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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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