Comparing odor-active compounds in three mango (Mangifera indica L.) cultivars by aroma extract dilution analysis and the method for evaluating odor interactions

Abstract

Volatile compounds in three mango (Mangifera indica L.) cultivars (Irwin, Carabao, and Nam Dok Mai) were investigated using aroma extract dilution analysis (AEDA) and AROMATCH. The AROMATCH analysis technique was applied to evaluate odor interactions: the relationships between a natural aroma and volatile compounds separated by gas chromatography (GC). As a result, 41 odor-active compounds were detected, 36 compounds of which were identified. Three volatile compounds (3-carene oxide, 4-acetoxy-2,5-dimethyl-3(2H)-furanone, and (Z)-6-dodecen-4-olide) were identified in mango for the first time. The flavor dilution (FD) factor for (Z)-6-dodecen-4-olide differed obviously between AEDA (FD: 243)and AROMATCH (FD: 59049) in Irwin. In addition, 3-carene oxide was described as plastic-like byAEDA but fruity by AROMATCH

Introduction

Mango (Mangifera indica L.) is one of the most popular tropical fruits globally. Its odor is especially attractive, and various products have a mango flavor. Asia is by far the largest producer of mangoes (i). Over 500 volatile compounds in mangoes have been reported (ii). Terpene hydrocarbons are the major volatiles of 20 cultivars (Delicioso, Haden, Super-Haden, Manga amarilla, Macho, Manga blanca, Ordonez, Obispo, Corazon, Delicia, Filipino, Huevo de toro, San Diego, Manzano, Smith, Florida, Minin, La Paz, Keitt, and Kent) (Pino et al., 2005). In addition, 4-hydroxy-2,5-dimethyl-3(2H)-furanone (HDMF) was reported as an important aroma compound in Haden, White Alfonso, Praya Sowoy, Royal Special, and Malindi by aroma extract dilution analysis (AEDA) (Munafo et al., 2014), which is used to determine the potency of odors in food extracts (Grosch, 1993).

Despite these many reports of cultivars, few studies focusing on Irwin, Carabao, and Nam Dok Mai, which have been popular varieties in recent years, have been reported with respect to odor-active compounds. For example, while Irwin especially has an attractive flavor for the market, there is only one study on its flavor, which focused on changes in the aroma constituents during storage (Shivashankara et al., 2006). Identifying the potent odor-active compounds of these three cultivars can help elucidate their characteristic aroma profiles.

Then, we applied AEDA and AROMATCH to determine the potent aroma components of each cultivar. AROMATCH was developed in our laboratory (Hattori et al., 2003, 2005). This method, originally termed as OASIS (Original Aroma Simultaneously Input to the Sniffing port), was renamed in order to avoid confusion with Oasis (Waters, MA, USA), the solid-phase extraction cartridge. AROMATCH makes it possible to evaluate odor interactions between natural aromas and the compounds separated from aroma concentrates by GC. Certain volatile compounds reportedly influence each other, even when their concentrations are below the odor threshold (Miyazawa et al., 2009; Ito et al., 2005; Kurobayashi et al., 2008; Takoi et al., 2010). Odor interactions are one of the most important aspects in the smell of natural products, and much research has been reported in various fields, especially of wine (Wu et al., 2015; Atanasova et al., 2004; Atanasova et al., 2005; Chaput et al., 2012; Lytra et al., 2013). Therefore, the aim of this study was to elucidate the potent volatile compounds of three mango cultivars with characteristic odors using AEDA and AROMATCH.

Materials and Methods

Mango fruit    All mangoes were purchased from domestic markets and stored at 4 °C until use. We used the following three mango cultivars: Irwin, Carabao, and Nam Dok Mai at the optimum commercial maturity.

Preparation of aroma concentrates    Fresh ripe mangoes were peeled and their seeds were removed. The flesh was placed into a blender, and mango extracts were prepared by stirring in 2.2 kg of dichloromethane in 1.1 kg of blended mango pulp at 25 °C for 20 min. After centrifugation (6000 × g, 4 °C, 20 min), the dichloromethane layer was isolated and distilled using a solvent-assisted flavor evaporation technique (SAFE) to obtain the volatile aroma concentrates (Engel et al., 1999). The aroma distillates were dried with an excess amount of magnesium sulfate using a rotary evaporator (40 °C, 600 mmHg) to approximately 2 mL. Then, 100 µL of the aroma concentrate was prepared using a gentle stream of nitrogen gas.

