North Carolina State University Study
The Search for a Better Tasting Tomato
With a growing number of consumers turning to cherry, grape and other specialty tomatoes in search of better taste, researchers are stepping up their efforts to improve the flavor of standard field- and greenhouse-grown varieties.
The Tomato Magazine
August 2005
By C.D. Harlow, M.M. Peet and E.S. Larrea, all with the Department of Horticultural
Science at North Carolina State University, Raleigh, N.D., have published a
study, “Modifying Fruit Quality in Greenhouse Tomato Cultivars with NaCl
Additions” presented during Plasticulture 05, held March 5-8 in Charleston,
S.C., sponsored by the American Society for Plasticulture.
The Search
Research was initiated in the fall of 2002 and repeated in 2003 to identify
greenhouse tomato cultivars and fertilization practices which would result
in superior fruit quality under Southeastern United States winter/spring
conditions, the group reported. A total of nine Lycopersicon esculentum
Mill. Cv. Cultivars were grown, with three cultivars (Trust, Elegance and
67) grown both years. Momotaro, S-630 and Diana were grown in 2002 only
and Calico, NC02127 and Octavio grown only in 2003. The beefsteak cultivar,
Trust, is widely grown in North America, especially in the Southeastern
U.S., and is generally considered to have good flavor and consumer appeal
compared to other beefsteak types. Flavor is especially important to direct
marketers, such as small greenhouse tomato growers. They were interested
in whether other cultivars, particularly the smaller cluster types, or
altered fertilization practices could improve flavor. In 2002-2003, fertilizer
concentration was raised to improve fruit quality, but in 2003-2004, fertilizer
concentration was kept the same but table salt (sodium chloride) was added.
Taste tests revealed differences among cultivars, with 67 and Elegance scoring
significantly higher than Trust both years. Adding salt or using more concentrated
fertilizer raised taste panel scores in some cultivars, but the effect was
not consistent. Highest total fruit production was seen in Calico, a large-fruited
cultivar and Elegance, a cluster type. Fruit weight was significantly lower
for both the higher fertilization (21 percent reduction) and the NaCl fertilization
(14 percent reduction) treatments.
After each harvest, fruit were frozen for later determination of Brix (a measurement
of soluble sugars), EC (a measurement of fruit saltiness) and total acidity.
Significant differences in Brix were seen between the standard and the elevated
or NaCl fertilizer treatments, with Brix in the elevated or NaCl treatments
higher at all harvest periods except the first, Harlow, Peet and Larrea noted.
Differences in Brix were seen among the cultivars, with NC02127 significantly
higher than all other cultivars in 2003 and Elegance and 67 significantly higher
than all remaining cultivars including Trust, which ranked lowest both years.
The remaining cultivars were significantly higher than Trust, but lower than
NC02127, 67 and Elegance, except for Momotaro in 2002, which was not significantly
different from Elegance. Cultivar differences in Brix were highly correlated
with taste panel ratings in 2002, but not 2003, when the cultivar with highest
Brix (NCO2127) was lowest rated by taste panels.
Significant differences in total acidity were seen between the standard and
NaCl fertilization treatments, with total acidity in the NaCl treatment higher
at all harvest periods except the first, they reported. Differences in total
acidity were seen among cultivars, with Elegance and 67 significantly higher
than all other cultivars. The cultivar with the highest Brix (NC02127) was
lowest in acidity, but Trust was also low. Cultivar differences in total acidity
and flavor ratings from taste tests were highly correlated. Total acidity was
not measured in 2002. Fruit pH was not significantly different between fertilization
treatments in 2002.
Not Been a High Priority
In contemporary breeding programs for field-grown tomatoes, Lycopersicon esculentum
Mill. traits such as yield, disease resistance, resistance to fruit compression
and long shelf life have been emphasized, while taste has been an incidental
and often ignored component, the research team emphasized. This is partly
because flavor is a complex and not well-understood characteristic with
important environmental, postharvest and cultural components as well as
genetic components. Flavor is not easy to measure, either in terms of chemical
constituents of the fruit or collecting meaningful sensory evaluations
from taste panels. Much of the distinctive ‘tomato’ flavor
comes from volatile organic compounds, but the amount of soluble sugars
(glucose and fructose) and the amount of organic acids are also important
components, as is their ratio. Both components should be high to produce
a tomato with excellent flavor. Finally the physical sensations experienced
while eating the tomato are important: firmness, ‘mouth feel’,
skin toughness, presence of a hard core, and ratio of wall and gel. In
all these characteristics, individuals have unique preferences.
“As a result, consumers are often disappointed in the flavor of tomatoes,
even those sold as ‘vine-ripe,’” the team said in their written
report. “Although breeding programs tomatoes are now beginning to address
this issue, consumers have been turning to cherry, grape, roma and greenhouse
tomatoes for better taste.”
