Euphytica
32 (1983)
845-855
IMPROVEMENT
OF TOMATO FLAVOR BY
GENETICALLY
INCREASING
SUGAR AND ACID
CONTENTS
R. A. JONES
Department
of Vegetable
Crops,
and S. J. SCOTT
University
Received
of California,
4 January
Davis,
CA. 95616,
USA
1983
INDEXWORDS
Lycopersicon
esculentum,
tomato,
flavor,
sensory
evaluation,
soluble
solids.
SUMMARY
Advanced
high sugar and acid breeding lines of tomatoes
(Lycopersion
esculentum
MILL.) were rated higher
in sweetness,
sourness and overall flavor intensity
than the standard
cultivars
Cal Ace or T,. Titratable
acidity and soluble solids content were major contributors
to differences
in overall flavor intensity.
The
results demonstrate
that significant
improvement
in tomato flavor can be attained by increasing
sugar and
acid contents
in tomato fruits by genetic manipulation.
Current
evidence indicates
that breeding for high
soluble solids in horticulturally
acceptable
tomato cultivars
is justified.
INTRODUCTION
Fresh tomato quality is a complex characteristic, involving not only physical appearance and firmness, but also flavor and nutritive value. Over the past decade considerable research effort has been directed towards understanding the contribution that
various fruit constituents make to the edible quality of fresh tomatoes. Continued
explorations into the role of organic compounds in determining flavor attributes are
pertinent amidst continued reports of consumer dissatisfaction with the quality of
fresh fruits and vegetables (MILLER, 1974; HANDY & PFAFT, 1975).
Changes in tomato fruit composition during maturation and ripening have been
reviewed (HOBSON &.DAVIES, 1971; SALUNKHE et al., 1974). Total solids and total sugars increase progressively during development and maturation (WINSOR et al., 1962a;
1962b). Generally, acidity in tomatoes increases during development and reaches a
maximum at the breaker stage, then decreases with further ripening (DAVIES, 1966;
STEVENS, 1972). Among the organic substances in tomato fruits, the sugars and acids
are major constituents important to tomato flavor differences and affecting tomato
flavor quality. SIMANDLE et al. (1966) compared the composition of 6 tomato cultivars
with their eating quality and found panel flavor scores to be signiticantly correlated
with soluble solids and pH. DE BRUYN et al. (1971) concluded that high sugar and
acid contents generally have a favorable effect on taste, and that flavor intensity was
more closely correlated with acid content than with sugar content of greenhouse tomato cultivars. Sugars and acids not only contribute to sweetness and sourness of tomatoes (DE BRUYN et al., 1971; STEVENS et al., 1977), but also appear to be important
845
R. A. JONES
AND
S. 1. SCOTT
factors in determining overall flavor intensity (STEVENSet al., 1979). In addition, numerous volatile compounds have a significant effect on flavor variation (DE BRUYN
et al., 1971; KAZENIAC &HALL, 1970; KADER et al., 1977).
Few investigators have evaluated the impact of fruit maturity at picking on flavor
quality of tomato fruits. The inability of table-ripe fresh market tomatoes to withstand
the handling necessary for long distance transport has led to the standard practice
of harvesting fruits at earlier stages of maturity which will permit handling, and subsequently ripen them in transit or at destination. Evaluating the impact of picking stage
on flavor quality of tomato fruits, BISOGNI et al. (1976) reported that vine-ripened
tomatoes had better flavor and overall quality than room-ripened tomatoes. Similarly
STEVENSet al. (1977) found that fruit picked mature green or at the breaker stage
(incipient red color) and ripened off the vine to table-ripe had poorer flavor quality
than fruit picked at the table-ripe stage. In the former study no significant differences
were found in pH, titratable acidity or soluble solids content, while in the latter study
tomatoes picked at earlier stages of ripeness had lower soluble solids content. It is
probable that problems with tomato quality, particularly flavor, result from harvest
and post-harvest handling procedures, and/or to genotypic differences in responses
to these procedures.
