Most people have tasted, or rather not tasted, hydroponically-grown tomatoes during the “off-season.” Dampened by the cold of winter, consumers are looking for that taste of warmer times only to be rewarded with a shell of the summer tomato.

Color, taste, firmness and selection are usually severally lacking. There is still much experimentation that is needed until we produce a great tomato in the winter, but research is truly a work in progress.

While breeding programs have brought us closer to a better-flavored tomato, the scope of this article will focus on the cultural practices that should enhance the palate response.

With that said, genetics should focus not only on storage qualities (shelf life, skin thickness, disease resistance for plants, etc.) but on promoting those factors associated with superior taste, which include but are not limited to soluble solids (measured by a refractometer in Brix, %), pH and organic acids. Now let’s dive into the chemicals and the factors influencing tomato flavor.

With plant genetics aside, let’s focus on certain compounds that impact flavor. Tomatoes are roughly 90% water with the solids representing 5% to 7% of the dry weight. Tomato fruit flavor is generally created from the ratio of carbohydrates (glucose, fructose, sucrose) to organic acids (citric and malic acid).

Generally, a good-flavored tomato possesses both a high amount of sugars and a high amount of acids. Likewise, bland tomatoes contain a high amount of sugars to a low amount of acids. As the tomato matures, the amount of sucrose (imparts a sweet flavor) decreases as starch (not so sweet) increases. Starch consists of many linked units of glucose acting as an energy storage reserve. From this reserve, the readily metabolizable glucose units can be cleaved off for the plant’s use.

When a traditional red tomato is changing from the orange to red color, the acidity drops, which increases the fruit’s pH. This has important ramifications to harvesting as tomatoes that are fully vine ripened may have a reduced blend of acid to sugar. However, the ripening process is ethylene dependent. This plant hormone gives rise to lycopene (antioxidant/carotenoid) production, the causative agent in red color change.

Therefore, the implication of early harvest may slightly reduce the rising pH, but will impact the flavor attributed to lycopene which is maximized during the deep red stage of ripening. In the book Tomato Plant Culture, it is stated that the recent genetic breeding programs of tomatoes have favored increased yields, shelf life and skin thickness with the negative side effect of an increase in fruit pH. Further stating, this is why so many consumers are shifting back to the heirloom varieties as the pH is generally lower than the recent hybrid cousins and therefore generally regarded as better tasting.

So now you may be asking, “How can I produce tomatoes that are high in both sugar and acid?” While some of these subsequent points below are not directly tied to flavor, they do promote healthier, more productive plants, which coupled with the right cultivar, should produce better tasting tomatoes.

Maintain ideal temperature range of 77°F daytime to 65°F nighttime

Ideal and constant temperatures will influence the biochemical reactions related to the water, carbohydrate and organic acid content of the ripened fruit. Proper growing temperatures will also improve yields by increasing pollination capability and decreasing fruit drop.

Expose plant to high/full light

Saturation of light by leaf receptors will maximize photosynthesis and subsequent carbohydrates that will be used for a myriad of biochemical reactions including fruit production.

Utilize a nutrient formula geared toward optimal tomato production

The nitrogen ratio should be derived from nitrates more than ammonia. In addition, keeping the ratio of potassium to nitrogen high will promote thicker skin that should reduce skin cracking.

Fertilization with a slightly higher EC at 3.5 dS/m

Ensuring a higher level of fertilizer is probably beneficial in a two-fold manner. In one regard, maintaining adequate levels of nutrients, like calcium, will not only improve plant vigor, but in the case of high cytosolic calcium, reduce the prevalence of blossom end rot. Secondly, the increased fertilizer (salt content) may contribute to the benefits of slight saline stress as detailed below.

Addition of sodium chloride at 35 ppm

Research studies have shown a correlation between slight salt stress and higher accumulation of soluble solids in particular sugars, organic acids and amino acids. At first one might be inclined to think this is merely a function of salt removing water from the fruit and thereby concentrating the sugar, organic acids, etc.

However, one study showed that during initial fruit development, salt stress caused a doubling of starch in the fruit that subsequently increased the soluble sugars in the ripened fruit. It appears that slight saline stress at the time of initial fruit development may be advantageous, but constant saline stress likely would lead to decreased vigor of the plant and decreased yield and size of the fruit.

During cultivation, avoid periods of drought stress followed by excess watering

Drought stress has been linked to fruit cracking and blossom end rot (BER). Proper irrigation will go a long way in reducing both.

Store ripening tomatoes in warm temperatures

Temperatures below 400°F will cease enzymatic activity necessary for maturity.

In conclusion, both genetics and cultural practices play a pivotal role in the overall taste of the tomato fruit. In general the smaller cherry and grape tomatoes have a higher total soluble solids (Brix %) relative to the larger ones like beefsteak.

Experimentation by growers with new and emerging cultivars coupled with ideal cultural practices will continue to improve the tomato flavor closer to its summer relative. For more information on growing tomatoes in the greenhouse or field, refer to the book Tomato Plant Culture in the literature cited.

References

  1. Jones Jr., J. B. Tomato Plant Culture: In the Field, Greenhouse, and Home Garden 2nd Edition. CRC Press, 2008.
  2. Peet, M. M.(Ed.) Sustainable Practices for Vegetable Production in the South. Focus Publishing, Newburyport, MA, 1996.
  3. Helyes, L., Dimeny, J., Pek, Z., and A. Lugasi. 2006. “Effect of Maturity Stage on Content, Color, and Quality of Tomato Fruit.” International Journal of Horticultural Science. 12(1).
  4. Helyes, L., and Z. Pek.2006. “Tomato fruit Quality and Content Depend on Stage of Maturity.”HortScience. 41(6).
  5. Fraser, P.D., Enfissi, E. M.A., Halket, J. M., Trusdale, M.R., Yu, D., Gerrish, C., and P. M. Bramley. 2007. “Manipulation of Phytoene Levels in Tomato Fruit: Effects on Isoprenoids, Plastids, and Intermediary Metabolism.” The Plant Cell. 19:3194-3211.
  6. Morgan, L. 1997. “Organic Fertilizers for Hydroponics.” Growing Edge, 9(2):32-39.
  7. Yin, YG, Kobayashi, Y., Sanuki, A., Kondo, S., Fukuda., Ezura, H., Sugaya, S., and C. Matsukura. 2010. “Salinity Induces Carbohydrate Accumulation and sugar-regulated Starch Biosynthetic Genes in Tomato Fruits in an ABA-and osmotic stress-independent Manner.” Journal of Experimental Biology. 61:563-574.