According to a recent USDA ERS study (Cook and Calvin, 2005), between the early
1990s and 2003, the North American greenhouse tomato area is estimated to have
grown 600 percent to 1,726 ha. Currently, 37 percent of fresh tomatoes sold
in U.S. retail channels are greenhouse grown. U.S. consumers also have an increased
interest in produce variety, and development of new types of tomatoes, such
as the tomato on the vine (TOV), is faster in the greenhouse than in the field.
The first producers of a popular new tomato product can garner substantial
profits, at least for a few years (Cook and Calvin, 2005).
Identification of cultural practices and cultivars giving superior taste is
the first step in meeting consumer demand, Harlow, Peet and Larrea said. As
demonstrated in other research, adjusting nutrient solution salinity allows
growers to improve fruit quality by modifying water availability. Under high
EC, fruit size is inversely related to EC, while the dry matter content of
the fruit is positively related to EC. The exact rate of yield decline depends
on cultivar, environment, nutrient solution and crop management.
Salinities higher than 2.3-5.1 mS.cm-1 improve tomato fruit quality. Other
research has also shown that marketable fresh-yield production efficiency decreased
by 5.1 percent for each dS m-1 in excess of 2 dS m-1, the researchers noted.
The number of harvested fruits was not affected; yield loss resulted from reduced
fruit weight (3.8 percent per dS m-1) and an increased fraction of unmarketable
fruit.
The object of the present study was to compare yield, fruit quality and taste
of large and small-fruited Lycopersicon esculentum Mill. cultivars grown under
standard fertilization, elevated fertilization or standard fertilization amended
with sodium chloride (NaCl), the team explained.
Methods and Materials
The researchers conducted their studies in project greenhouses at the North
Carolina State University Horticultural Field Laboratory in Raleigh, N.C.
Lycopersicon esculentum Mill. seeds were sown in September of 2002 and
2003. Then were then transplanted (two per Bato® bucket containing
perlite) in November 2002 and 2003.
Treatment design was a split-plot with fertilizer treatment as the main-plot
factor and cultivar as the sub-plot factor. Fertilizer treatments were standard
(modified Steiner solution (EC ˜ 1.90 dSm-1) or standard plus the addition
of a 35mM NaCl solution (EC˜4.65 dSm-1). A total of nine Lycopersicon
esculentum Mill. Cv. Cultivars were grown, with three cultivars (Trust, Elegance
and 67) grown both years. Momotaro, S-630 and Diana were grown in 2002 only
and Calico, NC02127 and Octavio were grown only in 2003. Experimental design
was a randomized complete block with six replications of 24 plants each. All
fertilization treatments were monitored, recorded and controlled by the Harrow
Fertigation Manager® (HFM, Climate Control Systems Inc., Leamington, Ontario)
and applied using two 3.78 l h-1 drip-type emitters per pot. Fertigation treatments
were initiated on Dec. 16, 2002, and Jan. 3, 2004, for the 2002 and 2003 experiments,
respectively. Water management was based on solar set points. All plants were
trellised and hand pollinated.
Harvest began on Jan. 30, 2003, and Feb. 9, 2004, respectively, and continued
twice weekly until April 28, 2003, and May 24, 2004, the researchers detailed.
At each harvest, fruit number and fruit weight per plant were recorded. All
fruit were bagged by plant and frozen for later analysis. At that time, the
samples of fruit from each treatment were homogenized and Brix, fruit pH (2002
only) and EC and total acidity (2003 only) values recorded. Data were analyzed
using General Linear Models (PROC GLM) with the Waller-Duncan K-ratio T-test
to separate means (SAS, Cary, NC). Taste panels were held March 14 and 24 and
April 28 for the 2002 study and May 11 for the 2003 study. Panelists were asked
to rate sliced fruit on a 1-5 scale for overall excellence, flavor and acceptability,
Results/Discussion
“Total yield per plant differed significantly between fertilizer treatments
(Fig. 1) for all fruit harvested in both years,” Harlow, Peet and Larrea
acknowledged. “In 2004, yields in the salt treatment were 14 percent lower
due to significantly different fruit size (p=0.0001, F=130.93), with fruit from
standard fertilization averaging 89.0 g/fruit, while fruit from the NaCl treatments
averaged 71.1 g/fruit. No significant difference in fruit number was seen between
fertilizer treatments. Similar results were seen in 2002, but the yield reduction
was greater (21 percent). Significant differences in total yield per plant were
seen between cultivars (Fig. 2) (p=0.0001, F=32.47), with Elegance and Calico
producing the highest yield per plant over the entire 2003 season. Elegance was
also the highest yielding cultivar in 2002, along with Momotaro. Momotaro, although
popular with taste panels, had a high incidence of cracking and was not included
in the 2003 study. Yields of Trust were intermediate in both years, with significantly
lower yields in 67 both years and NC02127 in 2003.