Gas chromatography-olfactometry/mass spectrometry (GC-O/MS)    GC-O/MS analyses were performed using a 7890 GC system combined with a 5975B mass spectrometer (Agilent Technologies, Santa Clara, CA, USA) and a sniffing port (GL Sciences, Tokyo, Japan) equipped with a TC-WAX column (30 m × 0.32 mm i.d., 0.50 µm film thickness; GL Sciences). We used helium as the carrier gas at 1 mL/min. The samples (0.2 µL) were injected using the splitless mode at 250 °C. The oven temperature was increased from 60 to 250 °C at 5 °C/min, and the mass spectrometer was operated in electron impact (EI) mode at 70 eV. The effluent at the end of the capillary column was divided between the mass spectrometer and the sniffing port. GC-O/MS analyses were performed by three expert panelists, and volatile compounds were identified by comparing their odor qualities, linear retention indices (RI), and mass spectra with reference compounds from our laboratory. RIs of the compounds were calculated from the retention times of n-alkanes.

AEDA    The flavor dilution (FD) factors for the odor-active compounds were determined by AEDA (Grosch et al., 1993). The FD factor of each odor-active compound in the aroma concentrates is the maximum dilution factor perceived by GC-O/MS. The higher the FD factor, the greater the contribution of the compound to the aroma of the sample. Stepwise dilutions of the mango aroma concentrates were done with ethanol to obtain dilutions ratios in the range of 1:31 to 1:3¹⁰. Each dilution was analyzed by GC-O/MS using the TC-WAX column.

AROMATCH    Compounds from the aroma isolates that interacted with the mango odor were screened using AROMATCH. This method is based on GC-O/MS and AEDA. A 100-g portion of fresh fruit from each mango cultivar was cut into ∼2 cm cubes and 10 g of 1% ascorbic acid solution was added. The mango was placed in a glass vessel at 25 °C, and the natural mango odor was released into the sniffing port at an air flow of 100 mL/min. AROMATCH was performed by using a combination of GC-O/MS analysis and sensory detection of the mango odors. We used serial dilutions of the mango aroma concentrates as described in the AEDA. The FD factors obtained by AROMATCH indicate the contributions of the compounds to the mango odors.

Results and Discussion

The aroma of freshly cut pieces of the three mango cultivars was evaluated by eight panelists; results showed a sweet, fruity, and lactone-like odor for Irwin, a green and sour odor for Carabao, and a fruity, sweet, sulfur, and green odor for Nam Dok Mai. The volatile aroma concentrates of each cultivar were obtained with SAFE. These odors mimicked the natural odors of the cultivars.

We used AEDA and AROMATCH to analyze the aroma concentrates from all three cultivars and 41 odor-active compounds were detected (Tables 1, 2). To identify the structures of these odor compounds, their odor characteristics, RIs, and mass spectra were compared to standard reference compounds from our laboratory. Thirty-six out of 41 detected compounds were identified, including three newly identified compounds (3-carene oxide, 4-acetoxy-2,5-dimethyl-3(2H)-furanone, (Z)-6-dodecen-4-olide). Furthermore, in Irwin, δ-3-carene and α-terpinolene were detected as abundant volatile compounds in a previous report (Shivashankara et al., 2006), while the compounds were not detected in AEDA and AROMATCH. Thus, volatile compounds with low odor thresholds may be detected by GC-O but not by GC (Taylor and Linforth, 2010).

Table 1. AEDA: Odor-active compounds determined from the prepared volatile aroma concentrates for the three mango cultivars: Irwin (I), Carabao (C), and Nam Dok Mai (N)

