Aroma perception of individual volatile compounds in fresh
tomatoes (
Lycopersicon esculentum
, Mill.) as affected by the
medium of evaluation
K.S. Tandon
a, E.A. Baldwin
b, R.L. Shewfelt
a,*
aDepartment of Food Science and Technology,Uni
6ersity of Georgia,Athens,GA30602,USA
bUSDA Citrus and Subtropical Products Laboratory,Winter Ha
6en,FL33883,USA
Received 7 December 1999; accepted 10 June 2000
Abstract
Odor thresholds of volatile compounds in fresh tomato (Lycopersicon esculentumMill.) were estimated and odor units calculated to determine whether the medium of evaluation affects aroma perception. The ‘ascending method of limits’ was used to determine odor thresholds of cis-3-hexenal, hexanal, trans-2-hexenal, hexanol, cis-3-hexenol, 2-isobutylthiazole, 6-methyl-5-hepten-2-one, geranylacetone, 2-pentenal, b-ionone, 1-penten-3-one, 3-methylbutanol, 3-methylbutanal, acetone and 2-phenylethanol in deionized water, an ethanol (100 ppm)/methanol (500 ppm)/water mixture and a deodorized tomato homogenate.Cis-3-hexenal exhibited the highest level of odor units in all three media. Odor thresholds were lower in deionized water for all compounds than in the ethanol/methanol/water mixture (simulating levels found in homogenized tomato) and the thresholds were even higher in the deodorized tomato homogenate for most compounds. Distinct differences were noted in aroma descriptors for the compounds in different media. The results suggest that both qualitative and quantitative changes are occurring in the perception of volatile compounds in the different media and that ethanol and methanol alter perception of tomato aroma. © 2000 Elsevier Science B.V. All rights reserved.
Keywords:Flavor interactions; Odor thresholds; Sensory descriptors
www.elsevier.com/locate/postharvbio
1. Introduction
Characteristic tomato (Lycopersicon esculen -tum) flavor represents a complex interaction of volatile and nonvolatile compounds (Petro-Turza,
1987). Some investigators have emphasized the importance of volatile compounds on the aro-matic component of flavor (Buttery et al., 1987; McGlasson et al., 1987; Langlois et al., 1996; Baldwin et al., 1998; Brauss et al., 1998; Krum-bein and Auerswald, 1998), while others have emphasized the contribution of sugars and acids (Stevens et al., 1979; Hobson and Bedford, 1989; Malundo et al., 1995).
* Corresponding author. Tel.: +1-706-5425136; fax: + 1-706-5421050.
E-mail address:shewfelt@arches.uga.edu (R.L. Shewfelt).
Tomato taste is attributed to a combination of sweetness (fructose and glucose) and sourness (cit-ric and malic acids) (Malundo et al., 1995). While over 400 volatile components have been identified in fresh tomatoes, the most important compounds are thought to becis-3-hexenal, hexanal,trans -2-hexenal, hexanol, cis-3-hexenol, 2-isobutylthia-zole, 6-methyl-5-hepten-2-one, geranylacetone, 2-phenylethanol, b-ionone, 1-penten-3-one, 3-methylbutanol, 3-methylbutanal (Buttery and Ling, 1993), acetone and 2-pentenal. The main precursors of these volatile compounds in tomato are the free amino acids, fatty acids and carotenoids (Table 1).
The C6 aldehydes (hexanal, cis-3-hexenal,
trans-2-hexenal) are known as ‘green’ compounds, as they impart a fresh, green character to tomato aroma, while ketones (acetone, geranylacetone andb-ionone) are contributors to a fruity aroma. These compounds have also been isolated from the nosespace of individuals consuming fresh tomatoes (Brauss et al., 1998)
It is not clear how much volatile compounds in tomatoes interact with other components in the fruit. The objective of this study was to determine the odor thresholds of the primary volatile com-pounds in fresh tomato aroma in three different media — deionized water, an ethanol/methanol/ water mixture (simulating levels normally found in homogenized ripe tomatoes) and a deodorized tomato homogenate — to determine if the medium affects aroma perception.