There were also consistent differences between cultivars in taste panel rankings
over the two years. In 2003 (Fig. 3) (p=0.0001, F=6.89), Elegance and 67 scored
higher in overall acceptance over all other cultivars, with Trust in the middle
and Octavio, Calico and NC02127 perceived as least acceptable. Taste panel
rankings of cultivars were similar in the 2002 study except that Trust was
rated in the lowest group by panelists. No difference between fertilizer or
salt treatments was seen in overall acceptance ratings in either year, although
in some cases in 2003, individual cultivars, such as Trust and Octavio were
rated higher in the salt treatment. Panelists identified similar trends for
flavor, acceptability and excellence.”
Both the high fertilizer and the salt treatments increased fruit quality as
measured by soluble sugars (Brix), but the overall increase, although significant
for all harvest periods, was only 5.4 percent in 2002. In 2003, Brix values
were higher overall than in 2002, probably because of better irradiance. For
all harvest periods except the first, processed fruit samples from the NaCl
fertilization treatments had significantly higher mean Brix than standard fertilization
fruit (Fig. 4) (p=0.0065, F=20.06),” and overall the increase was 12
percent, the researchers said. Additionally, Brix for both fertilizer concentrations
increased over time both years. These results could be explained by treatment
effects or by increased irradiance as the experiment continued into spring
in the case of controls. Significant differences between cultivars were seen
in mean Brix (Fig. 5) (p=0.0001, F=90.74).
In 2003, Brix was highest in NC02127, with 67 and Elegance higher than the
remaining cultivars, the three pointed out. All specialty cultivars except
Calico had higher Brix than Trust. Similarly in 2002, highest Brix was measured
in 67 with Elegance and Momotaro in the next highest group. Trust had the lowest
values of any of the cultivars grown. Significant differences between cultivars
were seen in relative total acidity of processed fruit samples (Fig. 6) (p=0.0001,
F=8.34), with Elegance and 67 higher than all other cultivars in 2003. In 2002,
total acidity was not measured and fertilization treatments did not affect
fruit pH significantly. A significant positive correlation was seen in 2003
between mean relative total acidity in the six cultivars and overall acceptance
(R2=0.946), supporting previous studies linking acid to taste in tomatoes.
In 2002, taste panel results were significantly and positively correlated with
cultivar Brix.
So, What Does the Study Show?
The addition of either higher fertilizer levels or NaCl to fertilizer solutions
decreased yield while increasing Brix, but taste panels in the study did
not consistently rate the NaCl or fertilizer treatments higher, the team
concluded, although for individual cultivars (Octavio and Trust in 2003),
rating differences between treatments were significant. Panelist acceptance
ratings of fruit were correlated with cultivar total acidity readings in
2003 and cultivar Brix in 2002. Lack of correlation of fertilizer treatments
with higher panelist evaluations may have been a result of variability
in panel make-up or seasonal changes. However, for the three cultivars
included in both the 2002 and 2003 trials, panel ranking was consistent
(Elegance=67>Trust).
“These studies indicate the potential for specialty cultivars as improved-taste
replacements to traditional cultivars without a decrease in yields,” Harlow,
Peet and Larrea said. Compared to Trust, all the specialty cultivars had higher
Brix, and all except NC02127 had higher total acidity and higher taste panel
ratings in both years. Yields were significantly higher in Calico and Elegance
than Trust, and only 67 and NC02127 had significantly lower yields.
The study also points out the importance of fruit acidity in assessing flavor.
NC02127 had significantly lower acidity than other cultivars and had the lowest
acceptance ratings even though it had significantly higher Brix than any other
cultivar. If this cultivar is omitted from the correlation of taste panel ratings,
the correlation with cultivar Brix was significant in 2003 as well as 2002.
NC02127 is also smaller and was the only orange-fruited cultivar, which may
have also affected ratings, as taste panels viewed the sliced materials.
Of the two methods of increasing fruit quality, NaCl addition was preferable
to increasing fertilizer concentration. Not only was it cheaper, and resulted
in less potential for nutrient run-off, but the yield reduction was less (14
verses 21 percent) and the increase in Brix was greater (12 verses 5 percent).
Increases in total acidity from NaCl addition were also significant, but since
this trait was not measured in 2002, no comparison can be made between the
two types of treatments. Since taste panels did not consistently rank cultivars
given salt or extra fertilizer higher, it is difficult to recommend them to
growers. However, the salt treatment did improve the acceptability of Trust
to taste panels, and higher Brix could be valuable during periods of low light
early in the season, so it is something to be considered.
Editor’s Note: This material has been edited from a formal presentation, “Modifying Fruit Quality in Greenhouse Tomato Cultivars with NaCl Additions,” made during Plasticulture ’05, held March 5-8, 2005, in Charleston, S.C. Funding for the 2002-2003 study was provided by the NC Specialty Crops Program. Seed was supplied by the following: NC02127, Dr. R. Gardner, NCSU tomato breeder; 67 and Diana, Sakata; Elegance and Trust, De Ruiter; Momotaro, Takaii; Calico, Enza Zaden; and Octavio, Johnny’s Selected Seed. Photos courtesy of North Carolina State University.
© 2006 Columbia Publishing
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