	RI			FD factor
No.	TC-WAX	compounda	odor qualityb	I	C	N
1	975	2,3-butanedione	milky	3	9	3
2	1024	α-pinene	green, tropical	729	243	1
3	1049	ethyl butanoate	fruity	81
4	1079	ethyl 3-methylbutanoate	fruity	81
5	1150	(Z)-3-hexenal	green		59049	59049
6	1087	hexanal	green	1	3	27
7	1165	β-myrcene	green, hop	81	3
8	1258	(E)-β-ocimene	green, terpene		3
9	1289	octanal	waxy, fatty		3
10	1342	2-acetyl-1-pyrroline	nutty, corn			2187
11	1368	(E,Z)-1,3,5-undecatriene	fruity	59049
12	1379	(Z)-3-hexenol	green, fiber-like		729	2187
13	1399	unknown	sulphor	59049		243
14	1455	3-carene oxide	plastic	19683
15	1467	3-(methylthio)propanal	cooked potato			3
16	1549	(E)-2-nonenal	green, fatty	243	81	3
17	1588	(E,Z)-2,6-nonadienal	green		243
18	1763	p-methylacetophenone	rice		729
19	1830	(E)-β-damascenone	honey, floral		243	243
20	1830	dihydro-β-ionone	waxy	6561
21	1837	geraniol	floral, rose	2187	243	2187
22	1909	2-phenylethanol	floral		3
23	1920	γ-octalactone	lactone, sweet, coconut	59049		2187
24	1926	(E)-β-ionone	floral		243
25	1934	unknown	sweet		243
26	1954	δ-octalactone	lactone, fatty, sweet, coconut	59049	243	6561
27	1965	caryophyllene oxide	spicy, terpeny			2187
28	1992	4-acetoxy-2,5-dimethyl-3(2H)-furanone	sweet	59049
29	1998	unknown	fruity		729
30	2016	γ-nonalactone	lactone, sweet	2187	243	19683
31	2017	4-hydroxy-2,5-dimethyl-3(2H)-furanone (HDMF)	sweet	59049	19683	59049
32	2052	unknown	spicy, tropical		59049
33	2059	p-cresol	animal	81	19683
34	2067	δ-nonalactone	lactone	2187
35	2128	γ-decalactone	lactone, peach	59049		2187
36	2195	3-hydroxy-4,5-dimethyl-2(5H)-furanone	sweet	9
37	2210	δ-decalactone	lactone	59049
38	2234	(Z)-7-decen-5-olide	lactone, butter	729		3
39	2405	unknown	bamboo, terpeny			243
40	2535	vanillin	sweet, vanilla		59049	59049
41	2550	(Z)-6-dodecen-4-olide	lactone	243

^a  Each compound was identified using expert agreement to determine its odor quality, retention index and mass spectrum compared to the standard reference compounds.

^b  Odor quality perceived at the sniffing port.

Table 2. AROMATCH: Odor-active compounds determined from the prepared volatile aroma concentrates for the three mango cultivars: Irwin (I), Carabao (C), and Nam Dok Mai (N)

	RI		FD factor and odor qualityb in AROMATCH
No.	TC-WAX	compounda		I		C		N
1	975	2,3-butanedione	1	milky	1	milky	9	milky
2	1024	α-pinene	243	green	2187	fresh, green	2187	fresh, green
3	1049	ethyl butanoate	1	fruity
4	1079	ethyl 3-methylbutanoate	9	fruity
5	1150	(Z)-3-hexenal			243	fresh, green, kiwi-like	2187	fresh, green
6	1087	hexanal	1	green	1	green	1	green
7	1165	β-myrcene	9	green	1	green
8	1258	(E)-β-ocimene			1	green
9	1289	octanal			1	green
10	1342	2-acetyl-1-pyrroline					2187	grainy
11	1368	(E,Z)-1,3,5-undecatriene	19683	green, fiber-like
12	1379	(Z)-3-hexenol			243	green	243	fresh, green
13	1399	unknown	59049	sulphor			2187	ripe
14	1455	3-carene oxide	59049	fruity
15	1467	3-(methylthio)propanal					243	flesh, body
16	1549	(E)-2-nonenal	81	green	3	green	3	green
17	1588	(E,Z)-2,6-nonadienal			3	green
18	1763	p-methylacetophenone			2187	green, fresh
19	1830	(E)-β-damascenone			3	potato, honey	3	floral
20	1830	dihydro-β-ionone	2187	green, floral
21	1837	geraniol	1	floral	3	floral, rose	6561	floral
22	1909	2-phenylethanol			1	floral
23	1920	γ-octalactone	59049	sweet, body			19683	body
24	1926	(E)-β-ionone			27	floral
25	1934	unknown			2187	fruity
26	1954	δ-octalactone	59049	sweet, body	3	sweet	2187	sweet
27	1965	caryophyllene oxide					27	spicy
28	1992	4-acetoxy-2,5-dimethyl-3(2H)-furanone	59049	sweet, sugar
29	1998	unknown			2187	body
30	2016	γ-nonalactone	243	sweet	243	sweet	19683	sweet
31	2017	4-hydroxy-2,5-dimethyl-3(2H)-furanone (HDMF)	19683	sweet, sugar	243	sweet, sugar	19683	sweet, sugar
32	2052	unknown			2187	tropical
33	2059	p-cresol	6561	ripe	243	animal
34	2067	δ-nonalactone	19683	sweet
35	2128	γ-decalactone	59049	fruity			9	fruity
36	2195	3-hydroxy-4,5-dimethyl-2(5H)-furanone	2187	sweet, sugar
37	2210	δ-decalactone	59049	fatty, sweet, milky
38	2234	(Z)-7-decen-5-olide	6561	sweet, fatty			243	sweet, fatty
39	2405	unknown					59049	ripe
40	2535	vanillin			59049	sweet	19683	sweet
41	2550	(Z)-6-dodecen-4-olide	59049	sweet, body