2. Materials and methods
2.1. Volatile compounds
Fifteen volatile compounds previously iden-tified as important contributors to the aroma of fresh tomatoes (Buttery et al., 1987; Petro-Turza, 1987) were studied. Authentic food grade samples of these compounds were obtained from reliable commercial sources. Compounds and sources fol-low: acetone (Sigma, 99% purity), geranylacetone (Sigma, 97%), hexanal (Sigma, 97%), trans -2-hex-enal (Sigma, 95%), cis-3-hexenal (Bedoukian Re-search, CT, 98%), cis-3-hexenol (Sigma, 98%),
b-ionone (Sigma, 95%), hexanol (Sigma, 98%), 3-methylbutanal (Sigma, 97%), 3-methylbutanol (Sigma, 97%), 6-methyl-5-hepten-2-one (Sigma, 98%), 2-phenylethanol (Sigma, 99%), trans -2-pen-tenal (Sigma, 95%), 1-penten-3-one (Sigma, 95%), 2-isobutylthiazole (Sigma, 99%), ethanol (Aaper Alcohol and Chemical Co., 98%) and methanol (Fischer, 99%).
2.2. Deodorized homogenate
Tomatoes were purchased at the red-ripe stage of maturity from local commercial vendors, cut into wedges and blended for 30 s to homogeneity in an Oster blender (Oster Corp., Milwaukee, WI). The homogenate was then frozen until evap-oration. After thawing in tepid water, a 500-ml aliquot was placed in a 1-l round-bottomed flask and evaporated at 50°C, 26 – 27 psi (rotation set-ting 5.5 – 6.0) for 4 h on a rotary evaporator (Glasapparatefabrik, Buchi, Switzerland). GC analysis revealed that volatiles had been reduced to non-detectable or trace levels except for methanol, which was most likely due to residual pectinmethylesterase activity (Baldwin et al., 2000). The procedure was repeated for all batches. Deodorized homogenate fractions were frozen un-til further use.
2.3. Sensory panel
A sensory panel, consisting of 11 members (six male, five female; aged 22 – 47) was convened. Participation in all sessions was required to com-Table 1
Origins of the volatiles considered important to the fresh tomato aroma (Buttery and Ling, 1993)
Amino acid
Lipid derived Carotenoid derived derived
plete the study. All panelists indicated that they were consumers of fresh tomatoes and none re-ported adverse reactions to tomatoes.
2.4. Test location
All evaluations were conducted at the Food Process Research and Development Laboratory of the Department of Food Science and Technol-ogy at the University of Georgia, Athens, GA. Round-table discussions were conducted prior to the beginning of the test to familiarize the panel with the test instructions and protocol. Parti-tioned booths were located in a temperature-con-trolled environment (:20°C) and equipped with fluorescent lights. Minimal distractions were per-mitted to interfere with panelist judgements.
2.5. Sensory method
Odor thresholds of the tomato aroma com-pounds were determined by a modification of the ‘ascending method of limits’ of the American Society for Testing and Materials (ASTM, 1991; Meilgaard et al., 1991). Panelists were asked to select the correct sample from one of two samples (one containing the compound and the other the control — test medium with no added com-pounds). The method provides a ‘best estimate threshold’ (BET) for each panelist, based on the geometric mean of the highest concentration missed and the next highest concentration. A group threshold is then calculated as the geomet-ric mean of the individual BETs. This method has been used to determine thresholds for wine vine-gar (Gonzalez-Vinas et al., 1996) and vanillin in water (Powers and Shinholser, 1988).