Tomato fruits picked green and ripened off the plant would appear then to have
lower flavor quality than vine ripened fruits. The purpose of these investigations were
to determine what impact high concentration of sugars and acids have on flavor quality
in advanced high solids breeding lines and in specially developed Ft hybrids and to
determine what effect maturity at picking has on tomato composition and flavor at
the table-ripe stage.
MATERIALSANDMETHODS
Genotypes. The genotypes studied included the control lines cv. Cal Ace or Ts, several
advanced high sugar breeding lines developed in our fresh market tomato breeding
program and selected Fi hybrids between the high soluble solids parents. The high
solids lines were: 1) HS89, derived from STEP375; 2) HS70, derived from an interspecific cross (L. peruvianurn); and 3) HS93. Plants were grown in the field at Davis,
California using standard cultural practices. Several plantings were made to provide
fruits throughout the normal harvest period (August through September).
Fruit sampling. Fruits were harvested at the table-ripe stage on the day of evaluation.
All samples were havested within a 5 day period to minimize possible environmental
effects on fruit composition. Excess fruits were picked, sorted and only fruits in good
condition with uniform color, size and firmness were selected for use. Each sample
contained a minimum of 20 fruits which were cut in half from stem-end to blossom
end. One group of half-fruit was diced (about 1 cm3) and thoroughly mixed for sensory
analyses. The remaining lot of half-fruit was retained for chemical analyses.
In a separate study, fruits of ‘Cal Ace’, HS70, HS89, HS93, and HS65 were harvested
at the full-size, mature-green stage. Fruits were held at 20°C under high relative humidity to ripen. The green fruits were sorted after 5 days in storage and those showing
color development were subjectively selected as mature-green (MG) when harvested.
When these fruits reached the table-ripe (TR) stage they were used for sensory and
846
Euphytica
32 (1983)
TOMATO
compositional
of each line.
FLAVOR
IMPROVEMENT
evaluations in comparison with freshly harvested, vine-ripened fruits
Compositional analyses. The fruit samples retained for chemical determinations were
each blended, filtered through Whatman # 1 paper, and the serum used for chemical
analyses. Titratable acidity was determined .by titrating 5 ml of the filtered tomato
serum to pH 8.1 with 0. IN NaOH and pH was measured with a Corning digital 109
pH-meter. Soluble solids content (SK) was determined as “Brix with a table-top model
ABBE-3L Bausch & Lomb refractometer. Reducing sugars were determined by the
method of SOMOGYI(1952).
Sensory evaluation and statistical analyses. Panelists were instructed in the use of the
scoring system, described previously (KADER et al., 1977) for the flavor characteristics
of sweetness, sourness, ‘tomato-like’, and overall flavor intensity. Genotype and treatment comparisons were presented in random order to panelists as a complete block
design, with blocking by days. For each flavor characteristic, standard deviations of
individual judges were calculated by genotype and 1.5judges were selected for lower
deviations from the aggregate mean response. Analysis of variance was performed
for the sensory scores and the compositional analyses. Significance levels were determined with Duncan’s New Multiple Range test (STEEL& TORRIE, 1980). Results of
the sensory evaluations were analyzed by multivariate analysis of multiple covariance
(COOLEY& LOHNES, 1971) in two ways. First, all chemical variables, soluble solids
(SSC), reducing sugars (RS, total titratable acidity (TTA), total acidity (TA), and
pH were used as covariates. Secondly, only SSC and TTA were used as covariates.
All four sensory variables were simultaneously evaluated as dependent variables. Results of these analyses were essentially identical as to significance of covariate terms
in the model and in adjusted R2 values for each dependent (sensory) variable. Therefore, results using only SSC and TTA as covariates are reported here, except for the
correlation matrix from the first analysis.
RESULTS
Cultivar studies. Comparisons between cultivars revealed significant variations in pH,
titratable acidity, reducing sugars and soluble solids content (Fig. 1). Genotypic differences in total soluble solids and reducing sugars showed a similar pattern. Fruits of
the high sugar breeding lines had significantly higher pH, reducing sugars and total
soluble solids content than ‘T3’. The control cultivar T3 had significantly higher total
titratable acidity and a lower pH than the three high sugar lines. HS93 had the highest
sugars and was intermediate in both pH and titratable acidity. HS89 and HS70 were
intermediate in sugars content. HS89 had the highest pH and the lowest titratable
acidity, while HS70 had the highest titratable acidity among the high sugar lines.