^a  Each compound was identified by expert agreement to determine its odor quality, retention index, and mass spectrum compared to the standard reference compounds.

^b  Odor quality perceived at the sniffing port for each original aroma.

AEDA    The AEDA results are shown in Table 1. HDMF (sweet) exhibited high FD factors in all cultivars: Irwin (FD 59049), Carabao (FD 19683), and Nam Dok Mai (FD 59049). HDMF has been identified in various mango cultivars, suggesting that it is a highly important volatile compound (Munafo et al., 2016; Chidley et al., 2016). In Irwin, lactones (γ-octalactone, δ-octalactone, γ-decalactone and δ-decalactone) showed high FD factors. The lactones had sweet, peach, and coconut notes, which might contribute to Irwin's sweet, fruity, and lactone-like flavors. In addition, 4-acetoxy-2,5-dimethyl-3(2H)-furanone (sweet) and (E,Z)-1,3,5-undecatriene (green, galbanum-like) also showed high FD factors. On the other hand, (Z)-3-hexenal (green) and vanillin (sweet) showed high FD factors in Carabao and Nam Dok Mai. In Carabao, p-cresol (animal) showed a high FD factor. Moreover, an unknown compound (No. 32) showed a spicy and tropical odor, which might also be important for the odor quality of Carabao. In Nam Dok Mai, γ-nonalactone (sweet) showed a high FD factor.

In summary, the AEDA analysis revealed compounds with low odor thresholds in each mango cultivar. It is well known that not only strong scents but also interactions between compounds with low odor thresholds are important factors that determine the smells of natural products. A study investigating beer aromas by Kishimoto et al. (2018) indicated that the overall synergistic contributions of multiple odorants were important for the beer aroma, none of which had any independent contributions to the overall characteristics (including sub-threshold components) of beer trademark odors. In other words, multiple odorants were required to interact with and balance each other in order to generate the well-equilibrated, characteristic beer scent. In this study we screened a number of volatile compounds from the aroma isolates of the three mango cultivars that interacted with natural mango odors by AROMATCH.

AROMATCH    The results of AROMATCH are shown in Table 2. In general, GC-O and AEDA allow the evaluation of odor qualities of separated compounds, while AROMATCH enables the evaluation of how these compounds act together with the natural flavors. Due to odor interactions, the FD factors and odor qualities of the compounds detected in AROMATCH were different from those in AEDA.

For Irwin, lactones and furanones showed high FD factors in both AROMATCH and AEDA. In AROMATCH, (E,Z)-1,3,5-undecatriene (green, fiber-like), 3-carene oxide (fruity), and (Z)-6-dodecen-4-olide had high FD factors (59049). Interestingly, 3-carene oxide was perceived as having a plastic-like odor by AEDA but fruity by AROMATCH. While 3-carene oxide is the oxidized form of δ-3-carene, the major compound for various mangoes including Irwin, it is unknown whether the formation of 3-carene oxide is related to an increase in the number of oxidants accompanying fruit ripening. In addition, p-cresol was described as having a malodorous animal note by AEDA, while it was described as having a ripe note by AROMATCH. To clarify whether 3-carene oxide and p-cresol are potential markers of mango ripening, more detailed studies are needed to investigate the relationship between ripening and the volatile compounds in Irwin, as in other cultivars (Marc et al., 2008; Pandit et al., 2009; Paulo et al., 2020).