2.6. Sensory e6aluation
Threshold evaluations were conducted daily be-tween 09:30 and 11:00 h. Samples were prepared within 1 h prior to evaluation in three media: deionized water, an ethanol/methanol/deionized water mixture (simulating levels normally found in homogenized ripe tomatoes) and the deodor-ized tomato homogenate. Volatile compounds were added to these media at concentrations
above and below the anticipated threshold, based on preliminary assessment. Aliquots (20 ml) of the samples were placed in 25-ml clear polyethylene vials and immediately capped. Samples were pre-sented to the panelists in partitioned sensory booths. Panelists were instructed to inhale the vapors from a pair of vials (one of which con-tained a specific volatile compound and the other a control) and choose the sample that contained the compound. The first pair in each set of sam-ples contained the highest concentration of the compound to alert the panelist to the specific odor being tested. The subsequent four samples (repre-senting four lower concentrations of the same compound in a single medium) were presented in random order to prevent habituation when sam-ples are presented in ascending or descending order (Pangborn, 1984). Panelists were forced to make a choice whether they were able to detect a difference or not. Each panelist evaluated each set of samples twice over a 3-week period such that all evaluations represent a composite of 22 evalua-tions. Panelists were also instructed to provide odor descriptors for each of the volatile com-pounds in each of the three media.
An odor unit (U0) is defined as the ratio of the
concentration of a compound to its threshold concentration (Guadagni et al., 1996). A com-pound with a positive log odor-unit value (occurs at a concentration above its threshold) is consid-ered likely to contribute to the aroma of a food, while one with a negative value is considered unlikely to contribute to the aroma.
3. Results
Table 2
Odor thresholds (ppb) of tomato aroma compounds in deionized water, methanol/ethanol/deionized water mix and deodorized tomato homogenate
Aroma compound Deionized water EtOH/MeOH/water Homogenate
170
Cis-3-hexenal 26 469
1047 2427
Cis-3-hexenol 479 1602
2-Pentenal 55 243 2265
1008
b-Ionone 23 197 431
2549 17433
832 Acetone
Table 3
Log odor units of tomato aroma compounds in deionized water, ethanol/methanol/deionized water mix and deodorized tomato homogenate
Deionized water EtOH/MeOH/water
Conc. in tomatoes* (ppb) Tomato homogenate
Compound
Cis-3-hexenal 12 000 2.66 1.85 1.41
0.81 0.47
3100 0.10
Hexanal
1900
Phenylethanol 0.59 0.14 −0.75
520
1-Penten-3-one 1.35 0.57 0.11
0.73 0.10
380 −0.53
3-Methylbutanol
270
Trans-2-hexenal 0.34 −0.02 −0.34
−0.36 −0.50
150 −1.03
Cis-3-hexenol
0.40 −0.24
2-Pentenal 140 −1.21
−0.60 −0.88
130 −1.60
6-Methyl-5-hepten-2-one
−0.51 −1.03
Geranylacetone 57 −1.08
0.09 −0.43
36 −0.45
2-Isobutylthiazole
27
3-Methylbutanal 0.65 0.25 1.13
7
Hexanol −2.03 −2.18 −2.18
−0.76 −1.69
4 −2.03
b-Ionone
* Buttery et al. (1987).
cis-3-hexenal. Similar increases were noted in odor thresholds determined in the deodorized ho-mogenate with a few notable exceptions. The threshold for 3-methylbutanal was actually lower in the homogenate than in deionized water or the ethanol (100 ppm)/methanol (500 ppm)/water mix (to approximate concentrations in ripe fruit) while similar to the threshold in deionized water for 6-methyl-5-hepten-2-one.
etha-nol/methanol/water mix were 2-phenylethanol and 3-methylbutanol.Trans-2-hexenal, 2-pentenal and 2-isobutylthiazole were above threshold only in deionized water. Other compounds such as cis-3-hexenol, 6-methyl-5-hepten-2-one, gerany-lacetone, hexanol and b-ionone were below threshold in all three media.