The F, hybrids had significantly greater levels of reducing sugars and soluble solids
content than ‘T3’ (Fig. 1). One hybrid, HS89 x HS70, had soluble solids content
comparable to both high sugar parents in the cross, while fruit of the hybrid between
HS93 and HS70 had levels similar to the lowest parent in the cross. HS93 and fruit
of the hybrid HS93 x HS89 had significantly greater soluble solids and reducing sugar
content than any of the genotypes examined. The hybrids had significantly greater
Euphytica
32 (1983)
847
R. A. JONES
REDUCING
SUGARS
SOLUBLE
CONTENT
AND
SOLIDS
S. J. SCOTT
PH
TITRATABLE
ACIDITY
d
i
d
z
I
a4
P
4.25
5
4
3
4.00
0
1234567
GENOTYPE
Fig. 1. Mean composition
of ‘Tj’ (l), three high sugar parents (HS93 = 2; HS89 = 3; HS70 = 4) and
3 high sugar hybrids
(2 x 4 = 5; 3 x 4 = 6; 2 x 3 = 7). Means among lines not labeled by the same
letter differ significantly
at the 5% level.
pH than ‘T3’, but lower than HS89. Titratable acidity in the hybrids was greater than
HS93 and HS89, comparable to HS70, but generally lower than ‘T3’. The exception,
HS93 x HS70 had levels similar to ‘T3’.
Panelist scores for sweetness, sourness and tomato-like flavor varied significantly
among the lines (Fig. 2). The differences for overall flavor intensity were not signiticant. HS90 had the highest sweetness score; HS89 and HS70 were intermediate. ‘T3’
had the lowest sweetness score and the highest sourness score. HS89 was equivalent
in sourness to ‘T3’ while that of HS90 and HS70 were lower. HS90 and HS70 had
the highest tomato-like scores, while HS89 was intermediate and ‘T3’ was lowest.
Generally, the Fi hybrids between the high sugar parents had greater sweetness,
sourness, tomato-like and overall flavor intensity than ‘T3’ (Fig. 2). In comparison
with the parental lines, the Fi hybrids tended to have greater sourness and overall
flavor intensity, but equivalent tomato-like flavor. The exception was hybrid HS89
x HS93 which was rated higher in tomato-like flavor. Hybrid HS89 x HS70 was
rated higher in sweetness than either of its parents. Sweetness in hybrid HS89 x HS93
was identical to that of the higher solids parent in the cross, while fruits of HS93
x HS70 were identical to the lower solids parent.
Differences in sweetness were related to variations in reducing sugars and soluble
solids content (Table 1). Sweetness also showed a strong correlation with tomato-like
and to a lesser degree, with overall flavor intensity. Tomato-like flavor was also highly
848
Euphytica32
(1983)
TOMATO
7
I
3
FLAVOR
SOURNESS
SWEETNESS
IMPROVEMENT
TOMATO-LIKE
OVERALLINTENSITY
6!
’
2 ’
OL
1234567
1234567
GENOTYPE
Fig. 2. Mean sensory scores for flavor character
of ‘T,‘, three high sugar parents and three high sugar
hybrids.
Refer to Fig. 1 for genotype
entries. Means not labelled with the same letter differ significantly
at the 5% level.
correlated with overall flavor intensity. In this study sourness showed a low correlation
with overall flavor intensity. Sourness was surprisingly not strongly correlated with
titratable acidity or with pH, but this probably reflects the lack of a definite trend
of acid concentration and the lack of a consistent relationship between titratable acidity and acid concentration among the genotypes.