Vanillin had the highest FD factor (sweet, FD: 59049) in Carabao, while an additional two compounds (α-pinene and p-methylacetophenone) also showed high FD factors (green, FD: 2187). A few unknown compounds (No. 25, 29, 32) also showed high FD factors (2187), and their odor qualities were fruity, body, and tropical, respectively, which suggested the effects of the aroma components in Carabao. In Nam Dok Mai, γ-nonalactone, HDMF, and vanillin also showed high FD factors (sweet, FD: 19683). The odor quality of γ-octalactone was described as sweet and coconut in AEDA, while it was described as body in AROMATCH.

On the basis of these results, the percent compositions of the identified odor-active compounds showing high FD factors (FD ≥ 19683) by AEDA or AROMATCH are shown in Table 3. Using AEDA and AOMATCH, the potent aroma components were identified in each cultivar: Irwin (No. 14, 23, 26, 28, 31, 35, 37, 41), Carabao (No. 5, 31, 33, 40), and Nam Dok Mai (No. 5, 23, 30, 31, 40). It appears that the odor qualities of these potent volatile compounds in AEDA and AROMATCH were associated with the aroma descriptions for the three mango cultivars by the eight panelists. The contents of the compounds in each cultivar were also related to their FD factors. This suggested that HDMF was one of the most important volatile components in all three cultivars, as reported previously, and the content in Irwin was more than 100 times higher than the two other cultivars. Furthermore, 3-carene oxide, 4-acetoxy-2,5-dimethyl-3(2H)-furanone, and (Z)-6-dodecen-4-olide were identified in mango for the first time, and are potent volatile compounds in mango, especially in Irwin. Interestingly, 3-carene oxide was described as a plastic-like odor by AEDA but fruity by AROMATCH. Moreover, the FD factor for (Z)-6-dodecen-4-olide in Irwin by AROMATCH (FD: 59049) was higher than that by AEDA (FD: 243). The biggest difference between AEDA and AROMATCH is whether or not odor-active compounds are evaluated in a complex odor. This suggested that the differences in those results were caused by odor interactions, e.g., 3-carene oxide and (Z)-6-dodecen-4-olide. To improve the detection of odor interactions in mangoes, it is important to examine the statistical sensory analyses of these results, focusing on changes in the odor thresholds and qualities.

Table 3. Percent composition of identified odor-active compounds (FD ≥ 19683) by AEDA or AROMATCH in three cultivars of mangoes: Irwin (I), Carabao (C), and Nam Dok Mai (N)

no.	compound	odor qualitya	I	C	N	FD ≥ 19683b
	Hydrocarbon
11	(E,Z)-1,3,5-undecatriene	fruity	tr.	-	-	I
Aldehyde
5	(Z)-3-hexenal	green	-	1.76	4.47	C, N
Epoxide
14	3-carene oxide	plastic	0.14	-	-	I
Furanones
28	4-acetoxy-2,5-dimethyl-3(2H)-furanone	sweet	0.02	-	-	I
31	HDMF	sweet	2.64	tr.	tr.	I, C, N
Lactones
23	γ-octalactone	sweet, coconut	1.06	tr.	tr.	I, N
26	δ-octalactone	sweet, coconut	1.54	tr.	0.46	I
30	γ-nonalactone	lactone, sweet	0.05	0.02	0.06	N
34	δ-nonalactone	lactone	0.03	-	-	I
35	γ-decalactone	lactone, peach	1.51	-	0.01	I
37	δ-decalactone	lactone	2.49	-	-	I
41	(Z)-6-dodecen-4-olide	lactone	tr.	-	-	I
Phenols
33	p-cresol	animal	tr.	0.03	-	C
40	vanillin	sweet, vanilla	-	0.13	0.04	C, N

tr.: < 0.01%, -: not detected.

^a  Odor quality perceived at the sniffing port in AEDA.

^b  FD ≥ 19683: the cultivars showed the FD factor ≥ 19683.

Acknowledgements    We are grateful to K. Kasai for the technical assistance and helpful discussions.

Funding    This research received no external funding.

Conflicts of Interest    The authors declare no competing financial interest.

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• i) http://www.fao.org/faostat/en/#data/QC/visualize (Jul. 16, 2020)
• ii) https://www.vcf-online.nl/VcfProdIndex.cfm?Catprodkey=10500 (Jul. 16, 2020)