Comments made by panelists during the threshold determinations suggested that the aro-matic character varied among media (Table 4). Some compounds showed similarities in all three media. For example, hexanal conveyed a ‘stale’ or ‘rancid’ note across the media, cis-3-hexenal was associated with ‘tomato-like’ aroma,b-ionone was associated with a ‘sweet’ or ‘floral’ note and 2-isobutylthiazole tended to be objectionable in each medium. In contrast, hexanol was described as ‘mint’ in deionized water, ‘alcohol’ in the etha-nol/methanol/water mix, and ‘glue’ or ‘oil’ in the deodorized homogenate. Acetone was described as ‘glue’ or ‘alcohol’ in deionized water, ‘sweet’ in the ethanol/methanol/water mix, and ‘green’ in the deodorized homogenate, while 3-methylbu-tanol was found to be ‘earthy’ in deionized water, ‘alcohol’ in the ethanol/methanol/water mix and ‘sweet’ or ‘fresh’ in the deodorized homogenate.
4. Discussion
The results of this study (Table 2) support earlier reports (Buttery et al., 1987, 1989; Brauss et al., 1998; Krumbein and Auerswald, 1998) that cis-3-hexenal is an important contributor to tomato aroma and flavor. Possible explanations for the differences in odor thresholds obtained in water between this study and earlier reports (But-tery et al., 1987, 1989, 1990) include greater sensi-tivity or more intense training of panelists in the previous studies and a smaller panel size in this study. The danger in interpretation of the two studies would be the rejection by this study of the contribution of a compound that does affect tomato aroma (Type I error) or the acceptance by previous studies of the contribution of a com-pound that does not affect tomato aroma (Type II error).
Of perhaps more interest is that during testing, different media produced different thresholds (Table 2) and thus, different odor units (Table 3). Therefore, in assessing the importance of a volatile compound in tomato aroma, it is neces-sary to first identify the best medium for testing. Results presented here show that ethanol and methanol tend to increase odor thresholds and
Table 4
Odor descriptors for tomato aroma compounds in deionized water, ethanol/methanol/deionized water mix and deodorized tomato homogenate
Tomato homogenate Aroma compound Deionized water EtOH/MeOH/water
Cis-3-hexenal Grass/tomato-like Alcohol/tomato-like Tomato/citrus Stale/grassy/green
Hexanal Grassy/green Rancid/stale oil
Floral/roses Alcohol/nutty
Phenylethanol Fruity/green/leafy
Glue/oil/pungent
1-Penten-3-one Pungent/rancid Fresh/sweet
3-Methylbutanol Earthy/watermelon rind Alcohol Sweet/fresh Trans-2-hexenal Floral/grass/apple Fruity/almond/vine Stale/green/vine
Leafy/cut grass
Cis-3-hexenol Fresh cut grass Green/celery
Stale/oil 2-Pentenal Vine/organic solvent Alcohol/nutty
Sweet/floral 6-Methyl-5-hepten-2-one Raw greens/nutty Alcohol/paint
Sweet/citrus/ester Geranylacetone Sweet/paint/sharp Glue/mint
2-Isobutylthiazole Fermented/plastic Acetone/medicine Pungent/bitter Nutty/glue/alcohol Stale/rotten Bug spray/alcohol
3-Methylbutanal
Glue/oil
Hexanol Mint/grass Alcohol
Sweet/perfume-like
b-Ionone Sweet/floral/leafy Sweet/floral
Glue/alcohol Green
Acetone Sweet
Ethanol Earthy/stale
thus suppress odor perception, but the effect is greater for some volatile compounds than others. When looking at changes in compounds with similar thresholds in water, increases due to etha-nol/methanol mixes for cis-3-hexenal and b -ionone tended to be greater than for 2-isobutylthiazole, while increases for hexanol and 6-methyl-5-hepten-2-one were less than for 2 -phenylethanol. Likewise, the deodorized ho-mogenate further suppressed aroma perception for all compounds except 6-methyl-5-hepten-2-one and 3-methylbutanal. Suppression of hexanol and 2-isobutylthiazole was minimal, while dramatic increases in thresholds were observed for 2-pente-nal, 2-phenylethanol, 3-methylbutanol, and ace-tone. Odor suppression by the ethanol and methanol mix lowered the number of compounds with positive odor units from nine in deionized water to six with little relative effect on order of odor units. These results suggest that accumula-tion of ethanol during ripening as reported by Ratanchinakorn et al. (1997) and changes in alco-hol content by modification of alcoalco-hol dehydroge-nase (Speirs et al., 1998) could have effects on tomato flavor. Thus, analysis of tomato flavor appears to be more complex than the interaction of taste components and odor-active volatile compounds.