As much as 59”/, of the variance in sweetness, adjusted for degree of freedom, was
Table 1. Correlation
matrix
for flavor
3 F, hybrids harvested
at the table-ripe
Variable
Sensory
Sourness
‘Tomato-like’
Overall intensity
PH
Titratable
acidity
Reducing sugars
Soluble solids
* and ** indicates
Euphytica
32 (1983)
characteristics
stage.
and chemical
components
cuhivars
and
parameter
sweetness
sourness
‘tomato-like’
overall
-0.07
0.53**
0.37*
0.08
-0.05
0.47**
0.49**
0.17
0.37*
-0.25
0.32
PO.12
-0.14
0.58**
0.10
-0.02
0.29
0.35*
4.10
0.13
0.11
0.1 I
significant
in 4 tomato
difference
intensity
at the 5% and 1% level, repectively.
849
R. A. JONES
IEDUCING
w
SUGARS
AND
S. I. SCOTT
SOLUBLE
SOLIDS
CONTENT
(%I
PH
TlTRATABLE
ACIDITY
(% CITRIC)
q IMG
u
TR
f
?
e
R5
c
0.4
0.3
12345
12345
12345
il12345
GENOTYPE
Fig. 3. Mean composition
of five tomato
cultivars
(1 = Cal Ace; 2 = HS89; 3 = HS93; 4 = HS65; 5
= HS70) harvested
at two stages of ripeness. Overall
means for each picking
stage not labelled with the
same letter differ significantly
at the 5% level.
described by the covariate model (Table 2). Analysis using soluble solids, total titratable acidity, reducin sugars, pH and total acidity as covariates did not increase the
adjusted or simple R !?values. The contribution of soluble solids to the taste characteristic of sweetness can be reasonably understood, but additional modifying factors
other than titratable acidity are apparently also involved but were not measured. Analysis of sourness indicated that the covariate model accounted for only 41% of the
variation observed in panelist responses. ‘Tomato-like’ is even more complex, since
Table 2. Multivariate
and concentrations
analysis of covariance
of sensory characteristics
of the covariates
in the model, acids and sugars.
Flavor
characteristic
(dependent
variables)
R2
simple
adjusted’
Sweetness
Sourness
Tomato-like
Overall flavor
intensity
0.75
0.64
0.63
0.59
0.41
0.39
0.63
0.38
’ Adjusted
of freedom.
850
of tomatoes
for degrees
Euphytica
32 (1983)
TOMATO
r SWEETNESS
FLAVOR
SOURNESS
IMPROVEMENT
TOMATO-LIKE
OVERALLINTENSITY
IJMG
rnTR
a
~
1
2
4
5
12
34
5
12345
GENOTYPE
Fig. 4. Mean sensory scores of five tomato cultivars
harvested
at two stages of ripeness.
3 for entries. Means not labelled with the same letter differ significantly
at the 5% level.
Refer
to Fig.
the model described only 39% of the variance observed for this taste attribute. The
covariates accounted for only 38% of the variation in overall flavor intensity of tomato
fruits.
Picking stage. Fruits picked at the mature green (MG) stage and ripened to table ripe
(TR) at 20 “C generally had significantly different levels of chemical components than
fruit ripened on the plant and picked at the TR stage (Fig. 3). The pH was significantly
lower in fruits picked at the MG stage than in the TR fruits in all genotypes, except
HS89 where the 2 picking stages were identical. The differences in titratable acidity
between the two stages were significantly higher in MG fruits of HS93 and HS65 and
although not significantly different in the remaining genotypes, also tended to be
higher in MG fruits. Reducing sugars and soluble solids content are also lower in
fruits picked at the MG stage than those left to ripen on the vine. Large differences
were observed among the lines for both soluble solids and reducing sugars for the
two stages of fruit ripeness. The high sugar lines showed a larger decrease in sugars
when fruits were picked MG compared to TR than did ‘Cal Ace’ which overall has
significantly lower levels of reducing sugars. Even though fruits of the high sugar lines
picked MG had approximately 1% less sugars or soluble solids than their respective
TR fruits, the MG levels were significantly greater than that found in fruits of ‘Cal
Ace’ left to ripen on the vine. The exception was fruits of HS93 picked MG which
were equivalent to TR fruits of ‘Cal Ace’.
