl nonanoate equivalents of fresh weight, with the response factors being taken as 1.0 for all compounds with reference to the internal standard.

GC-MS analyses were performed on a Perkin-Elmer Clarus 500 gas chromatograph with a similar fused capillary column as in GC-FID. The temperature program and carrier gas flow rate were the same, as in GC-FID, with EIMS, electron energy, 70 eV; ion source and connecting parts temperature, 250°C. The acquisition was performed in the scanning mode (mass range m/z 35–400 u). Compounds were preliminarily identified by use of NIST, Wiley, NBS, Adams 2001, and in-house Flavorlib libraries, and then the identities of most of them were confirmed by comparison of their linear retention indices with those of the reference standards or with the published data.^[4]

Odor detection threshold determination

A previously described multiple paired comparison test was used.^[5] Samples were prepared in capped, wide-mouthed, 50 mL glass bottles. A group of 30–40 unscreened and untrained assessors was used for determining the odor thresholds. In each case, panels were replicated a sufficient number of times, so that a minimum of 100 responses were obtained for each concentration used in determining a particular threshold. The test involved presenting the assessors with several samples, along with an aqueous solution for reference. Each sample was compared for smell individually with the reference to determine a possible difference. Six samples were presented to each judge during each session. The first bottle was the reference and the next five coded bottles contained four different dilutions and an aqueous solution identical to the reference. The four dilutions were placed in order of increasing concentrations to prevent fatigue. The position of the aqueous solution coded sample among the different samples was arbitrarily changed from day to day. The statistical analyses for determining the odor detection threshold values involved calculating the concentration corresponding to 50% positive responses from the total judgments. The calculation was made from the linear regression of percentage detection against log concentration. The 95% confidence limit calculated for the threshold values was used as a measure of error. The relative standard deviations were lower than 6%.

Results and discussion

All of the volatiles from choch fruit, isolated by SDE, were evaluated by three experts by smelling a drop of the extract onto a cardboard smelling strip as done by perfumers. After evaporation of the solvent, all three experts agreed that the distillate evoked the characteristic odor of the fruit, thereby indicating that the method used for aroma isolation was appropriate.

A total of 30 volatiles were identified, representing 2.31 mg/kg in choch fruit (Table 1). Compared with other tropical fruits, this is a low level, e.g. 12.8 μg/kg in papaya,^[6] 240–332 mg/kg in bullock’s heart,^[7] 18–123 mg/kg in mango,^[8] 6.5 mg/kg in sapote,^[2] and 18.8 mg/kg in yellow sapote,^[9] but higher than 1.5 mg/kg in star apple,^[10] determined with the same isolation technique. The composition of the fruit included 12 ketones (27.5% of the total volatile composition), seven terpenes (64.8%), four esters (4.1%), four alcohols (2.1%), two aldehydes (1.1%), and a sulfur compound (0.4%), all of them reported for the first time. Major compounds were (E)-β-caryophyllene (56.3% of the total volatile composition), with lesser amounts of 3-hydroxy-2-butanone (6.1%), 2-pentanone (5.6%), and (E)-3-penten-2-one (5.6%).

Table 1. Volatile compounds of choch fruit.

Compound	LRI	ID	Concentration (mg/kg)
Butanal	595	a	0.02
Ethyl acetate	612	a	0.04
1-Butanol	653	a	0.02
3-Methylbutanal	654	a	0.01
2-Butanol	676	a	0.00
2-Pentanone	688	a	0.13
3-Pentanone	703	a	0.02
Ethyl propanoate	714	a	tr
3-Hydroxy-2-butanone	718	a	0.14
(E)-3-Penten-2-one	735	b	0.13
3-Methyl-2-pentanone	753	a	0.01
3-Hexanone	784	a	0.04
2-Hexanone	792	a	0.05
3-Heptanone	880	a	0.04
2-Heptanone	894	a	0.01
3-Heptanol	902	a	0.02
Methional	905	a	0.01
3-Octanone	983	a	0.02
3-Octanol	991	a	0.01
5-Nonanone	1049	a	tr
Methyl 2-methyloctanoate	1158	b	0.04
Acetophenone	1063	a	0.05
(E)-β-Caryophyllene	1420	a	1.30
trans-α-Bergamotene	1435	a	0.01
allo-Aromadendrene	1461	a	0.08
β-Bisabolene	1508	a	0.03
(Z)-γ-Bisabolene	1515	a	0.04
(E)-γ-Bisabolene	1531	a	0.01
cis-Methyl dihydrojasmonate	1658	b	0.01
α-Bisabolol	1687	a	0.03

tr: ≤ 0.01 mg/kg; LRI: lineal retention indices in RTX-5MS column; ID: identification.

^aCompound definitely identified (comparison of LRI and mass spectra with reference compound)

^bBy mass spectra data and retention index data from the literature.

It has been shown for a considerable number of foods that all the volatiles present in a food cannot interact with human olfactory receptors. Instead, only a smaller number of the so-called key odorants is obviously detected by the human odorant receptors and, consequently, participates in the creation of the respective aroma impression in the brain.^[11] One approach to separating odor-active volatiles from the bulk of odorless food volatiles is to correlate the concentrations of the individual odorants with the respective odor thresholds using the odor activity value (OAV) concept.^[11–13] As summarized in Table 2, the results yielded six aroma-active compounds with OAV > 1, which have been arranged following their retention indices. By far, the highest value was found for (E)-3-penten-2-one with the characteristic ethereal-fruity odor. Other important odor-active compounds were (E)-β-caryophellene (citrus-like and fresh notes), methional (cooked potatoes), and 3-methylbutanal (malty notes) based on its higher OAVs (above 10). Other potentially important odorants obtained with the odor activity approach were 3-heptanone, butanal, and 3-hexanone. To confirm the aroma contribution of these compounds, aroma extract dilution analysis studies and aroma recombination experiments should be performed. However, the aroma-active compounds identified can already be suggested as indicators to assess the odor quality of choch fruit.

Table 2. Aroma-active compounds identified in choch fruit.

Compound	Odor threshold (mg/kg)	Odor note^a	OAV^b
Butanal	0.009^c	Fruity	2
3-Methylbutanal	0.0005^c	Malty	11
(E)-3-Penten-2-one	0.0015^d	Ethereal-fruity	88
3-Hexanone	0.0400^c	Fruity	1
3-Heptanone	0.008^c	Green-fatty	5
Methional	0.0004^e	Cooked potatoes	18
(E)-β-Caryophyllene	0.064^f	Citrus-like, fresh	20

^aOdor note from in-house database.

^bOdor-activity values were calculated by dividing the concentrations by the respective odor threshold.

^cLeffingwell and Assoc.^[14]

^dCzerny et al.^[15]

^fPino^[16]

Conclusion

A total of 30 volatiles were identified in choch fruit, which included 12 ketones, seven terpenes, four esters, four alcohols, two aldehydes, and a sulfur compound, all of them reported for the first time. Major constituents were (E)-β-caryophyllene (56.3% of the total volatile composition), with lesser amounts of 3-hydroxy-2-butanone (6.1%), 2-pentanone (5.6%), and (E)-3-penten-2-one (5.6%). Six compounds were considered as aroma-active volatiles, of which the most important were (E)-3-penten-2-one (fruity), (E)-β-caryophyllene (fresh), methional (fresh), 3-methylbutanal (malty), 3-heptanone (green fatty), butanal (fruity), and 3-hexanone (hexanone). The interaction of these notes contributes to the complexity of the flavor of choch fruit resembling that of sweet potato.