Perception of the compounds in tomato ho-mogenate further reduced the number with posi-tive odor-unit values to four. These changes showed an increase in relative importance for 3-methylbutanal, 2-isobutylthiazole and trans -2-hexenal with a decrease for 2-phenylethanol, 3-methylbutanol and 2-pentenal. Thus, panelists were less able to perceive volatile compounds in deodorized homogenate than in deionized water and the relative contribution of each compound was different.
Descriptors generated for compounds evaluated in deionized water using individual perceptions of panelists (Table 4) were in general agreement with those reported by Kazeniac and Hall, (1970). All C6 compounds (cis-3-hexenal, trans-2-hexenal,
hexanal, hexanol and cis-3-hexenol) were de-scribed as green, grassy or floral, withcis -3-hexe-nal also described as tomato-like. A spoiled fermentative odor was associated with
2-isobutylthiazole similar to that of spoiled, vine-like and slightly horseradish-type description by Kazeniac and Hall (1970). Two ketones (b-ionone and geranylacetone) were predictably described as sweet and fruity, while acetone and 1-penten-3-one connoted glue, alcohol or pungent. Ethanol, methanol and 2-methylbutanol were described as earthy and stale. The recurring descriptors among the ten compounds with positive odor units in deionized water were grassy, green, alcohol, earthy, vine-like and floral.
Of further interest was the perception of differ-ent types of odors as affected by evaluation medium. Cis-3-hexenal was described as tomato-like in all three media, but most compounds elic-ited different response in different media. Both hexanal and trans-2-hexenal connoted rancid and stale odors in the ethanol/methanol mix and the deodorized homogenate. Many compounds (hex-anol, 6-methyl-5-hepten-2-one, cis-3-hexenal, 2-pentenal and 3-methylbutanol) elicited an alcohol odor in the ethanol/methanol mix but not in deionized water, while others did not elicit the alcohol response. Several compounds (6-methyl-5-hepten-2-one, 3-methylbutanol and 1-penten-3-one) were described as sweet or fresh in the deodorized homogenate but not in deionized wa-ter. The six compounds with positive odor units in the ethanol/methanol mix were characterized by rancid, stale, alcohol, tomato-like, fruity, green, nutty, glue and pungent. Ethanol has been associ-ated with enhanced sweetness perception in fresh tomatoes (Rothe and Schrodter, 1996), while methanol does not appear to contribute to the flavor of tomato juice or paste (Kazeniac and Hall, 1970). The four compounds with positive odor unit values in the deodorized homogenates were characterized by stale, green, grassy, fresh, tomato-like and citrus. These data suggest that qualitative changes are occurring as well as quan-titative changes in the perception of the volatile compounds in different media. In addition, etha-nol and methaetha-nol may alter perception of tomato aroma.
perceived. There appear to be important interac-tions between aromatic components in fresh tomato flavor, with ethanol and methanol (at levels normally found in homogenized red-tomato tissue, but below their odor thresholds) serving as probable potentiators of sensory perception. An increase in compounds contributing to floral (6-methyl-5-hepten-2-one and b-ionone), fruity (cis -3-hexenal and geranylacetone) and fresh (3-methylbutanol and 1-penten-3-one) notes or a decrease in compounds contributing to stale (hex-anal, trans-2-hexenal and 3-methylbutanal), pun-gent (2-isobutylthiazole) and alcohol (2-phenyl-ethanol) notes would likely be beneficial to tomato flavor. These recommendations directly contradict those of Gray et al. (1999) who recom-mend increasing levels of C6aldehydes. Consumer
testing will be required to determine which set of recommendations will enhance acceptability of tomatoes.
Acknowledgements
Supported by NRI Competitive Grants Pro-gram/USDA award 9801482.
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