Fruits picked at the mature green (MG) stage and ripened to TR were significantly
different in flavor characteristics than those ripened on the plant and picked at the
TR stage. Significant differences were observed for all sensory characteristics evaluatEuphytica
32 (1983)
851
R. A. JONES
AND
S. J. SCOTT
ed except overall flavor intensity (Fig. 4). In general, fruits picked at the TR stage
were sweeter, less sour and had more tomato-like flavor. The high soluble solids lines
had considerably higher sweetness scores in fruits picked on the vine at the TR stage
than in fruits picked at the MG stage and ripened off the vine to TR. Large differences
in sourness were observed in all lines with fruits picked at the MG stage having signiticantly higher sourness scores than their respective TR fruits, but HS89 showed no
significant differences. All the lines showed a difference between fruits at the mature
green and the table ripe stage for ‘tomato-like’ flavor, with fruits picked at the TR
stage having significantly higher scores.
Generally, the high soluble solids lines did not differ among themselves in overall
flavor intensity, nor did individual genotypes differ between fruits picked at the two
stages of maturity. However, in comparison with TR fruit of ‘Cal Ace’, the high solids
lines exhibited significantly greater overall flavor intensity whether they were picked
MG or TR. The correlation matrix shows a strong positive relationship between sourness and titratable acidity and a negative relationship with pH (Table 3). Differences
in sweetness were related to variations in soluble solids and reducing sugars. Overall
flavor intensity was correlated with sourness, titratable acidity and sugars. The tomato-like flavor characteristic was correlated with sweetness and sugars and was also
related to overall flavor intensity.
DISUSSION
Great difficulty is encountered when attempting to improve flavor quality in a breeding
program. Perception of flavor is a complex sensation involving the tastes and aroma
of numerous compounds (BUTTERY et al., 197 1; STEVENS et al., 1977). The large number
of compounds involved and their subtle interactions underscores the difficulty in dealing effectively with flavor attributes. In addition, sensory analysis requires large sample
sizes for meaningful statistical comparisons, because of great variability in panelist
evaluations and variability in fruit composition. Thus, the ability to discriminate
among genetically different progenies becomes a burdensome task not easily accomodated within even a modest breeding program. Attempts to overcome such limitations
Table 3. Correlation matrix for mean flavor characterisctics and mean chemical components of five tomato
cultivars harvested at two ripeness stages.
Variable
Sourness
Tomato-like
Overall Intensity
PH
Titratable Acids
Reducing Sugars
Soluble Solids
Sensory Parameters
sweetness
sourness
tomato-like
overall intensity
0.13
0.60**
0.46**
a.10
0.14**
0.38**
0.44**
0.19
0.62**
-0.41**
0.52**
0.10
0.12
0.70**
-0.10
0.33*
0.58**
0.45**
Xl.34**
0.61**
0.44**
0.50**
* and **indicate significant difference at the 5% and 1% level, respectively.
852
Euphytica
32 (1983)
TOMATO
FLAVOR
IMPROVEMENT
have focused on identifying fruit constituents which have a major impact on flavor
and evaluating the effects of genotypic variation in these components. Objective evaluation of key components combined with subjective evaluations by sensory panels may
identify the components most responsible for flavor variation. Fruit composition may
then serve as useful objective selection criteria within a breeding program to improve
flavor attributes. The results presented here clearly underscore the importance of variation in sugar and acid content to genotypic flavor differences. While in agreement
with previous studies reporting that sugars and acids are important to tomato flavor
(DAVIES & WINSOR,
1969; STEVENS et al., 1979; DE BRUYN et al., 1971; SIMANDLE
et
al., 1966; LOWER & THOMPSON, 1967), this is the first study to assess quantitatively
the impact of significant increases in soluble solids content of commercially acceptable
tomato types on flavor qualilty. Changing the genetic determinants which condition
sugars assimilation resulted in dramatic improvements in tomato flavor quality. Not
only was sweetness improved but also, overall-flavor intensity and tomato-like flavor.
Previous reports evaluating tomato flavor quality have utilized a multiple regression
model in which tomato-like was the dependent variable and sweetness, sourness, overall intensity and all chemical components were independent variables. Such a model
assumes that sweetness, sourness, overall intensity and the chemical parameters are
all components of tomato-like flavor. This interpretation would appear reasonable,
but depends upon independence of the contributing variables. Examination of the
correlation matrix clearly indicates that this assumption is invalid (Table 1). A pattern
of correlation is clearly evident among the sensory variables. A more complex model
is needed to evaluate relationships between a group of correlated sensory variables
and a group of variables that partially represent fruit composition. This study therefore
employed multivariate analysis of multiple covariance. Fruit composition variables
were considered independent or predictor variables, while all four sensory variables
were considered to be dependent on fruit composition. This approach allows simultaneous evaluation of the sensory variables and may more closely describe the complexities of taste.
Soluble solids content and titratable acidity were selected as covariates in a model
to test whether they can adequately describe or predict flavor attributes. The covariance model accounted for 59% of the variance for sweetness, indicating that inclusion
of soluble solids content as a covariate was a major contributor to this flavor characteristic. Similarly the covariate model accounted for a significant portion (41%) of the
variance for sourness. It is important to note that inclusion of reducing sugars, pH,
and total acidity as additional covariates, failed to further increase R2 values, suggesting that they are either not important or are redundant. The low adjusted R2 values
suggest that variables other than those measured may also contribute significantly
to flavor. However, that soluble solids content and titratable acidity may serve as
relatively simple yet effective screening criteria is evident. Applied to segregating lines,
determination of these chemical parameters should help to identify potential progeny
with improved flavor attributes. Selection for horticultural traits may further reduce
the sample size to a manageable level for sensory analysis.
Picking tomatoes at the MG stage significantly alters their sugar and acid content
when ripened to table-ripe. These changes contribute significantly to variation in flavor
quality compared to fruits harvested table-ripe. Fruits left to ripen on the vine have
Euphytica
32 (1983)
853
R. A. JONES AND
S. J. SCOTT
higher sugar content and are sweeter than fruits picked at the MG stage and allowed
to ripen off the vine. This observation is consistent with previous studies indicating
that fruits picked at earlier stages of ripeness are lower in soluble solids and reducing
sugars than fruit ripened on the vine (DAVIES, 1966; HALL, 1968; KADER et al., 1977).
However, it appears that cultivars differ in TR fruit composition compared between
fruits picked at the 2 stages of ripeness. Previous work has indicated that some cultivars
show significant differences in fruit composition between room ripened and field-ripened tomatoes (KADER et al., 1977), whereas others demonstrate little or no change
(BISOGNI
et al., 1976). Many of the changes in chemical compostion which condition
improved tomato flavor are initiated with the onset of ripening (first color) and tend
to be cumulative throughout continued color development of fruits left on the vine
(SALUNKHE et al., 1974; HOBSON & DAVIES, 1971). General&, fruits removed from
the plant at earlier stages of ripeness do not allow these changes to reach their full
potential. Our data clearly indicate that picking fruits at the mature green stage of
ripeness adversely affects their TR flavor in a strong negative fashion relative to fruits
harvested when table-ripe.
In comparison with TR fruits of ‘Cal Ace’, the high solids lines exhibited significantly greater overall flavor intensity whether they were picked MG or TR. Even though
fruits of the high solids lines picked MG had approximately 1% less sugars or soluble
solids than their respective TR fruits, the MG levels were considerably greater than
those found in fruits of ‘Cal Ace’ allowed to ripen on the vine. While ‘tomato-like’
flavor is a complex characteristic, there is little doubt that genetically altering the sugars content in the TR fruits, as in the high solids lines presently studied, also dramatically improves ‘tomato-like’ flavor in comparison with TR fruits of ‘Cal Ace’. Important in this regard are MG fruits of lines HS70 and HS89, which were judged superior
to and HS93 which were judged equivalent in ‘tomato-like’ flavor to vine-ripened fruits
of ‘Cal Ace’. Sugar content is important to tomato flavor; harvesting and handling
techniques that reduce levels of this component have an adverse effect on flavor
(KADER et al., 1977). The present study indicates that the problem of reduced tomato
quality can be largely overcome by genetically increasing sugars and acids. The significant improvement of tomato flavor quality indicates that tomato breeders interested
in high edible quality should undertake the breeding of cultivars with higher levels
of soluble solids and acids.
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